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by Copyright Donna Louise Cathro 2002 Three-Dimensional Stratal Development of a CarbonateSiliciclastic Sedimentary Regime, Northern Carnarvon Basin, Northwest Australia by Donna Louise Cathro, B.Sc. (Hons.) Dissertation Presented to the Faculty of the Graduate School of The University of Texas at Austin in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy The University of Texas at Austin August, 2002 UMI Number: 3108481 ________________________________________________________ UMI Microform 3108481 Copyright 2004 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ____________________________________________________________ ProQuest Information and Learning Company 300 North Zeeb Road PO Box 1346 Ann Arbor, MI 48106-1346 Dedication This dissertation is dedicated to Uncle Murray, who first sparked my interested in geology, and to Graham Moss, who kept me going. Acknowledgements The road to a Ph.D. is a long one that requires stamina and enthusiasm not only from the candidate but also from their committee, family and friends. I have been fortunate during my graduate career at UT to receive continued support from all these avenues. First, the main influence in shaping this dissertation from start to finish has been Jamie Austin. Jamie is an energetic, enthusiastic supervisor that has a keen eye for detail, and is an excellent cheerleader. He also taught me to step back and look at the work as a whole, thank you. I am also indebted to Bill Fisher, Craig Fulthorpe, Garry Karner, Bill Galloway and Dick Buffler who have provided valuable insight during numerous discussions. Garry has been involved in this project since the very beginning and I would like to thank him and Lois for making my short visit to NY both enjoyable and successful. In addition, I have interacted with a number of professors and researchers at the UT Department of Geological Sciences and Institute for Geophysics during the course of this research. They include; Sharon Mosher, Larry Lawver, Hilary Olson, Ian Dalziel, Jack Sharp, Paul Mann, Tom Shipley, John Goff, Gail Christeson and Dennis Trombatore. This project would never have been possible without the technical support of Mark Wiederspahn, Kevin Johnson and Steffen Saustrup. They cheerfully handled my many questions and unique difficulties I encountered on the way. Geoscience Australia (formerly AGSO Geoscience Australia) generously provided the primary financial support for this project. This support was supplemented by the American Chemical Society-Petroleum Research Fund, the American Association of University Women-International Fellowship, the UTIG Ewing-Worzel Fellowship and the Geology Foundation. Geoscience Australia and Woodside Australian Energy supplied the seismic and well data investigated in this research. I have many mentors and friends at Geoscience Australia who v have provided me with geological and technical assistance. In particular, I am grateful to my primary mentor, Marita Bradshaw. Chris Pigram, Trevor Powell, Jane Blevin, Heike Struckmeyer, Lachie Hatch and Mark Webster have all helped me along the way and made it possible for me to stay to complete my studies. Ken Heighway and Janene Broere handled the very important logistical issues associated with my stay in the US. Last but not least are my friends and family. I would like to thank Karen Romine who first encouraged me to take on a Ph.D., and to look further afield than Australian universities. I am grateful to my friends Laurie, Bob and Sylvia have all shared the experience in the close quarters of our small office. Laurie, thanks for the many massage vouchers and margueritas. Dan, Ingo, Kathy (Morgan and Elena), Hongbo, Rob, Imtiaz, Tip, Marcy, Drew, Martha, Amanda, Rene, Susie and the Calibre soccer team, have seen me through the inevitable ups and downs with liquid refreshments, a soccer game or a dog walk. Cori, Brook and Kara have always been there for me, along with Keith, Deanna and Barbara. I wish all these friends the best with their personal endeavors and hope to see them in the southern hemisphere soon. Back home, I have received much encouragement from dad, Tischa, Graham A., Val, Stan, Jamie, Jules, Annie, Jane and Monique. Thanks for the constant supply of chocolate! Above all, I am grateful to mum and Graham M. for their unwavering support and faith in my ability at all times. vi Three-Dimensional Stratal Development of a Carbonate-Siliciclastic Sedimentary Regime, Northern Carnarvon Basin, Northwest Australia Publication No._____________ Donna Louise Cathro, Ph.D. The University of Texas at Austin, 2002 Supervisors: William L. Fisher and James A. Austin, Jr. Detailed stratigraphic interpretation of continental margin clinoforms is a necessary first step in understanding the link between this complex stratal architecture and large-scale processes resulting in progradation. Maps derived from a 3D seismic volume (40-55 Hz) nested within a regional 2D multichannel seismic grid (25-35 Hz), and tied to nine hydrocarbon exploration wells, show the detailed morphological evolution of five prograding clinoformal sequences in the Northern Carnarvon Basin (NCB), northwest Australia. Depocenters concentrated along northeast-southwest oriented, linear clinoform fronts are governed by latitudinal variations in sediment productivity. Fronts change from smooth to highly dissected, with intense gullying apparent after the mid Miocene optimum. Bottomsets remain relatively sediment-starved without the development of aprons on the lower slope and basin. Small-scale variability suggests heterogeneous sediment dispersal through these slope conduits. Along-strike sediment transport superimposed on northwest-oriented vii progradation changes from south-directed in the late Oligocene to north-directed in the late mid-Miocene suggesting a reorganization of circulation in the southeastern Indian Ocean. Prominent seismic discontinuity surfaces represent intervals of shallow paleo-water depth and flooding of the shelf. Exposure surfaces are subordinate. Rather than build to sea-level, progradation occurs with shelf paleo-water depths of 20-200 m. Therefore, onlap onto the clinoform front is not coastal and the sensitivity of the clinoforms to sea-level changes is muted. However, in the midMiocene, partial exposure of the shelf and development of karst topography indicate paleo-water depth falls of 60-180 m across two sub-sequence boundaries. The sequence stratigraphic framework is combined with a twodimensional, forward kinematic and flexural model for deformation of the lithosphere to determine the distribution, magnitude and history of CretaceousTertiary compression-induced inversion across the Dampier Sub-basin in the NCB. Inversion simultaneously creates and destroys accommodation space at different wavelengths superimposed on long wavelength subsidence and eustatic variations that impact the entire margin. Inversion anticlines focused along earlier rift fault systems are small, temporally and spatially variable relative to the total accommodation space created in the sub-basins during rifting and thermal subsidence. Santonian inversion represents ~4 km shortening, whereas the four modeled events in the Miocene each represent ~200-400 m of shortening across the modeled section. viii Table of Contents List of Tables........................................................................................................xiii List of Figures ...................................................................................................... xiv List of Supplemental Data.......................................................... ..........................xx Chapter One: Introduction and Geological Background.................................. 1 1.1 INTRODUCTION.................................................................................... 1 1.2 HETEROZOAN CARBONATE SHELVES ......................................... 10 1.3 GEOLOGIC SETTING.......................................................................... 15 Chapter Two: Data and Methodology............................................................... 31 2.1 DATA..................................................................................................... 31 2.1.1 Seismic Data............................................................................... 31 2.1.2 Wells........................................................................................... 35 2.2 METHODOLOGY................................................................................. 40 2.2.1 Seismic Analysis ........................................................................ 43 2.2.2 Well Information Lithology and Chronostratigraphy ............. 51 2.2.3 Synthetic Seismograms at well-sites .......................................... 54 2.2.4 Kinematic and Flexural Forward Modeling .............................. 72 Chapter Three: Clinoform progradation along a deeply submerged Oligocene-Miocene heterozoan carbonate shelf: Implications for sensitivity to sea-level variations. .............................................................. 82 3.1 INTRODUCTION.................................................................................. 82 3.2 GEOLOGIC SETTING......................................................................... 89 3.3 METHODOLOGY................................................................................ 95 3.4 RESULTS............................................................................................ 101 3.4.1 NCB Structural Trends............................................................ 103 3.4.2 NCB Clinoform Morphologies................................................ 105 3.4.3 Distribution of Depocenters .................................................... 119 ix Regional variability.................................................................. 119 Variability within the 3D volume ............................................. 122 Summary................................................................................... 124 3.4.4 Comparison of clinoform morphology and paleobathymetry . 129 Shelf (platform and ramp) landward of breakpoints ................ 133 Clinoform fronts seaward of breakpoints................................. 137 Seaward of clinoform toes-of-slope ......................................... 137 3.5 DISCUSSION ..................................................................................... 138 3.5.1 Sediment productivity as a primary control on clinoform distribution ............................................................................... 138 3.5.2 Reorganization of southeastern Indian Ocean circulation in the late middle Miocene ........................................................... 140 3.5.3 Downslope erosion and headward failure: mechanisms of submarine canyon formation on clinoform fronts.................... 142 3.5.4 Are clinoforms sensitive to sea-level changes?....................... 147 3.6 CONCLUSIONS ................................................................................. 156 Chapter Four: An early mid-Miocene, strike parallel shelfal trough and possible karstification in the Northern Carnarvon Basin .................... 160 4.1. INTRODUCTION.............................................................................. 160 4.2. GEOLOGIC SETTING...................................................................... 168 4.3 DATA.................................................................................................. 170 4.4 RESULTS............................................................................................ 171 4.4 DISCUSSION ..................................................................................... 176 4.5 CONCLUSIONS ................................................................................. 181 Chapter Five: Cretaceous-Tertiary inversion history of the Dampier Subbasin: Insights from quantitative basin modeling................................ 184 5.1. INTRODUCTION.............................................................................. 184 5.2. GEOLOGIC BACKGROUND ........................................................... 193 5.2.1 Tectonic Framework ................................................................ 193 5.2.2 Stratigraphic Framework.......................................................... 197 x 5.2.3 Previous Work: Pre-Tithonian stratigraphic modeling ............ 199 5.3. METHODOLOGY.............................................................................. 200 5.3.1 Seismic Interpretation .............................................................. 201 5.3.2 Biostratigraphic Data................................................................ 203 5.3.3 Tectonic induced accommodation: the thermo-mechanical model ........................................................................................ 203 5.3.4 The modeling approach ............................................................ 211 5.4. RESULTS: CRETACEOUS-TERTIARY TECTONIC AND STRATIGRAPHIC EVOLUTION ................................................... 225 5.4.1 Paleotopography at the end of Tithonian-Valanginian rifting; 135 Ma...................................................................................... 225 Seismic Observations ............................................................... 226 Modeling Results ...................................................................... 227 5.4.2 Passive margin formation Late Cretaceous inversion .......... 229 Seismic Observations ............................................................... 229 Modeling Results ...................................................................... 230 5.4.3 Passive margin formation Late Tertiary Inversion................ 231 Seismic Observations ............................................................... 231 Modeling Results ...................................................................... 233 5.4.4 Post-depositional modifications to clinoform morphology..... 241 5.5. DISCUSSION ..................................................................................... 244 5.5.1 Intraplate deformation of the Indo-Australian plate in the Santonian.................................................................................. 244 5.5.2 Collision as a mechanism for Miocene inversion?................... 246 5.5.3 Long- and short-wavelength modifiers to the stratigraphic response to regional thermal subsidence and eustasy. ............. 248 5.6. CONCLUSIONS ................................................................................ 251 Chapter Six: Implications................................................................................ 254 6.1 INTRODUCTION................................................................................ 254 6.2. GENERAL IMPLICATIONS ............................................................. 255 xi 6.3. IMPLICATIONS SPECIFIC TO THE NCB ...................................... 259 Appendix 1: Sequence biostratigraphy of the late Oligocene-early Miocene Mandu, Tulki and Trealla Limestone, Rankin Trend, western boundary of Northern Carnarvon Basin, West Australia, by Graham Moss............................................................................................ 262 Appendix 2: Samples collected for micropaleontological analysis ............... 360 Appendix 3: Checkshot surveys from well-completion reports .................... 366 Appendix 4: Preliminary research report into Neogene sediments of the North West Shelf and associated ostracod faunas................................. 371 Appendix 5: Synthetic seismograms ................................................................ 413 Appendix 6: Porosity vs. depth reports ........................................................... 422 Bibliography ...................................................................................................... 429 Vita ................................................................................................................... 455 Supplemental data............................................................................................. 456 xii List of Tables Table 1.1. Differences between siliciclastic, photozoan, and heterozoan carbonates......................................................................................... 12 Table 2.1. Acquisition parameters for the five 3D seismic volumes ................... 33 Table 2.2. Seismic acquisition parameters for the Geoscience Australia 2D ..... 36 Table 2.3. Well ties and age estimates for seismically defined major downlap surfaces............................................................................................. 56 Table 2.4. Bathymetric zones . ............................................................................. 57 Table 2.5. Typical mineral and matrix density and sonic velocity values. .......... 59 Table 2.6. Stretch/squeeze values used to enhance ties between synthetic seismograms and seismic data at each well. .................................... 70 Table 2.7. Assumptions, parameters and sources of error for calculating velocity from seismic, check shots and sonic data. ......................... 71 Table 3.1. Summary of seismic observations defining the five downlapping sequences and 19 sub-sequences. ................................................... 97 Table 3.2. Description of the five major sequences. . ......................................... 98 Table 5.1. Modeling parameters for the Dampier Sub-basin ............................. 217 Table 5.2. Modeled time-lines and their significance.. ...................................... 223 Table 5.3. Comparison between modeled foreset declivity and actual declivities observed on depth-converted 101r_09.......................... 236 xiii List of Figures Figure 1.1. Factors affecting the creation and destruction of accommodation space. .................................................................................................. 2 Figure 1.2. Seismic location map, Northern Carnarvon Basin. .......................... 3 Figure 1.3. The clinoform geometry. ...................................................................... 6 Figure 1.4. Worldwide distribution of clinoforms in the Neogene as imaged on seismic data. .................................................................................. 7 Figure 1.5. The Exxon model................................................................................. 8 Figure 1.6. Generalized cross-section of a cool-water, heterozoan carbonate shelf. ................................................................................................. 13 Figure 1.7. Reciprocal sedimentation in a mixed carbonate/siliciclastic environment...................................................................................... 16 Figure 1.8. Modern topography and bathymetry map of the North West Shelf (NWS), Australia and surrounding regions. ......................... 17 Figure 1.9. Proterozoic-Mesozoic structural trends superimposed on the modern physiography of the Northern Carnarvon Basin and adjoining onshore region.................................................................. 19 Figure 1.10. Generalized tectono-stratigraphic summary of the Northern Carnarvon Basin. .............................................................................. 22 Figure 1.11. Breakup of eastern Gondwanaland and plate tectonic factors that affected development of the North West Shelf. .............................. 23 Figure 1.12. Plate reconstruction at 15 and 5 Ma................................................. 27 Figure 1.13. Late Miocene inversion in the Browse Basin. ................................. 28 xiv Figure 1.14. Inversion in the Northern Carnarvon Basin. .................................... 29 Figure 2.1. 3D seismic coverage on the Rankin Trend. ....................................... 32 Figure 2.2. Flow chart of processing sequence for composite 3D seismic volume.............................................................................................. 34 Figure 2.3. Flow chart of processing sequence for the 2D MCS. ........................ 37 Figure 2.4. Interval velocity profile through part of Geoscience Australia line 136_11. ............................................................................................. 38 Figure 2.5. Methodological overview. ................................................................. 41 Figure 2.6. Representative 2D/3D MCS profile in TWTT. ................................. 42 Figure 2.7. Definition of seismic boundaries from downlap discontinuities.. ..... 44 Figure 2.8. Schematic representation of different types of seismic volumes....... 45 Figure 2.9. A portion of the 3D amplitude volume used in this study. ................ 46 Figure 2.10. The same region shown in Fig. 2.9 through the variance volume. .. 47 Figure 2.11. Location of approximate dip- and strike-oriented traverses in 3D volume.............................................................................................. 48 Figure 2.12. Examples of multiples recognized in 3D MCS................................ 52 Figure 2.13. Comparison between seismic data and synthetic seismogram through incised slope........................................................................ 53 Figure 2.14. Chronostratigraphic framework for the Tertiary interval. ................ 55 Figure 2.15. Flow chart of generalized procedure to create synthetic seismograms. .................................................................................... 60 Figure 2.16. Procedure used to create synthetic seismograms for comparison with seismic data .............................................................................. 61 xv Figure 2.17. Extracted wavelets from the 3D seismic volume at Goodwyn 2, Goodwyn 4, Goodwyn 6 and Goodwyn 7........................................ 65 Figure 2.18. Extracted wavelets from the 3D seismic volume at Eastbrook 1. .. 66 Figure 2.19. Extracted wavelets from 2D survey 136 at Dampier 1, Goodwyn 3 and Goodwyn 7 ............................................................................. 68 Figure 2.20. Extracted wavelets from 2D survey 101r at Goodwyn 7................. 69 Figure 2.21. RMS velocity profiles for Geoscience Australia line 101r_09........ 74 Figure 2.22. Present-day and decompacted sediment thicknesses at each well ... 77 Figure 2.23. Predicted time line stratigraphy across the Dampier Sub-basin at the end of Tithonian-Valanginian rifting. ....................................... 79 Figure 3.1A. Data in the Northern Carnarvon Basin............................................. 83 Figure 3.1B. Location map, 3D MCS and wells, Dampier Sub-basin. ................ 84 Figure 3.2. Generalized tectonostratigraphy of the NCB................................... 90 Figure 3.3 Representative MCS 2D/3D profile in two-way travel time (TWTT) ............................................................................................ 93 Figure 3.4. Uninterpreted and interpreted 2D seismic line 101r_09 illustrating broad incision up-dip from the clinoforms..................................... 102 Figure 3.5. Uninterpreted and interpreted 3D seismic traverse illustrating a mound in sequence MM1............................................................... 104 Figure 3.6. Uninterpreted 2D MCS line 136_15 (TWTT) and line drawing interpretation of sequence OL1 ...................................................... 106 xvi Figure 3.7. Uninterpreted and interpreted 3D seismic traverse illustrating subsequences within downlapping sequences OM1, EMM1 and MM1............................................................................................... 108 Figure 3.8. Uninterpreted and interpreted portion of 2D MCS line 136_10 illustrating concave-upward upper slope on DLS4 ........................ 110 Figure 3.9. Uninterpreted and interpreted seismic section and horizon slice through variance volume illustrating karst morphology. ............... 112 Figure 3.10. Strike-oriented uninterpreted and interpreted 3D seismic traverse illustrating asymmetric incision on the shelf at sub-sequence boundary 4.1................................................................................... 114 Figure 3.11. Uninterpreted and interpreted seismic sections illustrating the division of sequence MM2 into sub-sequences.............................. 115 Figure 3.12. Uninterpreted 2D MCS seismic strike section 136_19 and line drawing interpretation illustrating northeast progradation of quartz-sand rich shelf-restricted packages within and above the MM2 sequence. .............................................................................. 118 Figure 3.13. Gridded and contoured TWTT isopach maps for each of the five major sequences derived from the 2D MCS coverage ................... 120 Figure 3.14. Distribution of sub-sequences depocenters, shelf-edges and termination patterns interpreted within the 3D volume. ................ 123 Figure 3.15. Horizon slices through a variance cube illustrating morphologic variations along clinoform fronts through time.............................. 126 xvii Figure 3.16. Horizontal time slice through the 3D volume at 1592 ms TWTT, highlighting along-strike basin incision. ........................................ 130 Figure 3.17. Comparison of seismic stratal architecture and results of paleobathymetric analyses at wells ................................................ 131 Figure 3.18. Present-day bathymetry of the northern slope of the Little Bahama Bank ................................................................................. 144 Figure 3.19. Asymmetric submarine canyon fill along the SE Brazilian margin............................................................................................. 145 Figure 3.20. Model for submarine canyon development . ................................. 148 Figure 4.1. Location of the Northern Carnarvon Basin, including location of karst morphology............................................................................ 161 Figure 4.2. Uninterpreted, interpreted and line drawing of a seismic traverse within the 3D volume, illustrating the pronounced clinoform succession....................................................................................... 163 Figure 4.3. Well information adjacent karst horizon. ........................................ 165 Figure 4.4. Shaded contour map of early mid-Miocene boundary 4.1............... 172 Figure 4.5. Schematic illustration of proposed karst development of the observed trough .............................................................................. 182 Figure 5.1. Location of available seismic data, Northern Carnarvon Basin ..... 186 Figure 5.2. Geoscience Australia seismic line 175_03 showing late Miocene inversion structure in the Browse Basin......................................... 188 Figure 5.3. Portion of Geoscience Australia line 136_15 in the NCB showing inversion in the Northern Carnarvon Basin.................................... 189 xviii Figure 5.4. Uninterpreted and interpreted depth section and modeled time-line stratigraphy of clinoform architecture across the Rankin Trend.... 190 Figure 5.5. Generalized tectonostratigraphy ...................................................... 194 Figure 5.6. Generalized lithospheric extension and compression model. ......... 204 Figure 5.7. Modeled superposition of border faults and inversion .................... 209 Figure 5.8. Modeled superposition of border faults and inversion. .................. 210 Figure 5.9. Predicted time-line stratigraphy across the Carnarvon Basin (Dampier Sub-basin) from the Tithonian-Valanginian to the Present ............................................................................................ 213 Figure 5.10. Schematic of predicted stratal geometries from inversion uplift ... 219 Figure 5.11. Part of depth converted Geoscience Australia line 101r_09 located west of the Rosemary Fault Zone ...................................... 221 Figure 5.12. Part of depth-converted Geoscience Australia line 101r_09 located over the Rankin Trend. ...................................................... 228 Figure 5.13. Distribution of the stretching factor along 101r_09 used as a proxy for Santonian and Miocene inversion. ................................. 232 Figure 5.14. Map view of inversion anticline as represented by horizon ON1.. 234 Figure 5.15. Comparison between assumed paleo-water depth profiles used in forward modeling ........................................................................... 237 Figure 5.16. Total decompacted subsidence from Permian to Present .............. 239 Figure 5.17. Profiles illustrating deformation of Oligocene-late Miocene clinoforms as predicted from the forward modeling...................... 242 xix Supplemental Data (oversized tables and plates requiring 36"-wide plotter) Plate 1: Composite well log Dampier 1 Plate 2: Composite well log Goodwyn 2 Plate 3: Composite well log Goodwyn 3 Plate 4: Composite well log Goodwyn 4 Plate 5: Composite well log Eastbrook 1 Plate 6: Composite well log Goodwyn 6 Plate 7: Composite well log Goodwyn 7 Rangechart Goodwyn 2 Rangechart Goodwyn 3 Rangechart Goodwyn 4 Rangechart Eastbrook 1 Rangechart Goodwyn 6 Rangechart Goodwyn 7 xx Chapter One: Introduction and Geological Background 1.1 INTRODUCTION Stratal geometry is a preserved record of base level variations that result from a complex interplay of factors that act in three dimensions, such as eustatic (global sea level) changes, tectonics, sediment supply, and climate (Fig. 1.1). These factors are difficult to isolate, as they occur on overlapping timescales and may generate similar sedimentary responses (Galloway, 1989). As a result, there has been a gradual shift in research emphasis from determining a global sea level curve to investigating factors known to be important for sediment distribution at a particular location or time (Christie-Blick and Driscoll, 1995). The main objectives of this research are to first establish the temporal and spatial variability within a clinoformal succession, then evaluate the paleo-environmental significance of the stratal architecture on the shelf and shelf-break before trying to relate these observations to changes in sea-level, sediment supply, tectonics and paleoceanography. To achieve this goal, the mixed heterozoan carbonate and siliciclastic, Paleogene-Neogene succession of the Dampier Sub-basin and Rankin Trend in the Northern Carnarvon Basin (NCB; Fig. 1.2), are examined in three dimensions. This interval, dominated by stacked clinoforms prograding to the northwest, is imaged on nested 2D and 3D seismic data and intersected by nine hydrocarbon exploration wells (Fig 1.2). Clinoform stratal architecture is readily identified on seismic profiles world wide on a number of time and geographic scales through geological history 1 Figure 1.1. Factors affecting the creation and destruction of accommodation space. A three-dimensional approach is necessary to study sedimentary sequences because the factors affecting their deposition and preservation (eustasy, tectonics and sediment supply) act on overlapping time and geographic scales, they interact with each other, and can produce the same sedimentary response. Modified after Galloway (1989). 2 Figure 1.2. (following page). Seismic location map, Northern Carnarvon Basin. (A) The Northern Carnarvon Basin (NCB), showing its four predominantly Mesozoic depocenters (after Romine et al., 1997; Stagg and Colwell, 1994). Up to 3200 line-km of 2D MCS are interpreted as part of this study (large box). Two hydrocarbon exploration wells northeast of the 3D seismic volume provide lithologic and chronostratigraphic information in this region. The onshore area, with a maximum elevation of ~1200 m in the Hammersley Ranges, is drained by intermittent rivers with maximum flow after cyclone events (Semeniuk, 1996), otherwise little is known about the riverine input to the NCB. (B) The 1500 km2 3D multichannel seismic (MCS) volume, the centerpiece of this study, is embedded within the 2D MCS control (A). Triassic-Aptian structures define the Rankin Trend, the western margin of the Dampier Sub-basin (adapted after Newman, 1994; Stagg and Colwell, 1994; Romine et al., 1997). Seven wells tied to the seismic, provide paleoenvironmental and timing constraints. Superposition of these data addresses the need, identified by previous studies elsewhere (e.g., Fulthorpe and Austin, 1998), for more laterally extensive, higher density seismic coverage, combined with well control, to understand fully along-strike variations in clinoform stratal geometry. 3 study area limit of 2D interpretation Be L1 Fig. 1.2B EH Da Ba Fig. 1.14 Fig. 2.21 Gr ey R Ex N o r te n R. A NWC 0 R. o scue R . HA MM ER RA SL NG ES EY 200 km 112 E B WELLS 115 30'E Fi g o 116 E 116 00'E S C YN LIN o 120 E E o 2 - Goodwyn 2 3 - Goodwyn 3 4 - Goodwyn 4 6 - Goodwyn 6 7 - Goodwyn 7 E1 - Eastbrook 1 D1 - Dampier 1 E1 7 SG ER TER RA C E RK PA N KE N KIN AN RB KG R MB PA R GB 4 6 3 2 W TRO UG H RE D 10 km RR R TE KE E AC Da D1 North West Shelf - Mesozoic depocenters: Exmouth, Barrow, Dampier and Beagle sub-basins - Triassic-Aptian structures Well locations - biostratigraphy, lithology from well completion reports (WCR) only - wells studied for paleobathymetry (Appendix 1) + biostratigraphy and lithology from WCR Seismic data - Geoscience Australia 2D MCS136 - other Geoscience Australia 2D MCS - Woodside Energy 3D MCS volume LEGEND Ex, Ba, Da, Be Blocks: Malus (MB), Rankin (RB), Goodwyn (GB), North Rankin (NRB) Grabens: Keast (KG), Searipple (SG) NWC - Tertiary inversion features (A) - North West Cape (A) 4 19 50'S E E IN EL AD M TR D EN o o .6 19 30'S .2 A RI TO C VI ND RE NRB T 22 S o 20 S o 18 S WELLS L1- Lambert 1 EH - Eaglehawk 1 o De Y u le F R Ro Ash bu be rt o (e.g., Bartek et al., 1991). The clinoform geometry, recognized by shallowly dipping topsets, inclined fronts, and near-horizontal bottomsets (Fig. 1.3) is particularly common on continental margins throughout the Neogene, occurring at a variety of latitudes (Fig. 1.4), tectonic regimes, and sediments (carbonate, siliciclastic and mixed). Due to their broad distribution and perceived sensitivity of topsets to base-level variations, clinoform geometries have been of particular significance in the development of seismic sequence stratigraphy, and specifically the two-dimensional sequence stratigraphic model (Fig. 1.5; Vail et al., 1977; 1991). Glacio-eustasy was emphasized from the outset, as the only known globally controlling force for third order sequence development (Vail et al., 1977), and a global sea level chart was developed based on this assumption (Haq et al., 1987). However, many have since questioned the validity of the global sea level chart (e.g., Hallam, 1984; Miall, 1986; 1991; Hubbard, 1988; Christie-Blick et al., 1990); continuing the debate of eustatic versus tectonic controls on sealevel variations derived from outcrop studies that started with Lyell (1835) and Suess (1885). Despite the development of the sequence stratigraphic model of Vail et al. (1977, 1991), the water-depth in which clinoforms develop is poorly understood and the subject of much debate. While some workers imply the clinoform rollover and shoreline are equivalent (Posamentier et al., 1988; Lawrence et al., 1990; van Wagoner, 1990); others argue that the clinoform rollover remains submerged even during lowstands (Austin et al., 1998; Fulthorpe et al., 1999; Steckler et al., 1999). In an effort to understand their significance, prograding 5 topset clinoform front bottomset onlap toplap downlap truncation Figure 1.3. The clinoform geometry. Clinoforms generally consist of lowgradient (<10) topsets, more steeply dipping (1-150 for siliciclastics; Adams 0 and Schlager, 2000; 2->40 for carbonates; Kenter, 1990) fronts, and 0 bottomsets with very low (<<1 ) gradients. Seismic discontinuities (onlap, toplap, downlap and truncation) are more apparent on dipping surfaces such as the clinoform front. Expansion of sequences by progradation increase the resolution and the ability to define their internal geometry (Fulthorpe, 1991). Modified after Christie-Blick et al. (1998). 6 160oW 80oW 0o 80oE 160oE 0o Figure 1.4. Worldwide distribution of clinoforms in the Neogene as imaged on seismic data (red stars). Modified after Bartek et al. (1991) 7 (A) TION) TS HIGH EUSTASY SMST LSF LSW TST HST SB1 INCISED VALLEY (ivf) CANYON (H ST ) (LSW) (LSF) LOW SB1 LST SUBSIDENCE SB2 DISTANCE SHALLOW DEEP (B) SB2 HST (TST) (ivf) SMST mfs TS CONDENSED SECTION (LSW) (LSF) SUBAERIAL HIATUS SB1 HST DISTANCE Figure 1.5. The Exxon model (Vail et al., 1991). Idealized depth-space (A), and time-space (B) diagram for Type 1 and Type 2 sequences and systems tracts relative to eustatic sea-level changes and tectonic subsidence. The depositional sequence is defined as "a stratigraphic unit composed of a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities" (Mitchum et al., 1977, p.53). In this model, stratal architecture is directly related to eustatic changes. Type 1 sequence boundaries record exposure of the shelf as sea-level drops below the shelf break. Type 2 sequence boundaries occur when sea-level falls are insufficient to expose the entire shelf resulting in deposition on the outer shelf. HST = highstand systems tract, TST = transgressive systems tract, LSW = lowstand wedge, LSF = lowstand fan, SMST = shelf margin systems tract. SB1 = type 1 sequence boundary, SB2 = type 2 sequence boundary, mfs = maximum flooding surface, TS = transgressive surface. 8 GEOLOGIC TIME DEPTH mfs (TST) HST (COND ENSE D SEC SMST SB2 clinoform stratal architecture is the focus of global studies designed to investigate the response of depositional systems to relative sea-level variations. Examples of such studies include: Miocene siliciclastics off New Jersey (Greenlee et al., 1992; Mountain et al., 1993; Austin et al., 1998), Neogene siliciclastics off Alabama (Greenlee and Moore, 1988), Cenozoic mixed carbonates and siliciclastics off NE Australia (Davies et al., 1989; McKenzie et al., 1991; Shipboard Scientific Party, 2001) and Miocene-Holocene, tropical carbonate successions along the edges of the Bahama Banks (Austin et al., 1985; Eberli and Ginsberg, 1989; Swart et al., 1996). This dissertation is separated into three main chapters, which have, or will be published as separate papers. Each chapter is self-contained and includes background and references relevant to the topic discussed in the chapter. Chapter 3 describes the detailed seismic stratal architecture of the Oligocene to Miocene clinoformal succession. Interpretation is restricted to the Dampier Sub-basin, using the nested seismic data. Seismic interpretations are compared to paleoenvironmental predictions derived from a study of benthic foraminifera in five of the wells (Appendix 1, Moss et al., in prep). These five wells intersect the 3D volume located on the eastern edge of the Rankin Trend (Fig. 1.2). This chapter is submitted to the American Association of Petroleum Geologists, Bulletin. Chapter 4 incorporates the interpretation of elongate, strike-oriented depressions in the middle Miocene, with stable isotope analyses and information derived from onshore studies (Chaproniere, 1984) to look in detail at one of the two karst horizons identified in the succession. This chapter is published in Marine 9 Geology (Cathro and Austin, 2001). Chapter 5 investigates the distribution, timing, and amplitude of post-rift Cretaceous and Tertiary inversion/reactivation events, thermal subsidence, and compaction on the distribution and postdepositional modification of the interpreted sequences as represented on a 2D transect across the Dampier Sub-basin. This portion of the study utilizes the forward kinematic and isostatic modeling software developed by G. Karner (Lamont-Doherty Earth Observatory). This chapter is being prepared for submission to Marine and Petroleum Geology. 1.2 HETEROZOAN CARBONATE SHELVES Carbonate shelves are recognized to develop as a continuum of environments sensitive to temperature, salinity, light, oceanography and nutrient supply. The two end member associations are defined as 1) heterozoan, composed of sediments derived from organisms that are light-independent, with or without red calcareous algae, and 2) photozoan, sediments derived from the remains of light-dependent organisms, combined with non-skeletal particles (e.g., ooids), with or without a heterozoan component (James, 1997). The NCB fits into the heterozoan classification, as the biogenic component is predominately benthic foraminifera, with lesser amounts of bryozoa and rare coral fragments (Appendix 1). Although organisms of the heterozoan assemblage are recognized in a variety of marine environments ranging from tropical to polar, and shallow to deep water, they only dominate the sedimentary succession when the influx of other sediments such as siliciclastics or photozoan-derived material is low. Therefore, heterozoan assemblages will not only predominate in cool-water 10 environments where mean oceanic temperatures are <20o, but also in sub-photic zones, regions of high nutrient flux, or where salinity is >43-45 or <33 in tropical environments (Lees, 1975; James, 1997). Subtropical carbonate sediments contain a mix of the heterozoan and photozoan end-members, consisting of coralline algae, corals, symbiont-bearing foraminifers, bryozoans, mollusks and other foraminifers (James et al., 1999). The production and sediment distribution of heterozoan carbonates has received relatively little attention compared to carbonates of the photozoan Association common in the tropical realm. Consequently, sequence stratigraphic models adapted for carbonate sediments concentrate on the differences between siliciclastic shelves and these tropical carbonate platforms (Table 1.1; Sarg, 1988; Greenlee and Lehmann, 1993; Handford and Loucks, 1993; Fitchen, 1997). Heterozoan carbonates lie in-between these end members; carbonate sediment produced in situ is redistributed by the same processes that influence siliciclastic sediments - waves, swells, and tides (Table 1.1; Fig. 1.6; James, 1997). Heterozoan cool-water carbonates tend to develop unrimmed open platform and ramp settings without continuous reef trends (Table 1.1; Fig. 1.6). The faunal assemblage is primarily non-phototrophic, dominated by organismswith calcite mineralogies, e.g., bryozoa, benthic foraminifera and mollusks (Jones and Desrochers, 1992). As a result, these carbonate platforms are little affected by meteoric diagenesis during exposure (James, 1997). During lowstands, sediment accumulation is concentrated below wave-base, and active primary carbonate production occurs shelf-wide (Fig. 1.6). Sediments produced 11 Carbonates tropical (PHOTOZOAN + minor heterozoan) Sediment source Climate is no constraint, Restricted to shallow, warm water marine sediments occur worldwide environments with sediments derived in situ. Primarily aragonite and Mg-rich derived from terrestrial and marine sources calcite Grainsize Reflects the hydraulic energy Reflects the size of skeletons of the environment with and precipitated grains. Many sand bodies are formed in situ from currents and waves forming shallow water sand bodies physiochemical or biological processes. Abundant carbonate mud. Main biogenic Hermatypic corals, calcareous green algae, components foraminifers, mollusks, minor bryozoa Shelf and slope Can develop distinct shelfCan develop distinct shelfmorphology slope breaks. Slope angles slope breaks and rimmed margins. are generally low, <3o , ranging 1-15o) Slope angles 2->40o (Kenter, 1990), 90o on escarpment margins. Platforms tend to remain near ambient base level. Response to base-level variations Transgression Backstep (if rate sed supply < rate SL rise) Backstep, aggrade or prograde Highstand Aggradation followed by progradation Excess sediment production - highstand shedding into basin from line source Siliciclastics Carbonates temperate (HETEROZOAN) Non-phototrophic in situ sediment source. Shelf-wide primary carbonate production. Primarily calcite. Combination of skeleton size and hydraulic energy of the environment. Sediment is distributed by waves, swells and currents. Minor carbonate mud. Mollusks and foraminifers ubiquitous, bryozoa, barnacles, ahermatypic corals Typically unrimmed open platform and ramp without continuous reef trends. Slope angles 2-8o (this study). Relatively deep shelf. Lowstand Exposure and sediment redistribution into the basin from point sources Exposure and extensive diagenesis, physical products are minor Alternatively - increased calciturbidite frequency (Shanmugam and Moiola, 1983) Ravinement or progradation Wavebase depth dependent deposition may occur across entire shelf with lesser amounts on slope and basin. Relatively narrow submerged shelf, produces sediment. Exposed portion subject to minor meteoric diagenesis. Table 1.1. Differences between siliciclastic, photozoan, and heterozoan carbonates. Modified after James and Kendall (1992), and James (1997). 12 PHOTIC PRODUCTION APHOTIC PRODUCTION Pelagic mud Benthic skeletal mud Fine skeletal sand Coarse skeletal sand Gravels & hard substrates Sediment Transport Wave Abrasion Zone Swell Wave Base Storm Wave Base tat i on OUTER SHELF -suspension settling -storm reworking -bioerosion -burrowing -sediment production and accumulation MIDDLE SHELF -storm reworking -burrowing -bioerosion -?seafloor cementation -active production sediment transport up- and downdip INNER SHELF -wave agitation -particle abrasion -sediment production but no accumulation -kelp forest -coralline algae -shaved shelf -winnowing -high energy Re se d -pelagic fallout -gravity flows Figure 1.6. Generalized cross-section of a cool-water, heterozoan carbonate shelf. Aphotic carbonate production occurs across the shelf, although it may be overwhelmed by photic sediment production on the inner shelf. Sediment accumulation concentrates on the outer shelf; sediment produced on the inner and middle shelf is transported landward and basinward. Sediment grainsize decreases basinward reflecting the reduced energy of the hydrodynamic setting. Base of the wave abrasion zone ~30-70 m, swell wave base <120 m and storm wave base <250 m. Distribution of sediment production and accumulation will vary with the hydrodynamic energy of the shelf. Modified after James (1997). im UPWELLING en SLOPE 13 on the shelf may be transported to the slope and basin at lowstand (James, 1997). During transgression, in-situ shelf-carbonate production (Boreen and James, 1993; James, 1997) results in progradation despite increasing water-depths, e.g., Broken Ridge, southeast Indian Ocean (Driscoll et al., 1991). Ravinement of the partially exposed lowstand shelf is also common during transgression (James, 1997). Sediment distribution at highstand depends on the depth of the wave-base; accumulation may occur across the entire shelf, with lesser amounts on the slope and basin. Sedimentation rates on cool, temperate carbonate margins are historically thought to be low relative to tropical carbonate platforms (Schlager, 1981; James and Bone, 1991; Boreen and James, 1993). However, results from the recent ODP Leg 192 in the Great Australian Bight show that upper Pliocene-Quaternary accumulation rates exceed 40 cm/ky (Feary et al., 2000), comparable to the lower end of average tropical platform growth rates (James and Bone, 1991). Siliciclastics introduced into either photozoan tropical, or heterozoan subtropical to cool-water carbonate shelves such as identified in the NCB, add even further complexity to the response of stratal geometries to relative base-level variations. Terrigenous clastic sediments can reduce organic sediment production on photozoan shelves, and overwhelm the sediment production on heterozoan shelves, although there is a fine balance between clastic influx and increased nutrient supply (James, 1997). Such mixed environments are commonly described using a reciprocal sedimentation model, whereby siliciclastic material 14 restricted to nearshore areas during a high relative sea-level bypasses the shelf and is redistributed basinward within discrete depocenters when relative sea-level falls (Fig. 1.7). Shelf siliciclastic sediments may form thin deposits as dunes. The reciprocal sedimentation model does not address the possibility of accumulating thick submarine siliciclastic sediments on the shelf such as observed in the NCB. Examples where this model has been applied include the Permian Basin, West Texas (Wilson, 1967; Meissner, 1972) and the Devonian Canning Basin, Western Australia (Southgate et al., 1993). 1.3 GEOLOGIC SETTING The North West Shelf (NWS) extends approximately 2000 km between North West Cape and Darwin, with an area totaling 800,000 km2. The margin is separated into four basins, of which all but the Browse Basin were originally defined by onshore mapping. They were later (1950's to 60's) extended offshore with the advent of drilling and seismic data acquisition (Hocking et al., 1994). The offshore NWS is currently subdivided into the Northern Carnarvon, Roebuck, Browse, and Bonaparte basins (Fig. 1.8; Purcell and Purcell, 1994). Initiation of these basins probably took place in the Paleozoic, but the majority of the sedimentary fill is Mesozoic in age (Butcher, 1989; AGSO NWS Study Group, 1994). The Northern Carnarvon Basin (NCB) is composed of four en echelon sub-basins and a plateau, largely identified on seismic data (Fig. 1.9). Compartmentalization is controlled by the Jurassic and Cretaceous extension 15 Figure 1.7. Reciprocal sedimentation in a mixed carbonate/siliciclastic environment. High relative sea level (A) is dominated by carbonate sedimentation. Siliciclastic material is trapped near the shoreline, distal from the shelf margin. The carbonate platform can build laterally as overproduced shallow water material is transported into the basin. At low sea level (B), the shoreline migrates proximal to the shelf margin. Carbonate sedimentation is reduced by the influx of siliciclastic sediments that bypasses the shelf margin to be deposited in the basin. Modified after Meissner (1972). 16 Figure 1.8. (following page). Modern topography and bathymetry map of the North West Shelf (NWS), Australia and surrounding regions. The extent of the four Jurassic basins (Northern Carnarvon, Roebuck, Browse, Northern Bonaparte) of the NWS is shown. The Northern Carnarvon Basin (NCB) is flanked on three sides by abyssal plains initiated during the Mesozoic. Today, the northeastern boundary of the Indo-Australia Plate is characterized by subduction beneath the Sunda Arc, island arc collision at the Banda Arc, and continental collision at Papua New Guinea with approximate plate boundaries shown by the red dashed lines (Coblentz et al., 1995; Hillis et al., 1997). Absolute plate motion (red arrows) is after Halbouty et al. (1981). A Tertiary inversion anticline in the Browse Basin (red star) is superimposed over the Paleozoic boundary fault with the Leveque Shelf. The red box over the Northern Carnarvon Basin highlights the study area shown in Fig. 1.2 (Wessel and Smith, 1991; Smith and Sandwell, 1997) 17 110oE EURASIAN PLATE 120oE 130oE 140oE 150oE PACIFIC PLATE 0 oS SUNDA ARC BANDA ARC NE PAP W U GU A INE A TIM OR NORTHERN BONAPARTE BASIN ARGO ABYSSAL PLAIN GASCOYNE ABYSSAL PLAIN INDO-AUSTRALIAN PLATE Darwin 10oS BROWSE BASIN ROEBUCK BASIN NORTHERN CARNARVON BASIN AUSTRALIA KM -4.0 -2.0 0.0 2.0 4.0 6.0 8.0 20oS CUVIER ABYSSAL PLAIN North West Cape 18 Figure 1.9. (following page). Proterozoic-Mesozoic structural trends superimposed on the modern physiography of the Northern Carnarvon Basin and adjoining onshore region. The Mesozoic subbasins that define the Northern Carnarvon Basin do not make an impression on the present-day shelf. However, the Exmouth Plateau and Argo, Gascoyne and Cuvier abyssal plains to the northwest are recognized in the bathymetry. Onshore, north-south oriented Proterozoic-Paleozoic faults onshore are approximately in-line with the en-echelon offsets between the Exmouth, Barrow and Dampier sub-basins. The red box over the Northern Carnarvon Basin highlights the study area shown in Fig. 1.2. Modified after Petkovic et al. (1999). 19 112 E o 114 E o 116 E ARGO ABYSS AL PLAIN o 118 E o GASCOYNE ABYSSAL P LAIN 18 S BEAGLE S-B Sy nc line M OR TF A PL o Ka ng ar EXMOUTH PLATEAU oo 7 RA IN NK 9 D ER PI AM AH S S- B LAMBERT SHELF LF HE SB Pt. Hedland Dampier De G 20 S o BA E Scholl Is. F. XM AL E PE Deepdale F. TH OU AR RR S- B OW PH A M DA UL CH L For rey R. R. Yul e st Rou Ca gh Ran pe ge F . Gira lia We CUVIER ABYSSAL P LAIN No RA MM NG ERS ES L E F. cue R. HA te s 22 S o b Ro hb As u rt e Y R. 200 km rth on 20 R. directions oblique to the north-south oriented Proterozoic-Paleozoic lineaments of the Scholl Island, Giralia and Rough Range Faults. Abyssal plains flank the NCB on three sides (Figs. 1.8 and 1.9): the Argo to the north with Jurassic oceanic crust at the continent-ocean boundary (COB), the Gascoyne to the west, and the Cuvier to the southwest, both with Cretaceous oceanic crust at the COB (Veevers et al., 1985, 1991). The outermost edge of the basin, the Exmouth Plateau, is composed of stretched and subsided continental crust topped by 10 km of Paleozoic to Cenozoic sediments, at water-depths of 800-4000m (Exon et al., 1982; 1992). The central and southern portion of the Exmouth Plateau is dominated by north and northeast trending faults of Late Triassic age (Fig. 1.9). The plateau is separated from the inner complex of predominantly Mesozoic (Fig. 1.10) depocenters to the east, with 12-15 km of sediment fill (Exmouth, Barrow, Dampier and Beagle Sub-basins), by the Kangaroo Syncline and the Jurassic fault blocks of the Rankin Trend. The inboard margin of the NCB is separated into the Peedamullah Shelf to the south and Lambert Shelf to the north (Hocking et al., 1994). Mesozoic to Cenozoic development of the NWS is the result of inversion and reactivation after initial extension and basin development in the late Permian (Fig. 1.11A; Etheridge and O'Brien, 1994). The early phase of rifting (Fig. 1.11B) was the result of supercontinent breakup and separation of the `Sibumasu Terrane' (Sumatra, Burma, Malay, Kalimantan and Indochina) from northwest Australia during the Late Permian (Sengor, 1987). This rifting initiated the northeast-southwest trending depocenters that collectively formed the Westralian 21 Tectonic Events/ Seismic Horizons DELAMBRE FORMATION ~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~ ~~~ ~~~~~ TREALLA LIMESTONE TULKI BARE MANDU CALCARENITE CAPE RANGE BROUP Collision with MIOCENEBanda Arc Collision with PLIO-PLEIST Eurasian Plate Minor and creation of inversion Indonesian arc events Collision with E.Papuan Terrane Figure 1.10. Generalized tectono-stratigraphic summary of the Northern Carnarvon Basin. The Permian to Cretaceous interval is summarized from Driscoll and Karner (1996) and Romine et al. (1997). Tertiary Formations are derived from Chaproniere (1984), and Apthorpe (1988). Carbonate sediments increase in importance from the Cretaceous and predominate in the Tertiary. 22 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Collision along northern Australian margin commences (Papua New Guinea/Sepik Arc) SANTONIAN Inversion Rift 4 Tithonian rifting culminating in continental breakup in the Valanginian and creation of the Gascoyne-Cuvier abyssal plains Rift 3 Sea floor spreading creating Argo Abyssal Plain MUNGAROO FM. LOCKER SHALE Rift 2 Late Triassic-Early Jurassic extension - onset of rifting in Argo Basin Rift 1 Late Permian extension Figure 1.11. (following page). Breakup of eastern Gondwanaland and plate tectonic factors that affected development of the North West Shelf. (A) Early Permian configuration prior to supercontinent breakup. (B) Late Permian "Sibumasu" Terrane separated from northwest Australian margin creating the Neotethys Sea (NS). (C) Late Jurassic "Argo Landmass" separated from Australian margin and creates Argo Abyssal Plain. (D) Early Cretaceous Greater India separated from eastern Gondwana and creation of the Gascoyne-Cuvier abyssal plains (Lawver et al., 1999) 23 24 Superbasin (Yeates et al., 1987). Subsequent Late Triassic-Early Jurassic extension is associated with onset of rifting in the Argo Basin. The next phase of rifting occurred in the Late Jurassic (Fig. 1.11C), between Australia and the `Argo Landmass' (Southern Tibet). This phase of rifting culminated in sea floor spreading in the Callovian (163 Ma; Driscoll and Karner, 1998) to Oxfordian (155 Ma; von Rad et al., 1992), which produced the Argo Abyssal Plain. The thermal subsidence that followed this episode was interrupted by a fourth phase of rifting in the Tithonian, which resulted in continental breakup in the Valanginian (Fig. 1.11D) and creation of the Gascoyne-Cuvier Abyssal plains, as Greater India moved in a west-northwest direction away from the Australian margin (Boote and Kirk, 1989; von Rad et al., 1992; AGSO North West Shelf Study Group, 1994). This Tithonian-Valanginian event generated significant post-Valanginian regional subsidence along the margin (Driscoll and Karner, 1996; 1998). Subsidence was interrupted by a minor phase of inversion, relative to the extensional events, variously dated to occur in the Cenomanian (Romine et al., 1997), Turonian (Driscoll and Karner, 1996; 1998) or Santonian (Cathro et al., 2001; Chapter 5). This inversion event is approximately coeval with major plate reorganization to the west, as the Indian continent began to drift more to the northwest, and timing of breakup between Australia and Antarctica (Sayers et al., 2001). Inversion structures generated in the early late Cretaceous, concentrate along pre-existing rift faults and include: the Novara Arch, uplift of Barrow Island and hanging wall uplift on the Rosemary Fault (Chapter 5; Driscoll and Karner, 1996; 1998). 25 In the Cenozoic, the NWS is influenced by collision of the Indo-Australian plate with the Pacific and Eurasian plates to the north (Fig. 1.12). Initial interaction was with the Pacific plate in the late Oligocene (>25Ma) when the Sepik Terrane (Fig. 1.12A) docked with the northern New Guinea-Australia margin (Pigram and Davies, 1987). Continued northward movement of the IndoAustralian plate resulted in the accretion of successive terranes at this margin (Lee and Lawver, 1995). Latest Miocene to Pliocene collision (Fig. 1.12B) of the Australian continental margin with the Banda Arc (Audley-Charles et al., 1988; Lee and Lawver, 1995; Richardson and Blundell, 1996) has produced reactivation and inversion features along the NWS. Examples of such structures include 1) possible flower structures in the Timor Sea region (O'Brien et al., 1993; Schuster et al., 1998), 2) Miocene anticlinal, features such as the Lombardina structure, in the Browse Basin (Fig. 1.13, Symonds et al., 1994; Struckmeyer et al., 1998), and 3) an anticlinal feature along the Rosemary-Legendre trend in the NCB (Fig. 1.14), and 4) the Giralia and Cape Range anticlines in onshore adjacent the NCB (Fig. 1.9; Malcolm et al., 1991). The extent of reactivation has lead M ller et al (1998) to propose an additional influence caused by the incipient breakup of the Indo-Australian Plate into the Indian, Australian and Capricorn plates, creating an intra-plate deformational zone. Northward drift of the Australian Plate is reflected in the NCB as a progressive change from siliclastic to predominantly carbonate sediments in the Cenozoic (Apthorpe, 1988). The NCB as part of the Australian continent moved 26 A) 100oE 110oE 120oE 130oE 140oE 150oE 15 Ma Nort h Java 160oE 0o New Guin e aA rc Trench Sepik Arc New Guinea 10oS 20oS 30oS B) 100oE 110oE 120oE 130oE 140oE Nort h New Guin e 150oE 5 Ma aA rc 160oE 0o North Banda Basin South Banda Basin Sepik Arc New Guinea Java Trench 10oS 20oS 30oS Australia-New Guinea continent to the 1000m isobath. Figure 1.12. Plate reconstruction at 15 and 5 Ma. Red boxes indicate study area.(A) 15 Ma - collision along the northeastern edge of the Indo-Australian Plate, that began in the late Oligocene, continued with docking terranes such as the North New Guinea Arc. (B) 5 Ma - late Miocene to Pliocene collision with the Banda Arc. Collision of a microcontinent fragment a few hundred kilometers north of the Australian North West Shelf is estimated at 8 Ma (Richardson and Blundell, 1996). Collision of the continental margin was much later, estimated at 2.4 Ma (Lawver et al., 1992); Lee and Lawver, 1995; plate reconstruction supplied by the Plates Project, UTIG, 1998). 27 NW 0 SE V.E. ~17 (@1500 m/s) ~8 (@3200 m/s) 0.5 late Miocene Two-way time (s) base Tertiary 1.0 Turonian 1.5 0 2.5 km 14/OA/262 1.8 Figure 1.13. Late Miocene inversion in the Browse Basin. This example comes from part of the dip-oriented Geoscience Australia seismic line 175_03. Onlap (red arrows) onto the anticline occured at various times during the Tertiary, and truncation occurs over the crest. Location of the anticline is given in Fig. 1.8. Modified after Struckmeyer et al. (1998). 28 0.0 136_19 136_20 101r_02 136_24 TWTT (s) 1.0 2.0 3.0 0.0 SW136_19 ?PlioPleist. l. Mioc 136_20 101r_02 136_24 V.E. ~19 (@1500 m/s) ~ 8 (@3200 m/s) ON3 NE 1.0 TWTT (s) Sant. l. Olig bTer 2.0 bCret 3.0 Rosemary-Legendre 10 km Figure 1.14. Inversion in the Northern Carnarvon Basin. This example comes from a portion of the dip-oriented Geoscience Australia line 136_15. The inversion anticline is west of the Rosemary-Legendre trend. Onlap on the flanks and truncation over the crest occurs at several horizons, indicating multiple phases of contraction that continues into the ?Plio-Pleistocene. Location of the seismic line is given in Fig. 1.2. 29 northward, from a position 36-40o S in the early Oligocene to its current position at 18-22o S (Veevers et al., 1991; Lawver et al., 1999). The wedge-like Cenozoic geometry of the NCB is dominated by prograding carbonate margins with minor intercalated siliciclastics as documented in several wells (e.g., Goodwyn 6, Lambert 1, Eaglehawk 1 on Fig. 1.2). Siliciclastic material may be delivered to the margin via the ancestral equivalents of the modern drainage system (Figs. 1.2 and 1.9). In addition, seismically resolved late Miocene prograding carbonate buildups are well developed in the northern Dampier and Beagle Sub-basins (Romine et al., 1997, their Figure 16). Their timing is similar to buildup development in the Eucla Basin, Great Australian Bight (Feary and James, 1995). 30 Chapter Two: Data and Methodology 2.1 DATA This study integrates a variety of data that has been obtained during petroleum exploration of the Northern Carnarvon Basin, North West Shelf, Australia. The data comprise 2D multichannel seismic (MCS), 3D MCS and nine wells (Fig. 1.2). Geoscience Australia and Woodside Australian Energy provided these data. 2.1.1 Seismic Data The 3D MCS are the primary data used in this study; they are nested in the regional 2D MCS. The 3D volume was merged during processing from parts of five 1980's and 1990's vintage surveys: Goodwyn South/Tidepole, Goodwyn North, Echo-Dixon, East Dampier and Rosie (Fig. 2.1; Table 2.1), located on the Rankin Trend northwest of the Dampier Sub-basin. The data have been provided as a single, completely processed volume (Fig. 2.2), covering an area ~1500 km2 to two seconds two-way travel time (TWTT). Central frequency of the 3D volume ranges from ~55 Hz, in that portion of the volume originally acquired as the Rosie survey, to ~40 Hz in the other surveys (Fig. 2.1). Therefore, vertical resolution varies from 15 to 20 m (assuming 3200 m/s) in the composite volume. Horizontal resolution (first Fresnel zone of the spherical wavefront) ranges between 215 and 252 m. The composite volume has an in-line, north-south, spacing of 25 m, and a cross-line, east-west, spacing of 12.5 m. 31 115 30'E WELLS 2 - Goodwyn 2 3 - Goodwyn 3 4 - Goodwyn 4 6 - Goodwyn 6 7 - Goodwyn 7 E1 - Eastbrook 1 D1 - Dampier 1 EH - Eaglehawk 1 L1 - Lambert 1 C YN o 116 00'E LIN E o 116 30'E o C VI A RI TO S EH E1 2 7 SG E ER TER RA C RK PA N KE N KIN AN RB KG R MB PA R 4 6 GB 3 Dampier Sub-basin GH OU TR Y AR E TR ND TRO UG H EW DR E E IN EL AD TR 10 km RR R TE KE M LE D1 M SE RO LEGEND 3D seismic volumes Goodwyn South/Tidepole, 1985 Goodwyn North, 1990 Echo-Dixon, 1991 East Dampier, 1992 Rosie, 1996 2D Seismic data - Geoscience Australia 2D MCS136 - other Geoscience Australia 2D MCS (101R and 110) Composite 3D volume Well locations biostratigraphy, lithology paleobathymetry, lithology, biostratigraphy North West Shelf - Triassic-Aptian structures Blocks: Malus (MB), Rankin (RB), Goodwyn (GB), North Rankin (NRB) Grabens: Keast (KG), Searipple (SG) Figure 2.1. 3D seismic coverage along the Rankin Trend. The volume provided by Woodside Australian Energy for this study (bold outline) is a composite of parts of Goodwyn North, Goodwyn South/Tidepole, Rosie, East Dampier and Echo-Dixon surveys. The composite survey covers ~1500 km2. 3D seismic data were provided to two seconds TWTT. Also shown are 2D seismic lines and well locations adjacent the 3D surveys. 32 o IS W 19 50'S E AC D EN 19 30'S D EN TR NRB L1 o Survey Acquired Number of sources Number of cables Active streamer length,m Active group length, m Number of channels Shot interval, m Fold Record length*, s Sample interval Acquisition filters, Hz Cross-line cdp interval, m Shooting direction Size, sq.km. Vessel Goodwyn Sth/Tidepole Goodwyn Nth Echo-Dixon 1991 2 2 4500 18.75 240 37.5 40 5.5 2 3 - 218 50 N-S 573 M/V Pacific Titan East Dampier 1992 2 3 4000 12.5 320 37.5 40 6 2 2 - 196 25 E-W 2746 M/V Western Legend Rosie 1996 2 4 4000 12.5 320 37.5 50 6 2 6 - 180 37.5 SW-NE 1125 not available 1985 1990 1 1 1 2 3200 3200 13.33 13.33 240 240 26.67 26.67 60 60 6 6 2 2 12 - 375 6 - 188 50 50 NW-SE NW-SE 206 267 M/V Western Odyssey M/V Western Resolution Table 2.1. Acquisition parameters for the five 3D seismic volumes used in this study. * all data provided only to 2s TWTT. 33 3D PROCESSING SEQUENCE NMO/f-k filtering Gap deconvolution Zero-phasing 3D radon demultiple 3D DMO Stack 3D migration Deconvolution Residual zero-phasing to wells Bandpass Filter time passband (Hz) (ms) 5-90 0 5-90 2500 5-90 3500 5-90 6000 Figure 2.2. Flow chart of processing sequence for composite 3D seismic volume, as supplied by Woodside Australian Energy. Velocity functions used in the stack are proprietary. 34 The 2D MCS consists of a subset of regional transects acquired (Table 2.2) and processed (Fig. 2.3) by Geoscience Australia between 1991 and 1994 (Fig. 1.2). The 3200 line km interpreted includes lines from surveys 101r, 110 and 136, all collected over the Dampier Sub-basin (Fig. 1.2). Central frequencies of ~25-35 Hz result in a theoretical vertical resolution of ~32-23 m (assuming 3200 m/s). The horizontal resolution ranges from 275 to 320 m. Horizontal resolution of the 2D data is estimated in the line of section; lines are spaced ~10 km apart. A more inclusive study, using the entire 2D dataset and ~80 wells (Romine et al., 1997) provides the framework for 2D interpretations. Declivities calculated on the shelf, slope and basin through the succession assume an average velocity of 3200 m/s. Time-velocity pairs supplied with the 2D MCS indicate that seismic velocities are 3200 m/s 400 m/s in the mapped interval (Fig. 2.4). This range in velocity translates to a difference of <1o on a reflection dipping 6o and ~0.1o on a reflection dipping 1o. Velocities from the 3D volume are proprietary and therefore unavailable. 2.1.2 Wells Wells are concentrated along structural highs, including the Rankin and Madeleine trends, within and on the margins of the Dampier Sub-basin (Fig. 1.2). Exploration targets are primarily Cretaceous and Triassic reservoirs. In general, older wells tend to contain more information on the late Paleogene-Neogene section. Composite well logs for Goodwyn 2, 3, 4, 6, 7, Dampier 1 and Eastbrook 1 are included as Plates 1-7. Data available at each of these wells consist of (1) electric logs, (2) sidewall core and ditch cuttings (see Appendix 2 for a listing of 35 Survey Acquired Active streamer length, m Active group length, m Number of channels Airgun capacity, cu.in. Shot interval, m Fold Record length, s Sample interval, ms Acquisition filters, Hz Line kilometers, km Vessel 101R 1991 4800 25 192 3000 50 48 16 4 8-64 1659 R/V Rig Seismic 110 1992 4800 25 192 3000 50 48 16 2 8-64 2888 km R/V Rig Seismic 136 1994 3000 12.5 240 1200 18.75 80 5.5 2 4-180 4215 R/V Rig Seismic Table 2.2. Seismic acquisition parameters for Geoscience Australia 2D MCS surveys. 36 SURVEY 101R 1991 SURVEY 110 1992 Gun delay removed SURVEY 136 1994 70 ms 70 ms 60 ms Wavelet processing output nominal minimum phase bandwidth from 8/18 to 64/72 deconvolution using Taner's exponential method output nominal minimum phase shaped to 6-100 Hz (butterworth filter) CDP sort First pass velocity analysis f-k filtering Radon demultiple Second pass velocity analysis Predictive deconvolution Stack Static correction for gun/cable depth Post-stack deconvolution Migration Bandpass Filter time passband (ms) (Hz) 0 8-60 1000 8-60 2000 8-55 3000 7-50 4000 6-45 5000 5-36 6000 5-35 7000 5-30 8000 5-25 10000 5-20 time passband (ms) (Hz) 0 8-65 1000 8-65 2000 6-60 3000 4-50 5000 2-40 8000 1-30 16000 1-20 time passband (Hz) (ms) 6-95 1500 6-85 2500 6-45 3500 6-35 5500 DMO Figure 2.3. Flow chart of processing sequence for the 2D MCS, as supplied by Geoscience Australia. 37 6800 0 0 6600 0 6400 0 6200 E1 0 6000 0 5800 0 5600 0 G2 5400 G4G7 5200 0 5000 G3 0 1500 2037 0 4800 0 seafloor G6 SHOT POINT 200 1500 1500 1947 1500 1906 1500 1500 1500 1500 1500 2116 1500 2164 200 400 2006 1990 1861 1931 400 2318 2297 2379 2368 2372 600 1932 2391 2204 2189 2302 DLS_TOP 2472 2594 2801 3022 2588 2605 600 TWTT (s) 800 2390 2738 2720 2660 800 DLS5 1000 2667 2894 3051 3150 3202 3004 3230 3085 3276 3274 DLS4 DLS3 DLS2 1000 1200 2912 3190 3281 3268 3338 1200 1400 3589 3565 3362 3243 3317 3468 3644 4016 3580 3682 3870 DLS1 1400 1600 3612 3824 3712 1600 3389 1800 3608 1800 38 3564 3426 3287 3229 2000 136_11 2000 interval velocity 2800-3600 m/s interval velocity 3200 m/s interpreted sequence boundaries Figure 2.4. Interval velocity profile through part of Geoscience Australia line 136_11. Velocities in the interpreted interval (late early Oligocene to late Miocene), range from 2800 m/s to 3600 m/s, centered around 3200 m/s. The section encompasses that part of the line within with the 3D volume. Location given in Fig. 2.6, vertical lines show location of wells for reference. 38 all wells, except Dampier 1 and Goodwyn 3 outside the 3D volume), (3) check shot surveys (Appendix 3), and (4) chronostratigraphy and lithology (Plates 1-7). Two additional wells, Eaglehawk 1 and Lambert 1 (Fig. 1.2), are used for general lithological information derived from the well-completion reports. Common electric log types in the Paleogene-Neogene section are gamma ray, sonic and caliper. Some wells also have density, neutron porosity, spontaneous potential, resistivity and photoelectric factor (PEF) logs. Caliper logs reveal greater borehole rugosity in the shallower section. This adversely affects the quality of the electric logs, particularly those whose data represent shallow depth of penetration into the borehole wall, e.g., density (Rider, 1996). Ditch cuttings and sidewall cores have been collected from the various core repositories at Geoscience Australia, Western Australia Department of Mineral and Petroleum Resources, and Woodside Australian Energy. Samples are collected at ~10 m intervals in five wells (Appendix 2). The results of studies concentrating on two biostratigraphic elements, foraminifera and ostracods, are included as appendices 1 and 4, and are incorporated into the interpretation. G. Moss (UTIG) has analyzed a subset of the collected samples for paleoenvironment using benthic foraminifera (Moss et al., in prep; Appendix 1). Vicki Passlow (Geoscience Australia) has conducted a preliminary investigation of paleo-water depths using ostracods on samples from six wells (Appendix 4). 39 2.2 METHODOLOGY The temporal and spatial variations of the Paleogene-Neogene clinoform succession have been studied using (Fig. 2.5): 1) A sequence stratigraphic approach to interpreting the 2D and 3D MCS in order to understand the 3D seismic geometry on a variety of spatial scales (Chapters 3 and 4); a. Five sequences (2-5 m.y. duration) are mapped in the 2D and 3D MCS (Fig. 2.6); b. Nineteen sub-sequences (~0.5-1 m.y. duration) are mapped in the 3D volume; c. Details of structural and stratigraphic discontinuities are examined on horizontal time slices, and slices of interpreted horizons from both amplitude and variance volumes; 2) The interpreted seismic geometry is used as a framework for the lithologic, paleobathymetric and biostratigraphic information at each well (Fig. 1.2); 3) Synthetic seismograms (Appendix 5) have been constructed at each well to tie information between the seismic data (measured in TWTT) and well information (measured in depth). 4) Forward kinematic and flexural modeling of the post-rift interval since 135 Ma reveals the distribution, timing and amplitude of Cretaceous and Tertiary compression-induced inversion in the Dampier Sub-basin (Chapter 5). 40 Non -ma rine ne ri Ma ] Geologic History TWTT (paleoenvironment) SEQUENCE BIOSTRATIGRAPHY rine -ma rine Non Ma A ] vertical exaggeration - high TWTT t gic olo ge e im vertical exaggeration - high + 3D SEISMIC STRATIGRAPHY (3D geometry) [ Hypothetical Paleobathymetry at shelf break ] Increasing paleo-water depth A Figure 2.5. Methodological overview. Variations in 3D seismic stratal architecture interpreted at a variety of spatial scales using 3D and 2D MCS are compared to the results of a sequence biostratigraphic study conducted on selected wells (Appendix 1; Moss et al., in prep.) The wells lie on an approximate dip-oriented transect, with one well offset along strike. Interpretation of the seismic data provides a geometric framework in TWTT within which paleoenvironments are examined. Integration of these data provides an understanding of geologic history impossible if these studies were performed in isolation. 41 DEPTH TWTT (s) 2.0 1.0 0.0 E1 G4 G2 G7 G6 G3 D1 Extent of 3D volume 10 km V.E. ~4.6 @ 3200 m/s Fig. 2.4 1 2 3 5 10 NW 0.0 SE seafloor DLS_top inclination (0) @ 3200 m/s DLS5 DLS4 DLS3 DLS2 DLS1 TWTT (s) 1.0 MM2 MM1 EMM1 OM1 OL1 2.0 Rankin Trend LEGEND DLS1 downlap sequence boundaries Dampier Sub-basin MM2 major downlapping sequences sub-sequence boundaries internal reflections Figure 2.6. Representative 2D/3D MCS profile in TWTT and interpreted line drawing illustrating subdivisions of the late Paleogene-early Neogene NCB succession into five downlapping sequences and 19 sub-sequences (medium black lines). This image is a dip-oriented composite of 2D line 136_11 and a co-linear traverse within the 3D volume. The nature of the clinoform surface changes 42 from smooth in the southeast to incised in the northwest. Seven wells are projected onto the line of section. The thick black line over five of the well traces indicate the intervals studied for paleobathymetry (Appendix 1). The samples intersect the topset, front and bottomset of the clinoforms. For locations of the profiles and the wells, see Fig. 1.2. 2.2.1 Seismic Analysis Definition of the five seismic sequences and 19 sub-sequences is based on the identification of surfaces of seismic downlap discontinuities within the 2D and 3D MCS data (Fig. 2.6). The sequences are interpreted here to occur between seismic discontinuity surfaces of systematic basinward (distal) downlap that are coincident with changes in stratal architecture or seismic attributes (Fig. 2.7). Downlap, defined as a non-depositional hiatus, can occur both within and at the base of seismic sequences (Mitchum et al., 1977; Christie-Blick, 1991). Seismic discontinuities defined by onlap and truncation are less consistent for mapping purposes in this succession. Where identified, the onlap and truncation seismic discontinuities are coeval with the downlap surfaces (Fig. 2.7). Two different types of 3D seismic volume are interpreted as part of this study: (1) the standard amplitude volume (Figs. 2.8 and 2.9) and (2) a generated variance volume (Figs. 2.8 and 2.10). Primary interpretation is conducted on vertical sections of the amplitude volume (Fig. 2.9), using Geoquest interpretation software. In-line and cross-line orientations are oblique to the progradation direction, so approximate dip- and strike-oriented traverses have been defined at 2 km intervals (Fig. 2.11). Interpretation on these and intervening traverses is typically spaced 500-1000 m and reduced to 100 m in complex areas. Where necessary, traverses at a variety of orientations have also been examined. Auto picking between the manually interpreted traverses is often possible on the shelf, where reflections maintain consistent polarity. 43 1.0 1.5 TWTT (s) NW 2 km V.E.~5:1 SE 1.1 1.5 TWTT (s) V.E.~10:1 downlap truncation/?toplap onlap downlap sequence boundaries sub-sequence boundaries Figure 2.7. Definition of seismic boundaries from downlap discontinuities. Downlap discontinuity surfaces define both the sequences and sub-sequences. The downlap discontinuity surfaces in this example occur where there is a change from relatively high amplitude (below light blue and above green) to relatively low amplitude (between light blue and green). Onlap and truncation are also apparent on the shelf associated with the downlap discontinuity surfaces. 44 High variance Low variance (Variance) Variance TWTT slice Variance horizon slice Figure 2.8. Schematic representation of different types of seismic volumes and profiles interpreted as part of this study. Variance is a trace-by-trace measure of the lateral dissimilarity of traces. Dissimilar traces such as yellow and red give a high value, whereas the similar traces on either side give a low value. The advantage of the variance volume is that it highlights structural and stratigraphic discontinuities in the seismic data. Primary interpretation is conducted on vertical slices through the amplitude volume, supplemented by interpretations on horizontal TWTT slices and horizon slices along interpreted reflections in both volumes. See Figures 2.9 and 2.10 for examples. 45 NW SE 1.0 Amplitude Volume 1.0 1.3 1.5 21 k m 31 km 1.5 1.0 2.0 2.0 N TWTT (secon ds) Strike- oriented traverses Dip-oriented traverse 1.5 Data Gap cr o Horizontal timeslice at 1.3s through the amplitude volume Data Gap 2.0 lin nes Amplitude extracted at interpreted horizon ss -lin es i N N N Figure 2.9. A portion of the 3D amplitude volume used in this study. Interpretation has been conducted on vertical, horizontal and horizon-slice sections. Vertical sections may be extracted at any orientation. In this study, dip- and strike-oriented traverses are the primary vertical sections examined. The closely spaced reflections on the horizontal timeslice are related to 46 dipping reflections on the vertical section. Increased disruption by incision to the northwest is also apparent. The amplitude extracted from the interpreted horizon is complex, particularly in the northwest; however, a coherent pattern is not obvious. Variance Volume m 21 k 3 1 km 1.0 1.5 Data G ap cr os slin variance low Data Gap es i ne s 2.0 li n- N high Horizontal timeslice at 1304 ms through variance volume slope channels Variance extracted at interpreted horizon N N Figure 2.10. The same region shown in Fig. 2.9 through the variance volume, TWTT slice and horizon slice. High variance (low coherency) areas are dark. Slope channels are more apparent in the variance display, because of the change in seismic character at incision flanks. 47 TWTT (secon ds) 115o20'E 115o30'E 115o40'E 0 11 115o50'E 81 _0 36_ 116o00'E 116o10'E 2 Goodwyn 2 3 Goodwyn 3 4 Goodwyn 4 6 Goodwyn 6 7 Goodwyn 7 E1 Eastbrook 1 D1 Dampier 1 WELLS 12 N 19o20'S 19o30'S E1 2 4 6 7 19o40'S 3 6_ 19 0 km 10 13 13 23 6_ 13 6_ 11 n 10 io 1 r at n _09 ad gr tio ro irec P d 20 D1 13 6_ 10 19o50'S 1r_ 02 Figure 2.11. Location of approximate dip- and strike-oriented traverses at 2 km intervals within the composite 3D volume. Interpretation between these traverses has reduced typical spacing to 500-1000 m, and to 100 m in complex areas. 10 1r 13 6_ 8 _0 10 48 TWTT isochron and structure contour maps have been constructed to illustrate along-strike variations in sequence and sub-sequence thicknesses and morphology of the bounding reflections (Chapter 3). The interpretation has been gridded and contoured using a convergent algorithm within Geoquest , with a node spacing ~50% of the average spacing between interpreted traverses. Contouring of the widely spaced 2D profiles has been performed separately, due to the increased line spacing (~10 km). All maps are uncorrected for the effects of compaction, subsidence and isostasy, as the overall effects of burial during clinoform progradation are complex. However, as in the New Jersey margin, rotation resulting from differential loading, combined with compaction and thermal subsidence, tend to increase declivities in the clinoform succession (Steckler et al., 1993; Chapter 5). Therefore, slope measurements of clinoform geometries may overestimate pre-burial declivities. A Geoquest variance volume (Fig. 2.10) has been generated from the amplitude volume using a 5X5 horizontal window (i.e., a 25-trace operator) and a 50 ms vertical window. Variance is a measure of the similarity (cross-correlation) of adjacent traces in in-line and cross-line directions, integrated over a small vertical analysis window. Therefore, variance displays are useful for delineating structural (e.g. faults) and stratigraphic (e.g., channels) discontinuities (Bahorich and Farmer, 1995; Marfurt et al., 1998). Horizontal time slices and horizon slices can be extracted from both the amplitude and variance volumes (Figs. 2.8-2.10). Horizontal time slices, spaced ~50-100 ms, have been primarily used in this study to highlight the trend of 49 small-offset reactivation faults in the Oligocene and early Miocene. They are also used to highlight slope incision. The shortfall of time slices for studying clinoforms is that these reflections are inherently inclined, and are therefore crosscut by the horizontal slices. Widely spaced reflections on time slices are the result of near-horizontal reflections in the vertical section; more closely spaced time-slice reflections occur in dipping areas (Fig. 2.9). In the studied succession, reflections on the time slices get younger to the northwest as a result of progradation, and are widely spaced on relatively shallow dipping clinoform topsets and bottomsets. Horizon slices reveal the spatial distribution of seismic amplitude and variance over a single interpreted reflection, removing the effects of structure and surface relief present in horizontal time slices (Figs. 2.8-2.10). Horizon slices are therefore relatively time-synchronous, assuming that seismic sequence boundaries represent chronostratigraphic surfaces (Vail et al., 1977b). Features imaged on the horizon slices have been typically first identified on time slices, along with other features of differing stratigraphic age. Their morphology is then clarified using the horizon slices at selected reflections. Horizon slices are primarily used in this study to delineate downslope transport paths and shelf depressions (Figs. 2.9 and 2.10). In general, the quality of the seismic data interpreted for this study is high, particularly in the 3D volume and in Geoscience Australia survey 136. However, there are limitations caused by interference, multiples, and velocity distortions. Seismic surfaces defined in good data areas have been mapped through lower 50 quality data areas such that they represent consistent termination patterns. Multiple energy is partially suppressed during processing, although in waterdepths <200 ms (~150 m) the water bottom multiple obscures primary reflections and reverberations are present through the seismic section (Fig. 2.12). In addition, in Geoscience Australia survey 101R, the water bottom is removed during processing in water-depths <200 ms (~150 m). Velocity distortion effects are related variations beneath low-angle faults, around carbonate buildups and by varying water-depth, pull-up beneath salt domes and push-down beneath shale diapers and channels. In these data, minor velocity pull-ups have also been interpreted below small cross-cutting submarine canyons. To assess this effect, a 1D synthetic seismogram has been constructed for the simplified case of a single erosional surface filled with relatively high velocity material (Fig. 2.13; M. Sen, UTIG, personal communication, 2001). Sonic velocities used for the model have been derived from the well, Eastbrook 1 (Fig. 1.2), which intersects the side of one of the submarine canyons; morphology of canyon flanks has been estimated from the seismic data. The seismogram replicates the overall pull-up, highlighting the possible impact that rugged morphology has on the underlying acoustic data. 2.2.2 Well Information Lithology and Chronostratigraphy Lithologic and chronostratigraphic data have been extracted from hydrocarbon exploration well-completion reports (WCRs; Plates 1-7). In the Oligocene-Miocene interval, lithology information is derived from ditch cuttings 51 SW 136_10 0.0 NE 136_11 0.5 1.0 V.E. ~26 @ 1800 m/s 0.0 136_10 5 km 136_11 ? 0.5 1.0 water bottom multiple 1 multiple 2 Figure 2.12. Examples of multiples recognized in 3D MCS. Water bottom multiples and reverberations partially obscure primary reflections in the subsurface. Multiple energy is relatively easy to distinguish if it cross-cuts the primary reflections such as in the center of the section. However,when primary and multiple reflection dips are comparable, they are difficult to distinguish. Energy detected above the interpreted water bottom is probably a combination of water column reverberation, post-critical reflections and direct waves. In the very shallow water areas, the assumption of vertical incidence between source and receiver is not maintained, resulting in distortion in the shallow seismic section. 52 TWTT seismic section 1.2 TWTT (s) 1.4 A B Interval velocity model 3800m/s 115 m 400 m 3400 m/s 1D synthetic seismogram Figure 2.13. Comparison between seismic data and synthetic seismogram. The seismic data show numerous cross-cutting incisions on the clinoform front between ~1.2 and 1.3 s ( and ). Reflections below ~1.4 s are characterized by pull-ups ( and ) coincident with one or a number of the incisions higher in the section. Pull-ups are broad when incisions stack slightly offset to each other (A) and sharp beneath vertically stacked incisions (B). Information derived from sonic logs (Eastbrook 1) suggests that the incisions are filled with higher velocity material than what composes the slope into which the incisions are cut. A 1D synthetic seismogram, created through the interval velocity model (M. Sen, UTIG, personal communication, 2001) with a single incised surface, shows small pull-ups similar to those observed in the seismic data. 53 200 ms and sidewall cores. Chronostratigraphic data are from analyses of planktic foraminifera by various biostratigraphers; the sample interval generally increases up-section because the Neogene is deemed non-prospective for hydrocarbons. Scans of planktic foraminifera from samples near seismic downlap discontinuity surfaces have been integrated with the WCR results (G. Moss, UTIG, personal communication, 2001, see Plates 1-7). Chronostratigraphy is expressed in terms of planktic foraminiferal biozonations (Blow, 1969), calibrated to a global Cenozoic timescale (Berggren et al., 1995; Fig. 2.14). Ages given for the downlap discontinuity surfaces in any one well correspond to the age range of the samples examined above and below the interpreted horizon (Table 2.3). The age given for seismic horizons is the mean of the ages from all of the wells; error bars incorporate the entire age range from all wells (Table 2.3). (As chronostratigraphy was poorly resolved in Dampier 1, this well was excluded). The methodology used for the paleoenvironmental analysis of benthic foraminifera is detailed in Appendix 1. Water-depth zonations used in this analysis are those of Murray (1991; Table 2.4). These zones increase in range with increasing water-depth, thereby reducing sensitivity of the analysis, particularly in water-depths >200 m. 2.2.3 Synthetic Seismograms at well-sites Synthetic seismograms are generally used to relate depth-referenced well data to time-referenced seismic data at well-sites (Telford, 1985). Synthetics have 54 Figure 2.14. Chronostratigraphic framework for the Tertiary interval. Geomagnetic chrons, planktonic zones, calcareous nannofossils and planktonic bioevents after Blow (1969), Martini (1971, McGowran (1979, Heath and Apthorpe (1984) and Berggren et al. (1995). Formations and the oceanic oxygen isotope curve of Miller et al. (1996) are related to this timescale. Modified after Moss et al. (in prep.). 55 GEOLOGIC TIME (Ma) Miller et al. (1996) BIOEVENTS Well Seismic reflector TWTT (ms) DLS_TOP DLS5 DLS4 DLS3 DLS2 DLS1 421 487 580 663 679 744 min 454 574 732 848 872 955 (m) ss max 463 602 743 861 884 964 Dampier1 (m) kb min max 463.1 472.1 583.1 611.1 741.1 752.1 857.1 870.1 881.1 893.1 964.1 973.1 Goodwyn 3 (m) kb min max 809 828 1257.2 1276.2 1415.2 1437.2 1603.2 1630.2 1867.2 1887.2 2068.2 2083.2 Goodwyn 6 (m) kb min max 809 825 1231 1252 1410 1425 1514 1532 1956 1969 2045 2070 Goodwyn 7 (m) kb min max 877 904 1228 1258 1459 1484 1631 1652 2052 2070 2082 2102 Goodwyn 4 (m) kb min max 953 971 1293 1305 1452 1468 1628 1640 2060 2092 2121 2143 Goodwyn 2 (m) kb min max 957.5 982.5 1280.5 1299.5 1471.5 1485.5 1677.5 1691.5 2100.5 2117.5 2133.5 2150.5 Eastbrook 1 (m) kb min max 1092.9 1125.9 2056.9 2080.9 2129.9 2143.9 2220.9 2236.9 2243 2264 Age (Ma)* 13.8-16.4 16.4-23.8 16.4-23.8 16.4-23.8 16.4-23.8 hiatus 23.8-49 Well Seismic reflector TWTT (ms) DLS_TOP DLS5 DLS4 DLS3 DLS2 DLS1 736 1016 1114 1229 1387 1494 min 779 1227 1385 1573 1837 2038 (m) ss max 798 1246 1407 1600 1857 2053 Age (Ma)* NA NA 16.4-20.52 20.52-23.8 23.8-33.7(29.4) hiatus 29.4-33.8 Well Seismic reflector TWTT (ms) DLS_TOP DLS5 DLS4 DLS3 DLS2 DLS1 757 1008 1119 1211 1439 1489 min 801 1223 1402 1506 1948 2037 (m) ss max 817 1244 1417 1524 1961 2062 Age (Ma)* 3.58-16.4 14.8-15.1 15.1-16.4 >18-23.8 21.5-27.1 hiatus 29.4-37.9 Well Seismic reflector TWTT (ms) DLS_TOP DLS5 DLS4 DLS3 DLS2 DLS1 793 1005 1144 1250 1498 1515 min 860 1211 1442 1614 2035 2065 (m) ss max 887 1241 1467 1635 2053 2085 Age (Ma)* <16.4 11.2-15.1 15.1-16.4 16.4-18.8 23.8-29.4 29.4-38.1 inc hiatus 29.4-37.9 Well Seismic reflector TWTT (ms) DLS_TOP DLS5 DLS4 DLS3 DLS2 DLS1 833 1027 1132 1235 1506 1534 min 923 1263 1422 1598 2030 2091 (m) ss max 941 1275 1438 1610 2062 2113 Age (Ma)* 11.4-11.8 14.8-15.1 17.3-21.5 15.1 18.8-23.8 23.8-32(29.4) 23.9-40.1 inc hiatus 29.4-33.8 Well Seismic reflector TWTT (ms) DLS_TOP DLS5 DLS4 DLS3 DLS2 DLS1 852 1042 1162 1286 1548 1565 min 945 1268 1459 1665 2088 2121 (m) ss max 970 1287 1473 1679 2105 2138 Age (Ma)* 8.3-11.4 10.4-14.8 16.4-20.52 16.4-21.5 Oligocene late Eocene-Oligocene (long ranges) Well Seismic reflector TWTT (ms) DLS_TOP DLS5 DLS4 DLS3 DLS2 DLS1 963 1485 1539 1610 1620 (m) ss min max 1067 1100 2031 2055 2104 2118 2195 2211 2217.1 2238.1 Age (Ma)* NA 13.6-16.4 16.4-21.5 TRUNCATED 30.4-33.8 32.8-33.8 Clinoform Reflections DLS_TOP DLS5 DLS4 DLS3 DLS2 DLS1 Estimated Age (Ma)* 10 +/- 6.4 13.4 +/- 3 18.3 +/- 3.1 20.1 +/- 3.7 25.4 +/- 4 hiatus 29.4-37.9 * age estimates calculated from planktic foraminifera zones in WCRs and G.Moss (UTIG), calibrated to the Berggren et al. (1995) timescale. Table 2.3. Well ties and age estimates for the six major seismically defined downlap discontinuity surfaces shown in Fig. 2.6. 56 Bathymetric Zone Transitional Inner neritic Middle neritic Outer neritic Upper bathyal Middle bathyal Lower bathyal Abyssal Depth Range (meters) Coastal and paralic environments 0-20 20-100 100-200 200-500 500-1000 1000-2000 >2000 Table 2.4. Bathymetric zones after Murray (1979). 57 been created for seven wells: Goodwyn 2, Goodwyn 3, Goodwyn 4, Goodwyn 6, Goodwyn 7, Eastbrook 1, and Dampier 1 (Appendix 5). Ideally, synthetic seismograms are generated using both sonic and density logs. However, density logs are available only in Goodwyn 6, Dampier 1, and Eastbrook 1. Density logs are often ignored, as density variations are small relative to changes in velocity (Table 2.5; Telford, 1985). Density logs are also more susceptible to adverse affects of borehole rugosity, due to their smaller radius of penetration (<13 cm) compared to sonic logs (<25 cm; Rider, 1996). As logging runs are not continuous to the surface, the sonic log is calibrated to the seismic datum (sea level) using a 1D check shot velocity survey. Check shot surveys are provided in the WCR. The procedure used to create synthetic seismograms for this study (Fig. 2.15) is: 1) Log editing (Figs. 2.15 and 2.16A): The match between the synthetic seismogram and seismic data depends on the quality of the initial sonic log. Spurious peaks create false reflections in the synthetic seismogram, lowering quality of the tie. Sonic logs have been inspected with reference to caliper, bit-size, gamma logs and drilling reports (i.e., casing seat locations and drilling problems). Spikes in the sonic log are removed if coincident with washouts commonly near casing shoes. 2) Choose check shots and calibrate sonic log (Figs. 2.15, 2.16B and C) Sonic logs are then calibrated to check shot surveys so that the average velocities between check shots correspond to the average velocities on the sonic log. A single check shot placed at the top of a sonic log is the minimum required 58 Compound Quartz Calcite Dolomite common lithologies sandstones limestones dolomites shale Density (g/cc) 2.65 2.71 2.85 Sonic (ms/ft) 55.10 46.50 40.00 51.5-55 47.6-53 38.5-45 62.5-167 Table 2.5. Typical mineral and matrix density and sonic velocity values (from Gearhart, 1983; Rider, 1996). 59 Logs Edit curves Seismic Extract wavelet at well locations Calibrate sonic to check shot survey Create RC series Decide on theoretical representative wavelet Convolve RC series with wavelet Match synthetic seismogram with seismic at well Stretch/squeeze to enhance match Figure 2.15. Flow chart of generalized procedure to create synthetic seismograms for well ties with 2D and 3D seismic data.. 60 Figure 2.16. (following page). Procedure used to create synthetic seismograms for comparison with seismic data. (A) Log editing. Log spikes are removed in the vicinity of casing shoes. Choose checkshots and calibrate sonic log (B). Red stars show all the check shots available in a well, with an extra reference check shot placed at the seafloor. Green arrows indicate reference check shots used to calibrate the sonic log. These reference check shots correspond to breaks in the sonic log and are spaced ~100 m apart to avoid large interval velocity variations. The resulting drift curve (middle panel, green) is smooth compared to the original drift curve (red). (C) The edited sonic log (black curve) is calibrated to the check shots. Interval transit time of the sonic log is increased in areas of increasing drift, and decreased in areas of decreasing drift. After calibration, the resultant drift curve (blue) is at zero and the sonic curve is calibrated to the check shots (red sonic curve). (D) Generate RC series. The RC series is convolved with the wavelet to produce a synthetic seismogram, which is finally compared to seismic data at the well tie. 61 62 to make the depth conversion absolute from the surface. However, greater accuracy is achieved from multiple measurements down the borehole (Goetz et al., 1979). Check shots are usually acquired at arbitrary intervals in the well, not related to lithologic or velocity breaks. Velocity breaks generated by the check shots will cause unwanted reflections when the synthetic is generated. Therefore, reference check shots are created to correspond with sonic and/or gamma log breaks. The time-to-depth conversion of these reference check shots is calculated using the average velocity determined by the check shot survey. A reference check shot is always placed at the seafloor, with an assumed average velocity of 1524 m/s as used in the check shot surveys. Appendix 3 lists all the check shots available in the wells, highlighting the acquired and reference velocity-depth pairs used to calibrate the sonic log. Goodwyn 3 has no check shot survey, so the velocity profile of Goodwyn 6, ~3 km northwest of Goodwyn 3, is used, adjusted for the shallower water-depth. The sonic log is then adjusted to compensate for the overall velocity structure, as revealed by the check shot survey. Drift, defined as the difference between check shot interval time and integrated sonic log time, variably decreases, increases or remains constant (Goetz et al., 1979). A decrease in drift with depth indicates the sonic log needs to be decreased to match the check shot times (i.e., the sonic velocities are too slow), and vice versa for an increase in drift with depth. A constant drift indicates areas where the average integrated sonic log velocity and interval check shot velocity are equal; the sonic is then left 63 unchanged. The calibrated sonic curve is used to generate a reflection coefficient (RC) time series. 3) Generate RC series (Fig. 2.16D): RC(layer 1/layer 2) = Z2-Z1 Z2+Z1 (2.1) where Z is the acoustic impedance of layers 1 and 2, calculated as the product of density (r) and velocity (v). In these wells, density is assumed constant, because density logs are not everywhere available so RC is determined by variations in velocity as determined from the calibrated sonic log. 4) Create and match synthetic seismogram (Fig. 2.16D): To create the synthetic seismogram, a suitable wavelet is convolved with the RC series. Wavelets extracted from seismic data adjacent each well guide the choice of wavelet used to create the synthetic seismogram. The processing sequence for the 3D volume indicates that these data were processed to zerophase (Fig. 2.2). Wavelets extracted from the 3D volume at Goodwyn 2, Goodwyn 4, Goodwyn 6 and Goodwyn 7, range between 40 and 50 Hz and are approximated by a 40 Hz, zero-phase Ricker wavelet (Fig. 2.17). The wavelet extracted at Eastbrook 1, the only well located within the 1996 Rosie survey, is approximated by a 55 Hz, zero-phase Ricker wavelet (Fig. 2.18). The processing sequence for the 2D surveys indicates that they have been processed to minimumphase (Fig. 2.3). Wavelets extracted from survey 136 range in dominant frequency from 35 to 40 Hz, and can be approximated by a 35 Hz, minimum- 64 A m p l i t u d e -60 -40 -20 0 Time 20 40 60 A m p l i t u d e 50 100 Frequency Theoretical zero phase, 40 Hz ricker wavelet Wavelets extracted from older 3D at: Goodwyn 2 Goodwyn 4 Goodwyn 6 Goodwyn 7 Figure 2.17. Extracted wavelets from the 3D seismic volume at Goodwyn 2, Goodwyn 4, Goodwyn 6 and Goodwyn 7. A 40 Hz, zero-phase wavelet (red) approximates the extracted wavelets and is used to create synthetic seismograms at these wells. 65 A m p l i t u d e -60 -40 -20 0 Time 20 40 60 A m p l i t u d e 50 100 Frequency Theoretical zero-phase, 55 Hz ricker wavelet Wavelet extracted from 3D at Eastbrook 1 Figure 2.18. Extracted wavelet from the 3D seismic volume at Eastbrook 1. A 55 Hz, zero-phase Ricker wavelet (red) approximates the wavelet extracted from the seismic data, and is used to create the synthetic seismogram at this well. 66 phase Ricker wavelet (Fig. 2.19). The match is less satisfactory than those from the 3D volume, as a result of ~75 Hz energy detected in all extracted wavelets. The wavelet extracted at Goodwyn 7, in survey 101r, has been approximated by a 25 Hz, minimum-phase Ricker wavelet (Fig. 2.20). Once these wavelets are chosen, they are convolved with the RC series, a linear filter that alters the shape of the source wavelet. Once the synthetic seismogram can be constructed it is compared with the seismic data at the well location. The synthetic can be stretched and squeezed from top to bottom at particular locations by inspection, to improve the match. However, any shift in TWTT will affect the depth-time relationship below the depth shifted, and is therefore cumulative (Table 2.6). Nonetheless, the resulting synthetic ties offer a good match to the seismic data (Appendix 5). There are correlation resolution limits. Synthetic seismograms created from sonic logs are a composite response of closer spaced reflectors than can be detected using seismic frequencies (Yilmaz, 1987). This will result in inconsistencies in the detailed correlation between seismic data and the synthetic. Additional factors impacting velocities derived from seismic, check shot and sonic log data are summarized in Table 2.7. Overall, sonic velocities tend to be greater than seismic velocities, although the dependency on frequency is not fully understood (Goetz et al., 1979). Spread lengths, fold, S/N ratio and deviation from the assumed hyperbolic path all impact seismic velocity picks. The sonic is in contrast affected by borehole conditions, instrumentation and calibration errors. 67 A m p l i t u d e -60 -40 -20 0 Time 20 40 60 A m p l i t u d e 50 100 Frequency Theoretical minimum-phase, 35 Hz ricker wavelet Wavelets extracted from 2D survey 136 at: Dampier 1 Goodwyn 3 Goodwyn 7 Figure 2.19. Extracted wavelets from 2D survey 136 at Dampier 1, Goodwyn 3 and Goodwyn 7. A 35 Hz, minimum-phase Ricker wavelet (red) is used in these wells to create synthetic seismograms. 68 A m p l i t u d e -60 -40 -20 0 Time (ms) 20 40 60 A m p l i t u d e 50 100 Frequency (Hz) Theoretical minimum-phase, 25 Hz ricker wavelet Wavelet extracted from 2D survey 101r at: Goodwyn 7 Figure 2.20. Extracted wavelet from 2D survey 101r at Goodwyn 7. A 25 Hz, minimum-phase Ricker wavelet (red) is used in this well to create the synthetic seismogram. 69 Depth (m below SL) 391 555 982 1551 Dampier 1 (RT = 9.1 m) TWTT (s) TWTT (s) Cumulative (original) (final) Change (s) 0.375 0.371 0.004 0.476 0.471 0.009 0.776 0.771 0.014 1.171 1.144 0.041 Goodwyn 7 (RT = 9.1 m) - 3D tie Depth TWTT (s) TWTT (s) Cumulative (m below SL) (original) (final) Change (s) 724 0.684 0.688 -0.004 790 0.745 0.73 0.011 809 0.742 0.754 -0.001 920 0.829 0.826 0.002 1177 0.977 0.974 0.005 Goodwyn 7 (RT = 9.1 m) - 101r_09 tie Depth TWTT (s) TWTT (s) Cumulative (m below SL) (original) (final) Change (s) 911 0.822 0.826 -0.004 1227 1.012 1.001 0.007 1996 1.476 1.488 -0.005 2800 1.995 2 -0.01 Goodwyn 2 (RT=12.5 m) TWTT (s) TWTT (s) Cumulative (original) (final) Change (s) 0.865 0.86 0.005 Goodwyn 3 (RT = 30.2 m) Depth TWTT (s) TWTT (s) Cumulative (m below SL) (original) (final) Change (s) 786 0.706 0.738 -0.032 1051 0.936 0.932 -0.028 2102 1.528 1.535 -0.035 Goodwyn 6 (RT = 8.0 m) TWTT (s) TWTT (s) Cumulative (original) (final) Change (s) 0.729 0.755 -0.026 0.877 0.863 -0.012 1.025 1.009 0.004 Depth (m below SL) 818 973 1234 Depth (m below SL) 969 Goodwyn 4 (RT = 30.2 m) Depth TWTT (s) TWTT (s) Cumulative (m below SL) (original) (final) Change (s) 796 0.745 0.743 0.002 2057 1.512 1.506 0.008 Eastbrook 1 (RT = 25.9 m) Depth TWTT (s) TWTT (s) Cumulative (m below SL) (original) (final) Change (s) 430 0.481 0.49 -0.009 554 0.598 0.606 -0.017 833 0.796 0.8 -0.021 887 0.839 0.835 -0.017 930 0.871 0.868 -0.014 1137 1.001 1 -0.013 2203 1.612 1.608 -0.009 2.694 1.903 1.899 -0.005 Table 2.6. Stretch/squeeze values used to enhance ties between synthetic seismograms and seismic data at each well. Prior to stretch/squeeze, each synthetic is locked at the sea floor. The first data point stretches/squeezes the interval between it and the sea floor, consequently shifting the synthetic below that depth. The first data point then becomes the anchor for the second stretch/squeeze. The total amount of stretch/squeeze at the last data point is the cumulative sum of the intermediate values. Stretch is represented by a negative change and squeeze by a positive. Note Goodwyn 7 is tied to both 3D and 2D MCS. 70 SEISMIC DATA Hyperbolic normal moveout, i.e., horizontal formations with constant velocity SONIC Assumptions Refraction of signal through formation that is unaltered by drilling CHECK SHOTS Check shot times measured on vertical straight paths, i.e., vertical wells, small source offset and no formation dip Source frequency 10-40 kHz 10-50 Hz Vertical resolution 37.5-7.5 m @ 1500 m/s 61 cm (receiver distance) Radius of penetration 2.5-25 cm Acceptable error (conventional Instrument Precision stack/stratigraphic detailing) 2-10%/0.1% RMS 0.0002% 1-2 ms Sources of error Short spread lengths decrease Sonic velocities are Amplifier DC offset applied to velocity resolution. generally greater than reduce noise will increase sonic seismic velocities, although times. the frequency dependency is poorly understood. Partial stacking to reduce fold, Cycle skipping, if returning Check shot picks calculated on and therefore computational signal is attenuated by the the first trough rather than first time, prior to velocity analysis formation to below the break will increase sonic times. reduces accuracy due to detection level, results in distortion of hyperbolic path. long or short sonic times. S/N ratio limits the ability to Borehole rugosity will only Lateral formation changes pick events for velocity analysis. reduce amplitude of close to boreholes, due to e.g., returning signal unless salt domes or carbonate reefs, borehole variations are deep can result in short check shot and rapid possibly resulting times. in long sonic times. A low-resolution velocity Velocity inversion near the Refracted ray paths in dipping spectrum results if the TWTTborehole will result in short strata will result in short check gates used along the hyperbolic sonic times relative to the shot times. The same effect search paths are too wide. check shot. This will occur occurs in the sonic log, but is Conversely, computational cost if the invaded zone adjacent relatively small. increases with decreasing gate the borehole is faster than length. the uninvaded formation, e.g., gas-bearing formations flushed during drilling. Noise on receivers resulting Acquisition errors such as if in +ve or ve spikes on the the airgun is too deep or if the sonic curve. gun moves during acquisition 10-50 Hz Table 2.7. Assumptions, parameters and sources of error for calculating velocity from seismic, checkshots and sonic data. Sources: Schneider (1971), Goetz et al. (1979), Badley (1985), Yilmaz (1987), and Rider (1996). 71 Check shot velocity surveys use frequencies comparable to seismic data, but are influenced by noise, picking methodology, formation variability and acquisition logistics. 2.2.4 Kinematic and Flexural Forward Modeling Two-dimensional forward tectonic modeling of the sequences interpreted within the 3D and 2D MCS highlights distribution, timing and magnitude of inversion in the Dampier Sub-basin. Regional interpretation of the five Oligocene-Miocene sequences are incorporated with modified interpretations tied to Goodwyn 7 (Fig. 1.2) from Romine et al. (1997) and Driscoll and Karner (1998) of the Cretaceous to late early Oligocene interval, covering the entire post-rift basin development. When a conflict exists between the two studies, we favor the interpretation of Romine et al. (1997) due to its regional well control. Line 101r_09 is modeled using the forward kinematic and flexural routines developed by G. Karner at Lamont-Doherty Earth Observatory because: (a) 101r_09 is a dip-oriented transect, centrally located in the Dampier Sub-basin, traversing the Lambert Shelf in the southeast to outer Rankin Platform in the northwest and ties with the 3D seismic volume (Figs. 1.2 and 1.9). This transect is therefore suitable to assess the impact of regional tectonics on the stratal development mapped in the 3D MCS. (b) Cretaceous and Tertiary inversion on the northwest and southeast bounding faults of the Dampier sub-basin is recognized. 72 (c) 101r_09 was one of three transects used in earlier investigations of Permian-Recent rift, re-rift, inversion and subsidence history and subsequent stratigraphic development of the NCB (Driscoll and Karner, 1996, 1998). The starting point for this modeling (Chapter 5) is the completion of the final phase of rifting in the TithonianValanginian, with initiation of post-rift subsidence as depicted in this earlier model. Step 1: Convert 101r_09 from TWTT to depth using the RMS velocities from 2D seismic processing. The original velocity analysis (Fig. 2.21A), spaced at 2 km (160 cdps) intervals, has been edited to remove spikes, linearly interpolated and median filtered every 1000 cdps to maintain the overall shape of the velocity profile and remove high frequency variability. The resultant velocity profile (Figure 2.21B) is used within Focus for TWTT-depth conversion. The conversion from TWTT to depth uses a vertical stretch function described by: z = vt (2.2) where v = the velocity at the top of the layer t = the vertical TWTT z = thickness of a layer with a vertical velocity gradient 73 A SP 202 0.0 1.0 2.0 3.0 2202 4202 6202 8202 10202 12202 14202 16202 Depth (km) 4.0 5.0 6.0 7.0 8.0 9.0 10.0 B 202 0.0 1.0 2.0 3.0 2202 4202 6202 8202 10202 12202 14202 16202 Depth (km) 4.0 5.0 6.0 7.0 8.0 9.0 10.0 VELOCITY 1485 1885 2285 2685 3085 3485 3885 4285 4685 5085 5485 5885 Figure 2.21. RMS velocity profiles for Geoscience Australia line 101r_09 (A) Raw data with velocity picks every 160 cdps (2 km). Red circles highlight points where data are removed prior to filtering. (B) Final edited and filtered RMS velocity profile used for the TWTT-depth conversion of line 101r_09. A smooth profile is required so that artifacts are not created during the conversion. See Figure 1.2 for location of seismic line. 74 Step 2: Calculate progressive loss of porosity with time and depth from Goodwyn 2, 3, 4, 6, 7 and Eastbrook 1. An estimate of porosity from the sonic log is calculated using the Wyllie time-average equation (Wyllie et al., 1958): sonic = (tlog-tma)/(tfl-tma) where sonic = tlog tma = = porosity calculated from sonic log interval transit time reading from log interval transit time of matrix (see Table 2.5, the midpoint (2.3) of these values are used in the calculation) tfl water) The resultant porosity logs are given in Appendix 6. Least-squares linear and exponential functions are fitted to each of the logs. The goodness of fit of the functions are indicated by the R2 (coefficient of determination) value. In all wells, except Eastbrook 1, the first data point is at >300 m; the linear curve actually fits better than the exponential function. However, in Eastbrook 1, values starting at ~85 m illustrate the rapid reduction of porosity at shallow depths, and an exponential curve fits best. The negative exponential curve used to control compaction rates in this forward modeling is averaged over the entire sedimentary succession intersected in wells (Driscoll and Karner, 1996; 1998). Step 3: Estimate paleo-water depth profiles Central to the forward modeling are the paleo-water depth assumptions made at each modeled time interval. These profiles are created along the modeled = interval transit time of pore fluid (set at 189 s/ft for saline 75 section at each time step using geometric and paleoenvironmental information in the seismic profiles and wells. For example, water-depth on the shelf, landward of clinoform breakpoints, is initially set at sea level and then adjusted according to preliminary paleo-water depth estimates using benthic foraminifera from well samples (Appendix 1). Water-depth in the basin is estimated using the precompaction height of the clinoform front. To guide the initial estimate of clinoform height, the Oligocene-Miocene sequences are restored to depositional thicknesses at each well (Fig. 2.22), taking into account the effects of mechanical compaction, which occurs when water in pore spaces between sediment grains is expelled with increased burial depth. The thickness of a decompacted layer, z'2-z'1, is given by: z'2-z'1 = z2-z1 0 (e-cz1-e-cz2) + 0 (e-cz'1-e-cz'2) c c hs where hs = hw (2.4) sediment volume assuming a unit cross-sectional area (constant with burial depth) z2-z1 0 c hw = = = = thickness of compacted layer decompacted porosity at surface coefficient determining slope of -depth curve water volume assuming a unit cross-sectional area (decreases with burial depth) assuming the porosity at any depth follows an exponential path (Appendix 6) = 0 e-cz (2.5) 76 E1 G4 G2 G7 G6 G3 D1 0 500 depth (m sub-seafloor) 1000 1500 2000 2500 5 km Modern sediment thicknesses in each well Decompacted sediment thicknesses in each well Present day depth sub seafloor of downlap surfaces Decompacted depth sub-seafloor of downlap surfaces Figure 2.22. Modern and decompacted sediment thicknesses at each well calculated using the exponential porosity-depth curves in Appendix 6. Colors represent the five sequences mapped in the MCS as shown in Fig. 2.6. The wells are D1, Dampier 1; G3, Goodwyn 3; G6, Goodwyn 6; G7, Goodwyn 7; G4, Goodwyn 4; G2, Goodwyn 2 and E1, Eastbrook 1. The horizontal scale details the approximate dip separation of the wells, with Goodwyn 7 ~14.5 km northeast of Goodwyn 4. See Figs. 2.1 and 2.6 for location of wells. 77 The general decompaction equation (2.4) mathematically represents sliding an individual stratigraphic unit up the porosity-depth curve. G. Karner at LamontDoherty Earth Observatory provided the iterative decompaction computer code. Step 4: Operation of the forward kinematic and isostatic model. The ultimate goal of the forward kinematic and isostatic modeling is to replicate the modern stratigraphy as imaged in the depth-converted seismic line (Chapter 5). However, stratigraphy at intermediate time steps must also be geologically viable and consistent with the seismic and well data. For example, modeled uplift and erosion must correspond to an observed unconformity, truncation and/or onlap onto the uplifted area, and preferably also be associated with redistributed sediments, either in the modeled transect or elsewhere in the basin. The final modeled stratigraphy, although non-unique, is constrained by iterations between the seismic and well observations and the modeled time steps. We model along the single 2D transect the entire post-rift phase of basin margin development. Initial surface topography and thermal conditions in the Valanginian are the result of the multi-phase rifting history determined by Driscoll and Karner (1996; 1998)(Fig. 2.23). Tectonism is the principal driving force behind the creation and destruction of accommodation and large-scale sediment distribution and preservation. The model uses the thermal disequilibrium resulting from the passive rise of the asthenosphere during rifting to determine the temporal and spatial distribution of subsidence during the modeled post-rift phase. The advective heat flux at the base of the lithosphere is dissipated by conduction following rifting; during each modeled time step, the 78 kilometers 300.0 0.0 -1000.0 -2000.0 -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 320.0 340.0 360.0 380.0 400.0 Barrow Delta (Forestier Claystone) 420.0 440.0 460.0 480.0 500.0 520.0 (synrift 4) Mungaroo Fm. (post rift 1) Upper Dingo Clayst. (synrift 3) Locker Shale (synrift 1) Lower Dingo Clayst. (synrift 2) Figure 2.23. Predicted time line stratigraphy across the Carnarvon Basin (Dampier Sub-basin) at the end of Tithonian-Valanginian rifting (~135 Ma). The topographic relief and thermal structure (not shown) of the lithosphere is determined by the preceding rifting events and form the initial conditions for modeling in the Cretaceous and Tertiary. Chronostratigraphic significance of the formations is defined in Fig. 1.10. The horizontal scale is relative to the continent-ocean boundary to the northwest. Modified after Driscoll and Karner (1996; 1998). 79 lithosphere cools and subsides, and flexural strength increases. This thermal decay is assumed to decrease exponentially with time and is strongly dependent on the amount of stretch during rifting (McKenzie, 1978; Driscoll and Karner, 1996; 1998). Superimposed on these initial conditions are the effects of eustasy, sediment loading, compaction, and compression-induced inversion since the Valanginian, all of which also influence accommodation. Long-term eustatic variations are derived from changes in mid-ocean ridge volume, modeled as a gradual fall from a peak of 200 m in the Turonian (after Hays and Pitman, 1973). Short-term changes are input as required to increase or decrease accommodation along the entire modeled section at any particular time step. Sediment distribution, and hence loading, is controlled by water-depth profiles at each time modeled (Step 3). Once the water-depth profiles are defined, they are used by the model to maintain the shape of the upper surface at each time step as thermal subsidence creates accommodation. The resultant sediment thicknesses created then impose a load on the lithosphere. The model calculates the resultant subsidence, which is dependent on the thermally controlled flexural rigidity of the lithosphere at the time of loading. Compaction is governed by the depth- dependent porosity function averaged over the entire succession as sampled in wells (step 2; Driscoll and Karner 1996; 1998). The location, timing and magnitude of uplift caused by compressioninduced inversion are determined from seismic and well observations. Inversion is identified by development of an anticline, with or without truncation on the 80 crest. Timing of uplift is constrained by onlap on the flanks. Inversion created by in-plane compression (i.e., shortening) is modeled as an increase in lithospheric thickness at the site of an anticline. The distribution of increased lithospheric thickness along the modeled section will influence the shape of the anticline, with the crest occurring at the location of maximum thickness. If uplift is sufficient to expose the crest subaerially, erosion occurs and truncation is observed on the seismic data. Within the model, subaerial erosion is calculated at each time step as a function of relief above sea level along the transect. The rate of erosion is selected manually. The inversion anticline is a positive load and is therefore flexurally compensated in the same way as the sediment load. This results in the creation and destruction of accommodation at different wavelengths, the inversion anticline destroys space locally over the crest, and space is created regionally by flexural compensation. The final modeled transect (Chapter 5) represents the cumulative effects of loads caused by sedimentation, inversion, and erosion, combined with compaction, thermal subsidence and eustasy. The ultimate consequences of each modeled interval are only apparent once the modeling is completed to the Present. Comparison with the seismic transect highlights where discrepancies exist, e.g., if the flexural response to a modeled inversion anticline results in significant long wavelength subsidence which is not apparent on the seismic data, then the modeled inversion must be reconsidered. 81 Chapter Three: Clinoform progradation along a deeply submerged Oligocene-Miocene heterozoan carbonate shelf: Implications for sensitivity to sea-level variations. 3.1 INTRODUCTION Clinoform seismic architecture has been used as a proxy for eustatic sea-level variations for the past 25 years (Vail et al., 1977; Haq et al., 1988; Vail et al., 1991; Hardenbol et al., 1998). However, before any global sea-level signal can be derived from this geometry, mechanisms governing clinoform formation and progradation must be understood in three dimensions for carbonate, siliciclastic and mixed sedimentological regimes. The Northern Carnarvon Basin (NCB), on the southern boundary of the North West Shelf (NWS), Australia (Fig. 3.1A), is an ideal location to investigate the relationships among clinoform progradation, paleoenvironment, and eustasy. A late Paleogene-early Neogene, mixed heterozoan carbonate-siliciclastic sediment succession observed there takes the form of unrimmed, stacked clinoforms prograding to the northwest. These clinoforms are superbly imaged by a 3D multi-channel seismic (MCS) volume embedded within a 2D MCS grid (Fig. 3.1A). As a result, three-dimensional stratal patterns can be mapped. Furthermore, the significance of these patterns can be tested using paleobathymetric analyses of benthic foraminiferal assemblages from available wells, correlated with chronostratigraphy and lithostratigraphy from industry well-completion reports (Fig.3.1B). Integration of all of these data allows an unprecedented opportunity to understand one example of the complex geologic development of clinoforms worldwide. submitted to AAPG Bulletin with co-authors, J.A. Austin, Jr. and G.Moss 82 study area limit of 2D interpretation Fig. 3.12 Be Fig. 3.1B L1 EH Fig.3.6 Da Ba N Ex Ash bu Gr ey R o r te n R. NWC 112 E Well locations biostratigraphy, lithology paleobathymetry,biostrat., lith. Seismic data Geoscience Australia 2D MCS136 other Geoscience Australia 2D MCS 3D MCS volume - location, 2D MCS illustrations o R. scue R . HA MM ER RA SL NG ES EY 200 km 120 E o 116 E o LEGEND Ex, Ba, Da, Be NWC North West Shelf Mesozoic depocenters: Exmouth, Barrow, Dampier and Beagle sub-basins Tertiary inversions North West Cape Figure 3.1A. The Northern Carnarvon Basin (NCB), composed of four predominantly Mesozoic depocenters. Up to 3200 line-km of the 2D MCS available from this basin have been interpreted (large box). The NCB has also been influenced by Oligocene-Recent plate interactions to the north (Pigram and Davies, 1987; see Fig. 1.8). Onshore, a maximum elevation of ~1200 m occurs in the Hammersley Ranges, drained by intermittent rivers with maximum flow after cyclone events (Semeniuk, 1996). Otherwise, little is known about riverine input to the NCB. 83 22 S o 20 S o 18 S WELLS L1- Lambert 1 EH - Eaglehawk 1 o De Y u le R F Ro be rt o 115 30'E WELLS 2 - Goodwyn 2 3 - Goodwyn 3 4 - Goodwyn 4 6 - Goodwyn 6 7 - Goodwyn 7 E1 - Eastbrook 1 D1 - Dampier 1 3 g. Fi .3 o 116 00'E SY L NC E IN o Fi g. 3 E1 .11 A N B .11 .3 g Fi Fi g. 3. 5 2 Fig. 3.9 Fig. 3.10 NRB 7 SG ER A C E KER T R AR P KE N IN NK RB KG RA MB PA R GB 4 6 3 W TRO UG H RE D 10 km g. Fi 8 3. RRA R TE KE CE Da L DE MA D1 IS W H LE OUG TR Well locations biostratigraphy, lithology paleobathymetry,biostrat., lith. Seismic data Geoscience Australia 2D MCS 136 other Geoscience Australia 2D MCS 3D MCS volume - 2D/3D MCS composite illustration location, 3D MCS traverses LEGEND Da North West Shelf Dampier Sub-basin Triassic-Aptian structures: Blocks: Malus (MB), Rankin (RB), Goodwyn (GB), North Rankin (NRB) Grabens: Keast (KG), Searipple (SG) Eocene-Miocene structures Figure 3.1B. The 1500 km2 3D multichannel seismic (MCS) volume embedded within the 2D MCS grid (Fig 3.1A). Triassic-Aptian structures define the Rankin Trend, the western margin of the Dampier Sub-basin (adapted after Newman, 1994; Stagg and Colwell, 1994; Romine et al., 1997). Seven hydrocarbon exploration wells, tied to the seismic data (Plates 1-7; Appendix 5), provide paleoenvironmental (Appendix 1) and timing constraints. Locations of various seismic examples referenced in the text are also shown. 84 19 50'S E EIN D EN TR o 19 30'S IA OR CT VI D EN TR 3 g. Fi .7 o 3 g. Fi .4 Stratal architecture is the preserved record of base-level variations that result from the interplay of factors in three dimensions: eustatic changes (glacioeustasy/tectonoeustasy), tectonics (basin formation/reactivation, in-plane force variations), variations in sediment supply, climate changes/paleoceanography, paleo-topography and post-depositional compaction (Fig. 1.1). These factors operate on overlapping time-scales and interact with each other, often producing similar sedimentary responses (Galloway, 1989). There has been a change in research emphasis over the past decade, from simply using clinoform geometry alone to determine eustasy through geologic time to analyzing time- and/or location-specific processes that together control sedimentation and ultimately stratal development (Christie-Blick and Driscoll, 1995; Feary and James, 1998; Duncan et al., 2000). Clinoforms generally consist of low-gradient (<1o) topsets, more steeply dipping fronts (1-15o for siliciclastics; Adams and Schlager, 2000; 2->40o for carbonates; Kenter, 1990), and bottomsets with very low (<<1o) gradients (Fig.1.3). Seismic discontinuities (onlap, toplap, downlap and truncation) are also apparent because of these changes in dip. The downdip depositional limit of clinoform fronts, such as those recognized in the NCB, may be represented by inclined, downlap terminations onto less steeply dipping bottomsets of a preexisting depositional surface. Downlap, defined as a non-depositional hiatus, can therefore occur both within and at the base of seismic sequences (Mitchum et al., 1977; Christie-Blick, 1991). The most common shelf-to-basin clinoform profile is sigmoidal (Adams and Schlager, 2000; Schlager and Adams, 2001). 85 Rounding of the breakpoint or shelf-break is now interpreted as a combined result of storms as well as sea-level changes (Adams and Schlager, 2000), superimposed on an exponential reduction of sediment transport capacity and/or competence away from the source at the shelf break (Kenyon and Turcotte, 1985). Basin margins composed of prograding clinoforms are observed worldwide (Fig. 1.4) in both siliciclastic and carbonate settings, at a variety of scales, and at different times (Bartek et al., 1991). Clinoform topsets are still believed to be responsive to base-level variations, particularly in carbonates (Schlager, 1981). Clinoform progradation indicates limited vertical accommodation relative to sediment supply (Posamentier et al., 1988). As a result, these sequences expand laterally as they build seaward, enhancing their seismic resolution (Fulthorpe, 1991). For all these reasons, prograding clinoforms are the fundamental building block of sedimentary sequences and have been used to formulate the principles of seismic sequence stratigraphy (Fig. 1.5; Vail et al., 1977b), and continue to be investigated to understand the response of depositional systems to relative sea-level variations. Examples include studies of Miocene siliciclastics off New Jersey (Greenlee et al., 1992; Austin et al., 1998), Neogene siliciclastics off Alabama (Greenlee and Moore, 1988), Cenozoic mixed carbonates and siliciclastics off NE Australia (Davies et al., 1989; Shipboard Scientific Party, 2001) and Miocene-Holocene, carbonate successions along the edges of the Bahama Banks (Austin et al., 1985; Eberli and Ginsberg, 1989). Two-dimensional sequence stratigraphic models, initially developed for siliciclastic regimes (Fig.1.5; Vail et al., 1977b, 1991), have since been modified 86 to account for different responses to base-level changes by siliciclastic versus carbonate depositional systems (Handford and Loucks, 1993; Fitchen, 1997). Because in situ primary production of healthy, shallow-water tropical carbonate systems can exceed all but the most rapid rates of base-level rise (Schlager, 1981), carbonate margins can variously backstep, aggrade or prograde during transgression (Pomar, 1991). "Highstand shedding" from carbonate platforms increases the thickness and frequency of calciturbidites (Droxler and Schlager, 1985; Schlager, 1991); these move basinward through numerous small submarine canyons to form a carbonate apron, e.g., along the northern slope of Little Bahama Bank (Mullins et al., 1984). In contrast, point-sourced, submarine fan deposition occurs at the base-of-slope in siliciclastic systems, and may be active during lowstands, transgressions or highstands depending on sediment supply, shelf width and slope reworking (Mullins and Cook, 1986; Galloway, 1989; Dingus and Galloway, 1990). Both increased lowstand calciturbidite frequency (Shanmugam and Moiola, 1983) and off-platform carbonate deposition during both transgression and highstand (Glaser and Droxler, 1991) suggest that a complex relationship exists between base-level variations and stratal geometries in siliciclastic and tropical carbonate environments. Heterozoan carbonates, such as those found in the NCB, tend to develop unrimmed open platform and ramp settings without continuous reef trends (Fig. 1.6). The photic zone is less important for sediment production than for photozoan carbonates. Calcite-prone, heterozoan platforms are little affected by meteoric diagenesis during exposure, so they respond as siliciclastic margins do to 87 waves, open-ocean swells and currents (James, 1997). Sediments accumulate below wave-base, and shelfal sediments may be transported basinward (Fig. 1.6; James, 1997). During transgression, in-situ shelf-carbonate production results in progradation (Driscoll et al., 1991; James, 1997). Sedimentation rates on temperate heterozoan carbonate margins are thought to be low relative to tropical photozoan carbonate platforms (Schlager, 1981; James and Bone, 1991). However, ODP Leg 182 has shown that upper Pliocene-Quaternary accumulation rates exceed 40 cm/ky in the Great Australian Bight (Feary et al., 2000), comparable to the lower end of average tropical carbonate platform growth rates (James and Bone, 1991). In the NCB, siliciclastics mix with a predominant heterozoan carbonate system, which complicates the response of the observed stratal geometries to relative base-level variations. Such composite regimes have been described using a reciprocal sedimentation model, whereby siliciclastic material restricted to nearshore areas during high relative sea-levels bypasses the shelf to be redistributed basinward within discrete depocenters when relative sea-level falls (Fig. 1.7; Meissner, 1972). Shelf siliciclastic sediments may form thin deposits as dunes during lowstands. This model has been applied successfully to the Permian Basin, West Texas (Meissner, 1972), and to the Devonian Canning Basin, Western Australia (Southgate et al., 1993). However, reciprocal sedimentation does not account for thick submarine siliciclastic sediments on the shelf, such as those observed in the NCB (Moss et al., in prep.). 88 This chapter presents an interpretation of nested 2D/3D seismic and borehole data (Fig. 3.1) which characterize a progradational succession of late Paleogene-Neogene clinoforms. Superposition of these data addresses the need, identified by previous studies elsewhere (e.g., Fulthorpe and Austin, 1998), for more laterally extensive, higher density seismic coverage, combined with well control, to understand along-strike variations in clinoform stratal geometry. Our interpretations identify smooth to highly dissected clinoform fronts similar to those known from high-density bathymetry profiling of modern clinoform slopes (e.g., Twitchell and Roberts, 1982; Mullins and Cook, 1986). In addition, we combine the observed stratal architecture with estimated paleo-water depths from benthic foraminifera (Moss et al., in prep.) and geomorphic features on the buried shelf to assess the paleobathymetric significance of both the clinoform shelf and break-point. Finally, the spatial distributions of clinoform and secondary depocenters highlight the detailed evolution of this prograding margin as a complex response not only to changing sea-level, but also to sediment supply, tectonics, and changing paleoceanographic conditions. 3.2 GEOLOGIC SETTING The NCB has undergone a complex Mesozoic-Cenozoic history (Fig. 3.2) since initial extension in the Late Paleozoic (Etheridge and O'Brien, 1994). The rifting and re-rifting events that followed in the Rhaetian, Callovian and Tithonian-Valanginian created and reactivated the northeast-southwest oriented basins along the NWS. Post-Valanginian regional subsidence is interrupted by minor inversion that is variously dated between the Cenomanian and Santonian 89 Figure 3.2. (following page). Generalized tectonostratigraphy of the NCB, compared to seismic downlapping sequences and sub-sequences mapped as part of this study, the eustatic curve (Haq et al., 1987), and the oceanic oxygen isotope curve of Zachos et al. (2001). The Permian to Cretaceous interval is summarized from Romine et al. (1997), and Driscoll and Karner (1998). Tertiary formations are derived from Chaproniere (1984) and Apthorpe (1988). Tertiary (sub) tropical planktonic foraminiferal zones of Blow (1969) are calibrated to the Berggren et al. (1995) time-scale. Vertical error bars (far right column) indicate the age-range of a seismic horizon, defined by the following factors: uncertainties in depth conversion of seismic markers, nonuniform sampling intervals for biostratigraphic analysis in the well-completion reports, and ranges of planktonic foraminiferal age zones (Chapter 2). The interval studied encompasses the overall warming since the Oligocene, peaking with the Mid Miocene Optimum in sequence MM1. Following the climatic optimum is a rapid cooling trend. Effects of ongoing plate collision are recognized along the entire NWS, including the NCB, as reactivated inversion features superimposed on Mesozoic structures (Fig. 3.1; Chapter 5). The color-coding used here is maintained throughout the following chapters. 90 Planktonic Foraminiferal Zones Chronostratigraphy (Berggren et al.,1995) PLEIST. Formations Seismic Downlapping Sequences Long-term and short-term eustatic curves - Haq et al. (1987) Calibrated to Berggren et al., 1995 200 Seismic (see Table 3.1) sub-sequences 0 5 Long-term d18O record Zachos et al., 2001 (Pacific, Indian and Atlantic oceans) 4 3 2 1 TREALLA LIMESTONE TULKI BARE MANDU CALCARENITE CAPE RANGE BROUP Collision with E.Papuan Terrane Collision along northern Australian margin commences (Papua New Guinea/Sepik Arc) PLIOCENE ~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~ ~~~ ~~~~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ late Minor Inversion - SANTONIAN MIOCENE N10 N9 TREALLA LIMESTONE/ PILGRAMUNNA FM YARDIE GP. middle CAPE RANGE GROUP early Sea floor spreading creating Argo Abyssal Plain RIFT 3 N4 23.8 MANDU FORMATION late OLIGOCENE early ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ DELAMBRE FORMATION Collision with Banda Arc Collision with Eurasian Plate and creation of Indonesian arc 1.75 DELAMBRE FORMATION N19 N18 N17 5.3 N16 11.2 N15 N14 N13 ? BARE FM DLS_TOP N12 N11 MM2 DLS5 MM2.6 MM2.5 MM2.4 MM2.3 MM2.2 MM2.1 10 MM1.4 MM1.3 MM1.2 MM1.1 EMM1.3 EMM1.2 EMM1.1 OM1.6 OM1.5 OM1.4 OM1.3 OM1.2 OM1.1 16.4 Tithonian rifting culminating in continental breakup in the Valanginian and creation of the Gascoyne-Cuvier abyssal plains RIFT 4 N8 N7 N6 N5 91 RIFT 2 Late Triassic-Early Jurassic extension - onset of rifting in Argo Basin TULKI LMST MM1 DLS4 EMM1 DLS3 20 OM1 DLS2 P22 28.5 P21 ~N2 P20 P19 P18 OL1 DLS1 OL1 30 RIFT 1 Late Permian extension 33.7 warming 91 (Chapter 5; Romine et al., 1997; Driscoll and Karner, 1998; Cathro et al., 2001). Collision of the Indo-Australian Plate with the Pacific and Eurasian plates to the north (Fig. 1.8), possibly initiated in the late Oligocene (>25 Ma; Pigram and Davies, 1987), continues and appears to be causing reactivation and inversion of Mesozoic structural trends all along the NWS (Figs. 1.13 and 1.14; Chapter 5; Malcolm et al., 1991; Struckmeyer et al., 1998). Continued northward drift of the Australian Plate during the Cenozoic has produced a progressive change from siliciclastic to predominantly carbonate sedimentation along the NWS (Figs. 1.12 and 3.2; Apthorpe, 1988). The NCB, today located between 18o and 22oS, has migrated from ~36-40oS in the early Oligocene (~30 Ma) to 24-28oS in the late Miocene (~10 Ma; Veevers et al., 1991; Lawver et al., 1999). Prograding carbonate barrier complexes, seismically resolved in the northern Dampier and Beagle sub-basins (Fig. 3.1), are midMiocene in age, coeval with barrier development in the Eucla Basin, Great Australian Bight (Romine et al., 1997; Feary and James, 1998). Well evidence (Fig. 3.1) suggests that carbonates with minor, intercalated mid-upper Miocene siliciclastics dominate a wedge-like Cenozoic seismic geometry (Fig. 3.3). Siliciclastic material may have been delivered by ancestral equivalents of modern rivers. Today, the largest rivers in this region are the Fortescue, Ashburton and DeGrey, each discharging >200 million m3 annually, mainly during floods (Fig. 3.1A). These rivers are fed by intermittent storm and cyclone runoff in a tropicalsemi-desert climate, and have spectacular gorges (Semeniuk, 1996). However, deltas of these modern rivers are small and are restricted to the coastal region 92 Figure 3.3 (following page). Representative MCS 2D/3D profile in two-way travel time (TWTT) and interpreted line drawing illustrating subdivision of the late Paleogene-early Neogene NCB succession into five downlapping sequences (heavy black lines) and 19 subsequences (medium black lines)(see Fig. 3.2; Table 3.1). This image is a dip-oriented composite of 2D line 136_11 and a co-linear traverse within the 3D volume (Figs. 3.1 and 3.2). Common physiographic elements (inner platform, platform, ramp, and clinoform front) are illustrated using the OM1-EMM1 downlap discontinuity surface, DLS3, as a reference. Seven wells are projected onto the line of section. The five highlighted (bold italics) are the subject of an ongoing sequence biostratigraphic study (Moss et al., in prep.), while two others provide extra age control. The thick black line over the well traces indicates the interval investigated in each well. The wells are: Eastbrook 1 (E1), ~700 m SW; Goodwyn 2 (G2), ~5 km NE; Goodwyn 4 (G4), ~900 m NE; Goodwyn 6 (G6), ~800 m SW, Goodwyn 7 (G7), ~14.5 km NE. Goodwyn 3 (G3) and Dampier 1 (D1) tie with the 2D MCS line. All wells target Cretaceous and Triassic-Jurassic reservoirs within the Rankin Trend and Dampier Sub-basin. For locations of the profile and wells, see Fig. 3.1. 93 TWTT (s) 2.0 1.0 0.0 E1 G4 G2 G7 G6 G3 D1 Extent of 3D volume 10 km V.E. ~4.6 @ 3200 m/s Fig. 3.17 1 2 3 5 10 NW 0.0 SE seafloor inclination ( ) @ 3200 m/s 0 clinoform ramp front shelf platform inner platform dls_top dls5 dls4 dls3 dls2 dls1 TWTT (s) 1.0 MM2 MM1 EMM1 OM1 OL1 basin 2.0 paleo-highs basinal lobe LEGEND dls1 downlap discontinuity surfaces MM2 major downlapping sequences Rankin Trend Dampier Sub-basin (see Fig. 3.2; Table 3.1) sub-sequence discontinuities (see Table 3.1) internal reflections hinge-line of increased subsidence (see Fig. 3.13) 94 (Semeniuk, 1996). Unfortunately, their Miocene loads are unknown. Nonetheless, Australian climates were warm and wet through the early Miocene, reaching a warm optimum in the early-middle Miocene before becoming arid in the late Miocene (McGowran and Li, 1996). This suggests that mid-Miocene rivers may have been large enough to provide the observed siliciclastics to the NWS. 3.3 METHODOLOGY Seismic data interpreted for this investigation are of two types: 1) a 1500 km2, 1980-90's vintage, 3D MCS volume, 70 km along-strike and 30 km wide, provided by Woodside Australian Energy (Figs. 2.1 and 2.2; Table 2.1), and 2) 3200 line-kilometers of 2D MCS, acquired (Table 2.2) and processed (Fig. 2.3) between 1991 and 1994 by Geoscience Australia (Fig. 3.1). frequency range of the 3D data is 40-55 Hz. The central Resultant theoretical vertical A central resolution ( l) at 1 s two-way travel time (TWTT) is 15-20 m. frequency of 25-35 Hz for the 2D data results in a theoretical vertical resolution of 23-32 m. Theoretical horizontal resolutions for the two data types (1st Fresnel zone) are 215-252 m and 275-320 m, respectively. The 2D horizontal resolution is calculated in the line of section; profiles are spaced ~10 km apart. Unless otherwise stated, all calculated declivities and thicknesses in the 2D and 3D MCS assume an average velocity of 3200 m/s, determined from time-velocity pairs provided with the 2D data (Fig. 2.4). Depth conversions were facilitated by synthetic seismograms at each of the wells (Section 2.2.3; Appendix 5). 95 Five major seismic sequences between 29.4 Ma and 10 Ma are interpreted (Figs. 3.2 and 3.3; Table 3.1), using Geoquest interpretation software. The sequences are defined between seismic discontinuity surfaces of systematic basinward (distal) downlap coincident with changes in stratal architecture or seismic attributes (Fig. 2.7). Distal downlap is a seismic geometry that represents a non-depositional hiatus resulting from the basinward termination of strata. Therefore, downlap discontinuity surfaces such as mapped in the succession can occur both within and at the base of depositional sequences, particularly if basinfloor fans are absent (Mitchum et al., 1977; Christie-Blick, 1991). Onlap and truncation seismic discontinuities are coeval with the mapped downlap surfaces, suggesting they represent sequence boundaries (Fig. 2.7). Only DLS4 at the base of sequence MM1 is characterized by downlap perched on an antecedent shelf. Five of the downlap discontinuity surfaces (DLS1, DLS2, DLS3, DLS5 and DLS_top) are correlative with sequence boundaries of supersequence 4, identified by Romine et al. (1997) using the same 2D MCS grid and ~80 wells (Table 3.2). Four of the major sequences are further subdivided into 19 sub-sequences within the 3D MCS using seismic discontinuities defined by both internal downlapping reflections and slope incisions (Table 3.1). TWTT isochron and structure contour maps were constructed to illustrate along-strike and dip-oriented variations in sequence thickness, thereby defining the nature of bounding surfaces. A Geoquest convergent algorithm was used to grid and contour interpretations, with a node spacing of ~50% of the average spacing (up to 100 m) between interpreted 3D traverses. Contouring of widely 96 Sequence Subsequence Style Basal reflection Depositional locus Internal reflection configuration Topsets Clinoform Front Bottomsets Ampl Cont/Freq Ampl Cont/Freq Ampl Cont Stratal geometry Termination patterns+ Under:Over Tesl:On/?Dn Inclination (o) - base reflection# Platform Ramp Clinoform Basin Front 2 3.5-8 14* 5 7-12* 3-4 5-6* 2-3 8-13.5* 2-3 6-9* 3-5.5 8-14.5* 4-8 10* 8 10* 6-8 10* 1 Approx. age (Ma) <10.6 Clinoform breakpoint Progradation+ Aggradation+ (km) (ms:m) Clinoform front - base reflection # Relief Length (ms) m km N/A N/A N/A Geomorphic features from 3D seismic <MM2 MM2.6 MM2.5 progradation progradation 5.4 progradation 5.3 MM2.3 MM2.2 MM2.1 MM1.4 shelf aggradation slope front fill slope front fill progradation progradation shelf progradation backstepped shelf progradation ?slope front fill shelf ?progradation progradation 4.2 shelf 5.2 5.1 DLS5 4.3 clinoform frontbasin clinoform front basin outer ramp-clinoform front outer ramp-clinoform front shelf ramp l-h l-m l-m m m-h h m l m v h h hv hv hv v m h v v v v v m/h l m m m-h m-h l g g g g v l SP - contorted SP - contorted SP - contorted SP(topsets)/C(front)/ Ch (bottomsets) SP(topsets)/C(front)/ Ch (bottomsets) disrupted Sh toe-of-slope failure at antecedent (DLS4)shelf-edge DLS_top 5.5 clinoform front clinoform front clinoform front hv hv hv v v v N/A N/A N/A N/A N/A N/A C - contorted C - contorted C - contorted Tesl:On N/A Tesl:On/Dl N/A Tesl:On <1 Tesl:On N/A Tesl:On Te/Tesl:On/Dn Tesl/Te:Dn/In <1 Te N/A Tes:On/Dn <0.5 Te;Dn <1 1.5-3 2 Te;On/Dn Tb (through OM1 ) Te/Tes;On/Dn 0.5-2 1.5-3 1.5 <1 <1 7-13B 1 1-2.5 1 1 1 1 3-5 ? 4-6 MM2 MM2.4 13.4-10.0 ? ? 1 1 2-4 -2-0 2-3 2-3 1 .5-2 <0.5 MM1 MM1.3 MM1.2 18.3-13.4 1-2 3-5.5 -- (-)31-31 ms: (-)50-50 m (-)63-(-)156 ms: (-)100-(-)250 m (-)16-25 ms: (-)25-(-)40 m 88-156 ms: 140-250 (-)31-26 ms: (-) 50-42 31-52 ms: (-) 50-83 24-66 ms: 38-106 m 12-18 ms: 19-29 m 0 ms:0 m 296-335 404-448 448-460 384-408 386-404 302 292-312 474-536 646-717 716-736 614-653 618-646 483 467-499 10.5-13 13.5-17 15-18 7-8 6-7 3.5-4 <.5-1 large gulliesasymmetric assymetric fill large gullies small gulliesaggradational cuspate toe-of-slope karst & shelf incision smooth front toe-of-slope ?failure karst smooth fronts 2.1-3.1 headless gullies MM1.1 EMM1.3 4.1 DLS4 shelf ramp clinoform front h SR h-vh h/h h l h m/h v-SR ramp v SR v Sh C SP(topsets)/Sh(front) toe-of-slope ?failure at antecedent (3.1) shelf-edge C(front)/Ch(bottomsets) C - smooth C - smooth C - smooth p SR SR C - smooth C - smooth 2-2.5 6-8 12-26* 7 -- 40-52 (-)3.5 -(-)7 .5-1 34-50 ms: 55-80 m 0-24 ms: 0-38 m 25-45 ms: 40-72 m 30-50 ms: 48-80 m 10-20 ms: 16-32 m 2-16 ms: 3-26 m (-)15-15 ms (-)24-24 m 10-20 ms: 16-32 m 16 ms: 26 m --240-252 222-256 64-83 384-403 355-410 N/A 1.5-3 1-2.5 EMM1 EMM1.2 3.2 <1 1 shelf ramp m-g/l 20.1-18.3 .5-1 EMM1.1 OM1.6 OM1.5 3.1 DLS3 2.5 <1 Te;On/Dn <1 2 2-3.5 3 1.5 1.5 0.5 0.5 2.5 6 4-7 6 5.5 5.5 4.5 4 4 1 1 222-254 1.5-2 1 193-256 207-260 190-266 216-252 130-198 188 - 355-406 309-410 331-416 190-425 346-403 208-317 300 2-3 1.5-2 2-3 2-4 2.5-3 2.5-3 clinoform front shelf ramp-clinoform front shelf ramp-clinoform front clinoform front clinoform front clinoform front shelf ramp and clinoform front mid-ramp - adjacent increased declivity due to subsidence and flanks of antiform m-h SR h l h l-m/m l-m m-h p-m g m m-g/h g/h g g v l SR SR SR m p Te;Dn <1 Te;On/Dn Te:On/Dn <1 Te;On/Dn <0.5 1.5 <1-3 ?:Dn <0.5 Te/Tes:On/Dn <0.5 <1-2 1 <1 1 g/l p/l g/l m-g l-h l-m m-h l-m m 1-1.5 progradation OM1 OM1.4 OM1.3 OM1.2 OM1.1 2.4 2.3 2.2 2.1 DLS2 25.4-20.1 1 1.5-2 1.5-3.5 2-3 -- OL1 ramp progradation gradual shelf edge N/A N/A l-m N/A N/A Si - ramp, no shelf break Tes +;On/Dn + 29.425.4 -- DLS1 landward on ramp -- up to 5 -- <1 * concave profile failure scarp buildup measurements relate to reflection at base of (sub)-sequence, e.g., DLS2 at the base of OM1 + through the (sub)-sequence B # Reflection character Ampl-Amplitude: l-low, m-moderate, h-high, v-variable Cont - Continuity: p-poor, m-moderate, g-good, v-variable, rf - reflection free SR - single reflection Freq - Frequency:l-low, m-moderate, h-high Termination patterns underlying reflections:overlying reflections Te - low angle truncation on shelf Tes - shelf truncation Tesl - slope truncation Tb - basin truncation In - internal convergence Stratal geometry - prograding clinoforms Si - sigmoid Ob - oblique C - complex sigmoid-oblique Sh - shingled Stratal geometry - others P- parallel SP - subparallel D - divergent Ch - chaotic RF - reflection free M - mounded SR - single reflection Table 3.1. Summary of seismic observations defining the five downlapping sequences and 19 sub-sequences. Note all dip and thickness calculations assume a mean velocity of 3200 m/s, determined from stacking velocity in the 2D data (see Fig. 2.4). No corrections are made for subsidence or compaction. 97 small submarine canyons 5.3-5.5 400 640 6.5-11 Sequence Basal reflection DLS_top DLS5 DLS4 DLS3 DLS2 DLS1 * approximate change in location of depocenter of consequetive sequences ** assuming 3200 m/s ***shaded area in Fig. 10 represents 40-100% thickness of depocenter (given in column 8) MM2 MM1 EMM1 OM1 OL1 Age Corelative reflections Depositional Depocenter migration* Maximum thickness** (my) (Romine et al., 1997) Locus strike (km) dip (km) (ms, TWTT) (m, thickness) 10.0 +/- 6.4 late Miocene 13.4+/- 3 arcuate strike-elongate 110 SW 7-15 680 1088 late mid Miocene 18.3 +/- 3.1 NE 80 NE 1-7 420 672 20.1 +/- 3.7 strike-elongate/diffuse 7-10 320 512 early Miocene 25.4 +/- 4 strike-elongate 65 SW 8-20 (SW-NE) 260 416 late Oligocene hiatus 29.4-37.9 NE 275 440 middle Oligocene Width (km) - shaded (Fig. 10) (> 40% maximum thickness-TWTT) 20 (SW)-50 (NE) - (~280 ms) 5-15->30 (in NE) - (~160 ms) 10-20 variable - (~120 ms) <10-~20 - variable (~100 ms) 14(SW)-61 (NE) - (~100 ms) Orientation NE-SW NE-SW NE-SW NE-SW NE-SW Table 3.2. Description of the five major sequences. Age estimates are compiled from well-completion reports and Moss et al. (in prep.) (Appendix 1; Plates 1-7). Depocenter migrations in dip- and strikedirections are relative to the preceding sequence. Width to 40% maximum thickness describes the area thicker than represented by light green-yellow in Fig. 3.12. Maximum thickness of each sequence is provided. All sequences are oriented NE-SW, prograding to the northwest (see Figs. 3.13 and 3.14). The major downlap discontinuity surfaces interpreted in this study are correlated with sequence boundaries defined by Romine et al. (1997). 98 spaced 2D profiles was performed separately to allow for the greater line spacing, typically ~10 km. All maps are uncorrected for the effects of compaction, subsidence and isostasy because although simple compaction will decrease slope gradients (Fig 2.22), the overall effects of burial during clinoform progradation are complex (Chapter 5). Steckler et al. (1993) used 2D backstripping on the New Jersey margin to show that rotation resulting from differential loading, combined with compaction, tend to increase rather than decrease slope declivities in the clinoformal succession there. Similarly, slope measurements of clinoform geometries in the NCB may overestimate pre-burial slopes (Chapter 5). In the 3D data, a Geoquest variance volume generated from the standard amplitude volume highlights structural as well as stratigraphic features (Fig. 2.10). Variance, similar to coherence, is a measure of similarity (cross- correlation) of adjacent traces in in-line and cross-line directions, integrated over a small vertical analysis window (Bahorich and Farmer, 1995; Marfurt et al., 1998). The variance volume is calculated using a 5X5 horizontal window (i.e., a 25-trace operator) and a 50 ms vertical window. Horizontal time slices and horizon slices are also used to highlight structural and stratigraphic features (Figs. 2.8-2.10). Because the horizon slice displays spatial distribution of seismic amplitude or variance over a single interpreted reflection, it removes effects of structures that remain in horizontal time slices. In the variance volume, both horizontal time slices and horizon slices highlight shelf depressions and downslope transport paths. 99 Paleo-water depth estimates are based on statistical analyses of depthranges of all benthic foraminiferal species identified in samples from available wells: Goodwyn 2, 4, 6, 7, and Eastbrook 1 (Fig. 3.1; Moss et al., in prep.). Goodwyn 3, Dampier 1, Lambert 1 and Eaglehawk 1, outside the 3D volume were not included in the paleobathymetric study. Depth ranges of a complete assemblage are calculated from the relative abundance of each species, combined with published depth-range information. Such information is typically derived by comparison with the same taxa studied in modern environments. These analyses have been conducted by Moss et al. (in prep.) using the Integrated Paleontological System (IPS) software developed at The University of Utah (Gary, 1999). A total of 227 samples, consisting of ditch cuttings and sidewall core with an average spacing of 29 m, have been examined at topset, slope front and bottomset locations (Fig. 3.3; Appendix 1). The marine benthic depth zones are: transitional marine, 0 m; inner neritic, 0-20 m; middle neritic, 20-100 m; outer neritic, 100200 m; upper bathyal, 200-500; middle bathyal, 500-1000 m; lower bathyal, 1000-2000 m, and abyssal, >2000 m (Murray, 1991). At each well, synthetic seismograms (Appendix 5) allow the seismic stratigraphy measured in TWTT to be compared directly with the paleobathymetric, lithologic and chronostratigraphic data referenced to depth downhole. The seismograms are created using downhole sonic measurements calibrated with check shot surveys; the resultant reflection coefficient series is convolved with a wavelet derived from the seismic data at each well location (Section 2.2.3). 100 3.4 RESULTS The late early Oligocene (29.4 Ma) to early late Miocene (10.0 Ma) succession is dominated by stacked clinoforms prograding to the northwest (Fig. 3.3). Approximately 26 km of progradation is observed, with only ~300 m of shelf-edge aggradation during this interval. The five major sequences (OL1, OM1, EMM1, MM1, and MM2) each span ~2-5 m.y. (Table 3.1). While boundaries between sequences (DLS1-DLS5, DLS_top) are dominated by downlap basinward, onlap and truncations on the shelf are also observed (Fig. 3.3; Table 3.1). Four of the mapped sequences within the 3D volume are divided into 19 sub-sequences, defined by basinward downlap, landward onlap and truncation on the shelf and clinoform front. The oldest sequence, OL1, is excluded, as only its toe-of-slope portion is imaged within the 3D volume (Fig. 3.3; Table 3.1). Each sub-sequence represents ~0.5-1 m.y. allowing for an ~1 m.y. hiatus in MM1 and MM2 (e.g., at top Tulki Limestone, Fig. 3.2; Table 3.1; Chaproniere, 1984). Several stratigraphic features are common to all sequences as highlighted on DLS3 at the top of sequence OM1. First, the shelf region of each mapped downlap surface is characterized by multiple inflections (Fig. 3.3; Table 3.1): an inner platform where dips reach ~2o, generally decreasing up-section, and a subhorizontal platform (<1o) separated seaward from a shelf ramp dipping 0.5-3o. Truncation is commonly low-angle and persistent across broad areas on the shelf. Isolated U-shaped incisions are located at the landward limits of the 2D MCS control (Fig. 3.4), but the 10-100 km 2D line spacing makes it impossible to connect these incision features with each other and with the progradational 101 1.0 TWTT (s) 1.5 V.E. ~ 4:1 2 km 1.0 DLS2 DLS1 1.5 Figure 3.4. Uninterpreted and interpreted 2D seismic line 101r_09 illustrating broad incision up-dip from the clinoforms. Location of the seismic example is given in Figure 3.1B. 102 succession imaged in detail within the 3D volume. Second, each breakpoint is typically gradational, separating the shelf ramp from a steeper clinoform front (Fig. 3.3). The generally convex upper clinoform front dips 2-8o. However, local dip variations up to 26o are associated with concave-upward features interpreted as front failure or incisions. Third, the region basinward of the clinoform fronts is relatively sediment starved; depositional lobes are rare. Finally, seismically defined build-ups commonly seen proximal to prograding carbonate successions elsewhere, e.g., in the Bahamas (Eberli and Ginsberg, 1989), are not commonly observed in the NCB (Fig. 3.3). However, several mid-Miocene shelfal build-ups occur in the 2D MCS (Romine et al., 1997, their Fig. 16), and one isolated mound is observed in the 3D volume (Fig. 3.5). 3.4.1 NCB Structural Trends Northeast-southwest oriented Mesozoic structural trends in the NCB, some of which are reactivated in the Cretaceous and Tertiary, affect OligoceneMiocene sediment distribution (Figs. 3.1 and 3.3). For example, increased declivities occur along northeast-southwest oriented hinge-lines overlying the Madeleine and Rosemary trends (Fig. 2.1). Thermal subsidence, compaction and asymmetric loading engendered by the progradation of sediments may have resulted in the increased reflection inclinations observed on the inner shelf platform, which is superimposed over the Dampier Sub-basin. Inversion uplifts along these Mesozoic structures also occur, and will add to the increased declivity on the shelf platform. One such antiform overlies the hanging-wall block of the Rosemary Fault, the eastern boundary of the Dampier Sub-basin (Fig. 1.14). 103 NW 1.0 1.1 Goodwyn 2 136_19 SE 1.2 TWTT (s) 1.3 V.E. ~5:1 1.0 Goodwyn 2 136_19 2 km 104 2 50 1.1 0 10 0 MM1.1 EMM1.2 1.2 MM1.2 EMM1.1 DLS4 EMM1.2 DLS3 1.3 Figure 3.5. Uninterpreted and interpreted 3D seismic traverse illustrating a mound in sequence MM1. The feature, onlapped by shingled reflections from MM1.1, is initiated either late in EMM1 or early in MM1. Location given in Figure 3.1. 104 Onlap occurs on its flanks and truncation over its crest (Fig. 3.1; Chapter 5). Its western (seaward) flank also constitutes part of one of the northeast-southwest oriented hinge-lines. Depocenters along the eastern (landward) limb of this antiform indicate that it was active during the early-middle Miocene. MioceneRecent inversion features are also identified onshore (Fig.3.1; Malcolm et al., 1991) and in other basins of the NWS, e.g., Browse Basin (Fig. 1.13; Struckmeyer et al., 1998). Two broad (~10 km wide) paleo-highs of uncertain origin are associated with the Rankin Trend (Fig. 3.3). The eastern one corresponds to the eastern limit of that trend and is 12-15 km wide, ~17 km long and 60-85 ms (96-136 m) high. The western paleo-high, 10-17 km wide, 20-25 km long with 40-75 ms (64-120 m) of relief, is west of the deep-seated Victoria Syncline (Fig. 3.1). 3.4.2 NCB Clinoform Morphologies The base of the mapped progradational succession, represented by sequence OL1 and marked by boundary DLS1 (Table 3.1), is a broadly concaveupward to linear ramp with a maximum seaward gradient of ~5o (Figs. 3.3 and 3.6). This ramp is generally smooth, with the exception of isolated, 2-6 km wide, 30-100 ms (50-160m) deep, U-shaped incisions recognized in 2D MCS profiles, 5-15 km landward of the 3D volume. A breakpoint is absent (Figs. 3.3 and 3.6). DLS2, the top of OL1 and the base of OM1, is a generally smooth ramp that is distally-steepened in places to a maximum of ~4o (compare Figs. 3.3 and 3.6). As is the case with DLS1, broad, shallow incisions are located ~15-20 km to the east, on the 2D MCS profiles (Fig. 3.4). 105 NW 1.0 136_19 136_20 101r_02 SE TWTT (s) 1.5 V.E. ~4:1 1.0 DLS2 1.5 136_15 1 DLS 2 km Figure 3.6. Uninterpreted 2D MCS line 136_15 (TWTT) and line drawing interpretation of sequence OL1, which varies along-strike from a sigmoidal ramp in the southeast (Fig. 3.3) to a distally steepened ramp in the northwest. Arrows indicate reflection terminations. Note differences between seismic downlap surfaces DLS1 and DLS2 (Table 3.1). Location of the profile and wells is given in Fig. 3.1. 106 In comparison with DLS1 and DLS2, DLS3 has a well-defined breakpoint separating a shallowly seaward-dipping (2-3.5o) ramp from the convex-upward, smooth front (Figs. 3.3 and 3.7). Inclination on the front increases seaward to 7o (Table 3.1). OM1 sub-sequence downlap discontinuity surfaces between DLS2 and DLS3 are smooth and show a progressive increase in front inclination from 4 to 6o; front length, here defined as the distance between the breakpoint and inflection at the toe of slope, is typically 2 4 km (Fig. 3.7; Table 3.1). Northwest-directed progradation is maintained throughout OM1, with a horizontal to slightly down-stepping shelf-edge trajectory (Fig. 3.7). Both DLS2 and DLS3 are truncated in the basin by a broad, strike-oriented, U-shaped incision centered over the western paleo-high (Figs. 3.3 and 3.7). The incision surface is ~5 km wide and continues for ~10 km along-strike. Its fill consists of low-amplitude, discontinuous reflections. DLS4, the top of sequence EMM1, also maintains a smooth, convexupward profile, with a front inclination of 6-8o (Fig. 3.7 and Table 3.1). Greater variability, associated with rare, steeply dipping (up to 26o) concave-upward profiles, occurs on this boundary regionally, as identified on 2D MCS profiles (Fig. 3.8). When compared with OM1, shelf-edge migration in EMM1 is upward, suggesting aggradation as well as northwest-directed progradation (Fig. 3.7). Front declivity increases on EMM1 sub-sequence downlap surfaces 3.1 and 3.2; front length ranges between 1 and 3 km (Table 3.1). Numerous, isolated incisionlike features characterize sub-sequence downlap discontinuity surface 3.1. These are up to 38 ms (~60 m) deep and 170-250 m wide, and extend for ~30 km2 over 107 Figure 3.7. (following page). Uninterpreted and interpreted 3D seismic traverse illustrating sub-sequences within downlapping sequences OM1, EMM1 and MM1 (Table 3.1). Location is given in Fig. 3.1. The shelf-break (colored squares) is defined as the maximum change in inclination (i.e., the inflection point) between the shelf and clinoform front. Migration of the shelf-edge between the sub-sequences indicates that progradation rather than aggradation has dominated during their development. The shelf-edge trajectory is horizontal to slightly down-stepped in OM1, and upward-directed in EMM1 and MM1. A landward backstep is detected in MM1. Clinoforms defining OM1 and EMM1 are not incised, but they are overlain by densely incised clinoforms in MM1. A broad incision basinward of the clinoform toes and superimposed over the western paleo-high (see also Fig. 3.3) truncates reflections in the OM1 and EMM1 sequences. The crossing with 2D profile 136_19 is shown. 108 1.0 1.5 TWTT (s) NW 2 km V.E.~5:1 136_19 SE DLS5 DLS5 4.1 MM1.4 MM1.3 MM1.1 EMM1.3 EMM1.2 EMM1.1 OM1.6 OM1.5 OM1.4 OM1.3 OM1.2 TWTT (s) 109 1.0 4.3 4.2 .2 MM1 DLS4 3.2 2.5 2.3 2.2 3.1 DLS3 2.4 MM1 EMM1 OM1 2.1 OM1.1 1.5 discontinuous, lowamplitude reflections DLS2 DLS1 OL1 DLS1 2.1 paleo-high shelf edge shelf edge migration basinal truncation surface downlap discontinuity boundaries sub-sequence downlap boundaries internal termination patterns 109 NW 136_19 SE TWTT (s) 1.50 1.25 V.E. ~6 @ 3200 m/s 136_19 1 km DLS5 DLS4 TWTT (s) 1.25 DLS3 MM1 EMM1.2 and 1.3 OM1 DLS2 Figure 3.8. Uninterpreted and interpreted portion of 2D MCS line 136_10 illustrating steeply dipping concave-upward upper slope on DLS4. This morphology is interpreted as failure of the shelf edge, removing part of the EMM1.2 and EMM1.3 sub-sequences (compare with Figure 3.7). Red arrows indicate truncation beneath DLS4. Location is given in Fig. 3.1. The crossing with 2D profile 136_19 is shown. 1.50 110 10-12 km along-strike, 4-8 km landward of the shelf-break. Overlying reflections are disrupted (Fig. 3.9A). In map-view, these features form circular closed depressions. The landward limits are marked by arcuate depressions that possibly represent coalesced circular features preferentially developed along an arcuate trend (Fig. 3.9B). A single mound is located on the DLS4 shelf, onlapped by MM1.1 reflections (Fig. 3.5). This mound is slightly elongate along-strike, with dimensions of ~1200 m by ~2000 m and a total relief of 40 ms (~70 m). The height:breadth ratio varies between 1:17 and 1:28. Internal reflections mimic the outer surface, but exhibit lower relief (Fig. 3.5). Sub-sequence downlap surfaces between DLS4 and DLS5 (Table 3.1) divide the MM1 sequence into two parts. The older (MM1.1-MM1.2), shelf-restricted interval is represented by the shallowly dipping, shingled (<2.5o) to distally-steepened (6o-8o) ramp represented by 4.1 and 4.2. The younger, complex oblique-sigmoidal succession (MM1.3MM1.4) downlaps into the basin (Figs. 3.3 and 3.7). A backstep occurs southeast of the DLS4 shelf-edge in the lower part of MM1 (Figs. 3.3 and 3.7; Table 3.1). A northwest, upward shelf-edge trajectory is reestablished in sub-sequences MM1.3 and MM1.4; aggradation and progradation have resumed (Fig. 3.7). On 4.1, a U-shaped, incision-like feature, ~38 ms (60 m) deep and up to 500 m wide, is mapped for ~8 km along-strike (Chapter 4; Cathro and Austin, 2001). This feature truncates shingled reflections and is superimposed on a subtle break in slope on the ramp, from near-horizontal to 0.5o seaward (Table 3.1). This feature is morphologically similar to incision-like features identified in sub- 111 (A) 1.0 TWTT (s) 1.5 1.0 NW 20 50 0 10 V.E.~2.5:1 2 km C B A SE DLS4 3.1 DLS3 3 3.2 2 1 1.5 1 - EMM1.1 2 - EMM1.2 3 - EMM1.3 116oE (B) N CB A Fig. 3.9A Progradation direction Goodwyn 7 2 km Figure 3.9. (A) Uninterpreted and interpreted seismic section highlighting horizon used to generate horizon slice illustrated in (B). Depressions A, B and C, all characterized by truncation along sub-sequence downlap surface 3.1 (Table 3.1), are correlated with features identified on the horizon slice. The depressions are located ~4 km landward of the shelf-break. (B) Variance TM horizon slice at sub-sequence downlap surface 3.1. Location given in Figure 3.14B. Circular and arcuate features are identified over an area ~30 km2 and ~13 km along-strike. Circular features are 170-250 m in diameter and ~60 m deep. The arcuate nature of the up-dip limit is suggestive of a failure scarp (see text for details). 112 sequence EMM1 at surface 3.1 (Fig. 3.9). Sub-sequence boundary 4.1 is further disrupted by a ~2 km wide, 45 ms (~72 m) deep, asymmetric dip-oriented incision (Fig. 3.10). The incision is filled with high-amplitude, onlapping reflections of MM1.3 (Chapter 4). Both types of truncation occur landward of the onlap edge of reflection 4.2 that marks the landward depositional limit of sub-sequence MM1.2. A distally-steepened MM1.2 ramp progrades to the preexisting EMM1 shelf-break; both convex- and concave-upward slopes are identified at the toe-ofslope, downlapping onto the antecedent shelf (Fig. 3.7). Progradation basinward recommences with the volumetrically more significant MM1.3 and MM1.4 subsequences (Fig. 3.7). In contrast to underlying sequences, the fronts of MM1.3 and MM1.4 are dissected by numerous V-shaped aggradational incisions (i.e., vertically stacked) up to 300m wide, 40-70 m deep and <500 m apart. Front lengths defined by the tops of these sub-sequences, 4.3 and DLS5, respectively (Table 3.1), increase to 3.5 - 7 km. DLS5 has a more complex morphology than older downlap surfaces; it exhibits shelf build-ups and its front is also highly dissected (Fig. 3.11). Truncation along DLS5 is up to 115 m deep and 800 m wide. Declivities of 7o13o are associated with build-ups typically ~1 km wide and 60 ms (~100 m) high (Table 3.1; Romine et al., 1997, their Fig. 17). These build-ups are located on an otherwise shallowly dipping (<1o) shelf, 20-35 km landward of the shelf-break. Front inclination is variable along-strike (Table 3.1). Convex-upward clinoform fronts dip 4-8o, and locally steepened concave-upward regions, exhibit local dips up to 10o. Relief increases to >380 ms (600 m; Figs. 3.3 and 3.7; Table 3.1). 113 SW 1.0 NE TWTT (secs) 1.25 V.E. ~ 15:1 1.0 MM1.4 MM1.3 MM1.1 DLS5 4.3 4.1 DLS4 2 km 1.25 3.1 Figure 3.10. Strike-oriented uninterpreted and interpreted 3D seismic traverse illustrating asymmetric incision on the shelf at sub-sequence downlap surface 4.1. The incision is filled with high amplitude concave-up to linear reflections. Incision-like features at sub-sequence downlap surface 3.1 are interpreted as karst, forming closed circular depressions in plan view (see Fig. 3.9). Location is given in Figure 3.1B. Note the high vertical exaggeration. 114 Figure 3.11. (following page). Uninterpreted and interpreted (A) dip- and (B) strike-oriented seismic sections illustrating the division of sequence MM2 (unshaded) into six packages bounded by front incisions. Two downlapping packages are identified on the shelf. The thick black line indicates the tie point for the orthogonal sections. Approximate location of the shelf-break (red square) for each sub-sequence indicates that progradation rather than aggradation has dominated during their development. The shelf-edge trajectory is horizontal to slightly down-stepped in MM2.1 and MM2.2, backstepped in MM2.3 (with shelf aggradation) and down-stepped in MM2.4MM2.6. V-shaped front incisions are ubiquitous, filled with asymmetric and to a lesser degree symmetric reflections. The dominant northeast progradation direction of the incision-fill suggests interaction of downslope processes with a northeastdirected, along strike current. Location of the seismic traverses is given in Figure 3.1. 115 (A) NW SE TWTT (seconds) 1.5 1.0 V.E. ~4:1 DLS_TOP 4 2 1 3 5 km sand sand 1.0 DLS5 6 MM2 5 MM1 and older (B) 1.0 1.5 TWTT (seconds) 1.25 1.0 1.25 116 DLS5 5.1 downlap discontinuity surfaces sub-sequence downlap surfaces shelf edge shelf edge migration sub-sequences MM2.1-MM2.6 internal termination patterns incisions incision fill SW V.E. ~4:1 DLS_TOP 6 5 2 km 5.5 NE 5.4 1- 6 MM2 5.2 4 2 1 3 5.1 5.3 DLS5 116 DLS5 marks the boundary between smaller, stationary front incisions within MM1 and larger, migrating incisions within MM2 (Fig. 3.11). In fact, MM2's six sub-sequences, MM2.1-2.6, are all defined by incisions along their fronts (Table 3.1). The V-shaped incisions along sub-sequence boundaries, 5.15.5, are up to 1-2 km wide and 150-200 m deep. They are stacked either vertically or with slight offsets. Incision fill is generally asymmetric relative to flanks, building to the northeast (Fig. 3.11). Symmetric, vertically stacked fill patterns are less common in this complex interval. A basinward-stepping, slopefront-fill seismic architecture in MM2.1 and MM2.2 replaces the prograding clinoform morphology of earlier sequences. Front declivity decreases from ~4o8o (DLS5) to ~2o-3o on sub-sequence downlap surfaces 5.2 and 5.3 (Table 3.1). However, local declivities up to 14.5o are associated with incision flanks. A backstep during MM2.3 is followed by continued progradation to the end of MM2. Inclination on the MM2.6 front at DLS_top increases to 8o (Table 3.1). Front length increases from <4-7 km, identified on MM1 and older sequences, to 15-18 km at the base of sub-sequence MM2.3, but decreases again to 6.5-11 km at the top of the succession (Table 3.1). The MM2 shelf is characterized by two wedges of downlapping reflections within the MM2.3 and MM2.6 sub-sequences, ~10-30 km landward of the shelf-break (Fig. 3.11). These shelf-restricted packages are characterized by downlap and onlap in the dip direction (Fig. 3.11), with strike-oriented downlap and progradation to the northeast (Fig. 3.12). MM2 packages constitute the oldest part of a larger, northeast-prograding succession (Fig. 3.12). Declivities of the 117 SW 136_12 0.25 Eaglehawk 1136_13 Lambert 1 (7 km SE) NE 136_14 136_15 101r_10 0.5 TWTT (s) 0.75 0.25 V.E. ~18:1 (@ 2400 m/s) 10 km Mound 0.5 0.75 DLS_TOP MM2 Erosional Base Figure 3.12. Uninterpreted 2D MCS seismic strike section 136_19 (see Figs. 3.1 and 3.7 for location) and line drawing interpretation illustrating northeast (along-strike) progradation of quartz-sand rich shelfrestricted packages within and above the MM2 sequence. This prograding interval is overlain by a mounded package with bi-directional downlap, suggesting that the direction of progradation has become oblique to the line of section within that interval. Crossings with other 2D MCS profiles are shown at the top of the profile, as is the intersection with the Eaglehawk 1 well. Note the high vertical exaggeration. 118 prograding foresets decrease upward from 2o to <0.5o; their average amplitude is 90-130 ms (~100-150 m). Individual clinoforms downlap onto a high-amplitude, irregular scour contact that rises to the northeast. Stacked on the northeastern limit of these foresets is a symmetrical mound exhibiting bi-directional downlap, onlapped by adjacent reflections. 3.4.3 Distribution of Depocenters Regional variability Two-way travel time (TWTT) isochron maps constructed using the 2D MCS for the five major sequences (Table 3.1) illustrate regional variations in OligoceneMiocene preserved sediment accumulations (Figs. 3.13A-F). Depocenters for all mapped sequences are oriented along-strike, northeast-southwest, but their distribution varies. Northwest-directed progradation tends to be greater in the northeast, particularly between OL1 and OM1 (compare Figs. 3.13A and 3.13B) and between EMM1 and MM1 (compare Figs. 3.13C and 3.13D, see also Fig. 3.13F). OL1's depocenter has a maximum vertical thickness of 275 ms (~440 m) and a width of >60 km (Figs. 3.13A and 3.13F; Table 3.2). The thickest accumulation is restricted to the northeast, narrowing to ~5-15 km and thinning to 120-160 ms (~190-260 m) to the southwest. West of the main clinoform depocenter exists two depositional lobes (Fig. 3.13A). The southwestern lobe is mounded, ~20-40 km wide and up to 120 ms (~190 m) thick, and composed of both chaotic and transparent facies (Figs. 3.3 and 3.13A). The lobe to the 119 1150E 1160E 1170E 1150E 1160E Chaotic 1170E 1150E 1160E 1170E A 0 ms 275 truncation Chaotic lobe B 0 ms 275 C 0 200 ms 325 200 10 0 190S 190S 19 S 19 S 19 S 190S transparentchaotic lobe 100 100 200 200 1 00 20 0 0 10 100 0 100 10 10 10 0 0 100 20 S 20 S 20 S 200S 200S 200S N OL1 29.4-25.4 Ma 40 km 115 E 0 N OM1 25.4-20.1 Ma 40 km 0 N 20 0 EMM1 20.1-18.3 Ma 40 km 116 E 100 100 0 117 E 0 115 E 116 E 300 0 117 E 0 115 E 0 116 E 0 117 E 0 D 0 ms 425 0 20 2 190S 190S 190S 190S 190S 190S 190S 190S 190S E 0 ms 700 50 0 40 0 3 19 S 19 S 19 S 19 S 19 S 19 S 190S F OL1-MM2 29.4-10 Ma 19 S 0 1 600 10 0 0 20 3 10 10 0 20 0 0 M 100 10 0 N MM1 18.3-13.4 Ma 40 km N MM2 13.4-10 Ma 40 km N MM2 MM1 EMM1 OM1 OL1 40 km 20 S 20 S 20 S 20 S 20 S 20 S 200S 20 S Figure 3.13. Gridded and contoured TWTT isopach maps for each of the five major sequences (Figure 3.3; Table 3.1), derived from the 2D MCS coverage. All sequences exhibit strike-oriented depocenters prograding northwest. Progradation is generally greater in the northeast. Other along-strike variations in location, width and distribution also occur. For example, secondary depocenters are located east of the main prograding clinoforms in C-E. Note the color scale changes between panels. Superposition of depocenter trends and loci through time are also shown (F). The color scheme is the same as defined in Fig. 3.2. 120 200S 200S 200S 200S 200S 200S 200S 200S 200S 0 seismic examples, Figures 3.3 (southwest) and 3.6 (northeast) hinge-line of increased subsidence anticline 3 seismic build-up (D,E: smaller numbers =older) TWTT thickness secondary depocenters (C, D, E) increasing northwest (Fig. 3.13A) is ~40 km wide and consists of low-amplitude, mounded and chaotic reflections that may include both OL1 and OM1 sequences. OM1's main depocenter is up to 20 km wide and 260 ms (~416 m) thick, displaced ~8-20 km northwest and >60 km southwest of the OL1 depocenter (Figs. 3.13B and 3.13F; Table 3.2). OM1 is more evenly distributed than OL1, maintaining a thickness of >180 ms (290 m) for ~160 km along-strike. EMM1's maximum thickness is >300ms (480 m), occurring ~7-10 km northwest of the OM1 depocenter (Figs. 3.3, 3.7, 3.13C and 3.13F; Table 3.2). EMM1 maintains a relatively constant thickness, 100-140ms (160-220 m) over a region ~30 km wide. This depocenter is wider northwest-southeast than OM1, thinning only gradually landward. A secondary depocenter is located east of the interpreted inversion anticline beneath the shelf (Fig. 3.13C). MM1 looks similar to OL1, but is displaced basinward because of continued northwest progradation of the entire succession (Fig. 3.13D). Its maximum sediment thickness, 420 ms (~670 m), is concentrated in the northeast, thinning and narrowing to the southwest. The depocenter has prograded 1-7 km northwest and is displaced ~80 km northeast relative to EMM1 (Figs. 3.13D and 3.13F). The depositional locus is also basinward of observed shelf build-ups, mapped as prograding barriers northeast of the 3D volume (Fig. 3.13D; Table 3.1; Romine et al., 1997, their Fig. 17). As is true with EMM1, a secondary depocenter is interpreted east of the inversion anticline on the shelf. MM2 forms a linear to slightly arcuate depocenter (Fig. 3.13E). Its maximum thickness is 680 ms (~1080 m), shifted >100 km to the southwest and 121 7-15 km northwest relative to MM1 (Fig. 3.13F). Build-ups backstep ~30 km and are not associated with a depocenter basinward, as is true for MM1. A second, strike-oriented depocenter landward of the main depocenter corresponds to shelfrestricted wedges identified in MM2.3 and MM2.6 (Figs. 3.11, 3.12 and 3.13E). Variability within the 3D volume The distribution of sub-sequence depocenters within the 3D volume highlights variations at intervals of ~1 m.y. (Fig. 3.14). OM1.1-OM1.6 are linear and northwest-prograding (Fig. 3.14A). However, they become less separated upward, suggesting decreased progradation through time (Fig. 3.7; Table 3.1). In the basin, OM1 sub-sequences are truncated (Fig. 3.7). In contrast with OM1, EMM1.1-EMM1.3 depocenters are more complex (Fig. 3.14B). EMM1.1 is linear. However, its zero edge, represented by downlap of boundary 3.1 (Table 3.1), is lobate. Circular and concave-seaward, arcuate, incision-like depressions identified in vertical section are located landward (Fig. 3.9). EMM1.2 is evenly distributed on the shelf (Fig. 3.7), with only local increases in thickness along the shelf-edge. EMM1.3 is also linear, but basinward thinning is lobate. All EMM1 sub-sequences are truncated in the basin (Fig. 3.7). MM1.1 is generally characterized by shingled downlap onto the EMM1 shelf (Fig. 3.7). This sub-sequence is irregular but thickest in the northeast (Fig. 3.14C). Its zero edge, represented by downlap of 4.1 (Table 3.1), is landward of the EMM1 (DLS4) shelf-edge (Fig. 3.7). MM1.1 has therefore retrograded with 122 115040`E 1160E 115040`E 1160E OM1.1-OM1.6 10 km A 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 19030S EMM1.1-EMM1.3 B S4 .1 DL 3 G NIN IN TH 3 6 DN 4.1 3.1 DN E 6 5 4 3 2 1 youngest (lightest) KEY TO A-D 10 km A-C D 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19 30S 19030S sub-sequences 4 DLS3 2.5 2.4 2.3 2.2 115040`E 3 R. 3.2 TR M M 3 1. k Fig. 3.9B oldest (darkest) 2 1 Fig. 3.7 Fig. 3.15A,B 1160E MM1.1-MM1.4 Fig. 3.15D,E C 4 LSion D 4 in cis 0 0 0 0 0 0 19030S 5 km 4. 3 1 1 D N 3 S5 DL .1 4 .2 DN R 4 /T k ON Fig. 3.7 Fig. 3.5 M 10 km MM2.1-MM2.6 123 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 3.2 T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T T 5 2 1 3.2 LS3 D Fig. 3.7 seismic examples wells M - mound k - karst depressions truncated zone Termination patterns on mapped surfaces ON - onlap TR - truncation DN - downlap 10 km 1160E DLS5 Approximate shelf5.5 edge location Fig. 3.15F D 19030S 4 5 3 2 5.5 Incisions 5.5 5.4 5 5. 5.3 5.2 Fig. 3.11B 5.1 DLS5 S5 DL 1 DL S5 i gr on ati 3 DN 2 MM .6 DN .3 M2 M Fig. 3.11A Figure 3.14. Distribution of sub-sequences depocenters, shelf-edges and termination patterns interpreted within the 3D volume. Darkest gray represents the oldest sub-sequence in any interval. OL1 is not included, as only the toe-of-slope is imaged in the 3D volume (see Fig. 3.3). (A) Approximately linear depocenters and downlap edges of OM1. (B) Linear depocenters with lobate downlap edges and sub-sequence thinning patterns of EMM1. (C) Broadly distributed shelf sub-sequences with linear and cuspate downlap/truncation zero edges dominate the initial stages of MM1. In contrast, MM1.3 and MM1.4 depocenters are linear, with lobate downlap edges. (D) Clinoform front depocenters within MM2, determined by location of vertically stacked and slightly offset downslope incisions (Fig. 3.11). Shelf-restricted downlapping wedges, (see Figs. 3.7 and 3.12) in MM2, parallel the preexisting shelf-edge. See Table 3.1 for nomenclature. Ra Ra Ra Ra Ra Ra Ra Ra Ra Ra Ra Ra Ra Ra mp d d d d d d d d d d d d dg ed em respect to EMM1.3, and it is truncated by the only dip-oriented shelf incision recognized in the 3D MCS (Figs. 3.14C and 3.10). Linear to concave-seaward, arcuate depressions (Chapter 4) and an oval mound (Fig. 3.5) are all southwest of this depocenter (Fig. 3.14C). MM1.2 thins at the EMM1 (DLS4) shelf-edge (Fig. 3.14C). Embayments in its downlap edge, defined by 4.2 (Table 3.1), correspond to concave-upward reflections interpreted at the toe-of-slope (Fig. 3.7). In contrast, MM1.3 and MM1.4 prograde seaward of the EMM1 shelf-edge (Figs. 3.7 and 3.14C). Their depocenters are generally linear. The MM1.3 downlap edge, defined by 4.3 (Table 3.1), is lobate and varies along-strike. Preservation of MM2 sub-sequences is dictated by location of vertically stacked and prograding front incisions (Figs. 3.11 and 3.14D). Shelf-restricted downlapping packages interpreted within MM2.3 and MM2.6 parallel preexisting shelf-edge trends. Up to the middle of MM2, individual incisions tend to be longer bottom-to-top, corresponding to longer fronts with lower inclinations (Table 3.1). MM2.1 and MM2.2 are concentrated on the preexisting slope front. MM2.3 forms a sheet on the shelf, with thin lobes on the front preserved between downslope incisions on surfaces 5.2 and 5.3. Progradation between erosion/incision paths characterizes sub-sequences MM2.4-MM2.6. Summary Clinoform shelf platforms and ramps are characterized by low- to highamplitude, low frequency, relatively smooth reflections, which truncate underlying reflections at low angles (Figs. 3.3 and 3.7). Large, isolated incisions occur in up-dip locations on the inner platform (Fig. 3.4). Smaller, incision-like 124 features are interpreted on sub-sequence downlap surfaces 3.1 and 4.1 (Fig. 3.9; Table 3.1; Chapter 4). These features are ~38 ms (~60 m) deep, and range in width from 170-250 m for 3.1 to 200-500 m for 4.1, both 4-8 km landward of the shelf-slope break. In cross-section, incision-like features above EMM1.1 are characterized by truncation, with associated disruption of overlying reflections (Fig. 3.9A). These truncation surfaces are most commonly U-shaped, although V-shaped and irregular examples also exist. In map view, these features form closed, circular depressions that cover >30 km2. Concave-seaward, arcuate features define the up-dip limit of these depressions (Fig. 3.9B). Clinoform fronts all show a progression upward from generally smooth to heavily incised (Fig. 3.15). Incision is most pervasive on narrow fronts dipping at >6o. However, once started, gullies and small submarine canyons are also maintained on longer fronts with declivities <3o (Figs. 3.7, 3.9 and 3.15; Table 3.1). Oligocene and early Miocene clinoform fronts are relatively smooth (Figs. 3.15A and B). However, on DLS2 numerous single- and multi-trunk, dip- oriented, headless gullies occur seaward of the toe-of-slope (Fig. 3.15A). In cross-section, these U-shaped gullies are spaced 200-1000 m apart, are ~25-30 m deep and 130-200 m wide, and shoal both up- and down-dip. The linear upper limit of these gullies is located at the toe-of-slope, where the inclination is <1o. In contrast, DLS3 shows no evidence of dip-oriented gullying at the toe-of slope (Fig. 3.15B). 125 Figure 3.15. (following page). Horizon slices through a variance cube illustrating morphologic variations along clinoform fronts through time. Area represented is given in Fig. 3.14. The horizontal scale of each panel is the same, unless otherwise noted. Shading highlights erosion paths. Single and multi-trunk headless gullies at the toe-of-slope of DLS2, above sequence OL1. Relatively smooth shelf and front of DLS3, at the top of OM1. Cuspate morphology interpreted on sub-sequence downlap surface 4.2, adjacent to the shelf-edge defined by DLS4. Note the change in scale. Closely spaced, single trunk, slightly sinuous gullies developed on sub-sequence downlap surface 4.2, within MM1. Large, more widely spaced gullies developed on DLS5, the boundary between MM1 and MM2. Small submarine canyons developed on sub-sequence downlap surface 5.2 within sequence MM2. These are larger and more widely spaced than gullies in MM1 shown in (E). The canyon (shaded) widens at its head, breaching the shelf-break. (A) (B) (C) (D) (E) (F) 126 115 50'E 0 1160E 115050'E 1160E 115055'E A DLS2 B DLS3 C 4 km 4.2 19035'S 19035'S C SLOPE 7 19035'S BASIN 2 7 BASIN 19 40'S 19 40'S 19 40'S 19 40'S 19 40'S 19 40'S 19 40'S 19 40'S 19 40'S 19 40'S 19040'S 4 6 3 4 km 2 19040'S SHELF 4 6 3 4 km SHELF 2 115045'E 115055'E 115045'E 115055'E 115045'E 115055'E D 4 km 4.3 19030'S E 4 km DLS5 19 30'S 19 30'S 19 30'S 19 30'S 19030'S F 5.3 SLOPE 19 30'S 19030'S BASIN 19035'S BASIN 19035'S 19 35'S 19035'S E1 E1 E1 SLOPE progradation direction SHELF SLOPE 2 E1 well SHELF 2 clinoform break 4 km SHELF 2 toe-of slope 127 The MM1.2 shelf-edge is characterized by broad, U-shaped truncation surfaces, located where the toe-of-slope defined by 4.2 progrades to the DLS4 shelf-break (Figs. 3.7 and 3.15C; Table 3.1). The resultant cuspate morphology is superimposed on the antecedent EMM1 shelf-edge. Individual failure scars are ~1 km wide. Larger, irregular features may be the result of coalescence of these scars (Fig. 3.15C). Clinoform fronts between sub-sequence downlap surface 4.2 and DLS_TOP show abundant V-shaped gullies and small submarine canyons (Figs. 3.7, 3.11 and 3.15D-F). Sizes and characteristics of these gullies vary up-section, but their overall density on any one horizon does not vary along-strike. Relatively small gullies, developed within MM1.3 and MM1.4, are 100-300 m wide, 25-45 ms (~40-70 m) deep, and persist for ~2 km in the dip direction on the upper front (Fig. 3.15D). They are straight, closely spaced (200-400 m), and single-trunk features. Their spacing is less than that of the headless gullies with similar widths developed on DLS2 (compare Fig. 3.15D with Fig. 3.15A). DLS5 marks a change from aggradational gullies in MM1 (Fig. 3.15D) to larger, cross-cutting gullies with asymmetric, northeast-prograding fill in MM2 (Figs. 3.11, 3.14D, 3.15E and F). Gullies on DLS5 are up to 70 ms (~115 m) deep, 300-800 m wide and spaced at ~700-1000 m. They are up to 6 km long, slightly sinuous, and restricted to fronts (Fig. 3.15E). On boundary 5.3, slightly sinuous, small submarine canyons are 1-1.5 km wide and 95 ms (~150 m) deep, spaced at 2-2.5 km, and 13 km long (Fig. 3.15F). Inclination on the unincised portion of the front is 2o-3o (Table 3.1). One canyon widens to ~4 km at its head 128 at the shelf-break, and impinges ~1 km onto the shelf (Fig. 3.15F). Submarine canyons of similar dimensions persist through MM2.3-MM2.6; their upper limits migrate to the northwest, with the prograding shelf-edge (Fig. 3.14D). Length decreases along with the length of the clinoform front. Despite active gullying of the clinoform fronts within MM1 and MM2, there is no evidence of submarine fans in the adjacent basin. Little if any basinal accumulation is evident (Figs. 3.3 and 3.14). Basinward of these clinoforms, little sediment has accumulated. Truncation of thin deposits is apparent in OM1 and EMM1 (Figs. 3.7, 3.14 and 3.16). The resultant erosional scar is arcuate and concave-seaward, continuing along-strike for ~10 km (Figs. 3.14 and 3.16). Up- and down-dip boundaries are sharp; only minor amplitude variations characterize the fill. Along-strike, the boundaries coalesce with and truncate high-amplitude, northwest-oriented gullies on the basin floor (Fig. 3.16). 3.4.4 Comparison of clinoform morphology and paleobathymetry The Goodwyn 6, 4, 2 and Eastbrook 1 wells represent an approximate diporiented transect within the 3D volume (Fig. 3.1). Depending on the sequence examined, Goodwyn 7 is variably up- or down-dip relative to Goodwyn 4 (Figs. 3.1, 3.3 and 3.17). The Goodwyn wells intersect the progradational wedge, whereas Eastbrook 1 is at all times either in the basinward of or on the lower clinoform front (Figs. 3.1 and 3.3). Biogenic components are dominated by benthic foraminifera with lesser amounts of pelecypods, bryozoa, coral and ostracods (Moss et al., in prep.; V. 129 5 km E1 Fi g. 3 2 seismic example, Figure 3.7 intersection with younger reflection intersection with older reflection .7 Figure 3.16. Horizontal time slice through the 3D volume at 1592 ms TWTT, highlighting along-strike extent of the basin incision also identified in Figs. 3.3, 3.7 and 3.14. The erosion forms an arcuate depression ~5 km wide and continuous for ~10 km along-strike. Dip-oriented drainage surfaces are located to the northeast and southwest. The horizontal section intersects both older and younger dipping reflections, as shown. 130 Figure 3.17. (following page). Comparison of seismic stratal architecture and results of paleobathymetric analyses derived from benthic foraminifers recovered from wells along a dip-oriented transect (Fig. 3.3; Appendix 1; Moss et al., in prep.). Goodwyn 7 is projected ~14 km along-strike (Fig. 3.1). Also shown are lithologic and biostratigraphic interpretations derived from well-completion reports (Plates 1-7; Appendix 1). Major downlap discontinuity surfaces are shown on the seismic data and in the wells. The two data-sets are tied using synthetic seismograms constructed at each well (Appendix 5). The approximate location of sub-sequence downlap surfaces is highlighted, although they are not shown on the seismic data (Figs. 3.3, 3.7, and 3.11; see also Plates 1-7). The wells intersect at shelf, front and basin locations. Paleobathymetric zonations: transitional marine (0 m), inner (0-20 m), middle (20-100 m), and outer (100200 m) neritic, upper (200-500 m) and middle (500-100 m) bathyal. Paleo-water depths quoted in the text represent the shallow end of the range determined by the paleobathymetric analyses. The horizontal bar indicates the possible range of paleo-water depths while the small vertical bar represents the deepest possible estimate from a single species (Moss et al., in prep.). Black areas in the chronostratigraphy column indicate where no samples were examined. Paleobathymetric analysis is more regularly spaced, at ~ 29m (av.). 131 Passlow, Geoscience Australia, personal communication, 2000). Paleo-water depth estimates show a general shoaling-upward trend (Fig. 3.17). The trend is time-transgressive, occurring first in Goodwyn 6, followed by Goodwyn 4 and 7, and finally Goodwyn 2. The carbonate-clastic sediments also show a general coarsening-upward: calcilutite, calcisiltite and marl dominate the outer shelf (ramp) and front, with calcarenite landward; this change is also time-transgressive landward to seaward. Dolomite and quartz-rich sandstone are locally significant in Goodwyn 6 and 4. In comparison, both lithologies and paleo-water depth estimates are variable in the most basinward well, Eastbrook 1. Shelf (platform and ramp) landward of breakpoints Shelf topsets are intersected in all Goodwyn wells, specifically above subsequence downlap surfaces 2.2 in Goodwyn 6, 2.3 in Goodwyn 4 and 7, and 2.4 in Goodwyn 2 (Figs. 3.7 and 3.17; Table 3.1). The Goodwyn 4, 7 and 2 wells intersect the shelf-break at these horizons. Paleobathymetric analyses of the upper Oligocene to early upper Miocene section reveal paleo-water depths ranging from transitional marine (0 m) to upper bathyal (200-500 m), commonly fluctuating between inner (0-20 m) and outer (100-200 m) neritic (Fig. 3.17). Where the shelf-edge is intersected, paleo-water depths of middle (20-100 m) to outer (100-200 m) neritic are recorded. The increase in paleo-water depth from inner to outer neritic on DLS3 between Goodwyn 6 and 2 requires a shelf declivity of 1-3o, slightly less than that calculated from the seismic, 2-3.5o (Table 3.1). Therefore, we suggest that early Miocene (DLS3) paleo-water depths 133 projected to the shelf-edge, using the paleobathymetric estimates, ranged from 135-400 m. Higher frequency paleo-water depth variations are superimposed on the overall shoaling-upward and northwest/basinward deepening trends. For example, four transitional marine (0 m) to inner neritic (0-20 m) intervals are separated by middle (20-100 m) to outer (100-200 m) neritic environments in Goodwyn 6 (pink, Fig. 3.17). Both downlap discontinuity surfaces DLS3 and DLS5 also correspond to shallow-water intervals (Fig. 3.17; Plates 1-7). A transitional marine environment interpreted at sub-sequence downlap surface 3.1 in Goodwyn 6 coincides with the numerous circular and arcuate depressions observed in the seismic data landward of the shelf-break (Fig. 3.9). At Goodwyn 2, ~4.5 km basinward, outer neritic (100-200 m) paleo-water depths occur at the same boundary. This requires an inclination on the shelf (ramp) at 3.1 of 1-2.5o, similar to that determined from the seismic analysis, 2o (Table 3.1). Therefore, paleo-water depths projected to the shelf edge, 3.5 km northwest of Goodwyn 2, using the paleobathymetric analysis are 160-350 m. Flooding of the partially exposed shelf appears to occur in pulses. Directly above the relatively shoal-water estimates at 3.1 in all wells, outer neritic (100-200 m) to upper bathyal (200 500 m) paleo-water depths suggest a deepening at that time of at least 100 m. This transgression occurs within subsequence EMM1.2; the locus of deposition is the shelf ramp, thinning on the clinoform front and basin (Figs. 3.7 and 3.14B; Table 3.1). Shoaling detected in Goodwyn 6 and to a lesser degree in Goodwyn 4 and 2 precedes shelf-wide 134 flooding at DLS4. Paleo-water depths increase from inner (0-20 m) to outer (100200 m) neritic in Goodwyn 6, middle (20-100 m) to outer (100-200m) neritic in Goodwyn 4, and outer neritic (100-200 m) to upper bathyal (200-500 m) in Goodwyn 2, a minimum increase of 80 m. Alternating middle (20-100 m) and outer (100-200 m) neritic intervals, and a single upper bathyal (200-500 m) zone, are interpreted within MM1 (Fig. 3.17). Shingled clinoforms prograde across the submerged shelf (Figs. 3.7 and 3.17). Linear depressions interpreted as karst (Chapter 4), and a single diporiented shelf incision (Fig. 3.10), are associated with middle neritic (20-100 m) estimates at sub-sequence downlap surface 4.1, as opposed to middle (20-100 m) and outer (100-200 m) neritic intervals in bounding samples. These results suggest shoaling, but transitional environments similar to 3.1 are not sampled. Although general deepening above 4.1 is represented in all Goodwyn wells, paleo-water depth estimates in MM1 are anomalous between wells. For example, middle neritic (20-100 m) estimates in Goodwyn 4 are basinward of outer neritic (100-200 m) and upper bathyal (200-500 m) estimates in Goodwyn 6 and 7, respectively, but basinward shoaling is not supported by observed northwestdipping seismic geometries throughout this interval (Figs. 3.3 and 3.7). Progradation into the basin occurs associated with overall deepening, as is suggested by paleobathymetric results between 4.1 and 4.3 and shoaling above 4.3 (Fig. 3.17). However, shoaling above 4.3 to inner (0-20 m) and middle (20-100 m) neritic depths in Goodwyn 6 and 7 is suppressed in Goodwyn 4 and 2. These 135 results highlight increased sensitivity to paleo-water depth variations of inboard wells as progradation proceeds. Overall shoaling of the succession continues between MM1 and MM2 (Fig. 3.17). In Goodwyn 6, 7 and 4, inner (0-20 m) to middle (20 -100 m) neritic assemblages are interrupted by two outer neritic (100-200 m) intervals, indicating an increase in paleo-water depth of 80-200 m. These deeper intervals correspond to two dominantly fine to medium grained (up to coarse-granule in Lambert 1 and Eaglehawk 1; Fig. 3.1), unconsolidated to partially cemented quartz-rich sandstones, interbedded with calcarenite and associated with dolomite (Fig. 3.17; BOCAL, 1972; BOCAL, 1973a; BOCAL, 1973b; Woodside Offshore Petroleum, 1981). In Goodwyn 7, this same interval is represented by dolomite with minor sandstone (Moss et al., in prep.; Woodside Offshore Petroleum, 1985). Only the lower sand is intersected in Goodwyn 4. An increase from inner- (0-20 m) to outer (100-200 m) neritic paleo-water depths corresponds to the upper sand in Goodwyn 6. A similar signal is associated with the upper sand in Goodwyn 2. Goodwyn 2 does not intersect the lower sand body or show any increase in waterdepths; consistent middle neritic paleo-water depth estimates are anomalous with respect to deeper signals from more landward wells (Fig. 3.17). This bathymetrically stable interval is dominated by large foraminifera, with smaller benthic and planktonic species absent (Moss et al., in prep.). These sands prograde along-strike to the northeast (Figs. 3.3 and 3.12), and their paleo-water depth estimates concur with observed foreset relief of 100-150 m. The base 136 reflection of the lower sand body truncates accumulation of MM2.3 on the shelf, following slope-front filling in MM2.1 and MM2.2 (Fig. 3.11). Clinoform fronts seaward of breakpoints Paleo-water depth estimates from clinoform fronts below sub-sequence downlap surfaces 2.4 in Goodwyn 2, 2.3 in Goodwyn 4 and 7 and 2.2 in Goodwyn 6 are similar to the outer shelf ramp (Fig. 3.17), generally characterized by outer neritic (100-200 m) zones with minor middle neritic (20-100 m) and rare upper bathyal (200-500 m) assemblages. Calcilutites are interbedded with lesser amounts of calcisiltite, marl and rare calcarenite (Fig. 3.17). All of these results together suggest that there is either <100 m water-depth change from the shelfbreak to these wells on the front or that the bulk of front sediments are derived from the shelf (ramp). On sub-sequence downlap surface 2.2, the average front inclination is 4.5o (Table 3.1). As Goodwyn 6 (upper front) and Goodwyn 4 (lower front) are separated by ~2.6 km, the change in seismically observed height on the front between these wells is >200 m, more than enough to be detected by the foraminiferal analyses in neritic environments if they are not reworked. Such a water-depth change is not observed, so we suggest that the majority of claysized carbonate sampled from the front is derived from the outer shelf ramp (Fig. 3.17). Seaward of clinoform toes-of-slope Eastbrook 1 is the only well located basinward of the toe-of-slope. Paleobathymetric estimates and lithologies from that well are highly variable representing a mix of material transported into deeper water environments and in137 situ sediments (Fig. 3.17). Estimates range from transitional marine to middle bathyal (500-1000m). Lithologies are interbedded calcilutite and calcisiltite with minor calcarenite. The observed variability suggests periods of quiet basinal deposition interrupted by influxes of shallower-water, coarser sediments from the shelf and front. Transitional (0 m) and inner neritic (20-100) assemblages identified in the slope-front-fill seismic packages of MM2 confirm that these sediments are derived from shallower-water environments. They are interbedded with outer neritic (100-200 m) sediments. Paleo-water depths of >500 m interpreted between DLS2 and DLS4 compare favorably with the range of uncorrected clinoform heights of 190-500 m for these sequences (Fig. 3.17; Table 3.1). 3.5 DISCUSSION 3.5.1 Sediment productivity as a primary control on clinoform distribution The submerged shelf of the NCB, dominated by temperate, heterozoan, biogenic sediment (Appendices 1 and 4), appears to be actively producing clastic material at all times, less influenced by location of the photic zone than tropical carbonate shelves (James, 1997; Moss et al., in prep.). Only one mid-Miocene (?) biohermal mound is seismically identified (Figs. 3.5 and 3.14C). This mound is similar in size and shape to Eocene-Recent bryozoan mounds developed on the low energy inner shelf and slope identified in the Great Australian Bight (GAB) and sampled by ODP Leg 182 (Feary and James, 1998, their Fig. 17; Feary et al., 2000). The NCB mound is located on DLS4, a flooding surface associated with paleo-water depth estimates of 100-200 m, so it may indicate similar low-energy 138 biohermal development, prior to burial by calcarenite-dominated, shingled clinoforms (Fig. 3.5). Other build-ups identified in the 2D MCS and mapped as a shelf barrier by Romine et al. (1997, their Fig. 16) occur in MM1.4 and MM2.1 (Figs. 3.13D and 3.13E). A barrier system also developed in the GAB at this time, suggesting that climate and oceanographic conditions were widely optimal for carbonate framework-building organisms during the mid-Miocene (Feary and James, 1998). A larger number of carbonate build-ups in the northeast (Fig. 3.5; Romine et al., 1997, their Figure 16) suggest that environmental conditions were more conducive to primary carbonate production there. Prograding build-ups within MM1.4 that backstep in MM2.1 (Figs. 3.13D and 3.13E) correlate with an alongstrike shift to the northeast in the MM1 depocenter relative to EMM1. MM1 also coincides with both the Miocene climatic optimum and intensification of the southward-directed Leeuwin current, which transports warmer waters from the Indonesian Throughflow (McGowran et al., 1997). These could have enhanced biogenic carbonate sediment production. In contrast, sequences without observable build-ups, OM1 and EMM1 (Figs. 3.13B and 3.13C), or with backstepped build-ups like MM2 (Fig. 3.13E), are more evenly distributed alongstrike. OL1 has no observable build-ups, but is concentrated in the northeast like MM1. The change in geometry from distally steepened ramp in the north (Fig. 3.6) to ramp in the south (Fig. 3.3) may be a function of distance away from the major depocenter to the northeast (Fig. 3.13A). Landward-stepping onlaps in the south may represent sediment transport along-strike (Driscoll and Karner, 1999). 139 These observations are interpreted as a response to latitudinal variations in productivity, modified by along-strike transport, when the NCB was located south of 30oS (Veevers et al., 1991; Driscoll and Karner, 1999; Lawver et al., 1999). The northeast-southwest oriented clinoform front progrades in approximately linear fashion for almost 20 m.y. (Fig. 3.13; Table 3.2). However, the observed large-scale (~100 km) variations in depositional loci and smaller scale (~10 km) downlap lobes (Fig. 3.14) support three-dimensional development not normally attributed to progradation of a carbonate shelf in response to a line source of sediment (Mullins and Cook, 1986). These observed along-strike variations could be the result of changes either in sediment production, alongstrike transport of sediments, or both. However, although shelf-parallel currents are suggested by the prominent late Oligocene-early Miocene strike-oriented incision seaward of the succession (Figs. 3.7 and 3.16), Oligocene onlap patterns (Fig. 3.3) and gully fill patterns (Fig. 3.11), sediment drift geometries are not recognized seismically. Therefore, we suggest that the strongest driver of variations in depositional loci is productivity variations along the shelf. 3.5.2 Reorganization of southeastern Indian Ocean circulation in the late middle Miocene Terrigenous siliciclastic material is first introduced into the NCB in the late middle Miocene, during MM2. Sandstones are preserved on the shelf as northeast-directed foresets, each 100-150 m high (Fig. 3.12), approximately normal to the primary clinoform progradation direction and landward of the shelfedge. The base of this northeast-prograding sand body is a prominent scour surface (Figs. 3.11A and 3.12). Reworking has played a major role in sand 140 deposition, as suggested by its quartz-rich lithology. Middle and outer neritic benthic assemblages (Fig. 3.17) suggest that deposition occurred on a submerged shelf, rather than along the coast in brackish or fresh-water environments (Moss et al., in prep.; Apthorpe, 1988). Isolated shelf-sand bodies of less relief (5-40 m) than those recognized in the NCB have been reported on the Pleistocene-Recent Languedoc, France (Tesson et al., 2000) and Louisiana (Penland et al., 1986) shelves, where they are interpreted as transgressive ridges. Such ridges are typically associated with an underlying transgressive ravinement surface (Snedden and Dalrymple, 1999; Tesson et al., 2000). These ridges require a source of sand either transported onto the shelf by currents or derived locally. In the NCB, siliciclastic material must be derived from outside the area of seismic control. The Precambrian crystalline Pilbara terrane onshore is drained by a number of rivers thought to be initially incised during the Valanginian (Fig. 3.1; Romine et al., 1997). Their sediment loads may have been high in the warm, wet, early-mid Miocene (McGowran and Li, 1996). If the observed sands are reworked ridges, then their northeasterly progradation suggests shelf-parallel current transport. This in turn suggests a reorganization of ocean circulation in the southeastern Indian Ocean at this time, perhaps involving replacement of the south-flowing Leeuwin Current with the north-flowing West Australian Current, following the mid-Miocene optimum identified by McGowran et al. (1997). A north-directed current is consistent with incursions of cool-water planktonic foraminifera at this time in the Wombat and Exmouth plateaus to the west reported from ODP Leg 122 (Zachariasse, 1992). 141 3.5.3 Downslope erosion and headward failure: mechanisms of submarine canyon formation on clinoform fronts Observed clinoform fronts are variously smooth to intensely gullied (Fig. 3.15). Large gullies and submarine canyons in MM2 (Figs. 3.11 and 3.15) have been previously interpreted to represent karst topography and therefore indicative of exposure (Romine et al., 1997, their Fig. 18). However, spatial control provided by the 3D volume indicates instead that these incisions represent sinuous downslope erosion paths rather than karst. Lobate thinning (Fig. 3.14B) and zeroedge downlap terminations of sub-sequences (Figs. 3.14B and 3.14C) both indicate uneven sediment distribution into the basin, and that the majority of sediment is deposited on the front (Figs. 3.3, 3.7 and 3.11). Nonetheless, lack of slope aprons suggests either that sediment supplies were too low or that alongstrike basinal currents, perhaps indicated by the erosional scar truncating OM1 and EMM1, persist through the Miocene, sweeping toes-of-slope clean of sediment (Figs. 3.7, 3.14 and 3.16). Closely spaced, small, narrow, single- and multi-trunk headless gullies characterize the DLS2 toe-of-slope (Fig. 3.15A). Dugan and Flemings (2001) have suggested that spring-sapping, with water derived from compaction, can develop such features at the toes-of-slope of broad continental shelves. The small gullies are then healed by subsequent sedimentation, resulting in a smooth clinoform front, such as DLS3 (Fig. 3.15B). These observations contrast with broad, flat-floored canyons of the New Jersey margin modern toe-of-slope, which have been interpreted to evolve into submarine canyons through headward erosion (Farre et al., 1983). 142 Systematic slope-front failure is first detected on the upper slope on 4.2 as a series of broad, shelf-edge amphitheaters (Fig. 3.15C); these are similar to New Jersey flat-floored canyons (Fulthorpe and Austin, 1998). However, unlike the New Jersey canyons, these amphitheaters are perched on the antecedent (DLS4) shelf (Figs. 3.9 and 3.15C; Table 3.1). Shelf-edge instability is rare on DLS4 (Fig. 3.7), suggesting that in addition to the 6-8o declivity of the clinoform front (Table 3.1), vertical relief and front length may influence instability. Gullies located on steep (4-8o), >480 m high, moderate width (3.5-7 km) clinoform fronts at 4.3 and DLS5 (Figs. 3.15D and 3.15E; Table 3.1) are similar to those detected today north of Little Bahama Bank (Fig. 3.18; van Buren and Mullins, 1983; Mullins et al., 1984; Mullins and Cook, 1986). Up-dip limits of these modern gullies are governed by distribution of submarine cementation; slope failures are present only where cementation is incomplete. Whether a similar relationship is appropriate in the NCB is uncertain; authigenic calcite is prevalent in well samples from both front and shelf settings, but its spatial distribution is unknown (Moss et al., in prep.). Asymmetric gully and submarine canyon fills MM2 suggest that downslope processes are also influenced by along-strike sediment transport (Fig. 3.11). Such fill patterns are also recognized along the siliciclastic SE Brazilian margin (Fig. 3.19), where the southwest-oriented Brazilian Current interacts with gravity flows within canyons on the slope (Faug res et al., 1999). Direction of the strike-oriented sediment transport in the NCB is to the northeast as indicated 143 10 km N LBB Figure 3.18. Present-day bathymetry of the northern slope of the Little Bahama Bank (LBB). The upper slope is dominated by small-scale gullies (generally <100 m wide) of similar dimensions to those on the upper slope in MM1 (Fig. 3.15D, E). Sediment derived from these numerous gullies are deposited on the lower slope as a slope apron. Such a slope apron is absent in the NCB. Modified after van Buren and Mullins (1983). 144 S Canyon N 1.0 1.5 1 km S Brazilian Current N 1.0 1.5 1 km Figure 3.19. Asymmetric submarine canyon fill along the SE Brazilian margin interpreted to be the result of combined down- and across-slope sediment transport (g and c) due to gravity flows (tc) interacting with the Brazilian Current. Similar asymmetric fill building to the northeast is recognized in the much smaller canyons in MM2 (Fig. 3.11). Modified after Faug res et al. (1999). 145 by fill progradation direction (Fig. 3.11). This orientation matches that proposed for prograding siliciclastic foresets on the shelf. Along-strike transport in the NCB is also suggested by the slope-front fill morphology observed in MM2.1 and MM2.2 (Fig. 3.11). In line with results from the clastic-dominated U.S. east coast (Twitchell and Roberts, 1982), spacing between gullies and small submarine canyons increases as inclinations of NCB clinoform fronts decrease (Figs. 3.7, 3.11 and 3.15D-F). Eventually, gullies reach the dimensions of small submarine canyons, i.e., >100 m deep and >400 m wide (Stow and Mayall, 2000), and breach the shelf-break. Gullies are not detected on the clinoform front until inclination exceeds ~6o, e.g., in MM1.3. This is greater than the minimum declivity of dissected slopes reported for either New Jersey (3o) or Little Bahama Bank (4o). Modern relief exceeds 1000 m on the modern New Jersey margin and is ~700 m on the Little Bahama Bank, while dissected NCB clinoform fronts are ~470-740 m high (Table 3.1). Miocene canyon formation is rare in the New Jersey margin, despite declivities exceeding 3o (Fulthorpe and Austin, 1998). Miocene clinoform fronts there are <500 m high and ~7 km wide (Steckler et al., 1999). In contrast, small submarine canyons in the NCB are maintained on 5.2 and 5.3 fronts, which are 13-18 km wide with declivities <3o. However, local inclination on submarine canyon walls in the NCB ranges from 6o to 13.5o. This conforms to assertions by Farre et al. (1983) and Pratson and Coakley (1996) that canyons become the focal point for further slope failure due to the steepness of the bounding walls. In fact, a broad headwall amphitheater at the top of the 5.3 canyon is identified, despite 146 decreased inclination of the surrounding, non-incised front (Fig. 3.15F; Table 3.1). Small gullies initially restricted to the upper front in MM1 (Fig. 3.15D) extend basinward and enlarge into small submarine canyons in MM2 that traverse the entire clinoform front (Figs. 3.15E and 3.15F). Apparently, once erosion paths are established, they are maintained (Fig. 3.14D). Simultaneous mid-front failure is absent. Heads of gullies and small submarine canyons appear to occur consistently at or near the shelf-break, where the change in inclination is greatest. Only rare examples breach the shelf break, as along the modern New Jersey margin (Fig. 3.15F; Twitchell and Roberts, 1982; Farre et al., 1983; Pratson and Coakley, 1996). This transformation from gullies to submarine canyons in the NCB can be explained by the model for submarine canyon development on the New Jersey margin proposed by Pratson and Coakley (1996). The upper slope, oversteepened during progradation fails producing small rills. Rills are then extended, widened and deepened by downslope sediment flows derived from headward failure that ultimately breach the shelf-break (Fig. 3.20). 3.5.4 Are clinoforms sensitive to sea-level changes? While clinoforms have been targeted globally for studies of sea-level history because of their presumed sensitivity to base-level variations, the paleobathymetric significance of clinoform breakpoints has remained unclear. We know that all or part of the shelf may be subaerially exposed during some sealevel falls. For example, combined results of ODP Leg 174A, high-resolution seismic studies and 1D backstripping of Coastal Plain wells along the New Jersey 147 INITIAL INSTABILITY DOWNSLOPE SEDIMENT FLOWS HEADWARD EROSION AND DOWNSLOPE SEDIMENT FLOWS Erosive sediment flow Slope Failure Flow Direction Figure 3.20. The model for submarine canyon development of Pratson and Coakley (1996), from observations along the New Jersey margin. The progression from smooth clinoform fronts, to gullied, and finally incised by small submarine canyons with broad amphitheaters at the shelf-break as observed in the NCB (Fig. 3.15), is similar to this model developed along a siliciclastic margin. In the Pratson and Coakley (1996) model, oversteepening on the upper slope generates downslope eroding sediment flows, creating small gullies which are then widened, deepened and lengthened across the slope by further sediment flows generated by headward erosion. Redrawn from Pratson and Coakley (1996). 148 continental margin all suggest that Miocene eustatic sea-level fluctuations were 20-30 m, with the shelf-edge near exposure during lowstands and submerged perhaps tens of meters during highstands (Austin et al., 1998; Kominz et al., 1998; Fulthorpe and Austin, 1998; Fulthorpe et al., 1999). However, 2D backstripping of clinoforms from the same succession beneath the modern New Jersey shelf suggests instead that shelf-edge water-depths were 80-100 m (Steckler et al., 1999). Late Paleogene-early Neogene glacio-eustatic variations of 30-50 m suggested by oxygen isotope studies (Miller et al., 1991) appear insufficient to expose such deeply submerged shelf-edges, even at lowstands. Nonetheless, reduced water-depths during lowstands could still result in erosion of the shelf and bypass of sediments to the slope (Steckler et al., 1999). New data from the Australian margin are at variance with the New Jersey results. Preliminary results of ODP Leg 194 from the Marion Plateau suggest highstand paleo-water depths of 30 20 m on the carbonate platform, with a midMiocene (12.5-11.4 Ma) sea-level fall of 86 30 m, greater than that predicted for New Jersey at the same time (Isern et al., 2001; Shipboard Scientific Party, 2001). However, part of the disparity may be a result of different assumptions used in calculating sea-level variations on these margins (Karner et al., 2001). For example, New Jersey sea-level estimates, were calculated from backstripping at wells, assuming local isostasy, i.e., zero flexural strength. In comparison, the larger magnitude sea-level estimate from the Marion Plateau assumes no differential subsidence between the closely-spaced (~20 km) wells, i.e., infinite flexural strength. This assumption is supported by seismic observations of 149 undisturbed and consistently dipping sediments and horizontal basement geometry between sites. Interestingly, if local isostasy is assumed in the calculation of sea-level magnitude in the Marion Plateau, then the estimates are reduced to 62 50 m, similar to the estimates of 20-85 m determined from the New Jersey margin. Consequently, a lively debate continues over how paleowater depths deduced from shelf sections can be used to estimate amplitudes of eustatic sea-level variations, in conjunction with seismic data on continental edges around the world. In the NCB, the mapped seismic downlap discontinuity surfaces are all chronostratigraphically significant across the shelf (Fig. 3.17). However, the only downlap surface associated with a hiatus is DLS1, at which boundary the upper Oligocene is often missing (Fig. 3.17). Interestingly, deep-water affinities associated with this horizon, combined with toe-of-slope locations of the wells, indicate that the observed hiatus is submarine rather than subaerial. A relict soil corresponding to DLS1 at Dampier 1 (Fig. 3.1) indicates exposure did occur landward (Bocal, 1969). DLS3 and DLS5, characterized by inner-neritic intervals in Goodwyn 6, are represented by middle and outer neritic environments basinward. Reduced paleo-water depths just above DLS_top in Goodwyn 6 may correspond with overall shallowing in Goodwyn 2 and 7. In contrast, a paleowater depth increase of at least 80 m across the shelf, to outer neritic in Goodwyn 6 and upper bathyal in Goodwyn 2 is recorded across DLS4, consistent with the observed backstep and shelf downlaps onto this horizon (Fig. 3.7). In fact the only surfaces associated with definite exposure features in the entire mapped 150 succession, combined with transitional to inner neritic affinities on the shelf, are 3.1 and 4.1 (Fig. 3.17; Table 3.1). These are seismically subordinate to the mapped major downlap discontinuity surfaces (Figs. 3.7 and 3.9). Therefore, we suspect that genetically linked surfaces within this succession vary in both character and prominence: (1) boundaries related to subaerial exposure are not always primary stratigraphic discontinuities; (2) reduced paleo-water depths are sufficient to produce seismically mappable interfaces without definite exposure; and (3) deepening across the shelf can result in prominent seismic boundaries, particularly when combined with renewed sedimentation on the shelf. Variation in progradation versus aggradation in space along the prograding front is evident both from shelf-edge trajectories and clinoform geometries (Figs. 3.3, 3.6, 3.7 and 3.11). Sigmoidal clinoforms in OL1 (Figs. 3.3 and 3.6) indicate that accommodation is available on the shelf, allowing aggradation to occur coeval with progradation (Mitchum et al., 1977; ChristieBlick and Driscoll, 1995). The observed change in geometry from distally steepened ramp in the north (Fig. 3.6) to ramp in the south (Fig. 3.3) may be a function of distance away from the major depocenter to the northeast (Fig. 3.13A), while landward-stepping onlaps in the south may represent sediment transport along-strike (Driscoll and Karner, 1999). The horizontal to slightly downstepping shelf-edge trajectory in OM1 (Fig. 3.7) suggests reduced accommodation. However, inner neritic paleo-water depths are only observed in Goodwyn 6. The outermost shelf and shelf-edge are characterized by middle-outer neritic estimates (Fig. 3.17). Therefore, although 151 accommodation exists to be filled in this sequence, progradation dominates the sequence. By comparison, increased accommodation is inferred from the general upbuilding recognized in both EMM1 and MM1 (Fig. 3.7). The amount of progradation decreases relative to aggradation from EMM1 to MM1. However, along-strike variations in progradation, as indicated by migration of the depocenters, are variable (Fig. 3.13F). Alternating transitional to outer neritic environments in Goodwyn 6, with middle neritic to upper bathyal zones basinward (Fig. 3.17), indicate that accommodation available on the shelf is partially filled by aggradation in EMM1 and MM1. These sequences occur on either side of the postulated latest early Miocene peak in sea-level (Haq et al. 1988) associated with general oceanic warming during the Miocene climatic optimum (McGowran and Li, 1996). OL1 to EMM1 are deposited on the rising limb, while MM1 occurs at the peak and MM2 is emplaced on the falling limb. The gross stratigraphic response leading up to the peak (EMM1) is similar to that at the optimum (MM1), separated by the backstep and shingled shelf downlap onto DLS4 during MM1.1 and MM1.2 (Figs. 3.3 and 3.6). Within EMM1 and MM1, small circular and arcuate to linear, steep-sided, closed depressions identified on 3.1 and 4.1 are interpreted as karst (Fig. 3.9C; Table 3.1; Chapter 4). The near-circular features identified on 3.1 resemble dolines. Amalgamation of individual dolines could result both in the arcuate depressions on 3.1 and the linear trough on 4.1 (Cvijic, 1981). Interpretation of these features as indicative of exposure is supported by transitional marine 152 assemblages at 3.1, and shoaling middle neritic assemblages, at 4.1, in Goodwyn 6 (Fig. 3.17). The influence of percolating surface waters in MM1.1 is shown by slight meteoric d18O and d13C isotope signals below 4.1 (Chapter 4; Cathro and Austin, 2001). Chaproniere (1984) also identifies emergence and intense recrystallization during the early mid-Miocene from onshore wells in the North West Cape (Fig. 3.1A), coeval with the trough observed on 4.1. As with that trough (Chapter 4; Cathro and Austin, 2001), there is no evidence for faulting immediately associated with depressions identified on 3.1. However, the arcuate nature of their up-dip limit (Fig. 3.9B) strongly suggests the headwall of a slump scar (e.g., Currituck Slide, east coast U.S.; Prior et al., 1986). Although such slumps are generally restricted to the slope (Galloway and Hobday, 1996), karst morphology may in part be controlled by fractures, joints or bedding planes below the resolution of the seismic data (Chapter 4; Cathro and Austin, 2001). Development of such karst confirms at least partial exposure of the shelf at least twice during the mid-Miocene. Depth of the water table, at or near which karst features will form, approximates sea-level in coastal regions (James and Choquette, 1988). Therefore, the presence of 60 m-deep karst depressions suggests minimum paleowater depth changes of 60-80 m in EMM1 and 80-160 m in MM1, probably engendered by relative sea-level falls. These estimates are still smaller than the middle (20-100m) and dominantly outer (100-200) neritic paleo-water depths for the shelf and shelf-edge (Fig. 3.17), suggesting that the shelf must be only partially exposed during these early-middle Miocene sea-level falls. 153 Identification of these karst features no closer than ~4 km landward of the shelf break supports this assertion. Karst on 3.1 (Fig. 3.9), beneath the shingled EMM1.2 sub-sequence (Fig. 3.7), also indicates that the partially exposed shelf is submerged prior to ensuing progradation. A minimum water-depth increase of 160 m during such submergence is predicted by changes in paleo-water depths in Goodwyn 6 (Fig. 3.17). Following MM1, the clinoform geometry is replaced by slope-front fill in MM2.1 and MM2.2, suggesting that sediment is transported along-strike. Subparallel aggradation during MM2.3 (Fig. 3.11) represents the period when accommodation was filled during sequence MM2. These shelf reflections are truncated by the basal scour of the northeast-prograding sand body (Figs. 3.11 and 3.12). Transgression during deposition of the sands is indicated by the increase in paleo-water depth at Goodwyn 6 from inner neritic (0-20 m) to outer neritic (100 200 m) across the scoured basal surface. The minimum deepening is 80-200 m (Fig. 3.17). In contrast, coeval variations in paleo-water depths are subdued in basinward wells and absent in Goodwyn 2 (Moss et al., in prep.). Horizontal to slight down-stepping progradation in MM2 (Fig. 3.11) suggests that accommodation is reduced, and resultant aggradation is decreased on the shelf. Nonetheless, middle neritic paleo-water depths that dominate the outermost well, Goodwyn 2, indicate that accommodation space remains at the end of MM2 deposition (Fig. 3.17). The mapped succession is unrimmed (Figs. 3.3, 3.7 and 3.11), implying that late Paleogene-early Neogene shelf sediments behaved as terrigenous clastics. 154 Despite active progradation, middle neritic to upper bathyal paleo-water depths detected on the outer shelf (Fig. 3.17) are greater than fair-weather wave-base, ~ 20 m (Jackson, 1997). Base-level in these sequences may perhaps be defined by a deeper interface, which further reduces sensitivity of heterozoan carbonate clinoform geometries to changes in eustatic sea-level. Processes such as oceanic swells, storm and internal waves must play an important role in clinoform development and progradation on the NWS (Galloway and Hobday, 1996; Cacchione and Drake, 1986). Primary off-shelf transport is indicated by the unidirectional progradation of clinoforms, dominated by calcilutite on the outer shelf and front (Figs. 3.3, 3.7, 3.11, 3.13, 3.14 and 3.17). The observed sigmoidal curvature of the Oligocene-Miocene succession suggests influences by base-level variations engendered by storm and internal waves as well as sea-level changes (Schlager and Adams, 2001). Waves from storm events are able to influence sediment in water-depths up to 200 m (Komar, 1976), whereas long period (14-20 s) swells derived from open-ocean storms can re-suspend sediments in 100 m of water (Vincent, 1986). Internal waves are documented on the modern NCB (Holloway, 1984). They may (1) resuspend sediment, to be redistributed by weaker currents (Cacchione and Southard, 1974; Butman et al., 1979) and (2) generate and maintain nepheloid layers on the uppermost slope (Cacchione and Drake, 1986). Integrated over time, these processes round an otherwise sharp shelf-break, as observed (Figs. 3.3, 3.7 and 3.11; Schlager and Adams, 2001). Variations in paleo-water depths on the NCB (uncorrected for compaction) are large compared to Miocene sea-level fluctuations predicted from 155 backstripping available New Jersey well-control (Kominz et al., 1998), deep ocean oxygen isotopic studies (Miller et al., 1991) and the well-known eustatic sea-level curve (Haq et al., 1988). A possible reason is that reactivation of Mesozoic structural trends, commencing in the Oligocene, is apparent all along the NWS. In the NCB, faults were reactivated over the Goodwyn Block (Fig. 3.1), and a prominent inversion anticline developed on the hanging-wall block of the Rosemary Trend (Fig. 3.13; Chapter 5). Accommodation space was destroyed over the crest of the anticline but may increase on the flanks due to the longwavelength flexural response to the tectonic load created by the anticline, thereby increasing the paleo-water depth estimates in the wells, which are all located basinward of the anticline. Initial forward modeling results indicate that this inversion was pulsed at 1-3 m.y. intervals, with a magnitude of ~70 m through to the Plio-Pleistocene (Chapter 5). Ultimately, such inversion could influence ongoing effects of regression or transgression, thereby further modifying the relationship between the evolving clinoform geometry and its complex forcing functions. 3.6 CONCLUSIONS Topsets of heterozoan carbonate clinoform successions such as those observed in the NCB are sensitive to base-level variations, but less so than for tropical photozoan carbonate systems. On a broad, seawardsloping shelf, the shelf-break is at all times submerged. Exposure indicated by karst morphology on the mid-outer shelf is coeval with middle to outer neritic paleo-water depths basinward. 156 Sediment transport dominated by progradation to the northwest is also influenced by along-strike processes. Variations in stratal architecture suggest that along-strike sediment transport changes from southdirected in the Oligocene to north-directed in the late mid-Miocene. Large-scale, along-strike variations of clinoform depocenters is interpreted as due to a combination of latitudinal variations in sediment productivity and along-strike sediment transport. Landwardstepping onlap, combined with ramp geometries southwest of the major OL1 depocenter, indicate along-shelf sediment transport to the south. Small-scale toe-of-slope distribution patterns are related to uneven sediment dispersal through gullies on clinoform fronts. Gullying on clinoform fronts is only apparent after the mid-Miocene optimum. Low-relief, low-inclination, short clinoform fronts are unincised, whereas high relief, low to high inclination, long clinoform fronts are incised. Once initiated, gullies become the focus for sediment distribution across the front; they can be maintained at a lower inclination than required for initiation, suggesting that dip of the failure wall is more important than slope of the surrounding unincised front. Sediment accumulates primarily on the northeast-southwest-oriented clinoform front. Despite abundant gullying in the mid-Miocene, carbonate aprons are not apparent in the basin. Sediment supply may only have been sufficient to build the front, with sediment transported 157 downslope distributed laterally by along-front currents there. The basin may also have been swept clean by along-strike currents, apparent from erosional scars seaward of the toe-of-slope. Siliciclastic sedimentation shelf was submarine, with middle to outer neritic paleo-water depths indicating that the observed along-strike progradation occurred during transgression of the shelf. The sediment source for the transgressive shelf ridges was distal allowing emplacement of mature, reworked sands on the carbonate-dominated shelf. A north-directed current, indicated by northeast progradation, was coincident with an influx cool-water planktonic foraminifera in the late middle Miocene and late Miocene in the Wombat and Exmouth plateaus (Zachariasse, 1992). This requires a major reorganization of ocean circulation in the southeastern Indian Ocean, involving perhaps replacement of the south-flowing Leeuwin Current with the north-flowing West Australian Current. The primary stratigraphic discontinuity recognized seismically may vary from sequence to sequence. For example, boundaries developed in response to subaerial exposure may be less apparent than surfaces in fully marine shelfal and shallow slope basin fill where exposure is not recorded. The level at which the NCB shelves are in equilibrium is greater than the fair-weather wave-base. Progradation in shelf water-depths of 20 to >100 m indicates that sediment transport can occur off a relatively 158 deeply submerged shelf. Therefore, it is not correct to assume that onlap against the clinoform front records a downward shift of coastal environments. The sensitivity of clinoforms to sea-level change is muted by both continuous sediment production of these systems and by deep baselevels during their evolution. Progradation decreased relative to aggradation (OM1-EMM1) before the mid-Miocene optimum, after which progradation increased relative to aggradation (MM1-MM2). An interval of shingled clinoforms, MM1.1 and MM1.2, prograding onto the shelf marks the inflection point. These overall patterns do parallel the inferred gradual increase and decrease of eustatic sea-level in the early and middle Miocene, but only in a general way. Fluctuations in paleobathymetric estimates across mapped downlap surfaces range from 60 to 200 m, uncorrected for compaction. These magnitudes are larger than relative sea level fluctuations derived off NE Australia (86 30 m) and the east coast of the U.S. (53 32 m). Compaction will increase variations in paleobathymetry relative to sea-level changes, but some of the disparity may be related to reactivation and inversion of Mesozoic structures. Paleobathymetric estimates from the wells located ~40-60 km from the anticline crest may be increased by the flexural response to the tectonic load resulting in increased accommodation on the flanks. 159 Chapter Four: An early mid-Miocene, strike parallel shelfal trough and possible karstification in the Northern Carnarvon Basin 4.1. INTRODUCTION Carbonate-dominated Tertiary sediments of the Northern Carnarvon Basin, North West Shelf, Australia, have received relatively little study compared to the more hydrocarbon-prospective Cretaceous and older section, despite one Paleocene discovery (Sit et al., 1994). Published Tertiary sequence stratigraphic studies (e.g., Apthorpe, 1988; Romine et al., 1997; Westphal and Aigner, 1997; Hull et al., 1998; Mutch and Vail, 1999) have been restricted either to wells alone, or to a combination of wells and regional 2D seismic profiles spaced at ~10-50 km. In contrast, we are interpreting a 3D seismic volume within 2D control calibrated by exploration wells (Fig. 4.1) to understand the detailed relationship between the development of Oligocene-Miocene seismic stratal architecture and presumably associated base-level variations. The relevant seismic section consists of a pronounced stack of clinoform geometries prograding to the northwest (Fig. 4.2). The 3D data also allow unprecedented visualization of a northeast-southwest - oriented trough associated with an early mid-Miocene boundary, 4.1, at the top of a set of shingled clinoforms within the prograding succession (Fig. 4.3). Location, orientation and isolation of this trough are unlike anything previously reported from similar prograding Tertiary continental margins elsewhere. published in Marine Geology 178 (2001), p157-169 with co-author J.A. Austin, Jr. 160 Figure 4.1. (following page). (A) Location of the Northern Carnarvon Basin (NCB), at the southern end of Australia's North West Shelf. (B) The NCB is composed of four predominantly Mesozoic depocenters (combined dark shaded area), as imaged by an ~8000 line-km 2D seismic grid. Prograding, predominantly carbonate Cenozoic-Recent clinoforms are imaged on a ~1500 km2, 3D seismic volume (light shading). (C) Location of the seismically defined, shelf-edge parallel trough relative to Triassic-Aptian and Eocene-Miocene structural trends (Adapted after Newman, 1994; Stagg and Colwell, 1994; Romine et al., 1997; Kingsley et al, 1998). Six exploration wells that tie with the 2D/3D MCS help verify the seismic interpretation. Location of seismic example in Figure 4.2 is given by the red line. (D) Close-up view of seismically defined, early midMiocene strike-parallel shelfal trough on boundary 4.1. The trough cross-cuts and is sub-parallel with various Eocene-Oligocene fault trends (magenta lines). 161 0 o Fig. 1B Gr ey R o rt e 40 S o R. NWC n R. o scue R . HA MM ER RA SL E NG ES Y 110 E C WELLS o 130 E 150 E o 115 30'E o o 112 E o 116 E o 116 00'E SY L NC E IN 120 E 2 - Goodwyn 2 3 - Goodwyn 3 4 - Goodwyn 4 6 - Goodwyn 6 7 - Goodwyn 7 E1 - Eastbrook 1 E1 D EN TR SG NRB Fig. 4.1D 2 GB 4 6 3 CE 7 N MB IN NK RB KG RA PA R 0 km 10 RRA R TE KE R PIE ELEINE M AD M DA 115 50'E o KE ND ER TER RA C E RK RO PA W T UG H RE T 1 wells used in study Geoscience Australia 2D survey 136 D other Geoscience Australia 2D surveys 3D seismic volume (B,C,D) k karst location, Romine et al., 1997 (B) mF mouth, modern Fitzroy River (A) C onshore Canning Basin (A) NWC North West Cape (B) Triassic-Aptian structural trends Blocks: Malus (MB), Rankin (RB), Goodwyn (GB), North Rankin (NRB) Grabens: Keast (KG), Searipple (SG) Eocene-Miocene structural trends 4 6 GB 3 KE R T E CE RA R KG 162 PA R 2.5 km 19 40'S mid-Miocene (4.1) strikeparallel shelfal trough o 19 50'S IN AS -B UB REND S o 19 30'S A RI TO C VI o 22 S o o 20 S c 20 S o mF Fig. 4.1C k De o 18 S A o B 0 km 200 Y u le R F Ro As h bu be rt o Figure 4.2. (following page). Uninterpreted, interpreted and line drawing of a seismic traverse within the 3D volume, illustrating the pronounced clinoform succession in the Oligocene-Miocene in the NCB. Interpreted seismic horizons, defined largely by downlap terminations, range from early Eocene to late Miocene in age. Three exploration wells, Goodwyn 3, 4 and 6, are offset various stated distances from this dip section (see also Fig. 4.1D). Basinwarddipping topsets identified at a variety of stratigraphic levels may indicate increased accommodation (dotted lines) by differential, compaction-induced subsidence that could in turn influence the location of fracture-controlled karst. Location of profile given in Figures 4.1 and 4.4. EEOC, early Eocene; DLS1, late early Oligocene; DLS3, early early Miocene; DLS4, late early Miocene; 4.1, early mid-Miocene, DLS5, mid-Miocene; DLS_TOP, late Miocene. 163 164 Figure 4.3. (following page). (A) Lithologies and ages of local sediments, as described in well-completion reports (BOCAL, 1973a; 1973b; Woodside Offshore Petroleum, 1981). Gamma ray (GR) and sonic (DT) logs highlight the lithologic uniformity of the section. Ranges (shaded) around horizons indicate their optimal depth range relative to the sea floor, translated from seismic sections in TWTT using synthetic seismograms (Appendix 5). (B) Close-up, uninterpreted and interpreted seismic traverse within 3D volume. Location given in Figure 4.2B. Truncation and onlap at this location characterize the early mid-Miocene boundary 4.1 with which the shelfal trough is associated. Truncations and changes in amplitude characterize the trough. Late early Miocene (DLS4) and mid-Miocene (DLS5) aged reflectors are the same as interpreted in Figure 4.2. 165 A offset (km) m (ss) 0 Goodwyn 3 KB 30.2 m 0.0 Lithology 50 Goodwyn 6 KB 8.0 m 2.9 DT (ms/ft) GR (gAPI) 40 0 50 Goodwyn 4 KB 30.2 m 6.1 DT (ms/ft) 140 40 0 GR (gAPI) Age (Ma) 140 Lithology Age (Ma) GR (gAPI) Lithology 50 Age (Ma) 140 DT (ms/ft) 40 1200 1300 **N9 (14.8-15.1) ~1km sub SB N9 (14.8-15.1) (14.8-15.1) (14.8-15.1) (14.8-15.1) (14.8-15.1) (14.8-15.1) (14.8-15.1) (14.8-15.1) ~2km cs **N8 (15.1-16.4) 1 4. d Burdigalian Burdigalian Burdigalian Burdigalian Burdigalian Burdigalian Burdigalian Burdigalian (16.1-20.52) 1400 sub SB **N6/5 (17.3-21.5) **N4b (21.5-23.2) N8 N8 N8 N8 N8 N8 N8 N8 (15.1-16.4) DLS4 1500 LITHOLOGY calcarenite calcisiltite calcilutite dolomite } **yellow where abundant recrystallization ** pers. comm., G.Moss, UTIG, 2001 cs casing shoe KB kelly bushing ,d stable isotope side-wall core, ditch cutting SE basinward progradation NW B trough DLS5 4.1 DLS4 trough 1.25 m 166 Prograding clinoform geometries vary in 3D at many spatial scales, but strike-parallel, mappable seismic features like the observed trough are rare. Contour currents can produce strike-elongate channels (e.g., Blake Plateau; Pinet et al., 1981) or strike-parallel sediment drifts (e.g., New Zealand; Fulthorpe and Carter, 1991). Collapse at the shelf edge sometimes results in arcuate to linear scarp amphitheaters and associated strike-elongated slope-toe sediments (Galloway, 1998; e.g., west Florida, Mullins et al., 1986; U.S. east coast; Fulthorpe et al., 2000). Shelf drainage systems are also very occasionally strikeparallel, as indicated by paleo-channels offshore Namibia (Day et al., 1992). Reefs and biohermal mounds developing on carbonate-prone shelves can also be narrow, linear, and strike parallel. Examples occur along the modern northeast Australia margin (Davies et al., 1989). Carbonate platforms, such as the one recognized within the Tertiary of the North West Shelf, can undergo diagenesis which could produce the observed trough during exposure intervals (James and Kendall, 1992). The size and type of diagenetic features vary, depending primarily on moisture levels during platform exposure. In arid conditions, the main diagenetic product may be only a caliche/hardpan profile 1-6 m thick, capped by a soil layer (Esteban and Klappa, 1983). This would be an impossible target to identify on the industry seismic data we are interpreting. A terrain of multiple surface depressions, some of which are seismically resolvable, is usually produced by diagenetic alteration under temperate conditions. Examples include: 1) circular dolines or collapse sinkholes, 10-100 m diameter and 2-100 m deep, 2) uvalas, larger, elongate 167 depressions > 1 km long, and 3) poljes, or karst valleys, produced by the systematic formation and then connection of dolines (Cvijic, 1981). Karst development in response to wet, tropical weathering of carbonates is dominated by positive residual relief, commonly up to 300 m, which produces the polygonal cones and towers characteristic of parts of east China (Silar, 1965). Subterranean karst caverns, single- to multiple-passage cave systems, may develop and be tens of km long (Palmer, 1991). Positive karst, although topographically distinct enough to be identified seismically, is rarely identified as such in the subsurface and may also have poor preservation potential in the ancient record (Purdy and Waltham, 1999). Negative karstic features resolved by seismic data are also rarely reported, and are typically restricted to interpretation from 3D (e.g., Ordovician Ellenburger Formation; Hardage et al., 1996; Loucks, 1999) or high resolution surveys (e.g., Bermuda; Garrett and Hine, 1979). We have been able to identify and map, using 3D seismic data, a strike-parallel trough associated with an early mid-Miocene carbonate ramp. Due to the trend and singularity of the trough in this geologic environment, we interpret it to represent karst topography as a consequence of preferential dissolution along heterogeneities within the platform. 4.2. GEOLOGIC SETTING The Northern Carnarvon Basin (NCB) is located at the southern extreme of the North West Shelf (NWS) of Australia (Figs. 1.8, 4.1A), which has undergone complex Mesozoic-Cenozoic development as a result of multiple inversion and reactivation events (Fig. 3.2). Initial extension and basin formation 168 occurred in the late Paleozoic (Etheridge and O'Brien, 1994), followed by three phases of rifting (Rhaetian, Callovian and Tithonian-Valanginian) and subsequent post-Valanginian regional subsidence (Driscoll and Karner, 1996; 1998). Minor inversion is dated by different authors to occur in the Cenomanian (Romine et al., 1997), Turonian (Driscoll and Karner; 1996, 1998) or Santonian (Chapter 5). Collision of the Indo-Australian Plate with the Pacific and Eurasian plates to the north (Fig. 1.8) was possibly initiated in the late Oligocene (>25 Ma; Pigram and Davies, 1987), continues today and is thought to result in reactivation and inversion features along the NWS (e.g., Malcolm et al., 1991). Continued northward drift of the Australian Plate into lower latitudes has produced a progressive change on the NWS from siliciclastic to predominantly carbonate sedimentation during the Cenozoic (Figs. 1.12 and 3.2; Apthorpe, 1988). Well evidence (Figs. 4.1 and 4.3) suggests that carbonate sediments with minor intercalated siliciclastic sediments dominate the seismically observed, wedge-like Cenozoic geometry. Siliciclastic material may have been delivered to the margin via the ancestral equivalents of the modern drainage system (Fig. 4.1B). Today, the largest modern rivers in this region are the Fortescue, Ashburton and DeGrey, each discharging >200 million m3 annually, mainly during floods. These rivers are fed by intermittent storm and cyclone runoff in a tropical-semi-desert climate, and have incised spectacular gorges (Semeniuk, 1996). Unfortunately, their Miocene loads are unknown. Australian climates were warm and wet through the early Miocene, reaching a warm optimum that 169 straddled the early-middle Miocene boundary before becoming arid in the late Miocene (McGowran and Li, 1996). 4.3 DATA The seismic data used for this investigation comprise a 1500 km2, 1980's90's vintage, 3D seismic volume, 70 km along strike and 30 km wide, provided by Woodside Energy, Australia (Figs. 2.1 and 2.2; Table 2.1). The volume is nested within 8000 line-kms of 2D MCS profiles covering the entire basin (Figs. 4.1B and 4.1C). The 2D data were acquired (Table 2.2) and processed (Fig. 2.3) between 1991 and 1994 by the Geoscience Australia (GA) and consist of three discrete surveys, 101r, 110 and 136. Central frequencies of the 3D data range from 40 to 55 Hz. The resultant theoretical vertical resolution at 1 s two-way travel time (TWTT) is 15-20 m. All slope and thickness calculations discussed in this chapter assume an average seismic velocity of 3200 m/s, determined from time-velocity pairs provided with the 2D data (Fig. 2.4). Depth conversions from the surface are conducted using synthetic seismograms at wells (Appendix 5) Velocities from the 3D volume were unavailable. From a total of nine exploration wells (Fig. 3.1), three are focused on in this chapter. Goodwyn 3, 4 and 6, are directly tied to the 2D and 3D seismic using synthetic seismograms constructed from check-shot calibrated sonic logs (Figs. 4.1 and 4.3; Appendix 5; Section 2.2.3). Well completion reports provide lithologic, chronostratigraphic and well log information for correlation with the seismic architecture (Plates 1-7; BOCAL, 1973a; 1973b; Woodside Offshore Petroleum, 1981). Stable isotope analyses (d13C and d18O) were performed on 170 bulk samples from Goodwyn 6 above and below the early mid Miocene boundary 4.1. The five side-wall cores and one ditch cutting (see Fig. 4.3A for location of samples) all tested as non-ferroan calcite with a mixture of Alarazin Red S and 2% hydrochloric acid. Isotope ratios of the calcite were determined by reaction with 100% phosphoric acid at 25oC, followed by cryogenic purification of the CO2 (McCrea, 1950). Isotope ratio measurements were made on a Prism II dual inlet mass spectrometer calibrated to NBS 19 in the stable isotope laboratory at The University of Texas at Austin, Department of Geological Sciences. 4.4 RESULTS Boundary 4.1 discussed in this chapter is early mid-Miocene in age, ~14.8 16.4 Ma, based on ties with Goodwyn 4 and 6 (Fig. 4.3A); it is defined by toplap, truncation and onlap (Fig. 4.3B). Its general morphology approximates a gently dipping (<<1 ), distally steepened (~2 ) carbonate ramp, which progrades to the northwest along a linear to slightly lobate front (Fig. 4.4A). The ramp terminates by way of downlap onto an early to mid-Miocene shelf (DLS4). The age of the shelf, 18.3 3.1 m.y. is constrained by Goodwyn 2, 3, 4, 6, 7 and Eastbrook 1 (Fig. 4.3; Table 2.3). The southern limit of the ramp's lobate character is coincident with a low relief, <20 m, ~10 km-wide embayment. Beneath the boundary, mid-Miocene and older clinoforms also prograde to the northwest; they are shingled to elongate-sigmoid in cross-section (Mitchum et al., 1977). Above the 4.1 boundary, sigmoid clinoforms prograde 171 Figure 4.4. (following page). Shaded contour map of early mid-Miocene boundary 4.1 (see Figures 4.2 and 4.3), showing the areal distribution of the strike-parallel trough. Location of interpreted region is given with respect to outline of overall 3D volume (see Figure 4.1). Contour interval is 10 ms two-way travel time (TWTT), ~16 m @ 3200 m/s. The ramp, which steepens seaward, is linear to lobate and terminates at a NE-SW - oriented downlap edge. (B) Close-up of the trough, whose trend is approximately normal to the dominant clinoform progradation direction. Three en echelon, linear to arcuate segments occur over 8 km of the mid-shelf. See text for detailed descriptions. The three wells and seismic profile (Fig. 4.2 and 4.3B) are numbered as in Figure 4.1. 172 basinward as well. The younger section thickens beyond the late early Miocene DLS4 breakpoint (Fig. 4.2). The most singular characteristic of this early mid-Miocene boundary 4.1 is a trough, sub-parallel to the strike of the ramp, identified both within the 3D volume and available 2D coverage (Figs. 4.1 and 4.3B). The trough consists of three separate linear to arcuate segments, each up to 500 m wide, with a combined length of at least 8 km along a northeast-southwest trend, orthogonal to the general direction of Miocene clinoform progradation (Figs. 3.13, 3.14 and 4.4A). The northernmost segment is ~5 km long, 200 500 m wide, linear to the northeast, and arcuate (i.e., concave towards the basin) at its southwestern end. Two shorter segments, one linear and ~1 km long, the second concave towards the basin and ~2 km long, occur to the southwest. These are offset from each other in en echelon configuration, although a general northeast-southwest trend for the trough as a whole is maintained (Fig. 4.4B). The trough is up to 60 m deep. In cross-section, the trough is characterized seismically by a combination of truncation, reflector interruption, and related amplitude variations (Fig. 4.3B). It varies from U- to V-shaped, symmetric to asymmetric, and is also sometimes irregular. These variations all occur on The feature lacks multiple 3D seismic traverses perpendicular to its strike. seismically resolvable internal structure, but a single high-amplitude, slightly concave-up reflector caps the interpreted trough-fill (Fig. 4.3B). The early mid-Miocene ramp is horizontal to slightly concave-upward updip of the trough; downdip of the trough the ramp steepens (0.50 to a basinward 174 max ~20) and is slightly convex-upward. The northern end of the trough is directly superimposed on this very subtle break in slope, from near horizontal to 0.50. The amplitudes of reflectors down-dip of the trough tend to be higher than those updip. However, these observed amplitude variations, and the subtle break in ramp slope just described, are also noted where no trough occurs farther northeast (Fig. 4.4A), so there may be no relationship of these phenomena to the trough itself. There is no seismic evidence for faulting immediately associated with the trough (Fig. 4.2). However, the trough is sub-parallel to an underlying structural high known as the Rankin Trend, the northwest edge of the northeast-southwest oriented Oxfordian-Aptian Dampier Sub-basin (Fig. 4.1C). This trend consists of discrete north-south and northeast-southwest blocks: Malus, Rankin, Goodwyn and North Rankin. The intervening Keast and Searipple grabens are bounded by en echelon faults (Newman, 1994). Small-offset (<20 m), Eocene-Oligocene fault trends reactivated within the Goodwyn Block parallel older northeast-southwest and north-south structures. The observed trough straddles one of these northsouth reactivated faults, and is sub-parallel to and 0.5 to ~2.5 km seaward of another, larger Paleogene fault trend (Fig. 4.1D). The deepening ramp is characterized by coarser grained calcarenites, which interfinger with finer grained calcilutites basinward (Fig. 4.3A). Biogenic material is calcitic, dominated by recrystallized foraminifera and lesser amounts of bryozoa (Appendix 1; Moss et al., in prep.). A drop in authigenic calcite content occurs in samples from Goodwyn 4 and 6, above the 4.1 boundary (Fig. 175 4.3B; pers. comm., G. Moss, UTIG, 2001; Appendix 1). These two wells straddle the trough (Fig. 4.3B). In Goodwyn 6, d18O values are 0.45 0.29 and d13C values are 2.43 0.5. These values are not consistent with common evidence for meteoric diagenesis, which is more often characterized by a reduction in d13C directly (<10 m) below the exposure surface and with a d18O shift (Allan and Matthews, 1982). The isotopic homogeneity suggests buffering of the diagenetic system by host rocks. Furthermore, any shift in d13C values may have been missed as a result of the 5 to 30 m sample interval (Fig. 4.3A). The d13C values are in the range of marine to slight meteoric alteration, while the covariance of d13C with d18O suggests mixing of mineral end-members between marine and diagenetic calcite (e.g., Banner and Hanson, 1990, their Fig. 9). The isotopic evidence indirectly supports the probability that meteoric diagenesis that may result in karst topography has occurred in the vicinity of the observed trough. 4.4 DISCUSSION Possible explanations for the observed trough, very unusual in its location, orientation and isolation within a Neogene clinoform succession, include: 1) fluvial drainage, 2) contour-current erosion, and/or 3) a concentration of diagenetic processes. The trough looks similar in cross-section to some channel incisions, because of associated reflector truncations and a concave-upward basal surface (Macurda, 1989; Burnett, 1996). If this feature is such an incision, then it may represent a remnant of fluvial drainage developed when the associated boundary was subaerially exposed. However, such systems usually form diporiented trends, even if individual incisions are sinuous (Galloway and Hobday, 176 1996; Fulthorpe et al., 1999). An exception are strike-oriented paleo-channels, several hundred meters wide and 15-20 m deep, which parallel the paleo-coast off Namibia for ~50 km, as identified in boomer and airgun seismic surveys (Day et al., 1992). Northwestern Australia was drained by a number of rivers during the Miocene, including ancestral equivalents of the modern drainage system (Fig. 4.1; Langford et al., 1995). Many, however, have not flowed since the late Miocene, when the climate became arid (White, 1994; McGowran and Li, 1996). Paleochannels of late Miocene age are known to exist beneath desert sands of the onshore Canning Basin (Fig. 4.1A), at least 400 km to the northeast (Tapley, 1988;1990). Other paleo-channels are identified beneath the modern shelf. For example, the mid-Miocene mouth of the paleo-Fitzroy River, ~600 km to the northeast, is located 160 km west of its current shore position (Fig. 4.1A; Tapley, 1990; White, 1994). While the presence of such channels confirms that subaerial exposure occurred on the NWS during mid-Miocene relative sea-level falls, no seismic connection between this trough and any other recognized fluvial drainage feature on the associated early mid-Miocene boundary 4.1 has yet been made. The vertical seismic resolution of our data, 15-20 m, may restrict recognition of these channels, if they are small. Alternatively, channels may not be preserved during subsequent transgressions. The most important conclusion is that both the singularity and size of this feature argues against a fluvial origin. Contour-current incisions are another possible agent for creating the trough; they naturally parallel many coasts. Shifts in current axes could produce changes in locations of associated erosional gullies through time, as has been 177 documented for Gulf Stream erosion of the upper Paleocene-lower Eocene section on the Blake Plateau, east coast U.S. (Pinet et al., 1981). Along the NWS, the southward-flowing Leeuwin Current may have been active since the Eocene; fluctuations in its strength have occurred at scales of ~105 years (McGowran et al., 1997). However, if this long-lived current were responsible for erosion of the observed trough, we would also expect to see either stacked incisions affecting numerous reflections or multiple incisions spatially distributed along the midMiocene paleo-shelf. Neither phenomenon is seismically observed. Furthermore, observed dimensions of the trough are very different from known dimensions of the axis of the Leeuwin Current, which is ~50 km-wide and ~200 m-deep. The third possibility is that the trough represents a subaerial and subterranean karst, produced by pervasive carbonate diagenesis during exposure of the boundary. Mid- to late Miocene-aged karst topography has been interpreted by Romine et al. (1997) along a 2D seismic line (Fig. 4.1B) southwest of the 3D volume (Romine et al., 1997). Further investigation in the 3D data now suggests this topography is generated by slope gullies (Fig. 3.15; Section 3.5.3). However, circular depressions identified on the shelf of an older boundary, 3.1, within the 3D volume, are interpreted as karst topography (Chapter 3). These features coalesce on their up-dip limit to form arcuate depressions continuous along-strike for 6-8 km (Fig. 3.9). Chaproniere (1975, 1984) identifies two periods of emergence during the mid-Miocene from onshore wells on North West Cape (Fig. 4.1B). The older one within foraminifera zone N8 (16.4-15.1 Ma), is associated with intense recrystallization, calcrete and pisolites which may be the 178 result of subaerial diagenesis (Condon et al., 1955; Chaproniere 1975; 1984). The early mid-Miocene boundary 4.1 on which the trough is developed is coeval with this exposure surface. Known karst features, such as caverns, dolines, related collapse structures, poljes, cones and towers (Cvijic, 1981; Esteban and Klappa, 1983), are all large enough to be imaged seismically (e.g., Hardage et al, 1996; Loucks, 1999; Purdy and Waltham, 1999). Some, like karst valleys (Cvijic, 1981), resemble the trough morphologically, and a carbonate ramp model of Emery and Myers (1996; their Fig. 10.20) implies that similar karst features can be oriented along strike. Karstification would preferentially concentrate along heterogeneities, including fractures, joints, bedding planes, vegetative cover, lithologic contrasts, grain-size and grain-packing changes and variations in cementation. Any such change in rock properties that will increase relative permeabilities and the potential for karstification (Trudgill, 1985), but the overriding control on permeability is jointing and bedding planes within otherwise tight lithologies (Trudgill, 1985; Esteban and Wilson, 1993). Unfortunately, because joints and bedding planes do not exhibit displacement, they are not seismically imaged. Hunt et al. (1995) have suggested, using field evidence, that sequence boundaries can be modified, and location of fracture-related karst controlled, by compaction-driven differential subsidence. For example, greater compaction of finer grained sediments along a clinoform front would be recognized seismically by basinward rotation of initially horizontal topset/toplap surfaces, as observed (Fig. 4.2). Extension fractures developed at this rotation hinge might then localize karst development (Hunt et al, 179 1995). Such compaction is governed by 1) stratigraphic architecture, 2) relationship to underlying, non-compacting basement, and 3) lateral facies relationships. We suggest that the stacked progradational geometry of the Tertiary section in the NCB, combined with calcite-dominated sediments there, which are less susceptible to early diagenesis and therefore more susceptible to compaction on burial (Choquette and James, 1990), superimposed on a complex of underlying fault blocks and grabens, all make compaction-induced differential subsidence likely. Seaward-dipping topsets suggest subtle basinward-increasing accommodation on the shelf (Fig. 4.2), thereby increasing the potential for fractures to develop at or near hinges in the NCB (Fig. 4.2, line interpretation). However, if differential compaction were the overriding control on karst development, we would expect multiple karst features at a variety of stratigraphic levels. This is not observed (Fig. 4.2). Instead, the singularity of the trough suggests a more unique control. Karst will form in areas without significant jointing or bedding plane discontinuities, if intergranular porosity is present to control permeability (Trudgill, 1985; Esteban and Wilson, 1993). Interestingly, the trough is associated with just such a basinward change, from sand-sized calcarenite to finer grained calcilutite (Fig. 4.3A). Such facies transitions, commonly distributed in relatively linear, strike-oriented bands on carbonate shelves and platforms (e.g., modern Southern Carnarvon Basin; James et al, 1999) will control the location of karst development as a response to differential mechanical compaction of carbonate sands (<5%) and carbonate muds (up to 30%) in the upper ~100 m of section (Goldhammer, 1997). We suggest that such 180 a facies transition must be driving localized diagenesis along only this part of the early mid-Miocene boundary 4.1, creating the observed feature. 4.5 CONCLUSIONS We propose that the observed trough is a result of karstification of an exposed, early mid-Miocene, middle to outer shelf carbonate ramp (Fig. 4.5). Trough formation occurred at a time when exposure is documented to the northeast by the presence of paleo-channels (Tapley, 1988; 1990), and to the southwest by exposure diagenesis (Condon et al., 1955; Chaproniere, 1975; 1984). Exposed surfaces were subject to a warm and wet Australian climate (McGowran and Li, 1996), that is conducive to diagenesis. The singular nature of the trough, its linearity, and lack of observed structural association all suggest that it has resulted from a combination of mechanisms other than jointing, such as grain-size changes and packing/cementation variations. The predominance of calcite-producing organisms on the submerged ramp (Fig. 4.5A) increases the possibility that some porosity will be preserved beyond early diagenesis, through which water will percolate on exposure (Fig. 4.5B). This must be less likely to occur in finer-grained sediments landward and basinward, as a result of their higher rate of porosity occlusion during early burial. If this trough is the result of meteoric diagenesis, then its relief allows us to calculate the minimum magnitude of relative sea-level fall at this time. Middle ramp sediment forms at 20 100 m below ambient sea level (Fig 4.5A; Kennett, 1982). Depth of the water table, at or near which karst features will form, 181 Highstand 3 6 4 middle ramp outer ramp slope EXPOSE d) (Highstan Lowstand D RAMP A RSL outer 4 middle 6 h 3 troug peritidalshelf lagoon edge B slope collapse infiltration gravity percolation VADOSE calcilutite + marl calcilutite and calcisiltite calcarenite, interbedded with calcisiltite, skeletal, f-vc, poor-mod sorting cemented in part, in part recrystallized to dolomite (Goodwyn 3 only) sediment heterogeneities lowered sea level salt water trough fresh water WATER TABLE PHREATIC C Figure 4.5. Schematic illustration of proposed karst development of the observed trough. (A) Carbonate sediments are deposited as part of a northwest-prograding ramp, indicating a marine environment with restricted accommodation. Margin-parallel facies belts are dominant. Sediments shown are consistent with lithologies described in well-completion reports (Fig. 4.3A), but locations of wells are approximate. No scale is implied. (B) Relative sea level (RSL) falls. Active carbonate deposition in margin-parallel facies belts persists below sea level, but the earlier middle and outer ramp is exposed. Preexisting sediment heterogeneities determine the location of seismically observed karst, like the trough. (C) Presumed depth of karst represents the minimum depth to the ambient water table at lowstand, this approximates sea level and indicates a minimum relative fall in sea level of 80-160m. Adapted from the Caribbean karst model of James and Choquette (1988). 182 approximates sea level in coastal regions (Fig. 4.5C). Therefore the presence of a 60 m deep karst trough suggests a minimum relative sea level fall in this location of 80 - 160 m, during the early mid-Miocene. 183 Chapter Five: Cretaceous-Tertiary inversion history of the Dampier Sub-basin: Insights from quantitative basin modeling. 5.1. INTRODUCTION Passive continental margins are commonly targeted as sites to study sealevel history because of their relative tectonic stability, dominated by slow thermal subsidence (Miller and Mountain, 1994; Christie-Blick, Austin, et al. 1998). However, horizontal stresses presumably generated at plate boundaries propagate thousands of kilometers through the lithosphere and can result in intraplate deformation (Karner, 1984; Cloetingh et al., 1985; Lowell, 1995). The primary observed response to transmitted compressional in-plane forces is inelastic (brittle) reactivation of preexisting extensional structures, ranging from positive inversion of isolated individual faults, forming localized anticlines, to basin-wide inversion that turns a basin inside-out (Hayward and Graham, 1989; Karner et al., 1993; Lowell, 1995). Fault reactivation during inversion is selective: not all faults, or fault segments, will be reactivated, and the magnitude of inversion changes along structures (Sibson, 1995). This tectonic overprint must be identified and quantified regionally, before sea-level variations can be determined from the observed distribution of sedimentary unconformities and onlap in the preserved stratigraphic record. Accommodation along the northwest Australian continental margin is dominated by syn-rift and post-rift thermal subsidence. However, positive preparing for submission to Marine and Petroleum Geology with co-author, G.D. Karner 184 inversion of preexisting extensional structures is recognized in the Cretaceous (Driscoll and Karner, 1998; Tindale et al., 1998) and Tertiary (Malcolm et al., 1991; Keep et al., 1998) sections of the North West Shelf (NWS). Cretaceous inversion features in the Northern Carnarvon Basin (NCB) include (Fig. 5.1): (1) a northwest-southeast oriented, forced fold along the northwestern edge of the Exmouth Sub-basin (combined Resolution and Novara arches), (2) uplift of Barrow Island, and (3) minor hanging-wall uplift northwest of the Rosemary Fault Zone in the Dampier Sub-basin (Fig. 5.1; Driscoll and Karner, 1996; Karner and Driscoll, 1999). Tertiary reactivation is identified along the entire NWS as: 1) extensional fault reactivation due to Timor Trough foreland development resulting in flexure of the outer Australian plate in the Northern Bonaparte Basin (Fig. 1.8; O'Brien et al., 1999), 2) Miocene compressional anticlines, such as the Lombardina structure in the Browse Basin (Figs. 1.8 and 5.2; Symonds et al., 1994; Struckmeyer et al., 1998), and 3) an anticline along the Rosemary Fault Zone and the Giralia and Cape Range anticlines in the NCB (Figs. 5.1 and 5.3; Malcolm et al., 1991; O'Brien et al., 1999). We identify and model Cretaceous and Tertiary inversion features along the northwest and southeast Mesozoic basinbounding fault zones of the Mesozoic Dampier Sub-basin (Fig. 5.1). As shown on line 101r_09, the inversion features are small relative to accommodation created in the sub-basins during rifting and thermal subsidence (Fig. 5.4). Nevertheless, such deformation will modify the stratal response to other processes responsible for the generation of discontinuities (i.e., onlap and truncation; Karner et al., 1993). 185 Figure 5.1. (following page). Location of available seismic data, Northern Carnarvon Basin (NCB-shaded area delineates the location of subbasin depocenters as defined seismically). The 2D regional seismic surveys cover the entire basin and overlap with the 3D volume. Up to 3200 line-km of the 2D MCS have been interpreted (black dashed box). The 3D volume is located seaward of the Rankin Trend (dark blue dashed line). Compartmentalization of the depocenters is partially controlled by extension oblique to north-trending Proterozoic-Paleozoic fractures e.g., Scholl Island Fault (Romine et al., 1997). The eastern edge of the sub-basins is defined by the Lambert and Peedamullah shelves. Cretaceous (blue stars and dashed line) and Tertiary (red stars and dashed line) inversion features are identified. Numbers refer to text. We forward-model Santonian reactivation of the Rankin (blue dashed line) and Tertiary inversion of the Rosemary-Legendre (red dashed line) trends using 2D MCS line 101r_09 (red solid line). Five wells tied to the 3D volume (white circles) provide paleoenvironmental information through the succession (Appendix 1; Moss et al., in prep.). Three additional wells (squares) intersecting the 2D MCS provide extra timing and lithological information. 186 0.0 136_19 136_20 101r_02 136_24 TWTT (s) 1.0 2.0 3.0 0.0 NW136_19 136_20 101r_02 136_24 ON4 V.E. ~19 (@1500 m/s) ~ 8 (@3200 m/s) ON3 SE DLS_top/ON2 ON1 1.0 DLS5 TWTT (s) DLS4 DLS3 DLS2 DLS1 Sant. bCret bTer 2.0 3.0 Rosemary-Legendre 10 km Figure 5.3. Portion of line 136_15 showing asymmetric inversion anticline northeast of the modeled line, 101r_09. Solid lines are the horizons interpreted throughout the 2D and 3D MCS as prominent downlap surfaces in the basin. Discrete inversion events are indicated by onlap on the flanks and truncation over the crest of the anticline at several horizons (arrows). The crest of the anticline is west of the Rosemary-Legendre Fault. Location given in Figure 5.1. 189 Figure 5.4. (following page). Uninterpreted and interpreted depth section and modeled time-line stratigraphy of clinoform architecture across the Rankin Trend, west of the Dampier Sub-basin on 2D line 101r_09 (see Fig. 5.1 for location). Accommodation, primarily created by subsidence following Tithonian-Valanginian extension, was locally modified by Late Cretaceous fault reactivation (blue arrows) along the Rankin Trend to the northwest and repeated Late Cretaceous to Plio-Pleistocene inversion (red arrow) across the RosemaryLegendre Fault Zone to the southeast. Minor deformation is recognized in the Cretaceous along the Madeleine Trend (blue arrow). Although inversion was repeated along the RosemaryLegendre Fault Zone in the Tertiary (red arrow), the axis of the anticline is ~15 km northwest of the Santonian inversion (blue arrow). Wells projected onto the 2D seismic line are: Goodwyn 2 (G2), 9.7 km SW; Goodwyn 3 (G3), 14.6 km SW; Goodwyn 4 (G4), 13.5 km SW; Goodwyn 6 (G6), 15 km SW; Goodwyn 7 (G7), 150 m NE; and Eastbrook 1 (E1), 14.7 km SW. Rosemary 1 intersects the southeast end of the 2D line. The vertical orange lines in the forward-modeled section give the location of the total subsidence curves in Figure 5.16. 190 Distance (km) along modeled section (origin at COB) 110_08 136_19 136_20 101r_02 136_24 NW 0.0 1.0 310 320 330 340 350 360 370 380 390 E1 400 410 G4 G2 G7 G6 G3 420 430 440 450 460 ROSEMARY 1 470 480 490 500 510 SE AGSO 101r_09 Depth (km) 2.0 3.0 4.0 110_08 136_19 136_20 101r_02 136_24 ROSEMARY 1 0.0 310 320 330 340 350 360 370 380 390 E1 400 410 G4 G2 G7 G6 G3 V.E. 7.5X 480 490 500 510 420 430 440 450 460 470 Line drawing 1.0 seafloor ON2 Fig. 5.12 DLS_top Depth (km) 2.0 DLS5 DLS4 DLS3 DLS2 Fig.5.6 1 DLS iary ert b. T 3.0 4.0 Kendrew Trough Madeleine Trend ian ton incision San ian Albian gin an lan i Va thon Ti Lewis Trough Rankin Trend 310 0.0 320 330 340 350 360 370 380 390 400 410 420 Dampier Sub-basin 430 440 450 460 Rosemary Trend 470 480 490 Lambert Shelf 500 510 Forward modeled section 1.0 ?Bare-Delambre Delambre Fm MM2 MM1 OM1 EMM1 2.0 3.0 arl ne ia M ltsto Mir tee Si luti ear l G OL1 ci Cal ga lon rite Lambert-Giralia adiola Too dalia R Win a le g Sh Muderon Bar yst.) r Cla row Gro up (Fo estie r Upper Dingo Clayst. Depth (km) 4.0 Permian-Triassic Locker Shale Ladinian-Carnian Mungaroo Fm. Jurassic Lower Dingo Clayst. Upper Dingo Clayst. Upper Dingo Clayst. Cretaceous Barrow Group (Forestier Clayst.) Muderong Shale - Windalia Radiolarite Gearle Siltstone Toolonga Calcilutite to Miria Marl Tertiary Lambert Fm to Giralia Calcarenite OL1 - Cape Range Gp. (Mandu) OM1 - Cape Range Gp. (Mandu) EMM1 - Cape Range Gp. (Mandu) MM1 -Cape Range Gp. (Tulki) MM2 -Yardie Gp. (Trealla and ?Bare) Delambre Fm. (?late Miocene) Delambre Fm. (?late Miocene - Plio-Pleistocene) Cretaceous, Tertiary compressioninduced inversion 191 Structural interpretations derived from one-dimensional (1D) backstripping analyses of wells (M ller et al., 1988, 2000; Kaiko and Tait, 2001) provide only partial insight into the inversion history of the NCB. While seismic unconformities may indicate erosion, it is not clear if such truncation is subaerial or marine. Determining vertical movement of the crust in significant waterdepths (>200m) from the preserved stratigraphy recorded in wells is also problematic because of the general lack of paleobathymetric resolution with increasing water-depth (Table 2.4). Furthermore, uplifts that result in erosion leave no geologic record that can be sampled by wells. Therefore, erosion and processes responsible for uplift are not easily quantified. In comparison, twodimensional (2D) forward kinematic and isostatic modeling, when combined with seismic and well analyses, provide a tool to analyze both the processes causing uplift, e.g., extension resulting in rift-flank uplift vs. compression-induced inversion, and the sedimentary response, e.g., amount of erosion and redistribution of sediments. Modeling flexurally accommodates resultant positive and negative loads across the margin. In addition, forward modeling allows a stepwise assessment of these processes through time. So, the model output is a culmination of geologically viable intermediate steps that build on one another to give a synthetic section that simulates the observed stratigraphy and can be quantitatively compared with it (e.g., NCB, Driscoll and Karner, 1996;1998; Gulf of Mexico, Mello and Karner, 1996; west Africa, Karner et al., 1997). We combine seismic sequence stratigraphy, sequence biostratigraphy and such forward kinematic basin modeling to determine 1) the distribution and 192 history of Cretaceous-late Tertiary inversion across the Dampier Sub-basin, 2) the distribution and magnitude of inversions relative to preexisting structural trends, and 3) the flexural response to tectonic and sedimentary loading. We also compare the paleo-water depths of decompacted clinoform geometries derived from the forward-modeled stratigraphy to paleo-water depths estimated from analysis of benthic foraminifera at hydrocarbon exploration wells within the clinoform succession. The analysis presented here builds on earlier quantitative basin analysis that examined the tectonic significance of the extension and inversion history in the Exmouth, Barrow and Dampier sub-basins since the Permian (Fig. 1.8; Driscoll and Karner 1996; 1998; Karner and Driscoll, 1999). 5.2. GEOLOGIC BACKGROUND 5.2.1 Tectonic Framework The NCB has had a complex history (Fig. 1.8). Broadly distributed intracratonic extension in the Late Permian is followed by multiple, more localized, phases of extension, which created the NCB sub-basins in the Late Triassic-Jurassic and reactivated them in the late Middle Jurassic (Figs. 5.1 and 5.5; Driscoll and Karner, 1996; 1998). The fourth and final phase of extension, in the latest Jurassic, culminated in continental breakup in the Valanginian. This final extension is represented by minor fault reactivation of northeast-southwest oriented structural trends. Ensuing regional thermal subsidence is disproportionately large relative to the minor upper crustal brittle failure observed, suggesting significant lower crust and upper mantle extension (Driscoll and Karner, 1998). 193 Figure 5.5. (following page). Generalized tectonostratigraphy of the NCB (after Chaproniere, 1984; Apthorpe, 1988; Romine et al., 1997; Driscoll and Karner, 1998), compared to seismic downlapping sequences and sub-sequences mapped as part of this study (Chapter 3; Cathro et al., submitted). Tertiary sub-tropical planktonic foraminiferal zones (Blow, 1969) are calibrated to the Berggren et al. (1995) timescale. Light green on the timescale indicates the complete interval modeled. The red area highlighted in the Tertiary indicates the interval mapped in the 3D volume (Chapter 3). Vertical error bars indicate the age range of seismic horizons. In addition to the downlap surfaces, four (Miocene-?Pleistocene) onlap surfaces are mapped proximal to the inversion anticline (Fig. 5.3). The oceanic oxygen isotope curve (Zachos et al., 2001) highlights an overall warming to a peak in the middle Miocene (mid-Miocene Optimum) that corresponds with a high in the eustatic curve (Haq et al., 1987), followed by oceanic cooling. 194 Planktonic Foraminiferal Zones Chronostratigraphy (Berggren et al.,1995) PLEIST. Formations Seismic Downlapping Sequences Long-term and short-term eustatic curves - Haq et al. (1987) Calibrated to Berggren et al., 1995 200 Seismic (see Table 3.1) sub-sequences 0 5 Long-term d18O record Zachos et al., 2001 (Pacific, Indian and Atlantic oceans) 4 3 2 1 TREALLA LIMESTONE TULKI BARE MANDU CALCARENITE CAPE RANGE BROUP Collision with E.Papuan Terrane Collision along northern Australian margin commences (Papua New Guinea/Sepik Arc) PLIOCENE ~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~ ~~~ ~~~~~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ late Minor Inversion - SANTONIAN MIOCENE N10 N9 TREALLA LIMESTONE/ PILGRAMUNNA FM YARDIE GP. middle CAPE RANGE GROUP early Sea floor spreading creating Argo Abyssal Plain RIFT 3 N4 23.8 MANDU FORMATION late OLIGOCENE early ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ DELAMBRE FORMATION Collision with Banda Arc Collision with Eurasian Plate and creation of Indonesian arc 1.75 DELAMBRE FORMATION N19 N18 N17 5.3 N16 11.2 N15 N14 N13 ? BARE FM DLS_TOP N12 N11 MM2 DLS5 MM2.6 MM2.5 MM2.4 MM2.3 MM2.2 MM2.1 10 MM1.4 MM1.3 MM1.2 MM1.1 EMM1.3 EMM1.2 EMM1.1 OM1.6 OM1.5 OM1.4 OM1.3 OM1.2 OM1.1 16.4 Tithonian rifting culminating in continental breakup in the Valanginian and creation of the Gascoyne-Cuvier abyssal plains RIFT 4 N8 N7 N6 N5 195 RIFT 2 Late Triassic-Early Jurassic extension - onset of rifting in Argo Basin TULKI LMST MM1 DLS4 EMM1 DLS3 20 OM1 DLS2 P22 28.5 P21 ~N2 P20 P19 P18 OL1 DLS1 OL1 30 RIFT 1 Late Permian extension 33.7 warming 195 The dominant northeast-southwest oriented structural grain produced by extension influences orientation of Cretaceous-Tertiary inversions (Fig. 5.1). Thermal subsidence was interrupted by minor inversion, variously dated between the Cenomanian and Santonian (Romine et al., 1997; Driscoll and Karner, 1998; Cathro et al., 2001). This inversion may have resulted from major plate reorganization, as India moved northwest and breakup commenced between Australia and Antarctica. Thermal subsidence was further punctuated by ongoing collision of the Indo-Australian Plate with the Pacific and Eurasian plates (Fig. 1.8). Initial interaction was with the Pacific Plate in the late Oligocene (>25 Ma), when the Sepik Terrane docked with the northern New Guinea-Australia margin (Pigram and Davies, 1987). Continued northward movement of the Indo-Australian plate resulted in accretion of successive terranes (Fig. 1.12), and fault reactivation and inversion along the NWS (Audley-Charles et al., 1988; Lee and Lawver, 1995; Richardson and Blundell, 1996). Increased subsidence interpreted from geohistory analysis on the outer NWS, of up to 500 m since ~10 Ma, has been linked to breakup of the Indo-Australian Plate into the Indian, Australian and Capricorn plates to the west (M ller et al., 1998). Today, the northeastern boundary of the Indo-Australian Plate is characterized by subduction beneath the Sunda Arc, collision between the Australian continent and Banda Arc, and continental collision at Papua New Guinea (Fig. 1.8; Coblentz et al., 1995; Hillis et al., 1997). 196 5.2.2 Stratigraphic Framework Sediment input into NCB sub-basins has been controlled by the paleotopography created by extension. Cretaceous syn-rift sediments of the Barrow Group (Fig. 5.5) were transported northwestward into the Exmouth and Barrow sub-basins (Fig. 5.1; Romine et al., 1997). Further north, in the Dampier Sub-basin, the relatively thin Forestier Claystone is the distal equivalent of north and west prograding foresets of the Barrow Delta (Tait, 1985; Miller and Smith, 1996). Sediment input from the Robe, Fortescue and Yule rivers to the east was preferentially directed into the Barrow Sub-basin through the Barrow-Dampier relay zone (Romine et al., 1997). The resultant Flag Sandstone is an important reservoir in the Barrow Sub-basin, whereas the Dampier Sub-basin was largely bypassed (Fig. 5.1; Romine et al., 1997). The late Valanginian-early Santonian post-rift Muderong Shale, Windalia Sandstone, Windalia Radiolarite and Gearle Siltstone overlie the Barrow Delta (Fig. 5.5). The fine-grained, transgressive, Muderong Shale covers much of the NWS, including the Exmouth Plateau to the west (Driscoll and Karner, 1998; Karner and Driscoll, 1999). This transgressive interval was briefly interrupted by deposition of the Windalia Sandstone in the Aptian, restricted to the BarrowDampier relay zone (Romine et al., 1997). The overlying regional but condensed Windalia Radiolarite is downlapped by the Albian-Santonian Gearle Siltstone in the Barrow Sub-basin, and its equivalent, the Haycock Marl, in the Dampier Subbasin (Fig. 5.1; Romine et al., 1997). 197 In the late Cretaceous-early Tertiary the NWS entered a phase of open marine, interbedded carbonate and siliciclastic deposition. Widespread, finegrained, transgressive carbonates and calcareous shales interspersed between prograding siliciclastic intervals (Fig. 5.5; Romine et al., 1997). Continued northward drift of the Indo-Australian Plate during this time (Fig. 1.12) produced a progressive change from siliciclastic to predominantly carbonate sedimentation. The NCB (Fig. 5.1) has migrated from ~36oS-40oS in the early Oligocene to 18oS22oS today (Veevers et al., 1991; Lawver et al., 1999). The late lower Oligocene (29.4 Ma) to early upper Miocene (10 Ma) heterozoan carbonate succession of the NCB is imaged as a set of stacked clinoforms prograding to the northwest (Fig. 5.4). More than 26 km of progradation have occurred during this 19 m.y. interval (Figs. 3.3 and 5.4; Romine et al., 1997). The clinoform succession is characterized by seawardsloping shelves and fronts inclined at 2-8o. Basinward of the clinoforms, only rare depositional lobes occur. Seismically defined build-ups, commonly seen proximal to prograding carbonate successions elsewhere, e.g., in the Bahamas (Eberli and Ginsberg, 1989), are a minor component (Romine et al., 1997; Cathro et al., submitted; Chapter 3). Paleobathymetric estimates indicate that the shelfedge was at all times submerged (20-200 m; Fig. 3.17; Appendix 1). However, seismically defined karst morphology (Fig. 3.9, Chapter 4) indicates that the shelf was at least partially exposed twice in the middle Miocene (within sequences EMM1 and MM1; Fig. 5.5). Northwest-directed carbonate progradation has been interrupted in the middle-late Miocene by an influx of texturally mature, quartz- 198 rich sands occurring as north-prograding shelf foresets (Fig. 3.12; upper MM2 and younger). These foresets, with an average height of 100-150 m, are marine, bounded by shallow-water shelf calcarenites (Fig. 3.17; Appendix 1). They suggest a reorganization of ocean circulation in the southeast Indian Ocean at the start of the late middle Miocene (Chapter 3; Cathro et al., submitted). This reorganization may have involved replacement of the south-flowing Leeuwin Current with the north-flowing West Australian Current following the midMiocene Climatic Optimum (Wright et al., 1992; Chapter 3). 5.2.3 Previous Work: Pre-Tithonian stratigraphic modeling The starting point for modeling the Dampier sub-basin stratigraphic succession is the completion of Tithonian-Valanginian rifting with initiation of post-rift subsidence (Figs. 5.4 and 5.5; Driscoll and Karner, 1996; 1998). The evenly distributed nature of initial Permian extension, represented by laterally extensive syn- (Locker Shale) and post-rift (Mungaroo Formation) sections, contrasts with the geographically more isolated subsequent syn-rift sections deposited during multiple phases of Mesozoic extension. The Dampier Sub-basin was initiated during the Late Triassic and is bounded to the east by the Rosemary Fault Zone and to the west by the antithetic Madeleine and Rankin trends which are located in the hanging-wall block (Figs. 5.1 and 5.4). Extension was greatest in the Lewis Trough, with lesser amounts observed to the west in the Kendrew Trough (Fig. 5.4; Driscoll and Karner, 1996; 1998). Callovian and Kimmeridgian extension involved fault reactivation, with the Madeleine and Rankin trends acting as the foot-wall block to the extension (Fig. 5.4; Driscoll and Karner, 1996; 199 1998). The switch of the Rankin Fault Trend from hanging-wall to foot-wall resulted in collapse and westward rotation of the previously uplifted area east of the Rosemary Fault Zone. Uplift along the Rankin Trend resulted in erosion of Lower-Middle Jurassic sediments on the western flank of the Dampier sub-basin (Driscoll and Karner, 1996; 1998). The final phase of Tithonian-Valanginian rifting resulted in continental breakup and creation of the Gascoyne-Cuvier abyssal plains (Figs. 1.8 and 5.5). Early Cretaceous extension was accommodated by minor brittle failure, with significant thermal subsidence (Driscoll and Karner, 1996; 1998). The disparity between brittle failure and thermal subsidence is interpreted to result from differential extension in the upper and lower plates, separated by an eastward-dipping intracrustal detachment (Driscoll and Karner, 1996; 1998; Karner and Driscoll, 1999). The depth of this detachment controlled the magnitude of thermal subsidence, which decreases with increasing detachment depth. The initial ~135 Ma topography, i.e., the starting point for the model, for Cretaceous-Tertiary development is derived from modeling the multi-phase rifting history (Fig. 2.23; Driscoll and Karner, 1996; 1998). The predicted synthetic stratal architecture is consistent with the seismic observations and well ties for the various syn- and post-rift phases. 5.3. METHODOLOGY The "Quantitative Basin Analysis" (QBA) approach (Driscoll and Karner, 1998) integrates seismic stratigraphic interpretation with forward kinematic basin modeling to understand lithospheric deformation and distribution of sediments 200 during rifting, re-rifting, subsidence and inversion. Geoscience Australia line 101r_09 (Fig. 5.4) has been chosen as the transect to be forward modeled because: 1) Cretaceous-Tertiary inversion on the northwest and southeast bounding faults of the Dampier sub-basin is recognized (Fig. 5.4); 2) This profile represents an ~200 km dip-oriented transect, centrally located in the sub-basin (Fig 5.1). From the east, the profile traverses the Lambert Shelf, Rosemary-Legendre Fault Zone, Dampier Sub-basin (Lewis Trough, Madeleine Trend and Kendrew Trough/Terrace), Rankin Trend and the Exmouth Plateau. The profile also intersects the available 3D seismic volume (Figs. 5.1) and is therefore suitable to assess the impact of regional tectonics on the stratal development mapped in the 3D MCS (Chapter 3; Cathro et al, submitted). 3) This profile was one of three transects used in investigation of Permian-Recent rift, re-rift, inversion and subsidence history and subsequent stratigraphic development of the NCB (Driscoll and Karner, 1996; 1998). Initial topography and thermal conditions of the lithosphere include the influence of these earlier tectonic events. 5.3.1 Seismic Interpretation Seismic data interpreted for this investigation are of two types: 1) a 1500 km2, 1980-90's vintage, 3D MCS volume, 70 km along-strike and 30 km wide, and 2) 3200 line-kilometers of 2D MCS, acquired and processed between 1991 and 1994 by Geoscience Australia (Fig. 5.1; Section 2.1.2). As part of this study, we have interpreted five major, late early Oligocene to late Miocene seismic 201 sequences (Figs. 5.4 and 5.5; Chapter 3; Cathro et al., submitted). The sequences are defined here to occur between seismic discontinuity surfaces of systematic basinward (distal) downlap, coincident with changes in stratal architecture or seismic attributes (Fig. 2.7). Onlap and truncation are less consistent for mapping purposes in this succession, however when identified, these seismic discontinuities are coeval with the downlap surfaces (Fig. 2.7). Interpretations from Romine et al. (1997) and Driscoll and Karner (1998) of the Cretaceous-late lower Oligocene are incorporated to allow modeling of the entire post-rift stratigraphic development. Horizons used for this purpose were the Tithonian and intra-Valanginian unconformities, the Albian downlap surface, the early Santonian and base Tertiary unconformities (Figs. 5.3 and 5.5). When a conflict arose between the two studies, we used the interpretation of Romine et al. (1997) was favored because of its regional well control. Four additional horizons, ON1-ON4, defined by onlap and truncation, have been interpreted proximal to the inversion anticline along the Rosemary Fault Zone, to show how local deformation continued from middle MiocenePleistocene (Figs. 5.3 and 5.5). As there is limited biostratigraphic control, and a harsh processing mute has truncated the late Miocene-Pleistocene in the modeled line, only ON2 has been added to the sequence of modeled time-lines. To compare the predicted geological model with the observed stratigraphy, the modeled seismic line must be converted to depth. RMS velocities derived from seismic processing are sub-sampled, edited and smoothed before such a depth conversion (Fig. 2.21; Section 2.2.4). The modeled profile 202 and depth-converted seismic section are then geographically cross-referenced using distance (km) along the modeled section, with the origin located on the Exmouth Plateau, ~100 km southeast of the Gascoyne COB (Fig. 1.8). Synthetic seismograms allow seismic stratigraphic interpretations in TWTT to be compared directly with the paleobathymetric, lithologic and chronostratigraphic data referenced to depth downhole in wells. The seismograms are created using downhole sonic measurements calibrated with check shot surveys; the resultant reflection coefficient series is convolved with a wavelet derived from the seismic data at each well location (Section 2.2.3; Appendix 5). 5.3.2 Biostratigraphic Data Paleo-water depth estimates are based on statistical analyses of depthranges and abundances of benthic foraminiferal species observed in samples from the available wells; Goodwyn 2, 4, 6, 7, and Eastbrook 1 all located within the 3D volume (Fig. 5.1; Appendix 1; Moss et al., in prep.). Goodwyn 7 also lies on 101r_09, whereas information from other remaining wells is projected northeast to the profile. A total of 227 samples, consisting of ditch cuttings and sidewall core with an average spacing of 29 m, were examined at topset, slope front and bottomset locations (Fig. 3.3; Appendix 1). 5.3.3 Tectonic induced accommodation: the thermo-mechanical model Accommodation is created and destroyed during rifting and is a combination of brittle failure in the upper lithosphere and plastic (ductile) deformation in the lower lithosphere (Fig. 5.6; e.g., McKenzie, 1978). Assuming 203 Figure 5.6. (following page). Generalized lithospheric extension model used to predict development of rift basins and passive margins, followed by compression-induced inversion (modified after Karner et al., 1997). (A) Simple shear in the upper crust is balanced by pure shear in the lower crust/lithospheric mantle. (B) Morphology of the topographic low produced by brittle failure in the upper crust and slip along border faults, is controlled by the heave, H, and shape of the fault. Lithospheric mantle extension controls distribution and amount of heat added to the lithosphere at the time of rifting. (C) Flexural adjustment of the lithosphere occurs due to crustal unloading and heat influx at the base of the lithosphere. The rift basin shape is a combination of the kinematic depression created during rifting, and total flexural rebound of the lithosphere in response to crustal unloading and heating at the base of the lithosphere. (D) Initial morphology prior to inversion. The basin is filled with syn- and post-rift sediments. (E) Compression of the extensional bounding fault results in net contraction of the upper section, including development of an anticline. As the original extensional fault has increased displacement with depth, net extension at the base of the fault may not be fully cancelled by the inversion. (F) During inversion, accommodation is simultaneously created and destroyed. The lithospheric load of a short-wavelength inversion anticline is flexurally compensated on a longer-wavelength, resulting in a regional depression on the flanks of the anticline. 204 (A) Initial Conditions Simple Shear (D) Pre-Inversion Upper plate (hanging wall block) tc Lithosphere Pure Shear Lower plate (footwall block) Tc Lithosphere Moho Isotherms a Asthenosphere offset A Asthenosphere (B) Kinematic Extension O (x) H L ing-wall collapse g han (x) Moho (E) Kinematic Inversion i(x) tc' Lithosphere offset Tc' Isotherms Lithosphere Moho i(x) Isotherms a' Asthenosphere A' Asthenosphere (C) Isostatic Response foot-wall uplift O H L (F) Isostatic response flexural "wings" tc' Lithosphere offset Moho Tc' Lithosphere Moho Isotherms a' A' Asthenosphere Asthenosphere synrift postrift 205 that flexural strength is maintained during extension, the ultimate shape of the basin following rifting is controlled by 1) flexural isostatic adjustment of the lithosphere to mechanical unloading of the crust, and 2) density variations engendered by the input of heat at the base of the thinned lithosphere (Fig. 5.6; e.g., McKenzie, 1978; Weissel and Karner, 1989; Driscoll and Karner, 1998). The initial rapid stage of rift subsidence is replaced by relatively slow, exponentially decaying subsidence caused by the conductive thermal reequibrilation of the lithosphere (McKenzie, 1978). These processes result in short-wavelength, disparate rift basins superimposed on long-wavelength regional subsidence. Similarly, variations in accommodation during compression-induced inversion are a combined result of additional mass, often characterized by anticline development, and flexural isostatic adjustment of the lithosphere to mechanical loading by the new mass of the anticline. Inversion does not always reactivate optimally oriented structures: more shallowly dipping faults or faults lined with low-friction gouge (e.g., talc), or pressurized fluids, may be preferentially reactivated (Sibson, 1995). Therefore, inversion features will be unevenly distributed. The ultimate shapes of compressive inversion features are short-wavelength anticlines; they may affect entire basins depending on the intensity of inversion. We analyze the response of the lithosphere to extension and compression is analyzed with respect to the kinematically produced loads (Fig. 5.6). Extension of lithosphere with pre-rift thickness a, and crustal thickness tc, is facilitated by 206 brittle failure and hanging-wall collapse in the crust and equal, although not necessarily spatially coincident, amounts of ductile extension in the lower crust and upper mantle lithosphere (Fig. 5.6A). The ratios of pre- and post-rift thickness of the resultant hanging- and foot-wall blocks are (x) and (x), respectively. The value of (x) across the margin determines the magnitude and distribution of rift-induced subsidence, while (x) is the primary control on postrift thermal subsidence. Extension perturbs the thermal structure of the lithosphere and leads to passive shallowing of the lithosphere/asthenosphere boundary (Fig. 5.6B). Flexural uplift of the rift flanks occurs in response to mechanical unloading of the lithosphere during extension (Fig. 5.6C; Weissel and Karner, 1989). In the modeling by Driscoll and Karner (1996; 1998), each rifting is treated as a series of instantaneous events, rather than a single large event with subsequent cooling (c.f., McKenzie, 1978). This allows heat to dissipate during protracted periods of extension, results in increased subsidence during rifting, and a corresponding decrease in post-rift subsidence (Cochran, 1983; Karner et al., 1992). Prior to inversion, loading by syn- and post-rift sediments augments thermal subsidence that has occurred since rifting (Fig. 5.6D). Compression of an extensional bounding fault may result in net contraction in the upper section, including development of an anticline. Net extension at the base is not cancelled out by the inversion (Fig. 5.6E; Coward, 1994; Hayward and Graham, 1989). Similar to extension, the ratios of pre- and post-inversion thickness of the crust and lithosphere are i(x) and i(x), respectively. Whereas for extension, (x) and 207 (x) 1, for inversion, i(x) and i(x) are between 0 and 1. The combination of the kinematically created inversion anticline, and flexural compensation of the positive load, determines the resultant geometry prior to any subsequent sedimentation and erosion (Fig. 5.6F). Because inversion is a selective process, it results in complex superposition with preexisting fault trends. For that reason, we have calculated the effect of inversion in segments of two multiple border fault systems that approximate the style of inversion observed in the NCB (Figs. 5.7 and 5.8). The modeled segments are deliberately simplistic and the magnitude of the inversion anticlines is exaggerated. In the NCB, fault trends consist of diffuse sets of normal faults rather than single fault traces. For example, the Rosemary Fault Zone is ~20 km wide on the modeled profile (Fig. 5.4). Any or all of these faults may be activated during in-plane compression. The modeled examples highlight the temporal variations in basin geometry and depocenter distribution when extensional faults are reactivated by compression resulting in the development of inversion anticlines at the basin interior or margin. In Figure 5.7, rifting is accommodated by brittle failure across east-dipping faults and simulating development of the Madeleine Trend (Fig. 5.4). The second rifting phase is accommodated in faults east of the original basin bounding fault of rift phase 1. Synrift sediments thicken westward in both rift phases producing an onlap surface to the east that is diagnostic of rifting (Karner et al., 1993). Finally, compression across the original border fault is accommodated by development of an inversion anticline. Accommodation is simultaneously reduced over the crest of the 208 A W 2000.0 E footwall uplift -25.0 kilometers 25.0 75.0 125.0 meters -75.0 0.0 -2000.0 onlap surface -4000.0 Rift phase 1 B 2000.0 footwall uplift kilometers 25.0 75.0 125.0 meters -75.0 0.0 -2000.0 Post-rift onlap surface -4000.0 Rift phase 2 C 2000.0 kilometers meters -75.0 0.0 -25.0 25.0 75.0 125.0 -2000.0 Post-inversion -4000.0 Figure 5.7. Modeled superposition of border faults and ensuing inversion. (A) Rift phase 1, with deformation across an east-dipping fault. Heave (5 km) and dip (30o) of the fault, along with effective elastic thickness of the plate (20 km), determine the amplitude and wavelength of the foot-wall uplift. Syn-rift sediments thicken into the border fault. (B) Rift phase 2, accommodated by slip across an east-facing fault, west of the initial boundary fault. Hanging-wall sediments of phase 1 form the base of the collapsed hanging-wall of rift 2, onlapped by syn-rift sediments of the second event. Syn-rift sediments of rift 2 thicken into the new bounding fault. (C) Compression-induced inversion concentrates on the boundary fault associated with rift 1. Post-inversion sediments onlap the flanks of the anticline. 209 A 2000.0 W kilometers -20.0 0.0 0.0 20.0 40.0 60.0 80.0 E -2000.0 -4000.0 Rift phase 1 B 2000.0 kilometers -20.0 0.0 0.0 20.0 40.0 60.0 80.0 -2000.0 Post-rift Rift phase 2 kilometers -4000.0 C 2000.0 -20.0 0.0 0.0 20.0 40.0 60.0 80.0 -2000.0 -4000.0 Post-inversion Figure 5.8. A second example of modeled superposition of border faults and ensuing inversion. (A) Rift phase 1, with deformation across a west-dipping fault. Syn-rift sediments thicken into the border fault. (B) Rift phase 2, accommodated by slip across an east-facing fault, west of the initial boundary fault. Foot-wall uplift associated with rift 1 forms part of the collapsed hanging-wall of rift 2. Syn-rift sediments of rift 2 thicken into the new bounding fault. (C) Compression-induced inversion concentrates on the boundary fault associated with rift 1. Post-inversion sediments onlap the flanks of the resultant anticline. 210 anticline due to short-wavelength brittle uplift, and created on the flanks by the long-wavelength flexural response to the load of the anticline. A second example of multiple border fault interaction and inversion, representative of the development of the Dampier Sub-basin (Fig. 5.4), is illustrated in Figure 5.8. This example is similar to the series of events thought be responsible for the subtle Cretaceous anticline west of the Rosemary Fault Zone. Extension, initially accommodated by brittle failure on a west-facing fault (Fig. 5.8A), flips to an east-facing fault, west of the original basin-bounding fault (Fig. 5.8B). The last tectonic event shown is compression, accommodated by development of an asymmetric anticline just west of the initial fault, combined with uplift of the footwall block and onlap onto the flank of the anticline (Fig. 5.8C). 5.3.4 The modeling approach The ultimate goal of forward kinematic and isostatic modeling is to replicate the modern stratigraphy, as imaged in depth-converted seismic data. However, stratigraphy at intermediate time steps must also be geologically viable and consistent with the seismic and well data. For example, modeled uplift and erosion must correspond to an observed unconformity, truncation and/or onlap onto the uplifted area, and preferably also be associated with redistributed sediments, either in the modeled transect or elsewhere in the basin. In addition, the flexural load induced by the uplift will result in long-wavelength subsidence on the flanks that is directly related to the amount of uplift. Combining the brittle and flexural lithospheric responses with the stratal architecture constrains both the location and magnitude of the modeled inversion. 211 In this manner, the final modeled stratigraphy is constrained by iterations between the available data (seismic and well observations) and the modeled time steps. Along the single two-dimensional transect, we model the entire post-rift basin margin development of the Dampier Sub-basin (Fig. 5.9). Initial surface topography and thermal conditions in the Valanginian are the result of the multiphase rifting history determined by Driscoll and Karner (1996; 1998; Fig. 5.5). Tectonism is assumed to be the principal driving force behind the creation and destruction of accommodation associated with basin-wide sediment distribution and preservation. Advective heat flux at the base of the lithosphere is dissipated by conduction following rifting; during each modeled time step, the lithosphere cools and subsides, and flexural strength increases. This thermal decay is strongly dependent on the amount of extension during rifting, and is assumed to decrease exponentially with time (McKenzie, 1978; Driscoll and Karner, 1996; 1998). Superimposed on these initial conditions are the effects of eustasy, sediment loading, compaction, and compression-induced inversion since the Valanginian, all of which influence accommodation. Eustatic variations will by definition increase or reduce accommodation globally, regardless of the mechanism creating them, or the time period over which they operate (Emery and Myers, 1996). Therefore, eustatic variations used in our model must result in accommodation variations across the entire margin. First-order long-term eustatic variations are modeled as a gradual fall from a peak of 200 m in the Turonian (after Hays and Pitman, 1973). No assumptions are made about short- 212 Figure 5.9. (following two pages). Predicted time-line stratigraphy across the Carnarvon Basin (Dampier Sub-basin) from the TithonianValanginian to the Present, based on integrated forward tectonic modeling and seismic analysis. The horizontal scale is relative to the Gascoyne continent-ocean boundary to the northwest (see Fig. 5.1 for location). The locations of inversion uplifts are shown by blue (Santonian), and red (Miocene) arrows. Time-line cross-sections are shown for: (A) Valanginian (~135 Ma), after the TithonianValanginian rifting, (B) Albian (~112 Ma), base Gearle Siltstone, (C) Santonian inversion (~85 Ma), base Toolonga Calcilutite, (D) DLS 1 (~29 Ma), base Cape Range Group, (E) DLS3 (~20 Ma), (F) DLS4 Tertiary inversion (~17 Ma), (G) DLS 5 Tertiary inversion (~15 Ma), and (H) ON2 Tertiary inversion (~7 Ma). 213 (A) completion Tithonian-Valangian rifting (145-135 m.y.) NW 300.0 0.0 -1000.0 320.0 340.0 360.0 380.0 400.0 420.0 kilometers 440.0 460.0 480.0 500.0 Barrow (Forestier)-synrift 4Upper Dingo Clayst. -synrift 3- SE 520.0 depth (m) -2000.0 -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 Mungaroo Fm. -postrift 1- Locker Shale -synrift 1Lower Dingo Clayst. -synrift 2- Lambert Shelf Rankin Trend Kendrew Trough Madeleine Trend Lewis Trough Dampier Sub-basin (B) post-rift subsidence and fill (112 m.y.) 300.0 0.0 -1000.0 320.0 340.0 360.0 380.0 400.0 420.0 kilometers 440.0 460.0 480.0 500.0 520.0 Muderong Shale-Windalia Radiolarite -postrift 4- depth (m) -2000.0 -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 Rosemary Trend (~20 km wide) (C) post-rift Santonian inversion (87-85 m.y.) 300.0 0.0 -1000.0 320.0 340.0 360.0 380.0 400.0 420.0 460.0 480.0 Gearle Siltstone-postrift 4 kilometers 500.0 520.0 depth (m) -2000.0 -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 u Flex re (D) post-rift late-early Oligocene- DLS1 (29 m.y.) 300.0 0.0 -1000.0 320.0 340.0 360.0 380.0 400.0 420.0 kilometers 440.0 lo Too 460.0 ng a 480.0 500.0 520.0 DLS1 Lambert-Giralia depth (m) -2000.0 -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 214 (E) early Miocene- DLS3 (20 m.y.) NW 300.0 0.0 -1000.0 -2000.0 320.0 340.0 360.0 380.0 400.0 3 DLS OM1 kilometers 420.0 OL1 SE 460.0 480.0 500.0 520.0 440.0 depth (m) -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 (F) late-early Miocene- inversion DLS4 (17 m.y.) kilometers 300.0 0.0 -1000.0 -2000.0 320.0 340.0 360.0 380.0 DLS4 400.0 1 EMM 420.0 440.0 460.0 480.0 500.0 520.0 depth (m) -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 (G) middle Miocene- inversion DLS5 (15 m.y.) kilometers 300.0 0.0 -1000.0 -2000.0 320.0 340.0 360.0 380.0 DLS5 400.0 1 MM 420.0 440.0 460.0 480.0 500.0 520.0 depth (m) -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 (H) late Miocene- inversion ON2 (7 m.y.) kilometers 300.0 0.0 -1000.0 -2000.0 320.0 340.0 360.0 ?Bare-Delambre 380.0 ON2 400.0 420.0 440.0 460.0 480.0 500.0 520.0 p to S_ MM2 DL depth (m) -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 215 term eustatic changes (e.g., Haq et al., 1987; 1988), rather they are input as required to uniformly increase/decrease accommodation across the entire margin. The timing of eustatic changes is a function of the modeled time steps and is linearly extrapolated between each time step. Paleo-water depth envelopes are used to define sediment distribution, and hence loading each at time interval modeled. These envelopes are created along the modeled section at each time step, using geometric and paleoenvironmental information gleaned from the seismic profiles and wells. For example, water- depth on the shelf, landward of clinoform breakpoints, is initially set at sea level and then adjusted according to preliminary paleo-water depth estimates using benthic foraminifera from well samples (Appendix 1). Water-depth variations in the basin are estimated using the pre-compaction height of the clinoform front. To guide the initial estimate of clinoform height, the Oligocene-Miocene sequences have been decompacted at each well (Fig. 2.22), using exponential porosity-depth relationships (Appendix 6). Once the water-depth profiles are defined, they are used to maintain the shape of the upper surface at each time step, as thermal subsidence creates accommodation. The resultant sediment packages then impose a load on the lithosphere. The model calculates the resultant subsidence, which is dependent on the thermally controlled flexural rigidity of the lithosphere at the time of loading. Compaction is assumed to be governed by a negative exponential depthdependent porosity function averaged over the entire succession, as sampled in wells (Table 5.1; Driscoll and Karner 1996; 1998). We simulate the progressive 216 Parameter x,z (x) (x) i(x) i(x) t trift T(x,z,t) Tm K Cp tc t c' a a' a a' c (z) sg (z) 0 Kp-1 Kp D(x) E Te(x,t) Ze g h(x,t) h(x,t) ke-1 SSL Description Value horizontal and vertical coordinates crustal/upper plate extension factor lithospheric mantle/lower plate extension factor crustal/upper plate compression factor lithospheric mantle/lower plate compression factor time since rifting or re-rifting event finite rifting interval 5-20 m.y. lithospheric temperature structure asthenosphere temperature 13330C thermal expansion coefficient 3.28x10-5 K-1 thermal diffusivity 8x10-7 m2s-1 specific heat 1.05 Jg-1K-1 pre-rift crustal thickness 34 km post-rift crustal thickness equilibrium lithospheric thickness 125 km post-rift lithospheric thickness density of asthenosphere at 00C 3330 kgm-3 density of asthenosphere at Tm 3179 kgm-3 0 density of crust at 0 C 2800 kgm-3 sediment density sediment grain density 2650 kgm-3 bulk sediment porosity e-Kpz surface porosity 60% porosity decay constant 2.5 km 0.4 lithospheric rigidity ETe(x,t)/12(1-2) Young's Modulus 6.5x1010 Pa effective elastic thickness controlling isotherm for Te 5500C Poisson's Ratio 0.25 gravitational acceleration 9.82 ms-2 topographic relief topographic load removed by erosion erosion time constant crust: 82 m.y. sediment: 10-40 m.y. maximum eustatic fall 200 m Table 5.1. Parameters for the Dampier Sub-basin used to create the model in Fig. 5.9 217 loss of porosity with time and depth, using an exponential decay function (Athy, 1930). Therefore, porosity at any depth, z, is (z) = 0e-Kpz, (5.1) where 0 is the surface porosity and Kp is the coefficient determining the slope of the -depth curve (Table 5.1). The location, timing and magnitude of uplift caused by compressioninduced inversion are assessed by comparing seismic and well observations with modeled uplifts. Inversion is differentiated from regional subsidence and eustatic variations by development of local positive features, e.g., anticlines (brittle response), with or without truncation on the crest. Timing of inversion deformation is constrained by onlap on the flanks of the anticline. Such onlap will be absent if the positive feature is the result of differential compaction. Deformation can result from single, repeated or continuous in-plane force variations, resulting in stratigraphic responses that can be differentiated on seismic data (Fig. 5.10). A single, large inversion event may cause exposure of the anticline crest, resulting in erosion of pre-inversion stratigraphy (Fig. 5.10A). The timing of deformation is constrained by the first onlap on the flanks of the anticline. By comparison, the stratal relationships associated with pulsed (Fig. 5.10B) and continuous (Fig. 5.10C) compression-induced inversion are more complex. Pulsed compression can result in numerous periods of erosion of the anticline crest. In addition, originally horizontal onlap surfaces on the flanks of 218 T1 T1 T1 T2 T1 T2 T1 T2 T1 T3 T2 T1 T4 T3 T2 T1 T3 T2 T1 T4 T3 T2 T1 T3 T2 T1 T4 T3 T2 T1 A T1 T2 T3 T4 B T1 T2 T3 T4 C T1 T2 T3 T4 Figure 5.10. Schematic of predicted stratal geometries dependent on frequency of uplift, assuming regional subsidence, and sea-level are constant. Each scenario has the same total uplift and cross-sections are oriented the same. (A) represents a single inversion event. Uplift affects all stratigraphy similarly prior to T3, with significant truncation over the crest. Post T3-sediments onlap the resultant anticline. (B) represents pulsed inversion events. Between each event, flat-lying sediments onlap onto the flanks of the preexisting structure. Onlapping strata rotate during each subsequent uplift, resulting in a gradual decrease in declivity up-section. The pulsed inversion events are smaller than a single uplift. (C) represents continuous inversion uplift during sedimentation. If the inversion rate was low, accommodation would be gradually reduced over the crest, resulting in a thinning of sediments. Increased truncation would be observed if uplift rate was high relative to sedimentation rate. Declivity of onlapping strata gradually decreases up-section. 219 the anticline are rotated with each subsequent uplift event (Fig. 5.10B). Between the inversion events, sedimentation may occur over the crest of the anticline as accommodation is created by thermal subsidence. Such sediment may be removed by later inversion events. Continuous in-plane compression, resulting in an inversion anticline, may initially result in uplift without exposure and erosion. However, as the total deformation increases, so will the potential for subaerial exposure (Fig. 5.10C). For the pulsed inversion case (Fig. 5.10B), stratigraphy on the flanks of anticline will rotate as uplift continues. In reality, differentiating between pulsed and continuous inversion based on stratigraphic relationships alone is problematic, as other factors such as rates of thermal subsidence, sediment supply, and sea-level variations relative to the rates of uplift also need to be considered. However, independent of the frequency of uplift, the stratal discontinuity surfaces associated with inversion are locally distributed. Truncation discontinuity surfaces are restricted to the crest, where accommodation is decreased. Onlap occurs on the flanks, where accommodation is increased by flexural loading of the anticline (Fig. 5.6F) combined with thermal subsidence. Repeated truncation and onlap surfaces on the crest and flanks of the anticline indicate recurrent compressive events, rather than a single uplift (Figs. 5.3 and 5.11). The model presented here assumes instantaneous, pulsed inversion (Fig. 5.10) superimposed on thermal subsidence, sediment loading and sea-level variations. Inversion created by in-plane compression is modeled as an increase in lithospheric thickness (i.e., shortening) at the site of such an anticline. The 220 NW 136_20 0.0 SE 440 101r_02 450 460 136_24 Rosemary 1470 kilometers depth (km) 0.5 1.0 V.E. ~15X 136_20 0.0 101r_02 5 km 136_24 Mute SEAFLOOR depth (km) DLS5 DLS4 DLS1 0.5 ON2 DLS_top ON1 MM2 1 MM 1.0 Base Tertiary EMM1 DL S3 S2 DL 1 OM 1 OL Lambert-Giralia Rosemary-Legendre Figure 5.11. Part of depth-converted line 101r_09 located west of the Rosemary Fault Zone (see Fig. 5.4). Repeated uplift is characterized by onlap on the flanks of the anticline and truncation over the crest (red arrows). Reflections are more steeply dipping in older onlapping intervals (i.e., compare EMM1 with more flat-lying reflections above ON2). This is the same anticline shown in Figure 5.3 which is located along-strike, to the northeast. ON1 (dotted line) is used to map the extent of the anticline in Fig. 5.13. Note that the upper portion of the seismic line was muted during processing. 221 distribution of increased lithospheric thickness along the modeled section will influence the shape of the anticline, with the crest occurring at the location of maximum lithospheric thickness. If uplift is sufficient to expose the crest subaerially, erosion occurs and truncation is observed on the seismic data. Within the model, subaerial erosion is calculated at each time step as a function of relief above sea level along the transect. We select the rate of erosion. The inversion anticline is a positive load; therefore, it is flexurally compensated in the same way as any other load. The combined brittle and flexural response to inplane compression results in creation and destruction of accommodation at different wavelengths. The inversion anticline destroys space locally over the crest, and space is created regionally along its flanks by flexural compensation (Fig. 5.6). The final modeled transect is designed to represent the cumulative effects of loads caused by sedimentation, inversion, and erosion, combined with compaction, thermal subsidence and eustasy. The ultimate consequences of each modeled interval are only apparent once the modeling is carried forward to the Present. Comparisons with the seismic transect highlight where discrepancies exist, e.g., if a modeled inversion anticline results in a load that causes large flexural "wings" (Fig. 5.6F) that are not apparent on the seismic data, then the modeled inversion is likely incorrect and must be recalculated. Table 5.1 outlines the input parameters to our kinematic and isostatic model; modeled time-lines and their tectonic significance are summarized in Table 5.2. Many of the parameters required to model margin development during 222 Time Line (1) (2) 3* 4* 5* 6 7 8 9 10 11 12 13 Geological Age, Ma 145 135 112 85 64 29 24 20 17 15 11 7 0 Time Since Basin Initiation, m.y. 108 118 141 168 189 224 229 233 236 238 242 246 253 Geological Formation (base) Barrow Group Windalia Radiolarite/ Lower Gearle Toolonga Calcilutite Lambert Formation Cape Range Gp Mandu Fm. (DLS1) DLS2 DLS3 Cape Range GpTulki Lmst. (DLS4) Yardie Gp- Trealla Lmst. (DLS5) ? Delambre Fm. (DLS_top) Process rifting 4 complete rift 4 post-rift 4 inversion post-rift 4 post-rift 4 post-rift 4 post-rift 4 inversion inversion inversion inversion Table 5.2. Time-lines and their significance used to create the model shown in Fig. 5.9 (bracketed time-lines are modified after Driscoll and Karner, 1998; * modified after Romine et al., 1997). 223 the post-rift phase have been defined by Driscoll and Karner (1996; 1998). Changes to some of these parameters will impact the entire section, not just the Cretaceous-Present interval. For example, pre-rift crust and lithosphere thicknesses defined by Driscoll and Karner (1996; 1998) are 34 and 125 km, respectively, to ensure that the region is isostatically balanced above sea level prior to Permian extension. This is consistent with the size of the Paleozoic Westralian Superbasin encompassing the Perth, Carnarvon, Canning and Bonaparte basins (Yeates et al., 1987), the large areas dominated by undeformed parallel stratigraphy (e.g., Stagg and Colwell, 1994), and the extended time period since the Early Carboniferous Alice Springs Orogeny, the last tectonic event to impact this region (Warris, 1993; AGSO North West Shelf Study Group, 1994). The assumed thicknesses of the crust and lithosphere are non-unique, but they impact the response to subsequent rifting, i.e., smaller crustal thickness decreases the size of the kinematically produced depression for a given value of ; in turn, if the lithosphere is thicker, the amount of heat injected during rifting for a given value of is increased, which impacts both the amount of post-rift thermal uplift and the time it takes for thermal contraction and subsidence to occur. The flexural strength of the lithosphere determines the response to kinematically induced loads, either tectonic or sedimentary. Flexural strength is governed by the effective elastic thickness parameter, Te, which varies with temperature and is therefore time-dependent. A number of thermo-rheological models have been proposed for continental lithosphere and are highly debated. For example, Karner et al. (1983) have suggested that similar to oceanic 224 lithosphere (Watts, 1978; Bodine et al., 1981) the depth to the 450oC isotherm approximates the thickness that is capable of elastically supporting loads over geologic time. This implies that rigidity increases with time as the margin cools. However, Willett et al. (1985) suggest that the effective elastic thickness Te, can correspond to a wide range of isotherms from 300oC to > 900oC. In this study, we have chosen 550oC as the controlling isotherm for Te, because it provides the best match to observed stratigraphy. 5.4. RESULTS: CRETACEOUS-TERTIARY TECTONIC AND STRATIGRAPHIC EVOLUTION We have investigated the tectonic and stratigraphic evolution of the Dampier Sub-basin since completion of its fourth and final stage of rifting in the Tithonian-Valanginian (Fig. 5.5; Driscoll and Karner, 1996; 1998). Accommodation was primarily created by thermal subsidence and sediment loading following this phase of rifting interrupted by compression-induced inversion. Seismic observations help constrain the temporal and spatial creation and destruction of accommodation and thus the history and distribution of tectonic inversion on this margin. These observations, integrated with the forward kinematic and isostatic modeling define the significance of tectonic, flexural loading and compaction processes on the preserved stratigraphic record. 5.4.1 Paleotopography at the end of Tithonian-Valanginian rifting; 135 Ma Initial surface topography and thermal conditions in the Valanginian are derived from the multi-phase rifting history modeled by Driscoll and Karner (1996; 1998; Fig. 5.9A). The rifting history delineates the location of discrete 225 depocenters and uplifted rift flanks resulting from the isostatic adjustment to brittle failure in the upper lithosphere. The dominantly northeast-southwest oriented structural trends created and reactivated during Late Triassic-Early Cretaceous rifting are a primary control on the distribution of Late CretaceousCenozoic inversion structures. Ductile deformation resulting in the passive rise of the lithosphere/asthenosphere boundary sets up the thermal disequilibrium that drives post-rift subsidence as the lithosphere cools, contracts and strengthens. Lithospheric strength at the time of loading will determine the flexural response to the imposed positive or negative load (e.g., erosion, uplift, sedimentation). The Early Cretaceous topography and thermal structure at ~135 Ma are a summation of the brittle, elastic and isostatic impact of rifting events that have occurred since the Permian (Fig. 5.5). Seismic Observations Minor truncation of Mesozoic stratigraphy across much of the Exmouth Plateau, to the west and southwest, onlapped by Early Cretaceous sediments resulted in an angular unconformity in the Tithonian (Karner and Driscoll, 1999). This stratal architecture has been interpreted to suggest that subaerial to shallow marine environments persisted on the plateau prior to deposition of the Early Cretaceous Barrow Delta (Driscoll and Karner, 1998; Karner and Driscoll, 1999). On 101r_09, the syn-rift, distal Barrow Delta equivalent represented by the Forestier Claystone is confined to the sub-basin and is represented by a lensshaped interval onlapping the Rankin Fault. The claystone thins landward to <50 m, where it onlaps basinward of the basin-bounding fault of the Rosemary Fault 226 Zone (Fig. 5.4). Significant paleo-water depths are implied. Minor bi-directional onlap/downlap (Fig. 5.12) is identified in the Tithonian-Valanginian interval, oriented along-strike with respect to the north-northeast progradation direction of the delta. Foreset relief of the Barrow Delta on the Exmouth Plateau to the southwest suggests minimum water-depths of 200-500 m along the modeled transect during deposition of the deltaic front (Ross and Vail, 1994; Karner and Driscoll, 1999). Modeling Results The modeled syn-rift interval requires that the Forestier Claystone partially fill accommodation created during Tithonian-Valanginian rifting. Although the locus of rifting is concentrated on the eastern margin during this time (Driscoll and Karner, 1996; 1998), the modeled unit is thickest (~650 m precompaction) adjacent the Rankin Trend (~km 418) and thins to the southeast (Fig. 5.9A), possibly controlled by structures along-strike. The flanks of the sub-basin are at or near sea-level as a result of a long-wavelength, 50-100 km, rift-flank uplift. However, the erosion that removed Jurassic sediments over the Rankin Trend is thought to have occurred mainly during Callovian extension and flank uplift (Driscoll and Karner, 1996; 1998). The amount of lower lithospheric extension required to create the regional subsidence that accommodates Barrow Delta progradation exceeds the minor amounts of brittle upper lithosphere extension accompanying Tithonian-Valanginian rifting. Therefore, an east- dipping intracrustal detachment, constrained by the eastward decrease in 227 NW kilometers Goodwyn 7 400 410 136_19 420 SE 430 depth (km) 4.0 3.5 3.0 2.5 2.0 V.E. ~11X 5 km DL S4 Goodwyn 7 136_19 DL S3 DL S2 1 DLS 2.0 DLS5 r tia er eT s y 2.5 depth (km) 4 WEDGE Ba 3 1 3.0 ToolongaMiria Marl n Sa ton i an Al la Va bi an in ia n ng 3.5 2 Late Triassic Rankin Trend 4.0 th Ti i on an C U/ Kendrew Trough Figure 5.12. Part of depth-converted line 101r_09 located over the Rankin Trend. Red arrows highlight (1) truncation of Triassic sediments in the uplifted fault block, (2) bi-directional stratal terminations (onlap and downlap) in the Tithonian-Valanginian Barrow Group, (3) onlap onto the Santonian-age reflection on the western side of the Rankin Trend, constraining the time of uplift, and (4) onlap/downlap onto the base-Tertiary reflection, suggesting a second phase of uplift occurred over the Rankin Trend. Uplift of the Rankin Trend is characterized by short wavelength (<15 km) bending (dotted line) of Triassic reflections, and deformation of the overlying Cretaceous-base Tertiary sediments. Location of the seismic section is given in Fig 5.4. 228 subsidence from the COB, is modeled to separate minor brittle extension in the upper crust from more significant extension in the lower crust and mantle lithosphere (Driscoll and Karner, 1998; Karner and Driscoll, 1999). 5.4.2 Passive margin formation Late Cretaceous inversion Seismic Observations Sedimentation was more regional following deposition of the Forestier Claystone. The Valanginian-Albian Muderong Shale-Windalia Radiolarite succession is continuous over the Rankin Trend to the northwest and the Lambert Shelf to the southeast (Driscoll and Karner, 1996; 1998). Valanginian-Barremian sediments recovered from Goodwyn 7 on the northwestern side of the Rankin Trend, and Neocomian sediments intersected at Rosemary 1 (Fig. 5.4) to the southeast, support this interpretation. Seismic reflections are sub-parallel through this interval; they dip to the west in response to post-depositional sediment loading, regional subsidence that increases to the west, and inversion. The Gearle Siltstone is an Albian-Santonian progradational wedge that first thickens, and then thins seaward and downlaps the Windalia Radiolarite (Fig. 5.4). The Santonian upper boundary of the Gearle Siltstone features large, <5 km wide and 50-400 m deep, irregular incisions, filled with discontinuous reflections. East of one such incision, truncation of underlying reflections prevails, and smaller incisions ~1 km wide occur on the Lambert Shelf. On the western margin of the Rankin Trend, a mounded lens within the overlying Toolonga Calcilutite-Miria Marl onlaps the Gearle Siltstone (Figs. 5.4 and 5.12). This Upper Cretaceous-base Tertiary interval is thickest east of the Rankin Trend (~km 460, Fig. 5.4) and then 229 thins landward by onlap. The Cretaceous-Tertiary boundary is marked by regional downlap and incisions up to 8 km wide and 150 m deep. These incisions are filled by relatively continuous reflections terminating by onlap updip and downlap basinward (Fig. 5.4). Santonian uplift identified on the Rosemary and Rankin trends results in deformation of the Gearle Siltstone and older section (Figs. 5.4 and 5.12). On the Rosemary Fault Zone, an anticline develops at this time in the hanging-wall block and is modified by incision near its crest (Fig. 5.4). Short-wavelength, <20 km, bending of the foot-wall block of the Rankin Trend (Fig. 5.4; km 413), combined with development of an anticline in overlying Cretaceous sediments, highlight the uplift at this location (Fig. 5.12). The wedge onlapping the Gearle Siltstone on the northwestern side of the Rankin Trend indicates that the deformation is Santonian or younger in age. Downlap and onlap occur over the crest of the anticline at the base Tertiary, suggesting that more than one phase of deformation has occurred (Fig. 5.12). Modeling Results The Muderong Shale completely fills the sub-basin from the TithonianValanginian rifting event, with increasing paleo-water depths modeled to exist in the northwest. That sediment load increases the amplitude of the sub-basin sediment thickness (Fig. 5.9B). Regional thermal subsidence is suggested by the broad distribution of the Muderong Shale, beyond the confines of the Dampier Sub-basin and west of the Rankin Trend to the Exmouth Plateau (Figs. 5.1 and 5.9). Thermal subsidence results in water-depths at the top of the Windalia 230 Radiolarite (Albian) ranging from ~100 m to the southeast along the Lambert Shelf to ~ 500 m northwest of the Rankin Trend (Fig. 5.9B). The overlying Gearle Siltstone is deformed by fault reactivation and inversion, adjacent to preexisting northeast-southwest structures (Fig. 5.9C). The intraplate compression, resulted in ~4 km of shortening, and is required during the Santonian (top Gearle Siltstone; Fig. 5.13), on the east and west boundary faults, and above the internal Madeleine Trend to match the observed stratigraphy (Fig. 5.4). Approximately 200 m of uplift results in subaerial exposure along the Rosemary Fault Zone and on the Lambert Shelf. To the west, the small inversion anticlines (~km 416 and km 430, Fig. 5.9C) remain submarine and concaveupward flexure occurs locally on the foot-wall of the Rankin Trend (Fig. 5.9C). 5.4.3 Passive margin formation Late Tertiary Inversion Seismic Observations The Paleocene-late early Oligocene (DLS1) stratigraphic interval, consisting of alternating carbonates and siliciclastics of the Lambert Formation to Giralia Calcarenite (Fig. 5.5), downlaps onto the base Tertiary (Fig. 5.4). Carbonate dominates the succession from the late-early Oligocene (DLS1), with deposition of the Cape Range and Yardie groups (Fig. 5.5). Progradation of the shelf edge above DLS1 shifts it northwest from the Kendrew Trough area to west of the Rankin Trend (Fig. 5.4). Height and declivity of clinoform fronts increase to the northwest. However, decreased declivity within MM2 (Fig. 3.11; Table 3.1) is below the resolution of the modeling. Sigmoidal clinoforms with convex upper slopes dominate to the top of MM1 (DLS5), followed by concave-up 231 1.01 1 Rosemary -Legendre Trend DLS_top DLS4, DLS5, ON2 Santonian inversion factor (b(x)) 0.99 Total horizontal shortening I x=0 S(b(x) - 1) Ex, where Ex=3.125, i.e., 511 elements 1600 0.98 0.97 0.96 0.95 0.94 300 Madeleine Trend x (km) 400 Rankin Trend 500 600 Figure 5.13. Distribution of the stretching factor along 101r_09, used as a proxy for Santonian and Miocene inversion. Values <1 indicate compression. (x) represents vertical thickening or compression of the lithosphere, which translates to a horizontal shortening across the region involved in the compression. Smaller (x) values indicate greater inversion. Inversion is larger and more broadly distributed in the Santonian, reaching a maximum at km 475 (0.95I105% of original lithospheric thickness). Total horizontal shortening in the Santonian is ~4 km. Inversion modeled to the northwest (at km 413) was required to produce the Santonian uplift observed on the Rankin Trend. Two models of inversion were used for the Miocene, both centered at ~km 455, northwest of maximum Santonian inversion. Maximum inversion for DLS4, DLS5 and ON2 is 0.99I101% of original lithospheric thickness. Total horizontal shortening is ~400 m for each event. Maximum inversion for DLS_top is 0.995I100.5%. Total horizontal shortening is ~200 m. 232 profiles of DLS_top and ON2 (Fig. 5.4). The clinoforms develop primarily as strike-oriented depocenters with increased progradation to the north, though other variations in location, width and distribution of the depocenters also occur (Figs. 3.13 and 3.14; Cathro et al., submitted). Sequences thin by onlap and truncation on the flanks and crest of the anticline to the east (Fig. 5.11). This anticline is asymmetric, with the western flank forming part of one of the northeast-southwest oriented hinge-lines demarking increased subsidence basinward (Fig. 3.13). The northeast-southwest oriented antiform tightens to the northeast (Fig. 5.14). Its axis, located on the westernmost edge of the Rosemary Fault Zone, is ~15 km west of the Cretaceous inversion anticline (Fig. 5.4), suggesting different faults are reactivated at different times in this ~20 km wide fault zone. Secondary depocenters on its eastern (landward) limb occur during EMM1 and MM1, indicating it was active during the early-middle Miocene (Fig. 3.13; Cathro et al, submitted). Onlap on its flanks is apparent from DLS1 (late early Oligocene) through to ON3 (?Pleistocene), whereas truncation is recognized at DLS1, DLS2, DLS5 and ON1 (Figs. 5.3 and 5.11). Declivity on its northwestern flank decreases up-section, particularly in the Plio-Pleistocene, suggesting that uplift occurred over a protracted time period (e.g., as in Fig. 5.10B). Modeling Results The Tertiary stratigraphic section is reconstructed using eight time intervals (Figs. 5.9D-5.9H). The six time intervals from late early Oligocene (DLS1) to late Miocene (ON2) document progradation of the Oligocene-Miocene sequences over the shallowly dipping late-early Oligocene ramp defined by DLS1 233 116oE Time (ms) 750 500 250 117oE mid to late Miocene ? ? ? ? ? ? N 10 ? 1r _0 9 40 km Figure 5.14. Two-way travel time structure-contour map of inversion anticline as represented by horizon ON1 (see Figs. 5.3 and 5.11), which is slightly older than the DLS_top downlap discontinuity surface dated as ~10 6.4 Ma. The northeast-southwest oriented anticline tightens to the northeast, where deformation is represented by faulting rather than folding. The crest of the anticline lies northwest of the basin-bounding fault of the RosemaryLegendre trend, with the location shown here at the Tithonian-Valanginian level (see Fig. 5.4). 234 20oS 19oS 13 6_ 15 (Figs. 5.4 and 5.9D). Paleo-water depth curves representing this stacked clinoform geometry are constructed assuming the topsets shoal landward of the breakpoints between the shelf and more steeply dipping fronts (Tables 3.1 and 5.3). Shelf paleo-water depths correspond to preliminary paleobathymetric results from analyses of benthic foraminifera from wells (G. Moss, personal communication, UTIG, 2001). Finalized paleo-water depths from foraminifera generally range from 20-200 m on the shelf (Moss et al., in prep.), suggesting that these initially constructed paleo-water depth curves systematically underestimate paleo-water depths on the shelf (Fig. 5.15 e.g., G2, G4, G6, G7 above DLS2; Appendix 1). Basinward, the biostratigraphic analysis provides poorer constraints on paleo-water depths (Fig. 5.15 e.g., E1). Bathymetric range limits are up to 500 m (e.g., DLS3 at E1) or reveal spurious shallow water-depth estimates suggesting dilution of deep-water foraminiferal assemblages by shallow-water fauna (e.g., DLS5 at E1). Basinward of the well control, the maximum paleo-water depths are assumed to be >1000 m for each of the modeled clinoforms (e.g., at km 340). The succession between the base Tertiary to late early Oligocene (DLS1) surfaces encompass the Lambert Formation to Giralia Calcarenite and their equivalents (Fig 5.9D). Accommodation generated by thermal subsidence is sufficient to allow renewed sedimentation to the southeastern (landward) limits of the modeled transect during this time. Previously, uplift associated with the Santonian inversion (Fig. 5.9C) had precluded extensive sedimentation landward of km 465. The succession builds to sea-level southeast of the Lewis Trough by 235 Seismic reflection Initial foreset angle (o) used in forward model Final foreset angle (o) derived from forward model Paleo-water depths at clinoform breakpoint used in forward model (m) Foreset angle (o) measured from depth converted section 101r_09 (Bracketed values represent range of declivities measured on TWTT sections, assuming 3200 m/s for the depth conversion.) MODERN ON2 DLS_top DLS5 DLS4 DLS3 DLS2 DLS1 1.1 5.0 5.6 2.6 2.0 2.5 1.5 0.8 1.1 7.7 7.7 3.9 3.2 3.3 1.9 3.0 200 40 40 130 120 90 150 150 5-10 8 5.3 6.5 2.5 3.0 (3.5-8) (4-8) (6-8) (4-7) (2.5-4) (<5) Table 5.3. Comparison between modeled foreset declivity and actual declivities observed on depthconverted 101r_09 (bracketed values are those determined from time sections in 2D and 3D seismic, using simple 3200 m/s for the depth conversion; see Table 3.1). 236 kilometers 390 0.0 Paleo-water depths: dotted - used in model solid vertical - benthic foraminifera E1 400 G2 G4G7G6 420 DLS_top PALEO-WATER DEPTH (m) DLS5 DLS4 DLS3 DLS2 DLS1 -500.0 kilometers 300.0 0.0 -1000.0 -2000.0 340.0 380.0 420.0 460.0 500.0 Figure 5.15. Comparison between paleo-water depth profiles used in forward modeling of 101r_09 (Fig. 5.4) and finalized benthic foraminiferal analyses at wells (Moss et al., in prep.). The paleo-water depth profiles used in the forward modeling tended to underestimate the paleo-water depth on the shelf as suggested from the benthic foraminifera. Colors at each well show the depth range of a particular horizon. Location of the wells relative to the seismic profile are given in Fig. 5.1. 237 the end of Giralia Calcarenite deposition (Fig. 5.9D). The top of the Giralia Calcarenite corresponds to DLS1. A relict soil horizon occurs at this time in Dampier 1, suggesting subaerial exposure occurred ~16 km southwest of the modeled profile (Fig. 5.1; BOCAL, 1969). Accommodation remains limited during deposition of OM1 (DLS2-DLS3; Fig 5.9E). The combined load of OL1 and OM1 and thermal subsidence after DLS1 over-generates accommodation southeast of km 450. Adding a short-term eustatic sea-level fall across the entire margin at ~20 Ma reduces this excess accommodation. Amplitude of the fall reaches ~50 m at DLS4 time (~17 Ma; Fig. 5.16). Resultant accommodation is filled by EMM1 (DLS3-DLS4; Fig 5.9F). A short pulse of inversion, representing ~400 m of shortening at DLS4 time, results in ~70 m of uplift centered at km 455; this is combined with the maximum sea-level fall (Figs. 5.9, 5.13 and 5.16). These two factors interact locally: the eustatic sea-level fall reduces accommodation over the entire margin whereas the inversion removes accommodation preferentially over the deformation axis for at least 80 km along strike. In response to these competing factors, EMM1 is eroded over the crest of the anticline (Fig. 5.9F), thickening slightly east of km 455 before thinning again as it reaches its landward depositional limit. In the model, the MM1 sequence onlaps EMM1 northwest of the anticline crest (Fig. 5.9G), contrary to the seismic observations (Fig. 5.4). Insufficient accommodation southeast of the modeled anticline suggests that the applied sea-level fall at DLS4 (Fig. 5.16) is too large relative to continued thermal subsidence to allow accommodation to 238 Figure 5.16. (following page). (A) Total decompacted subsidence from Permian to Present at three locations on the modeled line (Fig. 5.4). Accommodation is created by a combination of rifting, thermal subsidence, compaction, sediment loading and eustasy. However, rifting followed by thermal subsidence, which is modeled to decay exponentially with time, modulates accommodation. Therefore, departures from the theoretical, sediment filled thermal subsidence curves of McKenzie (1978) must be the result of a combination of the remaining factors. The largest uplift, at km 413 (red), is associated with rift flank uplift of the Rankin Trend. Total accommodation is greatest at km 455, coincident with location of the Lewis Trough of the Dampier Sub-basin. The combined effects of inversion and eustatic variations in the Santonian and Tertiary are insignificant at this scale. (B) Total sediment accumulation from Oligocene to Present compared with eustatic variations (long- and short-term) required to replicate the observed stratal geometry. The long-term eustatic variations are derived from changes in mid-ocean ridge volume (Hays and Pitman, 1973), while short-term changes are input as required to increase or decrease accommodation along the entire modeled section. In comparison, the effect of inversion is variable in time and space, with the greatest influence at km 455 (green) and least at km 360 (blue). Flexural loading is not observed, possibly overwhelmed by the effects of differential sediment loading and subsidence. Results from this study indicate that inversion is pulsed at 2-4 m.y., with a maximum amplitude of ~50-70 m at the crest. In addition eustatic amplitudes of 30 m varying on time scales of ~8 m.y. were also input into the model. Sediment loading expected from deposition during the latest Miocene to Present is excluded from the subsidence curves with the last data point at ~6 Ma. Note the scaling is relative only. 239 Ma Decompacted subsidence (cumulative) (A) 250 0.0 R1 -1000.0 -2000.0 -3000.0 -4000.0 -5000.0 -6000.0 -7000.0 -8000.0 200 150 100 50 0 delta: 1.0 beta: 1.0 R2 R3 R4 Post-rift 4 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 360 km - Exmouth Plateau 413 km - Rankin Trend 455 km - Dampier Subbasin (Lewis Trough), HWB of Rosemary Trend Detail shown in B INVERSION DLS_TOP 11 Ma 6 Ma ON2 7 Ma (B) DLS2 24 Ma DLS5 15 Ma DLS4 17 Ma MODELED EUSTASY DLS1 29 Ma LONG TERM INVERSION DLS3 20 Ma Low SHORT TERM ACCOMMODATION TOTAL ACCOMMODATION 100 m High 240 develop on this part of the shelf. If the local inversion uplift were too large (Fig. 5.13), then accommodation would still exist on its landward flank. We repeatedly apply inversion pulses (Fig. 5.13) to each horizon up to ON2, superimposed on sea-level variations with a wavelength ~18 m.y. (Figs. 5.9H and 5.16). Sedimentation across the shelf was re-established during MM2 (Fig. 5.9H). Inversion just prior to DLS_top is revealed by onlap onto ON1 on the northwest flank of the anticline, within MM2 (Fig. 5.11). Inversion modeled at DLS_top is slightly less than that required for the other three Miocene time steps (Fig. 5.13), only ~200 m of shortening produces uplift of ~50 m at km 455 (DLS4, DLS5, and ON2; Figs. 5.4, 5.9H and 5.16). The late Miocene interval, between DLS_top and ON2, is modeled as covering the entire shelf. Basinward, the shelf edge is marked by failure, as indicated by the concave-up slope (Figs. 5.4 and 5.9H). From 7 Ma to the Present, we model with deposition of the thick Delambre Formation (Figs. 5.5 and 5.4). Inversion events, which clearly occur in this interval, e.g., onlap onto ON3 and ON4 (Fig. 5.3), are not modeled because their timing is poorly constrained and they are not well imaged on the modeled profile. Sedimentation during this time is modeled as a response to an interplay of continued thermal subsidence and sediment loading. 5.4.4 Post-depositional modifications to clinoform morphology Post-depositional modifications of clinoform morphology, due to the interactions of compaction and flexural subsidence, are shown in Fig. 5.17. Initial morphology of each modeled interval is progressively deformed as subsequent layers are deposited and their loads flexurally accommodated. Slight uplift at the 241 Figure 5.17. (following page). Profiles illustrating deformation of Oligocene-late Miocene clinoforms as predicted from the forward modeling. In each profile, the predicted original morphologies at time of deposition of the sequences are shown (gold dotted outlines indicate top and bottom of package at time of deposition). Each profile illustrates how this original morphology changes through time with each additional load, subsidence, compaction and inversion (arrows). The predicted location and shape of each sequence in the Present is also given (lowermost representation in each profile). The effect of sediment loading by each successive clinoform, combined with continued subsidence, has a more dramatic cumulative effect than the inversion to the west, which produces only a small anticline. Rotation in response to asymmetric loading caused by the progradation, combined with compaction, has increased declivities on the clinoform front and shelf since deposition. Note that the deformation caused by sediment loading of the MM2 sequence is largely equalized by ensuing sedimentation to the Present. 242 300.0 0.0 NW kilometers 320.0 340.0 360.0 380.0 400.0 420.0 440.0 460.0 480.0 500.0 520.0 SE PRESENT-ON2 300.0 0.0 NW kilometers 320.0 340.0 360.0 380.0 400.0 420.0 440.0 460.0 480.0 500.0 520.0 SE EMM1(DLS4-DLS3) -1000.0 -1000.0 -2000.0 -2000.0 -3000.0 -3000.0 kilometers 300.0 0.0 320.0 340.0 360.0 380.0 400.0 420.0 440.0 460.0 480.0 500.0 520.0 kilometers 300.0 0.0 320.0 340.0 360.0 380.0 400.0 420.0 440.0 460.0 480.0 500.0 520.0 ON2-DLS_top OM1 (DLS3-DLS2) -1000.0 -1000.0 -2000.0 -2000.0 -3000.0 -3000.0 kilometers 300.0 0.0 320.0 340.0 360.0 380.0 400.0 420.0 440.0 460.0 480.0 500.0 520.0 kilometers 320.0 340.0 360.0 380.0 400.0 420.0 440.0 460.0 480.0 500.0 520.0 300.0 0.0 MM2 (DLS_top-DLS5) -1000.0 OL1 (DLS2-DLS1) -1000.0 -2000.0 -2000.0 -3000.0 300.0 0.0 320.0 340.0 360.0 380.0 -3000.0 kilometers 400.0 420.0 440.0 460.0 480.0 500.0 520.0 kilometers 300.0 0.0 320.0 340.0 360.0 380.0 400.0 420.0 440.0 460.0 480.0 500.0 520.0 MM1 (DLS5-DLS4) DLS1-bTERT -1000.0 -1000.0 -2000.0 -2000.0 -3000.0 -3000.0 243 southeastern edge is associated with multiple phases of inversion between the middle and late Miocene. Deformation is most significant in older intervals, impacted by maximum burial compaction and multiple tectonic events. Rotation due to differential loading and compaction increases with time, steepening shelf and slope declivities. This is particularly apparent when the large sediment loads of MM2 and late Miocene (ON2) to Present are emplaced. As has been observed on the New Jersey Margin (Steckler et al., 1993), modeled estimates of clinoform slopes in the NCB (Chapter 3; Cathro et al., submitted) overestimate pre-burial slopes. The shapes of clinoform breakpoints are also modified. For example, the breakpoint modeled at the top of sequence OL1 is flattened by compaction and loading. The inversion in the southeast has had limited effect on the shape of clinoforms located 30-60 km northwest, except to contribute to increased declivity at their landward edges. Subsidence associated with flexural load of the inversion anticline cannot be separated from regional loading engendered by clinoform progradation, due to the relatively small amount of uplift induced by the inversion. Although the final modeled section replicates the overall shape of the clinoforms, it tends to underestimate the declivity of the clinoform front, particularly at DLS3-DLS5 (Table 5.3). 5.5. DISCUSSION 5.5.1 Intraplate deformation of the Indo-Australian plate in the Santonian Onlap onto the western flank of the Rankin Trend, combined with shortwavelength (~15 km) bending of the Triassic foot-wall block and deformation of overlying reflections (Fig. 5.12), are best explained in terms of compression rather 244 than rift-flank uplift engendered by extension (Driscoll and Karner, 1998; Karner and Driscoll, 1999). The observed deformation requires uplift along the Rankin Trend during the Santonian (Fig. 5.9D), superimposing these short-wavelength features (Figs. 5.9C and 5.12) on the longer-wavelength, ~60 km, uplift associated with flexural unloading of the rift flank (Fig. 5.9B). The amount of shortening associated with this reactivation is minor, ~4 km. The identification of this and other inversion features, e.g., the forced fold in the Exmouth Sub-basin (Driscoll and Karner, 1998) indicates a phase of compressional tectonics must occur in the late Cretaceous. However, the shortening occurs at a time when the northern Australian margin should be only subject to thermal subsidence. A possible explanation for Cretaceous inversion is intraplate deformation resulting from reorganization along the Indo-Australian plate boundary. Movement of the Indo-Australian Plate during the Cretaceous indicated by breakup in the central Great Australian Bight, is interpreted to occur at ~83 Ma, near the Santonian/Campanian boundary (Sayers et al, 2001). Such movement could result in reorganization along the Indo-Australian plate boundary generating stresses that could then be propagated thousands of kilometers through the lithosphere to the plate interior, assuming the lithosphere maintains flexural strength following rifting (e.g., Cloetingh et al., 1985; Weissel and Karner, 1989). Cretaceous northern Indo-Australian plate boundary interactions north are undefined, however, inversion of northeast-southwest oriented extensional structures suggests intraplate compression had component oriented southeastnorthwest in the NCB. 245 5.5.2 Collision as a mechanism for Miocene inversion? Models of foreland basin development along the convergent boundary north of the NCB (Fig. 1.8) predict that increased subsidence will occur as a response up to several hundred kilometers to the southwest, including the Timor Trough and outer NWS (Lorenzo et al., 1998). However, increased subsidence has also been interpreted from backstripping on the outer NWS southwest of this flexural window, e.g., in the NCB, Roebuck and Browse basins (Fig. 1.8; M ller et al., 1998; 2000). To account for this increased subsidence, M ller et al. (1998; 2000) suggested that fragmentation of the Indo-Australian Plate (into the Indian, Australian and Capricorn plates) at ~18 Ma, resulted in a diffuse intraplate deformational zone in the central Indian Ocean that included the outer NWS (Royer and Gordon, 1997; M ller et al., 1998; 2000). Unlike results from 1D backstripping in the NCB (e.g., Barber, 1988; Miller and Smith 1996; Westphal and Aigner, 1997; M ller et al., 1998; 2000), the forward modeling presented here does not require increased tectonic subsidence on the outer margin of the Dampier Sub-basin starting ~20 Ma to replicate the modern stratigraphy. Kaiko and Tait (2001) have highlighted the sensitivity of 1D backstripping analyses to assumptions of paleo-water depth. They have reworked the backstripping analyses of wells on the outer margin of the NWS, assuming a simple thermal sag model, without increased subsidence in the Miocene, and relatively deep paleo-water depth estimates (c.f., Barber, 1988 and others) for the outer NCB margin determined from micropaleontology studies (e.g., Apthorpe, 1979; Ujetz and McInerny, 1998). One well from their study, 246 Brigadier 1, ~50 km along-strike from km 360 on our modeled section (Fig. 5.4), shows post-rift paleo-water depths ~500-1000 m on the outer margin prior to arrival of the Oligocene-Miocene clinoform foresets (Kaiko and Tait, 2001). Similar paleo-water depths are required in our forward modeling, resulting from thermal subsidence since the Valanginian (Figs. 5.9D-H and 5.15; Table 3.1; Driscoll and Karner, 1996; 1998). Without the increased subsidence predicted on the outer NWS from the backstripping models, isolated anticlines (Figs. 5.2, 5.3, 5.11 and 5.14) comprise the primary Tertiary inversion signature predicted from the forward modeling. We suspect that observed deformation along the NWS could therefore be related to compressive plate boundary stresses from collision recognized at this time on the convergent margin to the northeast (Fig. 1.8). Finite element modeling of the regional Australian stress field has shown that the heterogeneous northern convergent margin has a significant effect on the orientation of modern horizontal stresses along the NWS, which typically range from northeast-southwest in the Bonaparte Basin (Fig. 1.8) to east-west in the NCB (Zoback, 1992; Coblentz et al., 1995; Hillis et al., 1997). The east-west directed maximum horizontal stress, as determined from field borehole breakouts, is favorable for development of inversion anticlines, as observed along preexisting northeast-southwest oriented normal faults in the NCB (Fig. 5.14; Hillis et al., 1997). Many models exist for the timing and number of orogenic events in the New Guinea region affected by ongoing plate boundary collision with Australia (e.g., Hamilton, 1979, Kroenke, 1984, Pigram and Davies, 1987; Richardson and 247 Blundell, 1996; and Quarles van Ufford, 1996). The timing for initial collisional orogenesis ranges from ~25 Ma (Pigram and Davies, 1987; Richardson and Blundell, 1996) to ~7 Ma (Quarles van Ufford, 1996). Initial compression- induced inversion in the Dampier Sub-basin is modeled to occur between these two end member times for onset of collision (~17 Ma; Fig. 5.9F). Pigram and Davies (1987) have proposed that New Guinea formed as a result of docking and accretion of multiple terranes, starting with the Sepik Terrane in the late Oligocene. Alternatively, Quarles van Ufford (1996) has suggested from onshore field studies, that the negatively buoyant northern limit of the Australian continental margin was subducted some time after 20 Ma, resulting in uplift and accretion of passive margin sediments prior to jamming of the subduction zone and onset of collisional orogenesis. The finite element modeling of Coblentz et al. (1995) has attributed the modern regional stress field along the northwest Australian margin to a balance between topographic forces (primarily mid-ocean ridge push) and resistance to those forces at collisional sections along the northeastern convergent boundary of the Indo-Australian Plate (e.g., New Guinea). Nonetheless, the exact relationship between these plate boundary interactions and related in-plane stress generation resulting in Miocene inversion structures is unclear. 5.5.3 Long- and short-wavelength modifiers to the stratigraphic response to regional thermal subsidence and eustasy. Total accommodation (i.e., tectonics, sediment loading, eustasy and compaction) since the Permian, as represented at three different sites on the modeled transect (Fig. 5.4), is modulated by a local rifting events (Fig. 5.16A). 248 For example, at the main depocenter of the Dampier Sub-basin, located at km 455 within the Lewis Trough, 65% of the ~11 km of total subsidence is a consequence of Triassic and Jurassic rifting events (Fig. 5.16A). Rift-induced accommodation, and therefore total accommodation, is less at the other two sites located on the western rift flank: the Rankin Trend (km 413) and the Exmouth Plateau (km 360). In addition, ~400 m of rift-flank uplift is detected on the Rankin Trend during the final rifting phase (Fig. 5.16A), corresponding with ~50 m of uplift at km 360 to the northwest. Such uplift decreases to zero at ~km 350. Between rifting events, accommodation is controlled by the combined effects of eustasy, thermal subsidence, inversion, compaction, and sediment loading/ unloading (erosion). The theoretical subsidence curves (Fig. 5.16A) show this decay following different amounts of extension in the lower crust and lithosphere mantle (as increases, so does the thermal subsidence). In general, a slow decay in subsidence, primarily since the final rifting in the Valanginian, is observed across the entire transect (R4; Fig. 5.16A). Negative deviations (i.e., increased subsidence) from the theoretical curves, associated with sediment loading, occur in each of the subsidence curves extracted along the modeled profile. For example, increased subsidence of up to 500 m between 112 and 85 Ma in the Lewis Trough is coincident with deposition of the Muderong ShaleWindalia Radiolarite in the basin (Figs. 5.9B and 5.16). Similarly, increased subsidence of up to 1000 m starting at ~29 Ma along the Rankin Trend, precedes the arrival of the prograding clinoforms at km 413 (Figs. 5.9E-H and 5.16B). This progradation induces flexural loading in front of the depocenter and 249 corresponds to ~200 m of increased subsidence at the Lewis Trough and Exmouth Plateau locations. Therefore, the flexural effect of sediment loading appears greatest at the locus of sedimentation. The wavelength of the flexural response is dependent on the strength of the lithosphere at the time of loading and the magnitude of the imposed load. In the case of the prograding clinoforms the flexural response reduces to zero ~65 km northeast and southwest of the prograding depocenter. By comparison, the stratigraphic effect produced by compression-induced inversion anticlines is relatively minor and areally limited. At each of the Miocene inversion events, ~50-70 m of uplift occurs in response to 200-400 m of shortening, along the anticline axis in the Lewis Trough northwest of the basin bounding fault of the Rosemary Fault Zone (Fig. 5.14). Uplift is not detected either at the Rankin Trend or Exmouth Plateau locations (Fig. 5.16B). Location of the Tertiary inversion centered at ~km 455, relative to the Santonian inversion at km 475, indicates that different faults are being reactivated within the Rosemary Fault Zone (Figs. 5.4 and 5.13). Sediment loading overwhelms the flexural response to these relatively small features; the long-wavelength flexural "wings" associated with the anticline (compare Figs. 5.6F and 5.14) are not discernible. The primary signature of the Miocene inversion is the broad anticline ~20 km wide observed in the seismic data (Figs. 5.4 and 5.11). Short-term eustatic variations in the Miocene, with a wavelength >18 m.y. required by the forward model, are 30 m relative to the postulated long-term fall since the Cretaceous (Fig. 5.16B; Hays and Pitman, 1973). Eustatic variations 250 affect the entire margin, with falls accentuated by uplift proximal to an inversion anticline. However, the clinoform shelf-break paleo-water depths in the NCB are produced in at least middle-outer neritic water depths (20-200 m; Fig. 3.17; Cathro et al., submitted). The shelf-break is likely to have remained submerged during eustatic sea-level falls. Nonetheless, shelf-wide reduced paleo-water depths, without exposure, are detected across DLS3, DLS5 and DLS_top, with a regional increase across DLS4 (Section 3.5.4; Fig.3.17; Cathro et al., submitted). 5.6. CONCLUSIONS We combine seismic sequence analysis (Chapter 3), kinematic and flexural forward tectonic modeling, and paleobathymetric analysis of benthic foraminifera (Appendix 1; Moss et al., in prep.) to define the history, distribution and magnitude of inversion in the Dampier Sub-basin during the Cretaceous and Tertiary. From this modeling, we have demonstrated that: 1) Discrete inversion events are defined in the Santonian, late early Miocene, middle Miocene, late Miocene, latest Miocene, and Plio-Pleistocene (two events observed on the seismic data, but insufficient biostratigraphic control for modeling). 2) Inversion tends to be focused along preexisting rift fault systems. However, the spatial distribution of inversion structures varies through time. Cretaceous inversion is concentrated along the northeast-southwest oriented Rankin, Madeleine and Rosemary trends. The locus of Miocene inversion is located ~20 km northwest of the Cretaceous inversion along 251 the Rosemary Fault Zone indicating a different fault within this zone was reactivated during the Miocene. 3) Compression-induced inversion creates and destroys accommodation space at different spatial wavelengths. Brittle deformation in the upper crustal lithosphere results in relatively short-wavelength uplift. Flexural response to this tectonic load produces longer-wavelength regional depression on the flanks of an inversion anticline. 4) Intraplate deformation resulting from interactions on the Indo-Australian plate boundary are a possible mechanism for Santonian inversion. This deformation represents ~4 km of shortening, at a time when the NCB should only be influenced by subsidence. 5) Tertiary inversion, modeled to commence at ~17 Ma, recurs through to the Plio-Pleistocene. Each inversion event represents ~200-400 m of shortening, resulting in 50-70 m of uplift at the crest of the resultant anticline. The flexural response to the amount of deformation during inversion is small compared to the flexural response to sediment loading. 6) Relationships between Neogene inversions and collisional orogenesis at the northern convergent plate boundary are complex. However, inversion of northeast-southwest oriented extensional structures in the NCB is favored by east-west oriented maximum horizontal stress interpreted from borehole breakout studies (Hillis et al., 1997). 7) Paleo-water depth estimates on the outer margin exceed 1000 m in the late Oligocene-Miocene. This accommodation is created by thermal 252 subsidence since the Valanginian, combined with limited sedimentation prior to progradation of Neogene clinoforms. 8) Post-depositional differential loading of the prograding clinoforms deforms and rotates the stratal architecture, generally increasing declivities on the shelf and clinoform front. 9) As the inversion structures in the Dampier Sub-basin occur on much shorter-wavelengths than thermal subsidence, sediment loading and eustatic sea-level variations, they can be spatially separated. Although inversion is highly variable, likely sites of deformation can be predicted from the preexisting structural fabric. In areas where inversion is more intense, both the brittle and flexural response will increase. Therefore, every margin must be assessed individually using regional geophysical datasets combined with well information to identify the extent and variability of inversion structures and their possible impact on stratigraphy. 253 Chapter Six: Implications 6.1 INTRODUCTION Seismic stratigraphy has risen in popularity as a mapping and interpretation technique since the 60's and 70's, when 2D MCS data, collected as part of the worldwide exploration for hydrocarbons, revealed similar clinoformal patterns in disparate and widely separated basins. Unfortunately, academic seismic stratigraphic studies have usually relied on the interpretation of widelyspaced dip profiles, usually with extrapolation of presumably related seismic packages in the strike direction only by jump correlation; two-dimensionality of those packages is often assumed. In the hydrocarbon industry, where 3D data is now commonplace, the seismic stratigraphic method is still largely treated as a predictive tool, providing information about the distribution of stratal architecture based solely on seismic data. Used in this way, where the prime goal has primarily been identification and retrieval of hydrocarbons, there has been little critical attention placed on the assumptions inherent to the seismic stratigraphic method. Until now, industry 3D seismic data have not been used by academicians to assess the development of globally common, albeit complex clinoform geometries. However, this study breaks from that trend by being based not only on interpretation of a 3D seismic volume (~70 X 30 km), but a volume nested within 3200-line kilometers of 2D MCS (Fig. 1.2). Therefore, both detailed and regional seismic interpretations within the same clinoformal succession can be attempted. 254 In addition, a total of nine wells are available within the combined seismic coverage; seven are tied to the seismic interpretation using synthetic well ties (Appendix 5). Lithologic and chronostratigraphic information from wellcompletion reports (Plates 1-7) are also augmented by a paleoenvironmental and paleobathymetric assessment using benthic foraminifera from five wells tied to the 3D volume (Fig. 3.17; Appendix 1; Moss et al., in prep.). These data are available only as a result of active hydrocarbon exploration in the NCB for the past ~50 years (primarily targeting the prospective underlying Mesozoic interval). Integration of these data has allowed unprecedented interpretation of a classic late Paleogene-early Neogene prograding clinoform succession. Implications of this research are separated into: 1) those relating to the sequence stratigraphic method, clinoform morphology, sediment distribution/preservation, and the quest for an understanding of eustatic variations, and 2) specific suggestions for further research into the clinoformal stratigraphic succession of the NCB, e.g., as a case study for a future proposal to the Integrated Ocean Drilling Program (IODP). 6.2. GENERAL IMPLICATIONS 1) Paleogene-Neogene sequences of both the NWS and New Jersey shelf do not conform to the seismic sequence stratigraphic model, even though both margins are classic progradational settings with well-defined clinoform geometries representing both carbonate and siliciclastic depositional environments. The division of such sequences into base-level diagnostic systems tracts (i.e., lowstand, transgressive and highstand) has been commonly used for 255 decades as a template for seismic interpretation, even though not all tracts are present on any given seismic profile or in all parts of a basin (Posamentier and James, 1993). In particular, recognition of a lowstand systems tract has been deemed essential for separating the aforementioned tripartite seismic stratigraphic depositional sequence (Vail et al., 1977; 1991) from a transgressive-regressive cycle (Johnson et al., 1985; Christie-Blick and Driscoll, 1995). In the Paleogene-Neogene NCB, systems tracts could not be identified, because the succession is dominated by progradation. In particular, lowstand basin-floor fans are largely absent, despite examination of continuous 3D seismic coverage ~70 km along strike. Similarly, neither lowstand systems tracts nor definitive basin-floor fans are identified in the Oligocene-Miocene clinoformal sequences on the New Jersey margin (e.g., Fulthorpe et al., 2000; Monteverde et al., 2000). While Oligocene systems tracts there are identified from well-based studies using continuous core, the lowstand systems tract is absent and the transgressive systems tract is <10 m thick (Pekar et al., 2001); this thickness is below the resolution of commercial seismic data (15-30 m). These results bring into question the general applicability of the sequence stratigraphic model (Vail et al., 1991), where systems tracts have been often genetically linked to the eustatic curve of Haq et al. (1987). Strict adherence to this model must therefore hamper an objective assessment of the three-dimensional variability of seismic stratal architecture and its usefulness for the study of eustasy. 2) Sediments composing clinoform topsets must be delivered to the shelf edge by deep-marine processes, including open ocean swells, currents, storm 256 and internal waves, rather than shallow shelf processes such as tidal currents, fair-weather waves and swells, or from river discharge at or near clinoform breakpoints. As part of the sequence stratigraphic model, clinoform topsets are commonly assumed to develop at or near sea level, with the breakpoint near sea level during lowstands (e.g., Posamentier et al., 1988; Vail et al., 1991). However, paleobathymetric estimates at the sequence boundaries from wells located on the NWS indicate that Oligocene-Miocene shelf edges are submerged to minimum water depths of 100-200+ m. 3) An accurate estimate of paleo-water depth basinward of clinoforms can only be obtained by combining depth-converted, decompacted clinoform amplitudes with the best biostratigraphic paleodepth estimates from adjacent shelf sediments. Clinoform amplitude is commonly used as a proxy for minimum paleo-water depth in the adjacent basin (e.g., Driscoll and Karner, 1996, 1999; Kaiko and Tait, 2001). However, paleobathymetric estimates employing this approach do not take into account water-depths on the adjacent shelf. Clinoform amplitude estimates are more accurate than relying on paleo-water depth assessments based purely on benthic foraminifera, as bathyal water-depth zones have vertical ranges from 300 m to >1000 m. In contrast, landward of the shelf break, paleo-water depth zones from benthic foraminifera are narrower, 20-100 m. To counter this inadequacy, in our study of the NCB, estimates from shelfal benthic foraminifera have been combined with those available from the clinoform geometry. 257 4) Interpretation of the 2D and 3D seismic data from the NWS indicates that well information cannot be projected along-strike for kilometers onto any other dip-oriented transects (e.g., Pekar et al., 2001), because clinoforms are inherently three-dimensional at a variety of scales. The "safe" extrapolation distance varies with geological setting and scale of seismic stratigraphic investigation. Even in this carbonate-dominated margin, characterized by diffuse sediment source(s), the clinoform geometry is clearly three-dimensional. In the NCB, the overall two-dimensionality of mapped 2-5 m.y. sequences 30-80 km along strike is the result of integration of small-scale sequences <0.5-1 m.y. in duration, with sedimentary lobes up to 10 km in diameter. Therefore, in the NCB wells should not be projected more than ~5 km along strike. Furthermore, interpretation of observed sedimentary lobes requires dip-oriented seismic profiles spaced at a maximum of 2 km. 5) There is no clear link between eustatic variations and submarine canyon formation. Local to regional factors, such as rate of sediment supply and/or increased tectonic activity, must also play important roles in submarine canyon development (e.g., Fulthorpe et al., 2000). On the NWS, submarine canyons identified along late middle Miocene (MM2) clinoform fronts are initiated as small gullies during the mid-Miocene climatic optimum (MM1). In contrast, submarine canyon development on the temperate carbonate shelf of the Gippsland Basin on the SE Australian margin has been linked to the postulated eustatic sealevel fall following the mid-Miocene climatic optimum (Haq et al., 1987; Bernecker et al., 1997). In contrast to both these southern hemisphere examples, 258 submarine canyons are relatively scarce in the Miocene on the siliciclastic New Jersey margin (Fulthorpe et al., 2000). 6.3. IMPLICATIONS SPECIFIC TO THE NCB 1) The northwest Australian margin is particularly well suited to the study of late Paleogene-Neogene sea-level changes. Seismic geometries in the NCB are similar to those observed on siliciclastic margins, e.g., New Jersey (ODP legs 150 and 174A), with multiple prograding clinoform sequences. However, carbonate-dominated sediments in the NCB contain a greater biogenic component than is possible within siliciclastics, increasing the potential for chronostratigraphic, paleoenvironmental and stable isotope information critical for understanding base-level variations (amplitude and timing). However, the NCB contrasts with other carbonate margins thus far targeted for sea-level studies. Some have a well defined, proximal platform as a sediment source, e.g., Bahamas (ODP legs 101 and 166) and NE Australian margin (ODP legs 133 and 194). Others exhibit numerous seismically defined biogenic buildups, e.g., Great Australian Bight, Australia (ODP leg 182). Because discrete shelf sediment sources are still undefined for the NCB, understanding the effect of diffuse shelfwide sediment production on clinoform development adds to the spectrum of passive margins currently drilled by ODP. This research will form the backbone of a future IODP proposal building on ODP proposal 550 (Bradshaw et al., 1999). Paleobathymetric variations, derived from widely-spaced (~29 m average) ditch cuttings and sidewall cores, are still sufficient to develop broad, consistent relationships with mapped seismic 259 stratal architecture. Continuous coring by IODP will provide even greater paleoenvironmental resolution, both within the sequences and at their boundaries. 2) On the NWS, investigation of the modern seafloor and shallow subsurface is essential for documenting processes of sediment production, transportation and accumulation crucial for understanding the buried Miocene succession. Unfortunately, existing seafloor data are sparse, comprising bathymetry grids at ~1 km cell size. Industry seismic site surveys exist only adjacent to hydrocarbon exploration wells, and these are unpublished. Available shallow seismic surveys, sea-bed photographs, samples (grab, dredge and sled) and oceanographic current meter moorings from various studies are widely spaced (e.g., Jones, 1973; Holloway, 1985; Collins et al., 2000). These data provide only a small amount of information on this 100-200 km wide continental shelf. 3) The maximum observed inversion-related uplifts, ~70 m along anticlinal crests, is of a similar scale to postulated eustatic variations, but result in only localized unconformities and onlap discontinuity surfaces. In contrast, eustatic variations must increase or decrease accommodation space across the entire margin. ODP drilling to document sea-level history has targeted passive continental margins, e.g., New Jersey, in an effort to minimize the effects of tectonism (subsidence or uplift) that could mask or mimic the stratigraphic effects caused by eustatic variations. In the NCB, slow thermal subsidence relative to sediment supply has also resulted in a well-developed prograding clinoform succession that maximizes sequence resolution and utilizes horizontal accommodation space (Fulthorpe, 1991). Inversion uplift anticlines, which can be 260 identified on the regional seismic data, are superimposed on this thermal subsidence. Regional MCS profiles with broad areal coverage are available to identify the location and distribution of inversion features. The stratigraphic effect of these inversion structures, i.e., their amplitude and region of influence, can be assessed using forward isostatic and flexural modeling. The regions influenced by uplift can then be avoided, when designing more detailed drilling transects for future sea-level history studies. 261 Appendix 1 Sequence biostratigraphy of the late Oligocene-early Miocene Mandu, Tulki and Trealla Limestone, Rankin Trend, western boundary of Northern Carnarvon Basin, West Australia. by Graham Moss Institute for Geophysics, University of Texas at Austin, Austin Texas, USA, 78759-8500. preparing for submission to Palios, lead author G.Moss with co-authors D.L. Cathro and J.A. Austin, Jr. 262 Abstract Verification of a global sea level curve for the Cenozoic based upon seismic stratigraphic analysis demands in-situ geological evidence of synchronous global sea level change. We tie sequence biostratigraphic analyses from industry wells in the Northern Carnarvon Basin (NCB) to seismic stratigraphic interpretations from an exceptional set of 3D and 2D seismic data. The biostratigraphic database is constructed from ~250 analyses of sidewall cores and ditch cuttings from Eocene to Pliocene intervals. A synthesis of some 286 benthic and 73 planktic taxa of foraminifera is supplemented with quantitative stratigraphic observations of other fossil groups, e.g., ostracods and fragments of bryozoans, corals and mollusks, and lithological components, such as, authigenic calcite, opaque minerals and quartz sands of variable maturity. Preservation of foraminiferal assemblages is extremely variable in Oligocene to latest Miocene stratigraphy, depending upon location of wells and interval investigated. Problems associated with sampling bias from facies control and poor preservation, inherent in studies of foraminiferal distribution in shallowmarine settings, are taken into account. Nonetheless, consistent, detectable faunal signals correlate between wells; some also correlate with prominent seismic horizons. A cluster of planktonic events above seismic horizon DLS4 in the middle Miocene, coincident with turnover in benthic foraminifera, is interpreted to record a regional flooding event and regression. We also suggest that observed diagenesis, particularly evident in Goodwyn 6, is a primary indicator of subsurface water mixing zones and possible emergence, associated shallowing 263 upward trend that dominated the NCB from the middle Miocene to the Pliocene. These detailed analyses of episodic diagenesis and faunal patterns provide evidence of higher-frequency sea level fluctuations (0.5-3Ma) within a secondorder (3-10Ma) major transgressive-regressive cycle. In summary, they suggest that the identified horizons are sequence stratigraphically significant. We compare these results to other, similar records on the southern Australian margin, the New Jersey Shelf and the Great Bahama Bank. Such comparisons are an essential step to understanding perturbations in global climate, including the likely eustatic response to an enhanced greenhouse effect. 264 TABLE OF CONTENTS LIST OF TABLES AND PLATES.................................................... 267 LIST OF FIGURES.................................................................... 268 OUTLINE OF THIS REPORT........................................................ 269 PART 1................................................................................. 269 1.1 INTRODUCTION......................................................... 269 1.2 STRATIGRAPHIC SETTING............................................. 270 1.3 THE CHRONOSTRATIGRAPHIC FRAMEWORK....................... 272 1.4 METHODS AND MATERIALS.......................................... 275 1.4.1 Data Collection and Preparation........................ 275 1.4.2 Integrated Paleontological System (IPS)............... 277 1.4.3 Data Organization......................................... 278 1.5 BENTHIC FORAMINIFERA AS PALEOENVIRONMENTAL INDICATORS.................................................................. 280 1.5.1 Infaunal Versus Epifaunal (i:e) Dominance........... 280 1.6 FACIES CONTROL ON THE STRATIGRAPHIC DISTRIBUTION OF TAXA.......................................................................... 282 1.7 FAUNAL CHANGE AND BIOFACIES PROFILES...................... 283 1.8 MEASURES OF FAUNAL CHANGE.................................... 285 1.8.1 Quantitative Stratigraphy and Depth-Age Curves.... 285 1.8.2 Cosine-Theta, Otsuka and Faunal Turnover........... 287 1.8.3 Diversity................................................... 288 1.9 PALEOENVIRONMENTAL CHANGE................................... 289 1.9.1 Reading the Bathymetry Record........................ 289 1.9.2 Infaunal to Epifaunal Ratios (i:e)....................... 291 1.9.3 Planktonic Percentages (p:b)............................ 291 1.9.4 Larger Foraminifera (lgFor)............................. 293 1.9.5 Agglutinated Foraminifera (aggl)...................... 295 265 1.9.6 Diagenesis................................................. 296 1.9.7 Sample-by-Sample Similarity Matrix.................. 296 PART 2: RESULTS AND DISCUSSION............................................ 298 2.1 2.2 GENERAL COMMENTS ON FAUNAS..............................298 RESULTS..............................................................300 2.2.1 Goodwyn 6................................................ 300 2.2.2 Goodwyn 7................................................ 305 2.2.3 Goodwyn 4................................................ 310 2.2.4 Goodwyn 2................................................ 313 2.2.5 Eastbrook 1................................................ 318 2.3 2.4 CONCLUSIONS...................................................... 325 ACKNOWLEDGEMENTS.......................................... 327 REFERENCES CITED................................................................ 338 APPENDIX A: SAMPLES INVESTIGATED........................................ 343 APPENDIX B: RANGE CHARTS FOR EACH WELL.................................344 APPENDIX C: SAMPLE DESCRIPTIONS ............................................345 APPENDIX D: ECOSTRATIGRAPHIC GROUPS USED IN PROFILES........... (INDEX NUMBERS AND DATUM NAME)....................................... GLOSSARY........................................................................ 351 355 358 APPENDIX E: STRATIGRAPHIC DATUMS USED IN QUANTITATIVE STRATIGRAPHY 266 LIST OF TABLES 1. Summary of benthic and planktonic diversities and sample intervals... 276 2. Bathymetric zones and depth ranges.......................................... 290 3. Generalized groupings of benthic foraminiferal genera.................... 292 4. Broad allocation of larger foraminifera to estimated depth ranges....... 294 5. Summary of sea level history from sequence biostratigraphy............. 324 LIST OF PLATES 1. Plate 1............................................................................. 328 2. Plate 2............................................................................. 330 3. Plate 3............................................................................. 332 4. Plate 4............................................................................. 334 5. Plate 5............................................................................. 336 267 LIST OF FIGURES 1. Location of wells in study..................................................... 271 2. Chronostratigraphic framework............................................... 273 3. Data organization............................................................... 279 4. Bathymetric determinations from depth ranges of key taxa............... 281 5. Example of an IPS output...................................................... 286 6. IPS similarity matrix............................................................ 297 7. Ecostratigraphic profiles from Goodwyn 6.................................. 301 8. Similarity matrix Goodwyn 6................................................. 302 9. Ecostratigraphic profiles from Goodwyn 7................................. 306 10. Similarity matrix Goodwyn 7................................................. 307 11. Comparison of lithostratigraphy and Tritaxia abundance between Goodwyn 4, 7 and 2................................................................ 308 12. Ecostratigraphic profiles from Goodwyn 4................................. 311 13. Similarity matrix Goodwyn 4................................................. 312 14. Ecostratigraphic profiles from Goodwyn 2................................. 314 15. Similarity matrix Goodwyn 2................................................. 315 16. Ecostratigraphic profiles from Eastbrook 1................................. 319 17. Similarity matrix Eastbrook 1................................................. 320 18. Comparison of depth-age curves (LOCs) for all wells..................... 322 268 OUTLINE OF THIS REPORT Part 1 consists of two parts. First, an introduction with a discussion of the regional stratigraphy in the NCB, previous studies and the chronostratigraphic framework. Next is a description of materials and methods, including a discussion of the Integrated Paleontologic System (IPS), and an explanation of the groups used in biostratigraphic analyses and correlations. Part 2 details the results of these analyses including individual well descriptions, discussions and conclusions. Appendices at the end of the report comprise all data collected including taxic lists and lookup tables. Plates 1-5 contain scanned images of selected specimens from all intervals in all wells studied. PART 1 1.1 INTRODUCTION Climate and tectonism are two dominant independent driving forces that control the amount and types of sediment that produce any stratigraphic framework (Vail et al., 1991). Exogenic climatic environmental and tectonic factors work in combination to produce the sequence stratigraphic architecture observed seismically on continental margins. If genetically related sequences forced, in part, by eustatic sea level change can be linked to global climatic variations, then they should be detectable in disparate yet coeval sedimentary basins. Stratigraphic data from five industry wells drilled on the eastern margin of the Rankin Trend, Northern Carnarvon Basin, Western Australia calibrated using 269 planktonic datums can be used to constrain the sequence stratigraphic architecture from 2D and 3D seismic data (Cathro in prep.). We use benthic foraminifera and other stratigraphic evidence to reconstruct paleoenvironments that can then be compared to interpretations from seismic stratigraphic analyses. By compiling disparate well-based data types, a coherent account of climate, sea level and clinoform development within a well-constrained chronological framework can be developed. 1.2 STRATIGRAPHIC SETTING The Cenozoic stratigraphy of the NCB preserves the record of three of the four major marine incursions that characterize sedimentation on the Australian continental margin (Fig. 1; Quilty, 1977; McGowran, 1979; Loutit and Kennett, 1985; Apthorpe 1988). The foraminifera-rich and often quartz-rich Giralia Calcarenite represents the middle Eocene to Oligocene transgressive phase with limonitic, glauconitic and ferruginous equivalents in the Merlinleigh Sandstone in the Merlinleigh sub-basin and the Robe Pisolite (Hocking et al., 1987). The latter is associated with fluvial drainage of the Hammersley Basin (Hocking et al., 1987; Heath and Apthorpe, 1987; Hocking, 1987). Records of the early Oligocene are largely absent, except for a prominent phase of duricrust formation onshore (Hocking, 1987). Quilty (1977) has argued that this hiatus extended from Zone P17 to Zone P21 (Fig.1), though it is clear that there exists a patchy distribution of lower Oligocene sediments (Apthorpe, 1988). 270 The second prominent marine incursion preserved in the NCB (Cycle III, Fig. 1) resulted in the late Oligocene to late Miocene transgressive sequence that produce the Cape Range Group, represented by the Mandu, Tulki and Trealla limestones. McGowran (1979) has suggested that this incursion, the most prominent interval of basin formation on the NWS, may have commenced around Zones N3-N4 and reached a maximum near the early-middle Miocene boundary (Fig.1). Apthorpe (1988) has divided this transgression into two subunits: cycle 3A commences around Zone P20 and results in deposition of the Mandu Calcarenite, while Cycle 3B begins around N8 and ends in the early late Miocene with the dolomitic and quartz sand-rich Bare Formation (Heath and Apthorpe, 1984). The two cycles are interrupted by a short hiatus near Zones N8 or N7 (Chaproniere, 1981; Apthorpe, 1988). This transgressive phase is punctuated by four excursions of tropical larger foraminifera from lower to higher latitudes (see Fig. 2) that fit well with the deep-sea derived oxygen isotope curve (McGowran, 1979). The larger foraminifera are conspicuous in carbonate lithologies along the southeastern margin of the NCB and are interbedded with dolomites (Apthorpe, 1988). 1.3 THE CHRONOSTRATIGRAPHIC FRAMEWORK Sequence biostratigraphic analysis relies on a firm chronostratigraphic framework in which to correlate stratigraphic horizons with seismic sequence boundaries. Numerous studies (e.g., Chaproniere, 1975; Quilty, 1974, 1977; 272 Wright, 1977; Heath and Apthorpe, 1987) have established a regional biostratigraphic succession for the NCB. Recent recalibrations of the Australian Cenozoic biostratigraphic record to global, combined with isotopic dating, provide the means to fit the NCB succession into a worldwide context magnetostratigraphy (Berggren et al., 1995; Chaproniere et al., 1996). Figure 2 sets out the framework of geomagnetic Chrons (from a revised Geomagnetic Polarity Timescale (GPTS)), planktonic foraminiferal and calcareous nannofossil zones to which we match planktonic datums for the middle Eocene to Pliocene in the NCB. The Eocene-Oligocene (33.7 Ma) and Oligocene-Miocene (23.7 Ma) boundary designations (Cande and Kent, 1992) are around a 2Ma shift from previously established chronologies for NCB faunas, though the ordination of planktonic bioevents remains largely unchanged. The Globigerinoides-Praeorbulina-Orbulina bioseries, as well as distinctive lineages, such as Cassigerinella, Globigerinoides triloba-sacculifer group and Neogloboquadrina, represent major components of the NCB planktonic succession. Highest diversities in most wells are encountered in the early and middle Miocene sequences. Dissolution resistant forms dominate assemblages in many of the Miocene sequences. For example, Globoquadrina and Neogloboquadrina are well represented but there are low relative abundances of susceptible genera, such as Orbulina and Globigerinella. With this in mind, our use of chronostratigraphic datums in the following sections uses as many last appearances as possible, from planktonic foraminifera and calcareous 274 nannofossils, in calculations of depth versus age curves and sediment accumulation diagrams. 1.4 METHODS AND MATERIALS 1.4.1 Data Collection and Preparation Samples were prepared by laboratory personnel at AGSO by soaking original material in a water and detergent solution, adding Hydrogen Peroxide, washing and collecting the residue in a 63-micron sieve. Dried residue from each sample was then sieved into three size fractions and a complete census was obtained (by G. Moss) from all fractions (Table 1 and Appendix A lists samples). The fractions and their common components are summarized thus: Greater than 600 microns - mostly larger foraminifera, e.g., Lepidocyclina, Heterostegina, Operculina, Miogypsina, Amphistegina and Gypsina spp.; larger macrofossil fragments such as bryozoan and rare coral fragments; and large lithified and recrystallized fragments, including chert and occasional pebbles. 600-150 microns - benthic and planktonic foraminifera with larger tests (including large foraminifera); some macrofossil fragments; calcareous algae; drilling contaminants; opaque minerals; glauconite; calcite rhombs; and some quartz sand. Less than 150 microns - smaller planktonic taxa, such as Cassigerinella and Streptochilus; small tests and juvenile stages of 275 WELL Goodwyn 7 Goodwyn 6 Goodwyn 4 Goodwyn 2 Eastbrook 1 Benthic Maximum 45 39 53 53 8 Benthic Average 17.8 11.9 25.5 20.8 1.8 Planktonic Maximum 21 24 23 19 12 Planktonic Average 6.4 6.3 10.0 9.1 2.8 # samples Interval (SWC) (m) 36 37.3 49(28) 51 54 32 28.6 24.3 23.9 31.9 Table 1. Summary of benthic and planktonic diversities and sample intervals 276 benthic and planktonic foraminifera; the bulk of quartz sand; and rare macrofossil fragments. 1.4.2 Integrated Paleontologic System (IPS) IPS has been developed by the Technical Alliance for Computational Stratigraphy (TACS), under the direction of Dr A. C. Gary of the Energy and Geoscience Institute at the University of Utah. The software was developed using input from a multi-disciplinary group of micropaleontologists, stratigraphers and geophysicists and was supported by a consortium of ten petroleum companies. The key attribute of the software is that it integrates apparently disparate stratigraphic data to document geohistory for a section or series of sections. Greater confidence in reconstructions comes from assembling and correlating various forms of data. Biostratigraphic and lithostratigraphic data, such as presence and absence or abundances of microfossil or macrofossil taxa are combined with geophysical logs or sediment characteristics. This enables the construction of cross-sections and ecostratigraphic profiles that can be compared and contrasted with seismic stratigraphic interpretations. Components of the software that we use in our analyses are: Quantitative stratigraphy: Planktonic datums for each well are matched to a recognized chronostratigraphic standard and depth-age curves and rates of sediment accumulation are calculated. Curves and calculations: Taxa are grouped according to key characteristics, calculations are made of ratios and faunal change, a 277 bathymetry curve is estimated from documented depth-ranges for taxa and then all results are profiled against depth in each well. Cosine q similarity matrix: Examination of a section, sample-bysample, is accomplished using this matrix. Each sample is compared to all others and the results are plotted as proportional (to Cos q value) squares. The result reveals repetitions of assemblages and highlights faunal discontinuities in the data sets that may, for example, reflect changes in sea level, sequence boundaries and climatic variations. Cross-sections: using planktonic and benthic bioevents (last and first appearances), cross-sections between wells are constructed. The biostratigraphic data is a collection of quantitative measurements of multiple paleoecological proxies (taxa) for environmental parameters, such as water depth (e.g., presence/absence of photic zone), changes in organic carbon supply, and salinity, etc. Some basic observations of key genera and their respective morphologies illustrate the value of quantitative measurements in the biostratigraphic data (Fig. 3). 1.4.3 Data organization Presence, absence and abundances of taxa (mostly per gram of sediment), as well as quantitative measurements of the relative abundance of other sample constituents (e.g., macrofossil fragments, quartz sand, authigenic calcite), were logged and stored in MS Excel (*.xls) files. These data were then converted to 278 279 Integrated Paleontologic System (IPS, discussed below) readable files (2nd tier, Fig. 3) that were used in data analysis and graphic output (3rd tier, Fig. 3). Spreadsheets are constructed as taxon presence/absence matrices with embedded annotations citing relevant taxonomic references and additional comments, see appendix B for layout. Data storage compiled the co-occurrence and relative importance of a taxon within each sample and, after slight modification and simplification, these data were imported into IPS as comma separated variable (*.csv for range charts) and text (*.txt for lithostratigraphic logs) formats (see Fig. 3). 1.5 BENTHIC FORAMINIFERA AS PALEOENVIRONMENTAL INDICATORS 1.5.1 Infaunal versus epifaunal (I:E) dominance Modern benthic foraminifera, as part of their competitive evolutionary strategies, have developed test morphologies to adapt to differing environments and a variety of substrates (Fig. 4). Tracking fluctuations in the abundance of morphotype groups can, with caution, be used as a proxy for changes in physical parameters (e.g., oxygen, organic carbon supply). Principal groups adopt an epifaunal (living on the surface), infaunal (within the sediment) and epiphytic (attached to plant surfaces) habit. Modern observations have established links between the development of elongate perforated or agglutinated wedge-shaped tests (e.g., Bolivina spp., Textularia spp., Eggerella spp.) with low-oxygen or high organic carbon supply and flattened trochospiral (e.g., Cibicides spp. Discorbis spp.) with higher oxygen oligotrophic conditions (Brasier, 1982; Jones and 280 281 Charnock, 1985; Murray, 1991). There are prominent exceptions to these associations, for example, some taxa are opportunistic free living scavengers (e.g., Sphaeroidina) that defy morphotype-environment classification and may change their life habit to capitalize on available resources (Linke and Lutze, 1993). Our approach is to use groups that satisfy an accepted consensus, as infaunal (i) or epifaunal (e), and avoid ambiguous or controversial taxa. Fluctuations and alternations may be more informative than absolute abundance measurements and rapidly changing ratios of morphotypes (e.g., i:e) signal environmental shifts. The faunal changes are correlated with other evidence, for example in the case of a shift from deeper to shallower depositional regime, decreased rates of sediment accumulation (? deepening and prevailing low-oxygen environments) or increased abundance of larger foraminifera (known oligotrophic epifauna). The aim is to reconstruct paleoenvironments from a consensus of the quantity, quality and resolution of all available data. 1.6 FACIES CONTROL ON THE STRATIGRAPHIC DISTRIBUTION OF TAXA The stratigraphic distribution of the foraminifera are to some extent controlled by post-mortem depositional and diagenetic processes (taphonomy). Species ranges and abundances may be secondarily controlled by taphonomic (e.g., hydrodynamic) processes such that clusters of first and last occurrences are concentrated during intervals of major flooding or basinward shifts of facies (Holland, 1995, 2000). The sequence biostratigraphic signal may be obscured, for example, by diagensis or the mixing of original assemblages by down-slope 282 transport. This has already been noted in the Miocene sequences of the NCB where Chaproniere (1984) has suggested that the extensive recrystallization in the Tulki Limestone may indicate emergence or the flux of subaqueous meteoric water. Therefore, differentiating final from temporary last and first appearances may isolate episodes of environmentally controlled faunal change from taphonomic overprinting. The Integrated Paleontologic System, used to analyze these biostratigraphic and lithostratigraphic data, is software that facilitates the comparison of apparently disparate information to assist the identification of 'real' faunal changes from taphonomic (e.g., secondary diagenetic) noise. We distinguish final and temporary last appearance datums and plot them against quantitative measurements of taxic abundance and lithological components in an attempt to isolate episodes of facies control. 1.7 FAUNAL CHANGE AND BIOFACIES PROFILES Ecostratigraphy can be described as the record of fluctuations in population and community structure resulting from regional changes in physical parameters, such as, temperature, climate, oceanography, water chemistry (e.g., salinity), substrate etc. (Kauffman et al., 1991, p.812). Furthermore, it is clear that most ecostratigraphic events involve rapid, short-term changes in community structure above background levels and synchronous changes have been shown to occur over a wide area (Moss and McGowran, 1995, 2001). The events may be integrated with other observations, such as lithological descriptions, including diagenesis, to produce a composite of event stratigraphy. Warm intervals, for 283 example, should show increases in the abundance of tropical-like species that have distinct correlative value (McGowran, 1986). They should also be synchronous with other evidence of warm-water conditions, such as, for example, stable isotope spikes in planktonic taxa or evidence for an intensified oxygen minimum layer (e.g., Cannariato and Kennett, 1999). One of the aims of analysis of stratigraphic data using the IPS is to reveal such associations for geohistorical reconstructions. Biofacies profiles are produced by IPS using the following methodology: 1. Collected data for each well, in the original MS Excel format (*.xls) is converted to a format that is easily read into IPS (see Fig. 3 and table Appendix C). 2. Within IPS, selected groups of informative ecostratigraphic groups (Appendix D), stratigraphic observations and seismic interpretations are compared. 3. A lookup table of observed and inferred depth ranges (nwsdepth.tbl) is imported into IPS and all taxa and their relative abundances in a given well are compared to the table; the consensus results in the calculated depth range for each sample. A short vertical line shows the deepest estimate for at least one taxon. A (red) marker, slightly offset from the right, indicates that for that sample the bathymetric estimate is based on less than 8 taxa (e.g., confidence of paleobathymetric estimate). The next section describes the elements of the biofacies profiles and how they are integrated to compile the ecostratigraphy. 284 1.8 MEASURES OF FAUNAL CHANGE The following discussion refers to Figure 5 and is an explanation of the parameters used in individual plots for wells in Part 2. 1.8.1 Quantitative Stratigraphy and Depth-Age Curves Collected biostratigraphic data provides two types of information that can be summarized as: 1. Non-unique, recurring abundance fluctuations of taxa (ecostratigraphy) and, 2. Unique final first and last appearances tied to a timescale (chronostratigraphy). The quantitative stratigraphy component of IPS uses the biostratigraphic data, in the form of recognized and unique correlative bioevents in each well, to compare with an composite geological timescale constructed from a Cenozoic standard (i.e., Berggren et al., 1995). Initially, last appearance datums (LADs) for key bioevents are plotted against the Cenozoic standard (listed in Appendix E). The next step is to fit a Line of Correlation (LOC) that best accommodates available data. More reliable datums, as determined by interpretation of preservation of sample or abundance of specimens, are sometimes weighted and the LOC adjusted accordingly. The final LOC must satisfy all stratigraphic data, included weighted (more confident) datums, and is therefore not a simple regression line (see Edwards, 1989 for discussion). Individual lines of correlation have been calculated for all wells from available data and are matched to selected 285 286 stratigraphic profiles (discussion follows). If the LOC is accepted a reasonable depth-age estimate, then it can be seen that last appearances that fall to the far right may be much higher in the stratigraphy than their ages suggest and, conversely, last appearances plotting to the far left of the LOC are possibly environmentally suppressed. Explanations for these anomalies may involve, in the case of right displacement, basin inversion and consequent erosion of older sediments that are then included into younger stratigraphic sections and, for left, 'pseudo-extinctions' forced by facies control. These conditions are discussed in more detail the following sections with reference to relevant examples. 1.8.2 Cosine-Theta, Otsuka and Faunal Turnover Three metrics are used to track faunal change in biofacies profiles. These are: 1. Cosine-Theta calculates dissimilarity between adjacent samples based upon the angle between sample vectors in n-variable space. Sample vectors with no components in common will have a value of zero (cosine 900) and samples that equivalent will have a value of 1 (cosine 00). Values are scaled to 100 and a maximum value signifies two adjacent assemblages are exactly the same. 2. Otsuka compares two adjacent samples by dividing the number of species in common by the square root of the product diversities in each sample. This measure is much more sensitive to sample size and can thus give some indication of the true faunal change when compared 287 with Cosine-theta. When both show sharp changes the faunal break may be a function of sample size, e.g., from poor preservation. Values are scaled to 100. 3. Turnover (Last plus first appearance datums) - Simple sum of last and first appearances of benthic taxa. 4. LADs (Last Appearance Datums) - Raw counts of benthic last appearances in each sample. 1.8.3 Diversity Benthic diversity, when shown (i.e., Eastbrook 1), are presented as a Shannon-Weaver information function (H) that is essentially a simple heterogeneity measure that ensures that taxa with low proportions contribute more to H than those with higher proportions. It is calculated using the formula: H= -Spi ln (pi), where pi is the proportion of taxon i. This takes into account not only the number of species but also the distribution of individuals between species (Murray, 1991). In these data diversity is often controlled by preservation and diagenesis and, therefore, is seen as limited use for paleoenvironmental determinations. It is reproduced here as a rough indicator of available information in each sample. Values approaching indicate high diversity assemblages. 288 1.9 PALEOENVIRONMENTAL CHANGE 1.9.1 Reading the Bathymetry Record IPS estimates a paleobathymetric range for each sample based upon a consensus (i.e., statistical analysis) of the reported depth ranges of all benthic species present in a sample. The ranges, generally derived from modern studies of the same taxa, are stored as designated upper and lower depths in a data file (nwsdepth.tbl, see Fig. 3 and CD). These data are combined with the recorded abundance of each species to calculate a possible depth range for the complete assemblage. Table 2 provides depth ranges discussed in this report. Figure - 5 Vertical lines show limits of bathymetric zones. On each profile the estimated lower limit of the photic zone (less than 130m) is marked with a heavy vertical line. The length of horizontal bar suggests the reliability of the estimate; the longer the bar, the less reliable the estimate (based upon sample standard deviation and variance). An unusually long bar generally suggests species mixing, perhaps from down hole caving or downslope transport (Gary, 1999). Accumulated bars, with limits joined, form an envelope of upper and lower limits for bathymetry calculated from all samples. The next short vertical mark to the right of the range estimate shows the deepest upper depth limit recorded by any species in the sample. When there is a low confidence, perhaps from a small sample or an unusually long depth range estimate, this mark may be the most appropriate estimate of paleobathymetry. 289 Bathymetric Zone Transitional (Tr) Inner neritic Middle neritic Outer neritic Upper bathyal Middle bathyal Lower bathyal Abyssal Depth Range (meters) Coastal and paralic environments 0 - 20 20 - 100 100 - 200 200 - 500 500 - 1000 1000 - 2000 greater than 2000 Table 2. Bathymetric zones and depth ranges discussed in the text (Haynes, 1981; Murray, 1991). 290 The mark on the far right of the paleobathymetric columns gives the indication of the number of species used in the paleobathymetric calculation. Red marks show that the estimate is based upon less than eight species and left-shifted means that it is significantly less than eight. 1.9.2 Infaunal to Epifaunal Ratios (i:e) Microhabitat preferences of benthic taxa are informative proxies for physical parameters, such as, available organic carbon in the sediment, changes in oxygen concentrations or types of substrate. Table 3 lists benthic genera according to microhabitat preference and morphotype. It is a generalized list that is derived from contemporary research and is therefore dynamic and controversial. The major point is that the most prominent taxa (greatest abundance) in the database, grouped here into their respective categories, are the principal components in the final i:e. These taxa are relatively well established in the literature (see Murray, 1991 for discussion). Bolivinids, for example, are generally accepted as infaunal and Cibicidids as epifaunal. See Table 3 and appendix D for allocations. 1.9.3 Planktonic Percentages (p:b) The relative abundance of planktonic foraminifera been used to estimate paleobathymetry following a study by Grimsdale and Morkoven (1955), who recorded a positive correlation with higher abundance of planktonic species and 291 INFAUNA Rounded planispiral Astrononion, unkeeled Elphidium, Melonis, Nonion, Nonionella, Pullenia Flattened ovoid Cassidulina*, Fissurina, Parafissurina Spherical or globular Globocassidulina Tapered and cylindrical Amphicoryna, Bolivina, Bolivinella, Brizalina, Bulimina*, Chilostomella, Dentalina, Eggerella, Nodosaria, Pleurostomella, Reussella, Stilostomella, Trifarina*, Uvigerina* Uniserial lagenids (unilocular) Biconvex trochospiral Ammonia, Hoeglundina EPIFAUNA Rounded trochospiral genera Gyroidinoides planispiral Keeled Elphidium Planoconvex trochospiral Alabamina, Anomalina, Anomalinoides, Cancris, Cerobertina, Cibicides, Cibicidoides, Discorbis, Gavelinella, Hanzawaia, Patellina, Pararotalia, Valvinulineria Discoidal Planorbulina, Spirillina Biconvex trochospiral and planispiral Eponides, Lenticulina All miliolids Agglutinated Gaudryina, Textularia Notes: 1. References: Corliss & Chen, 1988; Thomas, 1990; Murray, 1991; 2. Corliss & Chen classify Elphidium spp. as infaunal while Murray argues that keeled forms are epifaunal and unkeeled infaunal. * Perhaps associated with oxygen minimum zone and high productivity (Mullins et al.,1985; Sen Gupta & Machain-Castillo, 1993). Uvigerina mostly infaunal, some epifaunal (Murray, 1991). Table 3. Generalized groupings of benthic foraminiferal genera according to morphotype and habitat preference. Controversial taxa are omitted from groups used in data analysis. 292 depth. The measure is usually expressed as the number of planktonic specimens as a percentage of the total foraminiferal population. The signal deteriorates due to taphonomic and diagenetic effects, such as down-slope transport and dissolution as well as variations in rates of sediment accumulation (Douglas, 1973; Murray, 1991). Another complexity is that, even above the lysocline, synsedimentary carbonate dissolution may be forced by organic carbon decay (Diester-Haass, 1991), thus altering the p:b ratio. Biofacies analysis that include the analyses of benthic morphotype distribution (i.e., high infauna equal increased organic carbon supply) may go some way to disentangling the a regression-forced planktonic decline from a signal of dissolution. P:B ratios are included here for comparisons with benthic metrics in an attempt to illuminate water-column characteristics (e.g., productivity changes), rather than a simple gauge of bathymetry. 1.9.4 Larger Foraminifera (lgFor) Species belonging to genera such as Amphistegina, Heterostegina, Operculina and Cycloclypleus have symbiotic diatoms within their tests that enable a semi- or fully autotrophic feeding strategy. They are particularly adapted to nutrient-poor, shallow, warm tropical or extratropical waters less than 130m water-depth (Leutenegger, 1983; Murray, 1991, see Table 4). Many of these taxa are migratory, are significant indicators of environmental changes, and are particularly useful shallow-water indicators (Li et al., 1996). These larger foraminifera are grouped and imported into IPS and resulting profiles have been 293 Genus Gypsina Lepidocyclina Miogypsina Marginopora Amphistegina Cycloclypleus Depth Range (m), typical environments Lagoonal, shallow-subtidal 0-60/70, inner shelf, coral reefs and lagoonal 0-130, lagoonal, shallow sub-tidal, inner-shelf Heterostegina Operculina Table 4. Broad allocation of larger foraminifera to estimated depth ranges and observed/inferred habitat (after Murray, 1991) extrapolated to observations in the NCB. 294 matched to other indices (see Part 2). The records are simple raw counts of specimens and their paleoenvironmental relevance is discussed in Part 2. Appendix D lists groups of taxa with larger foraminifera labeled in the epifaunal category. Plates 1-5 are scanned images of selected specimens. Local biostratigraphic observations are first tied to a global chronostratigraphy (Fig. 2) and then extended to a regional context. This permits the synthesis to be correlated with similar studies from the southern margin of Australia and the Northern Hemisphere. 1.9.5 Agglutinated Foraminifera (aggl) Agglutinated foraminifera, though largely uncommon in the investigated samples, are dominated by Gaudryina, Eggerella, Tritaxia, Textularia and Verneuilinina spp. These genera are mostly infaunal and commonly associated with a wide bathymetric range, but predominantly inner shelf to upper bathyal environments (Jones and Charnock, 1985). When present they often appear in pulses associated with trends to shallow-water assemblages. In Goodwyn 7, the distinct episodic swings appear to coincide with inflections in the bathymetry curve to shallow-water assemblages. Mitchell and Carr (1998) studying Cretaceous chalks and marls in the Anglo-Basin have argued that peaks in the abundance of the genus Tritaxia (common in Goodwyn 7) were forced by restricted bottom-water ventilation during late highstand. 295 1.9.6 Diagenesis Percentages of Foraminifera (%f), authigenic calcite as mostly sparry calcite (%ac), shell (%sf) and macrofossil fragments (%mf) have been recorded for samples and logged in profiles. The percentage of foraminifera and authigenic calcite is a useful primary indicator preservation of the foraminiferal assemblage. Secondary diagenesis is a prominent component in all investigated wells. 1.9.7 Sample-by-Sample Similarity Matrix The exam function in IPS allows the simultaneous comparison each foraminiferal assemblage in the well with all other assemblages. This may be useful to help recognize intervals of dissimilarity, faunal changes and repeated assemblages. Figure 6 shows a sample-by-sample matrix that compares all samples in an investigated well. It can be seen that the diagonal compares all samples against themselves so are 100% similar (therefore shown as small dots). The size of the square is proportional to similarity. Larger squares on the matrix show similar benthic assemblages. The example shows that if an assemblage at 2090m is compared to all other assemblages in the well then, based upon foraminiferal components, there are specific assemblages similarities up-section (i.e., at 1914 and 1649m). 296 PART 2: RESULTS AND DISCUSSION 2.1 GENERAL COMMENTS ON FAUNAS Assemblages were generally diverse though preservation was extremely variable, particularly in Goodwyn 6. Infaunal taxa were dominated by the Bolivina, Bulimina and Globobulimina and Uvigerina and Trifarina spp. While epifauna are largely represented by Cibicidides, Cibicidoides, Heterolepa, Discorbids, Rosalina, and, particularly from the late early Miocene to the latest Miocene the important symbiont-bearing larger foraminifera, such as, Operculina, Cycloclypleus, Heterostegina, Lepidocyclina and Amphistegina. Table 1 lists maximum and average diversities for benthic and planktonic taxa in all wells investigated General summary of observations: Goodwyn 6, the more landward well, revealed lower benthic diversities but possessed similar planktonic diversities to the generally better preserved sequences in Goodwyn 7, situated some 16 kms. along strike to the northwest. Species richness tended to show local variations, such that, more diverse and well preserved assemblages were encountered in Goodwyn 4 and Goodwyn 2. Benthic assemblages in Oligocene to early Miocene intervals are dominated by infaunal taxa, consisting mostly of Bolivina, Brizalina, Globocassidulina, Nonion and Nonionella spp. These genera are characteristic of deeper shelfal environments with muddy and sandy substrates. Ecostratigraphic 298 interpretations environments. strongly suggest deeper-water outer neritic to bathyal Eastbrook 1 was poorly preserved and exhibited profound diagenesis through the investigated intervals. Representative specimens from all wells were picked for detailed identification using scanning electron microscopy (Plates 1-5). Extratropical excursions by larger foraminifera are a heuristic component of the stratigraphic succession in the NCB. The middle Eocene, correlating with the Kirthar Transgression, an isochronous transgression common to the Australia and India (McGowran, 1979), records a larger foraminiferal assemblage correlated with Letter Stage Tb. In Zones P22-N4 in the late Oligocene on the southern margin of Australia, another widespread transgressive phase is marked by the immigration of Amphistegina spp., an influx of the elphidid Parrellina spp., and pronounced endemism the Murray and Otway Basins (Li et al., 1996; Moss and McGowran, 2001). Li and McGowran (1994, 2000) have documented the occurrence of these 'third order warm intervals' in the Janjukian (late Oligocene-Miocene), the mid-Longfordian (early Miocene) and climatic optimum of the Batesfordian-Balcombian (early middle Miocene) Stages in southern Australia (see Fig. 1 for correlations). The early and middle Miocene match with our observations is more convincing with the late Oligocene-Miocene. The middle Miocene optimum appears as prominent increases in the abundance of preserved larger foraminifera and coincidentally there is an overall increase in last appearances of benthic foraminifera and swing to shallower depths in paleobathymetric curve. On the southern Australian margin, this significant event 299 appears to have commenced around Zone N15 and marks a major eustatic sealevel fall at the end of the middle Miocene (TB2.1/2.3) (Li and McGowran, 2000). These prominent middle Miocene events are followed by quartz-rich horizons in the late Miocene corresponding with a paleobathymetric low in all wells. 2.2 RESULTS As Goodwyn 6, the most nearshore well, appeared to show the most profound variability of assemblage preservation, the wells are discussed in the order of the nearshore to offshore. 2.2.1 Goodwyn 6: Figures 7 and 8 - See Goodwyn 6 - descriptions (datafile G6desc.xls) Sequences sampled in Goodwyn 6, the most nearshore of the investigated wells, spanned from late Early Cretaceous poorly fossiliferous glauconitic claystones to Pliocene foraminiferal-rich, calcareous sandstones. Samples revealed intervals of prominent recrystallization, particularly at the base in the late Oligocene to Miocene, ranging to abundant and well-preserved benthic and planktonic foraminifera in the middle Miocene part of the sequence. Mature quartz sands are common in the upper part of the middle Miocene interval. General observations: The investigated interval spanned some 1,300 m, ranging in age from latest Eocene to the latest Miocene. 300 301 302 Preservation was extremely variable with pervasive recrystallization common in most intervals but most obvious in the lower part of the investigated sequence. The late Oligocene and early Miocene intervals (1765-1600 m) revealed abundant infaunal genera, such as Bolivina and Brizalina, that were replaced by epifauna (e.g., Cibicides and Cibicidoides) in adjacent younger intervals. Early middle Miocene (EMM1) and middle Miocene samples reveal lower diversity assemblages with common larger foraminifera, together with macrofossil fragments and well-rounded and polished quartz sands. Concentration of authigenic calcite alternates strongly with foraminiferal abundance in the upper part (early middle Miocene and younger) of the investigated sequence. The change from relatively stable to erratic calcite diagenesis coincides with a the conspicuous change in the bathymetry curve at DLS3 (see below). Figure 6 shows assemblage dissimilarity profiles (Cos, Otsuka) against a derived bathymetric curve (from all benthic taxa encountered) ecostratigraphic groups and quantitative stratigraphic observations. The depth-age curve for Goodwyn 6, shows a wide scatter of LADs suggesting a large component of facies control in this landward well. The suppressed upper range of Catapsydrax parvulus (18), Zone N17 (20) and the calcareous nannofossil Sphenolithus heteromorphus (41), appears to be a direct result of condensed horizons around DLS1 and at the top of sequence OM1. 303 A line that accommodates the available data produces a higher accumulation rates in sequences EMM1 and MM1 through the late early Miocene to the middle Miocene (~20 to 15Ma). Ecostratigraphic profiles show: The most prominent faunal change, signaled by concurrent sharp falls in Cosine-theta (Cos) and Otsuka curves, appear to be around 1,600, 1,400m and 1,200 m This is reinforced by the inspection of the similarity matrix for Goodwyn 6 (Fig. 8) that shows significant gaps in the sample-by-sample matrix (areas 1 and 2) between the OM1 and EMM1. The bathymetric curve suggests three prominent deeper-water cycles interrupted by intervals dominated by shallower-water assemblages, these are, from the base: 1. Between 1,700 m and 1,600m; 2. 1,500 and 1,300m; and 3. 1,100 to 800 m The richness of the infaunal group match the deeper signals in the bathymetric curve, while planktonic to benthic ratios (p:b) alternate with larger foraminifera. Following the final peak in the percentage of total foraminifera around 1,300 m, the percentage of authigenic calcite observed alternates succinctly with the percentages of quartz sand. This is interpreted to indicate a marked shift in environments from MM1 to MM2 across the DLS5. This marked environmental shift is further exemplified by a distinct dissimilarity between lower part of the section below DLS4 and the upper part (see Fig. 7, area 4). 304 The sample-by-sample similarity matrix (Fig.7) shows that the substantial gaps in similarity in adjacent samples occur at significant shifts in the bathymetry curve to lower sea levels (i, ii and iii). At iv and v large patches of dissimilarity (blank spots) argue that the interval 1435 to 1165 m and the 1005 to the top of the well are starkly different from assemblages at the base of the investigated intervals. This supports the interpretation that the most profound faunal change occurred at DLS4 around 1435 m. 2.2.2 Goodwyn 7: Figures 9, 10 and 11 - see 'Goodwyn 7 - descriptions' (datafile G7desc.xls) In comparison to Goodwyn 6, foraminiferal assemblages are generally well preserved in the more basinward well Goodwyn 7, some 16 kms to the north. The Tertiary sequences are essentially two-part; they are characterized by distinct intervals of calcilutite and claystones in the Paleogene and show distinct switch to sandstone and calcarenites in the Neogene. The middle to late Miocene sequences are variably dolomitic and preservation of foraminiferal assemblages is extremely variable. Inspection of Figure 9 suggests that: The depth-age curve reveals a slower rate of accumulation in the early part of OM1 that rises steadily to maxima in EMM1 and MM1. By the time of the introduction of larger foraminifera in sequences MM1 and MM2 few foraminiferal datums provide control and rates are extrapolated. From evidence supplied by other curves that show marked changes, it is doubtful whether rates of 305 306 307 308 sediment accumulation remained constant in sequence MM2 (later middle Miocene). The Cosine-theta curve signals two prominent deviations to lower values at around 1700 m and 1500 m. The first matches a sharp decline in infaunal to epifaunal ratios and in the p:b ratio, the introduction of larger foraminifera and the beginning of a minor flourishing of agglutinated numbers. This appears to signal a prominent environmental change at the end of sequence OM1, signaling a faunal change perhaps consistent with a shift to shallow-water faunas. The most prominent bathymetric change in the section appears to coincide with the trend to shallower-water assemblages near 1,300 m There is a coherent transition marked by falling i:e, p:b and agglutinated curves, and a sharp increase in larger foraminiferal numbers at the top of sequence MM1. The change succinctly terminates an interval dominated by deeper-water assemblages (1,400-1,300 m), that is interpreted to signal maximum flooding associated with a local manifestation of the middle Miocene optimum. This apparent shallowing precedes the influx of well-rounded and polished quartz sand in MM2. Agglutinated foraminifera, dominated by Tritaxia victoriensis, though contributing a small but persistent component to assemblages prior to MM1 did not recover following its acme near DLS3, a persistent signal in Goodwyn 7, 4 and 2 (see Fig. 11). This may reflect deeper-water lower-oxygen conditions consistent with a stratified water-column (see Mitchell and Carr, 1998 for a 309 discussion of the sequence stratigraphic implications of Tritaxia distribution), on the eastern margin of the Rankin Trend during the early part of sequence EMM1. Another significant group, the Elphidids, makes an appearance in larger numbers following the decline of larger foraminifera at around 1,200 meters. This group of semi-endemic species are common to inner to mid-shelf environments and are characteristic of modern mid-latitude faunas (Hornibrook et al., 1989; Li et al., 1999). 2.2.3 Goodwyn 4: Figures 12 and 13 - see 'Goodwyn 4 - descriptions' (datafile G4desc.xls)'. Stratigraphy in Goodwyn 4 shows a generally well-preserved faunal succession until the upper 300m of the of the 1216m interval investigated. The latest Eocene interval at the base has common agglutinated benthic taxa and late Oligocene to the middle Miocene (from around 2000-1500m) revealed diverse and abundant benthic and planktonic assemblages. In the late Oligocene-early Miocene common taxa included abundant infaunal unilocular and bolivinid species alternating with epifaunal cibicidids. Significant events are recorded below. Summary for Goodwyn 4: At around 1450m (near DLS4) there is a prominent fall in the Cosine-theta and Otsuka metrics (signaling poor sample preservation) and is matched by an interval of intense diagenesis (high authigenic calcite-low foraminifera). 310 311 312 There is a conspicuous rise in the number of last occurrences (LADs) and an influx of larger foraminifera. All strongly suggesting a significant faunal change. There is also marked shift to lower infaunal to epifaunal ratios combined with the arrival of larger foraminifera at the beginning of sequence MM1, strongly supports the notion of ensuing oligotrophic (low nutrient) bottom-water conditions forced by increased ventilation and the removal organic carbon. The percentage of quartz sand increased late in sequence MM2, coinciding with a decrease in larger foraminifera. At the same time there is a transition to deeper-water environments from evidence provided by the bathymetry curve and a gradual rise in the percentage of planktonic taxa (p:b). These observations coincide with a sharp change in bathymetric curve in Figure 13 with a shift to shallower-water assemblages around 1,400m in the MM1 sequence. 2.2.4 Goodwyn 2: Figures 14 and 15 - see 'Goodwyn 2 - descriptions' (datafile G2desc.xls). The base of the investigated section is characterized by greenish, Festained and poorly preserved assemblages of probably middle to late Eocene age. From 2164.1 to 2106.1 m, the planktonic faunas remain poorly preserved and are extremely glauconitic, but suggest Zone P18 at the base to P22 and N4 at the top of the interval. Assemblages from 2106.1 to 1898.9 m are more diverse, are marginally better preserved and characteristically show late Oligocene age. The 313 314 315 upper part of this interval is dominated by rich assemblages of infaunal forms that include bolivinids and unilocular taxa (see Table 3). Summary of investigations: Depth-age and rate curves show a generally increasing rate of sediment accumulation that reached a maximum in sequence MM1 before beginning to decline after DLS5. Inspection of the total percentages of foraminifera and authigenic calcite (Fig. 14) suggest that Goodwyn 2 is more consistently affected by postdepositional diagenesis. Last appearance datums (LAD) reach their peak in upper part of MM1, immediately prior to the shift to lower sediment accumulation rates. The bathymetric curve shows a steady upper bathyal signal prior to DLS3, that is only interrupted by a sharp divergence to poorly preserved (less than 8 taxa) inner neritic assemblage near 2020 m. Above DLS3, the volatile bathymetric signal corresponds to rapid fluctuations in the number of last appearances that may be a response to sea level fluctuations. The most dramatic swing to shallow-water assemblages above DLS5 is preceded by the marked increase in last appearances. The beginning of the decline in the rate of sediment accumulation after DLS4, also shows a shift in the Cosine-theta curve suggesting a faunal change, a decrease in the p:b ratio and the initial occurrence of larger foraminifera in the lower part of sequence MM1. These changes are interpreted to be in response a generally falling sea level. 316 The early Miocene to mid Miocene sequence, from around 1770.9 to 1408.2 m, show evidence of gradual increase diagenetic alteration, with abundant, diverse planktonic and benthic assemblages showing sharp fluctuations interpreted be a response to sea level changes. The paleobathymetric signal is comparable to the shift to shallow-water assemblages in Goodwyn 6 at DLS3 and in the upper part of MM1 in Goodwyn 7. It appears to be delayed to the lower part of the sequence MM2 in Goodwyn 2, as is the marked introduction of larger foraminifera. This also matches a sharp decline in infaunal to epifaunal ratios near 1,300 m, and a slightly delayed a decrease in the percentage of planktonic taxa. The above ecostratigraphic changes are preempted by a shift in the cosinetheta curve, but not the Otsuka, that suggests a prominent faunal change (rather than a preservational artifact). It appears that faunal change began in sequence MM1, signaled a significant turnover of species and that were replaced by assemblages typical for low-nutrient, low-food (oligotrophic) environments (increased numbers of larger foraminifera). As authigenic calcite percentage in samples declined there were increases in macrofossil fragments (chiefly bryozoans) and mature quartz sand. This is interpreted as further evidence of lowered sea levels. The similarity matrix (Fig.15) shows the major break in the faunal succession is consistent with the swing in the bathymetric curve to shallow-water (inner neritic) assemblages, dominated by larger foraminifera, above DLS5. 317 The is also a conspicuous repetition of like assemblages through much of the section up to around 1300m. (episodic appearance of high similarity red larger squares). This may be signaling regular short-term (e.g., parasequence) flooding events. 2.2.5 Eastbrook 1: Figures 16 and17 - see 'Eastbrook 1 - descriptions' (datafile EB1desc.xls) Around 990 meters of section were investigated in Eastbrook 1 at intervals averaging near 30 meters. The oldest (late Eocene) sediments consisted of gray calcareous claystones and the youngest (middle Miocene) were dominated by calcilutites. Moderate to severe diagenetic alteration dominated all examined samples. Foraminifera were few to very rare, shell fragments were common and fine mature quartz was more abundant at the top. The bathymetry curve reflects frequent mixing of with alternations of apparently neritic with bathyal assemblages. Foraminiferal specimens were few and so interpretations of paleobathymetry are presumed to be extremely unreliable. Examination of curves in Figure 16 reveal: Depth-age and rate curves suggests slower and more uniform accumulation rates compared to the other more easterly wells on the shelf. The younger datums, top Gq. Dehiscens (21) and D. Altispira (7), appear to be suppressed in the well (large shift to the right from the LOC and therefore last appearance far deeper in the section than expected). Facies control on the 318 319 320 distribution of species ranges may account for the condensation of LADs above DLS2. The succession may also reflect mixing due to down slope transport. The Cosine-theta curve shows only rare continuity of assemblages with only episodic appearance of benthic foraminifera. Diversity and last appearance datums are low reflecting impoverished assemblages. Planktonic abundance reached its highest at the base (middle-late Eocene. In the late middle Miocene (MM2), following DLS4, benthic foraminifera outnumbered planktonics, perhaps as a result of increased downslope transport. The high percentage of authigenic calcite strongly suggest that diagenesis obscured bathymetric changes, particularly in the lower part of the section, where percentages of foraminifera recovered reaches only 5% of the total sample. A pulse of quartz sand, interpreted to be a regional event, appears sharply at the top of the interval at around 1,750 m. Shell fragments, that match foraminiferal percentages very well, are interpreted to be meaningful indicators of intervals of downslope transport. The similarity matrix for Eastbrook 1 (Fig. 17) highlights the sparse nature of the foraminiferal data. Clusters of species occurrence appear in the lower (Eocene) and upper part (MM2) of the investigated interval. Almost no occurrences are registered near picked down-lap surfaces. Figure 18 shows accumulated LOCs for all wells. The most striking contrast in this composite plot is the much slower rate of rock accumulation in 321 322 Eastbrook 1, the most distally located well to the northwest; other wells are all updip on the prograding Oligocene-Miocene clinoforms. The next observation is the general sigmoidal shape of all of the depth-age curves. There appears to be a change to higher rates of rock accumulation in Goodwyn 6, 7, 4, and 2 beginning in the late Oligocene that slowed once again in the middle Miocene. The circles mark estimated inflection points equivalent to approximate time for increases in the rate of accumulation. They mark a progression older to younger beginning with Goodwyn 6 then Goodwyn 7 and Goodwyn 4 followed by Goodwyn 2, that may be consistent with a stepping-out of the prograding sequence. The generalized summary in Table 5 catalogues a general shift higher to lower sea level that appears as a progression from Goodwyn 6 (EMM1), followed by Goodwyn 4, Goodwyn 7 (MM1) and Goodwyn 2 (MM2). The series of events appear to mark the migration of clinoforms from the beginning of the early to the late Miocene. From evidence provided by the similarity matrices in Goodwyn 4 and 2, there is some evidence for cycles high frequency sea level fluctuations. All wells, with the exception of the very poorly preserved record Eastbrook 1, provide firm evidence for a regional sea level event (regression) in sequence MM1 or near DLS5. Though there is a suggestion that this significant change was registered earlier (near DLS3) in Goodwyn 6. 323 INTERVAL AGE (Blow Zones) DLS-top LM1 late Miocene (N15-N18) DLS5 - MM2 Middle Miocene (N9-N15) DLS4 - MM1 middle Miocene (N7-N9) DLS3 -EMM1 early middle Miocene (N4 - N6) late Oligocene -early Miocene (P21 - N4) DLS 2 - OM1 SUMMARY OF SEA LEVEL HISTORY Low shallow sea levels recorded in all wells, influx of quartz sands, larger foraminifera Marked fall to shallow-water assemblages and clear increase in larger foraminiferal abundance in Goodwyn 2, shallow (<150m) sea levels recorded in Goodwyn 2, 4, 7 and 6 Marked shift to shallow-water assemblages, with an marked influx of larger foraminifer and elevated benthic turnover in Goodwyn 7 and 4; first appearance of larger foraminifera and faunal turnover in Goodwyn 2 Shift to shallow-water assemblages in Goodwyn 6, Generally high planktonic and infaunal taxa abundance, Transgression Highstand in all wells. Table 5. Summary of sea level history from sequence biostratigraphy 324 2.3 CONCLUSIONS Larger foraminifera are the dominant component of assemblages from the middle to late Miocene, indicating widespread shallow neritic conditions that suggest a profound fall in relative sea-level in the middle Miocene. These data are supported by evidence from ostracod assemblages that show a progression to shallower-water assemblages in sequence MM1 (personal communication V.Passlow, AGSO, 2000). Li and McGowran (1994, 2000) have documented the occurrence of these 'third order warm intervals' in the Janjukian (late OligoceneMiocene), the mid-Longfordian (early Miocene) and climatic optimum of the Batesfordian-Balcombian (early middle Miocene) Stages in southern Australia (see Fig. 1 for correlations). Southern margin records of early and middle Miocene larger foraminiferal incursions match our observations more convincingly than in the late Oligocene-Miocene. The middle Miocene optimum appears as prominent increases in the abundance of preserved larger foraminifera and coincidentally there is an overall increase in last appearances of benthic foraminifera and swing to shallower depths in paleobathymetric curve. On the southern Australian margin, this significant event appears to have commenced around Zone N15 and marks a major eustatic sea-level fall at the end of the middle Miocene (TB2.1/2.3) (Li and McGowran, 2000). These prominent middle Miocene events are followed by quartz-rich horizons in the late Miocene corresponding with a paleobathymetric low in all wells 325 Late middle Miocene to late Miocene (MM2-LM1) facies are characterized by an increased and widespread mature quartz sands of in all wells examined. Faunas characteristic of deeper-water and more eutrophic environments were dominant below downlap surface 4 (DLS4, below middle Miocene sequence MM1); Goodwyn 6, the most landward of the investigated wells, appears to provide the most sensitive record of fluctuations in sea level as exemplified by profound episodes of diagenesis at 1600-1300m, 1250-1050m and 1000-900m. There is a strong suggestion of recurrent emergence, though a preliminary analysis of isotopic profiles (see Fig. 4.3) are ambiguous probably due to large sampling intervals; Eastbrook 1, the most basinward well, shows extreme recrystallization and very poor preservation of foraminiferal assemblages probably due to proximity to the lysocline during sediment accumulation. There is some evidence that the top of the interval (1,900 to 1,700m) is condensed; Biostratigraphic and ecostratigraphic profiles from Goodwyn 7, an isolated well some 14 kms along strike in the northeast, more closely resembles Goodwyn 4; A relatively slow rate of sediment accumulation in the Oligo-Miocene to early middle Miocene sequence is followed by increasing rates of accumulation in the early middle Miocene to late Miocene. These patterns are consistent with a 326 general fall in relative sea level and a steadily increasing terrigenous sediment supply. There are strong faunal similarities with records from the southern margin of Australia and New Zealand, particularly with the timing of influx of larger foraminifera, Elphidids, Notororalids and the establishment of 'modern' assemblages by the end of the late Miocene. Synchronous and widespread prograding sequences appear to be a dominant regional signal, overriding local tectonics through much of the Oligocene-Miocene to late Miocene. 2.4 ACKNOWLEDGEMENTS Thanks to the Technical Alliance for Stratigraphy, particularly Tony Gary (EGI, University of Utah) and Gary Sjogren for the use of and assistance with IPS. Thanks to AGSO Geoscience Australia, Western Australian Geological Survey and Woodside Australian Energy for the loan of side-wall core and ditchcutting samples. Christian Thun and Richard Brown of AGSO Geoscience Australia prepared all samples for investigation. Dr. K. Milliken assisted with SEM and image processing facilities. GM was supported a postdoctoral appointment at UTIG and by an American Chemical Society Petroleum Research Fund Grant #35451-AC8. 327 PLATE 1. (following page) Infauna: a. Bolivina reticulata Hantken, Goodwyn 4, 1877.6m, b. Bolivina bassensis Parr, Goodwyn 4, 1978mm, c. Bolivina pseudoplicata d. Bolivina sp. Goodwyn 7, 60mm, e. Bolivina acerosa Cushman, Goodwyn 4, 1932.4m, f. Siphotextularia awamoana Finlay, g. Bolivina lapsus Finlay, Goodwyn , h,l. Bulimina marginata d'Orbigny, Goodwyn 7, l. 60mm, i. Reussella attenuana Hornibrook, j. Dentalina albatrossi (Cushman), Goodwyn 2, 2011.7m, k. Tubulogenerina mooraboolensis Cushman, l. Bulimina marginata d'Orbigny, , m. Reussella ensiformis (Chapman), 60mm. n. Bulimina sp. 1, Goodwyn 2, 1344.2, o. Bulimina pupoides p. Reussella spinulosa (Reuss), Goodwyn 7, EMM1, 60mm, q. Tritaxia victoriensis (Scale bar = 100mm unless otherwise stated) 328 a b c d e f g h i j k l nn oo pp qq 329 PLATE 2. (following page) Infauna: a,b. Pseudononion victoriense (Cushman), Goodwyn 4, 1438.7m, b. Goodwyn 4 1895.9m, c. Melonis centroplax (Carter), Goodwyn 4, 2005.6m, d. Astrononion stelligerum (d'Orbigny), e. Anomalinoides colligerus , Goodwyn 7, 1765m, f. Planularia cf. gemmata, Goodwyn 7, 1685m, g. Pullenia bulloides (d'Orbigny), Goodwyn , h. Cassidulina laevigata d'Orbigny, Goodwyn 2, 1682.5m, i. Saracenaria altifrons (Parr), Goodwyn 7, 2080m, 50mm, j. Palliolatella sp. 60mm, k. Oolina apiculata, l. Lagena sulcata (Walker & Jacob), n. Oolina globosa (montagu), o. Lagena striata (d'Orbigny), Goodwyn 7, 1645m, 60mm, p. Sigmoidella elegantissima (Parker & Jones), Goodwyn 4. (scale bar = 100mm unless other stated) 330 a b c d e f g h i j k l m n o p 331 PLATE 3 (following page) Epifauna: Rosalina bradyi (Cushman), 50mm, b., Cancris intermedius Goodwyn 4, 1932m, 100mm, c. Rosalina vilardeboana d'Orbigny, Goodwyn 60, d.100, e. Cibicides umbonatus200, f. Cibicides lobatulus (Walker & Jacob), 60mm, g,h. Cibicides boueana (d'Orbigny), 60mm, i. Planodiscorbis irregularis Carter, 100mm, j. Heterolepa brevoralis (Carter), Goodwyn 7, 60mm, k. Cibicides mediocris Finlay, Goodwyn 4, 1895.9m, 200mm, l. Karreria maoria (Finlay), 100, m.100, n. Heterolepa subhaidingeri (Parr), 100mm, o. Discanomalina vermiculata (d'Orbigny), 60mm, p., Discorbis cf. balcombensis 200mm, q. Usbekistania charoides 100mm, r. Svratkina shauni (Quilty), Goodwyn 4, 1786.1m, 60mm, s. Lenticulina lucida (Cushman), 200mm, t. Sigmoilina obesa Heron-Allen & Earland, Goodwyn 4, 999.7m (scale bar = 100mm unless other stated) 332 a b c d e f g h i j 100 k l m n o p 333 PLATE 4 (following page) Epifauna (Larger foraminifera): a. Operculina complanata, Goodwyn 7, 1295m, b. Amphistegina hauerina, Goodwyn 4, 1292.4m, c. Amphistegina lessoni d'Orbigny, c. , d. Operculina cf. victoriensis Chapman & Parr, Goodwyn 7, 1295m, e. Gypsina globulus, g. Miogypsina sp., Goodwyn 2, 1088.2m, 600mm, h. Biloculina bulloides , Goodwyn 6, 60mm, i. Peneroplis sp., Goodwyn 2, 1088.2m, , 500mm j. Quinqueloculina sp. Goodwyn 6, k,l. Celleporiform bryozoans, Goodwyn 6, EMM1 (scale bar = 200mm unless other stated). 334 a b c d e f g h i j k l 335 PLATE 5 (following page) Planktonic foraminifera: a. 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Goodwyn 6 (m) 715SWC 730SWC 750-755DC 760-765DC 780SWC 800-805DC 810SWC 850SWC 880-885DC 920-925DC 950SWC 970.1SWC 1000-1005DC 1020-1025DC 1040-1045DC 1050-1055DC 1075-80DC 1085-90DC 1135SWC 1165SWC 1200-05DC 1230SWC 1260SWC 1290SWC 1320SWC 1350SWC 1370-75DC 1410SWC 1430-35DC 1470SWC 1490SWC 1500-1505DC 1525-30DC 1570SWC 1600SWC 1630SWC 1675SWC 1715SWC 1755SWC 1765SWC 1805SWC 1830-35DC 1875-80DC 1915SWC 1935-40DC 1975-80DC 2015SWC 2045-50DC 2075-90 Goodwyn 7 (ft) 1968.5DC 2181.8DC 2378.6DC 2559.1DC 2690.3DC 2821.5DC 2952.8DC 3084DC 3248DC 3362.9DC 3559.7DC 3707.3DC 3789.4DC 3953.4DC 4084.6DC 4248.7DC 4363.5DC 4527.6DC 4626DC 4757.2DC 4872DC 4954.1DC 5085.3DC 5183.7DC 5397DC 5528.2DC 5593.8DC 5790.7DC 5905.5DC 6053.1DC 6102.4DC 6200.8DC 6282.8DC 6446.9DC 6496.1DC 6561.7DC 6610.9DC 6692.9DC 6824.1DC 6971.8DC Goodwyn 4 (ft) 3010DC 3090-100DC 3160-70DC 3270-80DC 3330-40DC 3430+40DC 3480-90DC 3550DC 3610DC 3670DC 3730DC 3790DC 3840DC 3940DC 4090DC 4240DC 4270DC 4420DC 4510DC 4570DC 4630DC 4660DC 4720DC 4820DC 4900DC 4960DC 5080DC 5200DC 5260DC 5350DC 5410DC 5500DC 5590DC 5680DC 5740DC 5800DC 5860DC 5920DC 5980DC 6070DC 6160DC 6220DC 6280DC 6340DC 6430DC 6490DC 6580DC 6790DC 6880DC 6970DC 7000DC Goodwyn 2 (ft) 2960-70DC 3050-60DC 3110-20DC 3170-80DC 3260-70DC 3320-30DC 3380-90DC 3440-50DC 3560-70DC 3620-30DC 3710-20DC 3800-10DC 3890-900DC 3950-60DC 4010-20DC 4100-10DC 4160-70DC 4250-60DC 4310-20DC 4400-10DC 4490-500DC 4580-90DC 4610-20DC 4640-50DC 4700-10DC 4820-30DC 4900-10DC 4960-70DC 5040-50DC 5120-30DC 5210-20DC 5300-10DC 5390-400DC 5510-20DC 5620-30DC 5710-20DC 5800-10DC 5890-900DC 5950-60DC 6010-20DC 6130-40DC 6220-30DC 6310-20DC 6400-10DC 6490-6500DC 6590-6600DC 6710-20DC 6740-50DC 6850-60 6900-10DC 6960-70DC 7030-40DC 7060-70DC 7090-100DC 7120-30DC Eastbrook 1 (m) 1690DC 1720DC 1740DC 1790DC 1840DC 1860DC 1890DC 1920DC 1940DC 2000DC 2050DC 2080DC 2100DC 2120DC 2140DC 2180DC 2200DC 2220DC 2240DC 2280DC 2300DC 2320DC 2340DC 2380DC 2420DC 2460DC 2500DC 2540DC 2580DC 2600DC 2640DC 2680DC 343 Appendix B: Range charts for each well See supplemental data 344 Appendix C: Sample descriptions 345 Eastbrook 1 - descriptions depth(m) 1690DC 1720DC 1740DC 1790DC 1840DC 1860DC 1890DC 1920DC 1940DC 2000DC 2050DC 2080DC 2100DC 2120DC 2140DC 2180DC 2200DC 2220DC 2240DC 2280DC 2300DC 2320DC 2340DC 2380DC 2420DC 2460DC 2500DC 2540DC 2580DC 2600DC 2640DC 2680DC Comments Foraminifera absent; ? black terrigenous material Impoverished sample; forams. rare; abund. recryst. calcite; few shell frags. Foraminifera rare; preservation poor; black opaque material pres. Forams. rare; occ. qtz. grns.; poor preservation bathy ? ? 20-100 20-100 ? 100-200 ? 20-100 20-100 TR TR ? ? 100-200 100-200 ? 1000-2000 TR 100-200 ? ? ? ? ? ? ? 100-200 ? 100-200 ? 1000-2000 ? environment MN MN ON MN MN TR TR ON ON LB TR ON Fe-stained calcite; few forams. poor preserv. Preservation poor; few forams.; shell frags. Rare forams.; shell frags. common; foram. preserv. very poor Forams rare; grains rounded, abraded; poor preserv.; freq. Fe-stained grains Grey coloured, abraded; Foram. preserv. poor; Fe-stained grains common Abund. abraded shell. frags.; dark grey to grey calcite; few poorly preserved forams with abund. calcite overgrowths Grey abraded calcite; reworked, few forams. Grey to dark-grey calcite, Fe-stained, foram. preserv., fine fraction diverse; poor, rare fine sand Grey, Fe-stai P3-P4 ON ON LB 346 Goodwyn 2 - Descriptions depth in mComments P:B 905.3 Echinoid frags.(Lovenia) abund.; shell frags. common; preservation good; ang. to sub-angular qtz.; Late Miocene, N16 or younger, ?extremu 0.454 932.7 Angular qtz. grains abund. in fine fract.; Elphidids common; Sph.globulus common; common planktonic 0.465 951.0 Poorly preserved tests; abund. angular qtz.; larger forams. common 0.494 969.3 Well-polished and rounded qtz. sand; larger forams. common; well preserv. tests 0.456 996.7 Preservation poor; forams. common; N16 or younger 0.464 1015.0 Well lithified, lg. forams common; free qtz. rare 0.000 1033.3 Well-rounded and polished qtz. grains; larger forams. common; bryozoan frags.; well lithified pale seds 0.000 1051.6 0.000 1088.1 0.000 1106.4 Well-rounded and polished qtz. grains; recryst. common 0.000 1133.9 0.000 1161.3 Abund. large foram. frags.; skeletal frags.; well lithified 0.000 1188.7 Lithified, large foram. dominated; abund. skeletal frags 0.000 1207.0 0.000 1225.3 Large forams. common; Operculina common (large foram) 0.279 1252.7 0.468 1271.0 0.676 1298.4 Preservation poor; shell frags., lge foram frags? common 0.707 1316.7 Foram. frags. common; preserv. poor; recryst. abund. 0.556 1344.2 0.541 1371.6 Poorly preserved; recryst.; larger forams common 0.456 1399.0 Foram. preservation poor; sm. forams dominate; N16 or younger 0.327 1408.2 N8 or younger 0.374 1417.3 Forams. common to abundant; preserv. poor; Fe-staining present 0.523 1435.6 Very poor preservation; forams. comparatively few; angular qtz. common 0.276 1472.2 Preservation mod.; forams. common; Fe-staining common 0.478 1496.6 0.609 1514.9 Cream colored, 'powdery' limestone; foram. tests poorly preserved 0.508 1539.2 0.500 1563.6 Powdery' cream-colored limestone; forams. poorly preserved; recryst. abund 0.790 1591.1 0.500 1618.5 0.812 1645.9 Agglutinated forams. common; preservation fair; larger forams. present; gray calcite-rich sed 0.556 1682.5 N7-N8? (Cassigerinella abund.); tests reasonably well preserved; high plankt. abund.; diverse and extremely rich fine fraction 0.503 1716.0 Poorly preserved, diverse benthic assemb.; shell frags. common; ?high energy redepos.; recryst. apparent 0.517 1743.5 0.356 1770.9 Gray-green, well-lithified; forams. poorly preserved; Fe-staining common 0.766 1798.3 Specimens reasonably well preserved;Anomalinoides/Nonion common; unilocular common 0.475 1816.6 Gray-green, lithified, recryst.; forams. preserv. poor; plankt. common 0.362 1834.9 Preservation 'low'; abundant auth calc.; Fe-stained grains; opaques common 0.388 1871.5 Gray-green, lithified; forams. sparse and poorly preserved in coarse fract 0.555 1898.9 Fine fraction diverse and well-preserved; globigerinids common to abund.; high productivity; unilocular and nodosar. abund 0.561 1926.3 0.627 1953.8 Diverse fine fraction; preservation poor, recryst. common; Fe-staining.S.angipor., ?P18-P21 0.633 1981.2 Preservation poor; diverse small fraction; plankton dominate lge. fraction; skeletal frags. common; Olig. warming diversifaction stage, equiv. Janjukia 0.590 2011.7 Diverse assemblage; preservation mod.-good; Bolivinids diverse 0.474 2048.3 Oligocene assemblage; forams. poorly preserved, Fe-stain., overgrowths 0.699 2057.4 0.500 2090.9 0.811 2106.2 Oligocene; Cassigerinella common; preservation poor 0.438 2124.5 Gray-green, poorly preserved sediment; recryst.; foram. tests poorly preserved; abund. plankton (incl.Cassigerinella ) in fine fraction. 0.809 2145.8 Poorly preserved assemblage; occasional foram.; C. chipolensis present ? contamination. 0.563 2154.9 Glauconitic; abund. recryst.; foram. assembl. poor. 0.899 2164.1 Poor fauna; greenish-grey grains (glaucony); abund. recrys.; preserv. poor 0.860 0.469 2173.2 Greenish, Fe-stained grains, ?glauconitic; forms. poorly preserved, few; occas. opaque min.; ?P12 mid. Eoc age N18-N19 ?N18 N18 N16 or younger N16 ?N16 N11-N8 N8 N5-7? N5 N4 N4-P22 N4 N4-P22 ?P18-P22 ?P18-P22 P18 P12 or younger DEPTH ENVIRON 100-200 ON 100-200 ON 100-200 ON 100-200 ON 200-500 UB 0-20 IN 100-200 ON 0-20 IN 0-20 IN 0-20 IN 0-20 IN 0-20 IN 0-20 IN 0-20 IN 100-200 ON 100-200 ON 1000-2000 LB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 500-1000 MB 200-500 UB 200-500 UB 500-1000 MB 200-500 UB 100-200 ON 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 20-100 MN 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 200-500 UB 347 Goodwyn 4 - descriptions depth in m. 917.4 944.9 966.2 999.7 1018.0 1048.5 1063.8 1082.0 1100.3 1118.6 1136.9 1155.2 1170.4 1198.5 1246.6 1292.4 1301.5 1347.2 1374.6 1392.9 1411.2 1420.4 1438.7 1469.1 1493.5 1511.8 1548.4 1585.0 1603.2 1630.7 1649.0 1676.4 1703.8 1731.3 1749.6 1767.8 1786.1 1804.4 1822.7 1850.1 1877.6 1895.9 1914.1 1932.4 1959.9 1978.2 2005.6 2069.6 2097.0 2124.5 2133.6 description P:B 0.235 Poorly preserved frag. forams; skeletal frags. common; angular qtz grns; larger forams common 0.219 Well-preserved, abund. foram and bbryozoan rich assemblage; occassional qtz. sand grns. polished; lge. forams. abund 0.376 Well-rounded polished qtz; abraded forams.; Elphidids common; echinoid spines common 0.200 Well-rounded, polished qtz. grains; bryozoan frags.; foram. frags., poor preserv 0.173 Foram. preserv. poor, abund. frags., recryst.; macrofossil frags. common, echinoid spines, bryozoans; qtz. sand polished and well-rounded common to a 0.479 0.578 0.341 Well-rounded and polished qtz. sand; preservation poor; abraded forams.; Elphidids common; echinoid spines pres 0.043 Very poor preservation; cibs. in fine fract.; larger forams. and elphidids in coarse fract.; abund. well-rounded and polished qtz. sand 0.218 Poorly preserved assemblage; Sphaeroidina replaced by glauconite; larger forams. present. 0.222 Poorly preserved assemblage; plankton rare; larger forams common; sand abund 0.280 Fe-stained grains; common poor preserv. Amphistegina (larger foram); abund. well-rounded and polished qtz 0.301 Abundant larger forams; preservation mod-poor; abundant micas; green ? glauconitic grains; N14-18 0.087 Larger forams. common; mica common to abund.; fe-stained grains, ?siderite; preservation poor, foram. frags. abund 0.080 Abundant larger forams.; abund. mica 0.049 0.253 Larger forams. common to abund.; mica abund. (drilling);Orbulina common. 0.409 0.302 Poorly preserved, low diversity assemblage; abund. recryst.; Fe-staining common 0.600 Well-rounded and polished qtz. sand grains; forams. poorly preserved, plankton common; Fe-staining and mica presen 0.000 Extremely lithified, ?dolomitized; forams. rare, lge. forams present; occ.Fe-staining 0.579 Poorly preserved assemblage; recryst. abund. 0.160 Fe-stained carbonates; chelated materials? 0.537 Preservation mod-poor; well-polished qtz. sand rare; rare larger forams 0.490 Mod. preserv.; recryst. common; cibs. common. 0.568 Preservation poor; few forams sm. fract.; larger forams common;Globigerinoides (plankton) common. 0.646 0.620 Variably preserved tests; plankton abund.; agglutinated common; ostracods common; Fe-staining and Fe-rich minerals common 0.390 Abraded forams.; abund. plankt.; larger forams present 0.325 Abund. diverse assemblage; recryst. common; ostracods, Fe-rich 0.538 0.440 Forams. poorly preserved; Fe-stained/Fe-rich grains present 0.606 0.487 Diverse benthic assemblage; Polymorphinids common; ?high productivity 0.421 Diverse benthic assemblage; preserv. poor; pyrite, Fe-stained frags present; common unilocular, lenticulina; ?dysoxic, high-productivity assemb 0.524 Rich, abundant benthic assemblage; preservation good; shell frags. common 0.305 Fine fraction diverse benthic assemblage; unilocular abund.; well-preserved, occas. qtz well-rounded and polished grns 0.376 0.584 Shell frags. common; diverse Cibicidid (epifaunal benthic) assembl., test well-preserved; ?well-ventilated, lower-org. carb 0.427 Poorly preserved forams.; abund. recryst.; Fe-stained, hematite? 0.534 Well-preserved assembalge; abundant plankton, Cibicidids; abundantC.chipolensis in fine fraction. 0.584 Forams. abundant and well preserved; Fe-staining common; rich abundant fauna 0.683 Abund. forams.; agglutin. common; plankton abund.; ostracods common 0.516 Abund. and diverse fine fraction; well preserved tests; Bolivinids (infauna) and small planktonics abund 0.646 0.457 Diverse, well-preserved assemblage; common ostracods; freq. Fe-stained grns.; agglutin. common; rich fine fraction, sm. planktonics abund 0.700 Preservation good; abund. plankt.; epifaun. dom. 0.746 Abundant forams.; preservation mod.; ?P17 latest Eocene (micra) 0.726 Abraded, Fe-stained; planktonics common, preservation poor 0.574 age N14 N15-N14 bathy 200-500 100-200 200-500 20-100 100-200 500-1000 200-500 200-500 100-200 100-200 200-500 20-100 100-200 20-100 20-100 100-200 100-200 100-200 100-200 100-200 200-500 200-500 200-500 200-500 500-1000 200-500 200-500 200-500 200-500 200-500 200-500 200-500 500-1000 200-500 500-1000 200-500 200-500 200-500 500-1000 500-1000 200-500 100-200 200-500 200-500 ? 500-1000 500-1000 500-1000 100-200 500-1000 environ UB ON UB MN ON MB UB UB ON ON UB MN ON MN MN ON ON ON ON ON UB UB UB UB MB UB UB UB UB UB UB UB MB UB MB UB UB UB MB MB UB ON UB UB ? MB MB MB ON MB N9 N8 N6-N5 ?N4b ?P22-N5 (see Gq.binaiensis N9 or younger N4 P22-N4 P22 P19-P22 P17 - latest Eocene 348 Goodwyn 6 - descriptions Depth Comment depth bathy 750-755DC abundant rounded fine qtz.; foram. preserv. poor; shell frags. abraded; fecal pellets 730 20-100 760-765DC 755 20-100 780SWC Abundant well rounded fine-grained sand; forams. extremely rare. 765 20-100 800-805DC Glauconitic; angular-subangular qtz. grains; forams. common, well preserved; echinoid spines common 780 ? 810SWC Poor plankton; age Miocene indeterminate 805 100-200 850SWC Abundant fine grained rounded qtz. sands;sponge spicules 810 200-500 880-885DC Frosted and polished qtz. grains; Eponides abund.; abraded shell and bryozoan frags. common; N8-N17/N18 850 ? 920-925DC Larger forams. common; bryzoan frags.; poorly preserved, abraded, abundant recrystal.; well-rounded qtz. grns. polished and frosted 885 200-500 950SWC Common angular terrigenous qtz.; abund. recryst. 925 20-100 970.1SWC Abundant rounded qtz.; 950 100-200 1000-1005DC Poorly-sorted sands; few bryozoan frags.; no forams. 970.1 ? 1020-1025DC 1005 ? 1040-1045DC Well-rounded, frosted and polished poorly-sorted qtz. grains; forams. common in fine fraction, rare in med. fraction. 1025 200-500 1050-1055DC Well-rounded fine frosted qtz. grains; few forams. 1045 200-500 1075-80DC Abundant well-rounded, polished and frosted qtz. grains; abund. calcite cementation 1055 200-500 1085-90DC Exremely well cemented; foraminifera rare; well-rounded qtz sand 1080 100-200 1165SWC Angular authigenic qtz.; barren 1090 ? 1230SWC Forams rare; preservation poor; recrystal. calcite abund.; age inderminate ? mid Miocene 1135 20-100 1260SWC Larger forams. abundant; tests abraded; ostracods present. 1165 ? 1290SWC 1205 20-100 1320SWC 1230 20-100 1350SWC 1260 20-100 1370-75DC 1290 100-200 1470SWC Few forams., poor preservation, abund. recryst., authigenic qtz., ostracods 1320 200-500 1490SWC 1350 200-500 1500-1505DC Poor preservation, abund. recryst., occ. polished qtz., ostracods 1375 20-100 1570SWC Common forams., preservation mod., larger forams. common. 1410 200-500 1600SWC 1435 20-100 1675SWC Preservation mod.; abundant cibs.; N5-N8 (early-Mid. Miocene) 1470 200-500 1765SWC Poor preservation, abund. Brizalina, Bolivina, abund. cibs., poor plankton ass., abund. recryst., occ. polished qtz., ostracods 1490 0-20 1875-80DC Poor preservation, abund. recryst., lithics pres., occ. polished qtz., plankt. ass. diverse ostracods 1505 0-20 1915SWC Few poorly preserved forams.; abund. recrystal.; depauperate assemblage 1530 100-200 1975-80DC Poor preservation, abund. recryst., occ. polished qtz., ostracods 1570 20-100 2015SWC 1600 20-100 2075-90 Poor preservation, abund. recryst., occ. polished qtz., diverse plankt. ass., ostracods 1630 200-500 1675 500-1000 1715 500-1000 1755 200-500 1765 500-1000 1805 200-500 1835 500-1000 1880 500-1000 1915 0-20 1940 500-1000 1980 500-1000 2015 100-200 2050 20-100 2090 100-200 2237.6 ? 2252 ? 2310.1 ? environ MN MN MN ? ON UB ? UB MN ON ? ? UB UB UB ON ? MN ? MN MN MN ON UB UB MN UB MN UB IN IN ON MN MN UB MB MB UB MB UB MB MB IN MB MB ON MN ON ? ? ? .349 Goodwyn 7 - descriptions depth in m 780 820 860 900 940 990 1025 1085 1130 1155 1205 1245 1295 1330 1380 1410 1450 1485 1510 1550 1580 1645 1685 1705 1765 1800 1845 1860 1890 1915 1965 1980 2000 2015 2040 2080 2125 comments Rounded and sub-angular frosted qtz., abund. sponge spicules?; Operculina dominant. Qtz. angular to sub-angular, low diversity assemblage dominated byOperculina Abundant well-rounded and polished/frosted quartz sand; larger forams well preserved Larger forams abundant; well-rounded and polished qtz. sands abund Larger forams. common; poor preservation Qtz. common Well lithified, poor assemblage. Qtz. grains well-rounded and polished; foram. tests abraded and fragmented Assemblage dominated by Operculina , Amphistegina and Elphidids; poorly preserved. Very poorly preserved assemblage; larger forams. common to abund.; Lenticulinids, Notorotalids common Diverse larger foram. assemblage; preservation reasonable; Amphistegina abund. Poorly preserved diverse assemblage; plankton abund.; Lenticulinids common Very poor preservation; few forams.; recryst. material common Lithified; Lenticulina common. Impoverished assemblage; tests fragmented and recryst.; few plankton Poorly preserved Cibicides and Agglutinated dominated assemblage; shell frags. common; reworked Benthos diverse; preserv. poor; shell frags. and Fe-mins. common Poor preservation; recryst.; pyrite present Poorly preserved assembl.; recryst. evident Diverse assembl.; well-preserved; Praeorb. sicana present. Diverse and abund. assembl. in fine fraction; good preserv Gray-green, lithified; forams. well-preserved Gray-green well-lithified; plankt. well preserved Gray sed. with occ. preserved forams.; evident recryst. Gray-green, lithified; well-preserved diverse ass. common to abund.; Miliolids present Pyrite present; foram. preserv. poor; latest Eocene, ?earliest Oligocene (C.chipolensis ) Gray-green, well-lithified sed.; few forams. poorly preserved p:b age N17 or younger bathy 20-100 20-100 20-100 20-100 20-100 100-200 200-500 100-200 20-100 20-100 20-100 20-100 500-1000 500-1000 200-500 200-500 200-500 200-500 100-200 200-500 200-500 100-200 200-500 200-500 200-500 ? 200-500 200-500 200-500 200-500 200-500 200-500 200-500 200-500 500-1000 500-1000 environ MN MN MN MN MN ON UB ON MN MN MN MN MB MB UB UB UB UB ON UB UB ON UB UB UB ? UB UB UB UB UB UB UB UB MB MB 0.031 0.000 0.000 0.000 0.000 0.048 0.250 0.000 0.000 0.000 0.000 0.078 0.226 0.496 0.282 0.469 0.458 0.294 0.389 0.213 0.356 0.037 0.500 0.460 0.413 #DIV/0! 0.537 0.556 0.360 0.681 0.572 0.591 0.581 0.494 0.594 0.898 undifferentiated mid Miocene N9-N10 N9 N8-9? N9-N7 N9-N7 N8 N7 N7-N6 N5 N5 N5-N4 N4-P22 P17-P18? P12 350 Appendix D: Ecostratigraphic groups used in profiles Epifaunal Taxa (L= larger Foraminifera) Alabamina tenuimarginata Ammobaculites agglutinans Amphistegina hauerina (L) Anomalinoides colligerus Anomalinoides macraglabra Anomalinoides nonionoides Anomalinoides planulata Austrotrillina striata (L) Baggina philippinensis Cancris intermedius Cellanthus craticulatus Ceratobulimina jonesiana Cibicides boueana Cibicides cf. umbonatus Cibicides cygnorum Cibicides lobatulus Cibicides mediocris Cibicides pseudoconvexus Cibicides refulgens Cibicides robustus Cibicides victoriensis Cibicides vortex Crespinella umbonifera Cribroelphidium poeyanum Cyclammina cf. incisa Cycloclypeus eidae Discorbinella bertheloti Discorbinella complanata Discorbis cf. balcombensis Discorbis cyclopleus Discorotalia tenuissima Dorothia goesi Dorothia minima Elphidium advenum Elphidium hughesi foraminosum Elphidium pseudoinflatum Eponides lornensis Eponides repandus Gaudryina convexa Glabratellina sigali Gypsina globulus (L) Gyroidina allani Gyroidina prominula Gyroidinoides danvillensis Haplophragmoides sp.2 Heronallenia parri Heterolepa brevoralis Heterolepa novozelandica Heterolepa subhaidingeri Heterostegina spp.(L) Hoeglundina elegans Lenticulina calcar Operculina sp. Lenticulina foliata Lenticulina formosa Lenticulina gibba Lenticulina gyroscalpra Lenticulina orbicularis Lenticulina cf. papillosa Lenticulina sp.1 Lenticulina thalmanni Lenticulina vortex Lepidocyclina badjirraensis (L) Lepidocyclina howchini (L) Miogypsina globulina (L) Neoconorbina terquemi Neoeponides berthelotianus Neoglabratella wiesneri Nonion pauperatum Nonion scaphum Nonionella turgida Notorotalia clathrata Notorotalia spinosa Operculina cf. victoriensis Operculina complanata Oridorsalis tener Oridorsalis umbonatus Osangulariella umbonifera Planorbulinella plana Pyrgo lucernula Pyrgo murrhina Quinqueloculina akneriana Quinqueloculina cf. seminula Quinqueloculina venusta Rosalina bradyi Rosalina vilardeboana Siphonina australis Siphotextularia awamoana Siphotextularia saulcyana Svratkina australiensis Tretomphalus concinnus Usbekistania charoides Valvulineria sp. Ammonia beccarri 351 Amphistegina lessonii (L) Anomalina cf. orbiculus Anomalinoides fasciatus Anomalinoides macralabra Buchnerina sp.1 Cancris auricula Cibicides neoperforatus Cibicides parki Cibicides psuedoconvexus Clavulina sp. Crespinella sp. Cribrononion sydneyensis Cycloclypeus spp.(L) Delosina complexa Discogypsina howchini Discorbis sp.1 Dyocibicides uniserialis Elphidium chapmani Elphidium crispum Elphidium discoidalis multiloculum Epistominella cassidulinoides Eponides cribrorepandus Eponides sp.2 Glabratellina sp. Gyroidina danvillensis Heronallenia lingulata Nonion cassidulinoides Nuttalides sp. Pararotalia verriculata Parrellina hispidula Parrellina imperatrix Patellina corrugata Planorbulinella sp. Planulina wuellerstorfi Plectofrondicularia advena Pyrgo vespertilio Pyrulina sp.1 Rosalina vitrevoluta Semivulvulina capitata Sphaerogypsina cf. globulus Spiroloculina canaliculata Stomatorbina concentrica Svratkina shauni Trochammina sp. Anomalinella rostrata Asterocyclina sp.(L) Cancris oblonga Cibicides cf. karreriformis Cibicides ihungia Cribrorotalia sp. Discanomalina vermiculata Discorbinella araucana Discorbinella scopos Dyocibicides biserialis Dyocibicides cf. uniserialis Epistominella decorata Epistominella iota Epistominella sp. Eponides sp. Frondicularia villosa Gypsina howchini Haplophragmoides sp. Heterolepa opaca Lenticulina altifrons Lenticulina cf. loculosa Lenticulina pusilla Lepidocyclina sp. Maslinella sp. Miogypsina sp. Operculina cf. granulosa Osangularia bengalensis Pararotalia sp. Planulina foveolata Pyrgo sp. Quinqueloculina sp.1 Quinqueloculina tropicalis 352 Infaunal Taxa Angulogerina esuriens Astacolus judyae Astrononion stelligerum Bigenerina nodosaria Bolivina acerosa Bolivina bassensis Bolivina cf. victoriana Bolivina limbata Bolivina decussata Bolivina moodysensis Bolivina pseudoplicata Bolivina pseudospissa Bolivina reticulata Bolivina retiformis Bolivinopsis sp. Brizalina albatrossi Brizalina lapsus Brizalina spathulata Bulimina marginata Bulimina pupoides Bulimina sp.1 Bulimina spicata Bulimina truncana Cassidulina crassa Cassidulina laevigata Cassidulina margareta Chilostomella oolina Dentalina cf. subsoluta Dentalina sp. Eggerella bradyi Eggerella cf. decepta Eggerella ihungia Fissurina annectens Fissurina aperta Fissurina globosa Fissurina kerguelenensis Fissurina marginata Fissurina quadrata Fissurina staphyllearia Fursenkoina complanata Gaudryina convexa Glandulina laevigata Globocassidulina subglobosa Globulina inaequalis Guttulina yabei Guttulina problema Guttulina regina Hoeglundina elegans Homalohedra carteri Karreria moaria Lagena semilineata var spinigera Lagena striata Lagena sulcata Liebusella soldanii Marginulina sp Marginulinopsis sp. Melonis centroplax Melonis obesa Nodosaria lamnulifera Nodosaria simplex Nonion cf. fabum Nonion scaphum Nonionella turgida Oolina apiculata Oolina globosa Palliolatella bradyiformis Palliolatella semialata Parabrizalina porrecta Parrellina sp. Planularia cf. gemmata Pleurostomella tenuis Pseudononion victoriense Pullenia bulloides Pullenia quinqueloba Pullenia subcarinata quinqueloba Pygmaeoseistron hispidula Pygmaeoseistron nebulosa Pyrulina angusta Pyrulina fusiformis Pyrulina sp.A Rectoglandulina comatula Reussella spinulosa Saracenaria italica Sigmoidella elegantissima Sigmoidella pacifica Sigmomorphina subregularis Siphotextularia awamoana Siphotextularia saulcyana Sphaeroidina bulloides Stilostomella cf. fistuca Textularia pseudogramen Textularia semicarinata Trifarina bradyi Trifarina parva Tritaxia victoriensis Uvigerina miozea Uvigerina proboscidea Vaginulinopsis pacifica 353 Vaginulinopsis sp. Angulogerina anguloca Bolivina subaenariensis Bulimina cf. inflata Buliminella elegantissima Cassidulina carapitana Dentalina albatrossi Dentalina cf. communis Dentalina cf. subemaciata Dentalina sp. 1 Fissurina crassianulata Fissurina submarginata Globocassidulina cf. williami Guttulina cf. spicaeformis Karreriella bradyi Lagena cf. blomaeformis Lagena hexagona Lagena substriata Melonis obesus Nodogenerina pomuligera Oolina emaciata Oolina hexagona Oolina seminuda var. Planularia planulata Pyrulina gutta Sagrinella rugosa Siphonina tubulosa Uvigerina bortotara costata Uvigerina semiteres Bolivina sp. Brizalina limbata Bulimina aculeata Bulimina alazanensis Bulimina exilis Bulimina lanceolata Bulimina pyrula var spinescens Bulimina striata mexicana Cassidulina minuta Cassidulina narcrossi australis Cassidulina neocarinata Dendrophyra sp. Guttulina communis Pseudopolymorphina sp. Saracenaria altifrons Sigmoilopsis sp.1 Textularia gramen Triloculina oblonga Tubulogenerina mooraboolensis Turrilina cf. browni Valvulinaria sp. 354 Appendix E: Stratigraphic datums used in quantitative stratigraphy (index numbers and datum name) 4 Globigerinoides extremus 7 Dentoglobigerina altispira 17 N18 18 Catapsydrax parvulus 19 Dentoglobigerina larmeui 20 N17 21 Globoquadrina dehiscens 24 N16 25 N15 28 N14 35 Tenuitella clemenciae 41 Sphenolithus heteromorphus (nanofossil) 43 Fohsella peripheroronda 44 Praeorbulina sicana 45 Praeorbulina glomerosa 46 N9 47 N8 48 Globorotaloides suteri 49 Globorotalia miozea 55 Catapsydrax dissimilis 57 Catapsydrax unicavus 58 N5 61 N4 65 Globigerina euapertura 66 P22 71 Globigerina labiacrassata 78 Subbotina angiporoides 79Globigerina ampliapertura 83 P18 84 Ericsonia formosa (nanofossil) 86 Turborotalia cerroazulensis 87 P17 91 Globigerapsis index 92 Globigerapsis index 100 Subbotina linaperta 103 Acarinina primitiva 108 Acarinina bullbrooki 110 Subbotina linaperta 112 Subbotina velascoensis 117 P4 119 Acarinina mckannai 355 Composite Standard used for calibrations (Cenozoic.age), after Berggren et al. (1995). LAD AGE (Ma) DATUM 0.22 Globoquadrina pseudofoliata 0.65 Globorotalia tosaensis 1.77 Globigerinoides fistulosus 1.80 Globigerinoides extremus 2.30 Globorotalia pseudomiocenica 3.09 Globorotalia multicamerata 3.09 Dentoglobigerina altispira 3.12 Sphaeroidinellopsis spp. 3.20 Globorotalia conomiozea 3.58 Globorotalia margaritae 3.80 Pulleniatina primalis 4.18 Globoturborotalita nepenthes 4.18 Pulleniatina spectablis 4.18 N19 4.50 Globorotalia crassaformis 4.60 Globorotalia cibaoensis 4.60 N18 5.60 Catapsydrax parvulus 5.60 Dentoglobigerina larmeui 5.60 N17 5.80 Globoquadrina dehiscens 6.00 Globorotalia lenguaensis 7.40 sinistral Globorotalia menardii 8.30 N16 10.90 N15 10.10 Neogloboquadrina nympha 11.40 Neogloboquadrina mayeri 11.40 N14 11.80 Globorotalia panda 11.80 N13 11.90 Globorotalia praescitula 11.90 Globorotalia fohsi robusta 12.10 Globorotalia fohsi lobata 12.10 N12 12.30 Tenuitella clemenciae 12.30 Tenuitella minutissima 12.30 Tenuitella pseudoedita 12.30 Tenuitella selleyi 12.50 N11 12.70 N10 13.60 Sphenolithus heteromorphus 14.60 Globorotalia peripheroacuta 14.60 Fohsella peripheroronda 14.80 Praeorbulina sicana 14.80 Praeorbulina glomerosa 14.80 N9 15.10 N8 15.10 Globorotaloides suteri 15.90 Globorotalia miozea 15.90 Globoconella miozea 16.40 Globorotalia incognita 16.40 N7 17.30 Globorotalia zealandica 17.30 Paragloborotalia semivera 17.30 Catapsydrax dissimilis 17.30 N6 17.30 Catapsydrax unicavus 18.80 N5 21.40 Tenuitella munda 21.50 Globorotalia kugleri 21.50 N4 21.60 Globoturborotalita angulisuturalis 21.60 Globorotalia pseudokugleri 22.80 Globoquadrina globularis 23.80 Globigerina euapertura 23.80 P22 23.90 Reticulofenestra bisecta 24.30 Tenuitella gemma 26.10 Chiasmolithus altus 27.10 Paragloborotalia opima 27.10 Globigerina labiacrassata 27.10 P21b 27.50 Sphenolithus distentus 27.50 Sphenolithus predistentus 28.50 Chiloguembelina cubensis 28.50 P21a 29.40 P20 30.00 Subbotina angiporoides 30.30 Globigerina ampliapertura 30.30 P19 31.80 Isthmolithus recurvus 32.00 Pseudohastigerina spp. 32.00 P18 32.80 Ericsonia formosa 33.70 Hantkenina spp. 33.80 Turborotalia cerroazulensis 33.80 P17 34.00 Cribrohantkenina inflata 34.00 P16 34.20 Discoaster saipanesis 34.30 Globigerapsis index 356 34.30 Globigerinatheka index 34.30 Discoaster barbadiensis 35.00 Reticulofenestra reticulata 35.30 Turborotalia pomeroli 35.30 Porticulasphaera semiinvoluta 35.50 P15 37.10 Chiasmolithus grandis 37.70 Acarinina collactea 37.70 Subbotina linaperta 38.10 Morozovella spinulosa 38.50 Planorotalites 39.00 Acarinina primitiva 37.50 Acarinina spp. 39.30 Subbotina frontosa 40.10 Globigerapsis beckmanni 40.40 Chiasmolithus solitus 40.50 Acarinina bullbrooki 43.60 Morozovella aragonensis 52.50 Morozovella marginodentata 52.70 Morozovella lensiformis 53.50 Subbotina velascoensis 53.60 Morozovella aequa 54.70 Morozovella acuta 54.70 Morozovella velascoensis 55.90 Globanomalina pseudomenardii 55.90 P4 56.30 Acarinina acarinata 56.30 Acarinina mckannai 57.10 Acarinina subsphaerica 59.20 Parasubbotina varianta 59.20 Parasubbotina variolaria 59.20 P3 357 Glossary Term Agglutinated Assemblage Benthic Biogenic Cosine-theta (Cos q) Definition Foraminifera that build test (shell) wall by cementing extraneous particles, such as quartz grains, shell fragments, opaque minerals, other foram. Test fragments, sponge spicules together. Populations of all species in a sample. Living on or in the sea bed. Produced by the physiological activity of organisms. Dissimilarity coefficient treats two samples as vectors in n-variable space and is the cosine of the angle between vectors. Sample vectors that have no components (taxa) in common (i.e., orthogonal) will have a Cos q of 0 (cos 900). Values scaled from 0-100. Sensitive to differences in proportions of taxa. Case-hardened soil formed in a semiarid environment. May be siliceous, calcareous, ferrigenous or aluminium rich. Number of different taxa in sample. Living on the sediment surface or other substrates. Living on plant substrates. Rich in nutrients/food. Generally less than 25m water depth. Multi-chambered foram with symbiotic algae or dinoflagellates. Taxa that form an evolutionary series. Depth at which rate of carbonate dissolution increases sharply. Low in nutrient/food. Duricrust Diversity Epifaunal Epiphytic Eutrophic Inner-shelf Larger foraminifera Lineage Lysocline Oligotrophic preparing for submission to Palios, lead author G.Moss with co-authors D.L. Cathro and J.A. Austin, Jr. 358 Otsuka Planktonic Planispiral Quinqueloculine Succession Taphonomy Taxon (plural = Taxa) Test Triserial Trochospiral Unilocular Uniserial Dissimilarity coefficient. Number of taxa in common divided by square root of number of taxa in sample 1 multiplied by number in sample 2. Equivalent to Simpson similarity. Stresses poor sample size as gives weighting to all taxa regardless of number of specimens. Living in the water column. Flattened, coiling and multi-chambered. Chambers arranged in one-half coil in length, e.g., miliolid genus Quinqueloculina. Series of faunal or floral assemblages. Branch of paleoecology concerned with manner of burial and the origin of plant and animal remains. Named group of organisms of any rank. Shell of a foraminifera made of calcareous, siliceous, agglutinated or chitinous material. Chambers added in three vertical columns. Spiral, cone-shaped and mulit-chambered. Single-chambered Test formed of chambers added in a single row. 359 Appendix 2 Samples collected for micropaleontological analysis Key: Dc: Swc: AGSO: DME: WOOD: ditch cuttings side-wall core Geoscience Australia Western Australia Department of Minerals and Energy Woodside Australian Energy 360 Goodwyn 2 Sample Location Type (ft) Source Sample Location (ft) Type Source Sample Location (ft) Type Source Sample Location (ft) Type Source 2000-10 2030-40 2060-70 2090-2100 2120-30 2150-60 2180-90 2210-20 2240-50 2270-80 2300-10 2330-40 2360-70 2390-400 2420-30 2450-60 2480-90 2510-20 2540-50 2570-80 2600-10 AGSO AGSO AGSO AGSO AGSO 2780-90 2810-20 2840-50 2870-80 2900-10 2930-40 2960-70 2990-3000 3020-30 3050-60 3080-90 3110-20 3140-50 3170-80 3200-10 3230-40 3260-70 3290-3300 3320-30 3350-60 3380-90 3410-20 3440-50 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO 3470-80 3500-10 3530-40 3560-70 3590-600 3620-30 3650-60 3680-90 3710-20 3740-50 3770-80 3800-10 3830-40 3860-70 3890-900 3920-30 3950-60 3980-90 4010-20 4040-50 4070-80 4100-10 4130-40 4160-70 4190-200 4220-30 4250-60 4280-90 4310-20 4340-50 4370-80 4400-10 4430-40 4460-70 4490-500 4520-30 4550-60 4580-90 4610-20 4640-50 4670-80 4700-10 4730-40 4760-70 4790-800 4820-30 4850-60 4870-80 4900-10 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO 4930-40 4960-70 4990-5000 5020-30 5040-50 5090-5100 5120-30 5150-60 5180-90 5210-20 5240-50 5270-80 5300-10 5330-40 5360-70 5390-400 5420-30 5450-60 5480-90 5510-20 5540-50 5570-80 5590-600 5620-30 5650-60 5680-90 5710-20 5740-50 5770-80 5800-10 5830-40 5860-70 5890-5900 5920-30 5950-60 5980-90 6010-20 6040-50 6070-80 6100-10 6130-40 6160-70 6190-200 6220-30 6250-60 6280-90 6310-20 6340-50 6370-80 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO 6400-10 6430-40 6460-70 6490-500 6520-30 6560-70 6590-600 6620-30 6650-60 6680-90 6710-20 6740-50 6780-90 6810-20 6850-60 6870-80 6900-10 6930-40 6960-70 7000-10 7030-40 7060-70 7090-100 7120-30 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO 361 Goodwyn 4 Sample Location (ft) Type Source Sample Location (ft) Type Source Sample Location (ft) Type Source Sample Location (ft) Type Source 1580 1600 1630 1660 1690 1720 1750 1780 1810 1840 1870 1900 1930 1960 1990 2020 2050 2080 2110 2140 2180 2200 2230 2260 2290 2320 2350 2380 2410 2440 2470 2500 2530 2560 2590 2620 2650 2680 2710 2740 2770 2800 2830 2860 2890 2920 2950 2980 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME 3010 3030+40 3070+80 3090+3100 3110+20+30 3160+70 3180+90 3220+30 3250+60 3270+80 3310+20 3330+40 3350+60+70 3380+90+4000 3430+40 3450+60 3480+90 3520 3550 3580 3610 3640 3670 3700 3730 3760 3790 3810 3840 3870 3910 3940 3970 4000 4030 4060 4090 4120 4150 4180 4210 4240 4270 4300 4330 4360 4390 4420 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME 4460 4480 4510 4540 4570 4600 4630 4660 4690 4720 4780 4810 4840 4900 4930 4990 5110 5140 5170 5200 5230 5260 5290 5320 5350 5380 5410 5440 5470 5500 5530 5560 5590 5620 5650 5680 5710 5740 5800 5830 5860 5890 5920 5950 5980 6010 6040 6070 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME 6100 6130 6160 6190 6220 6250 6280 6310 6340 6370 6400 6430 6460 6490 6520 6550 6580 6610 6640 6670 6700 6730 6760 6790 6820 6850 6880 6900 6940 6970 7000 7030 7060 7090 7120 7150 7180 7210 7240 7270 7300 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME 362 Goodwyn 6 Sample Location (m) Type Source Sample Location (m) Type Source Sample Locations (m) Type Source Sample Locations (m) Type 495-500 510 525 540 555 570 585 600 615 630 644.9 660 675 690 700 715 730 740-745 750-755 755 760-765 770-775 780 790-795 800-805 810 820-825 830-835 840-845 850 860-865 870-875 880-885 890-895 900-905 910-915 920-925 925 935-940 945-950 950 960-965 970.1 980-985 990-995 1000-1005 1010-1015 1020-1025 1025-1030 1030-1035 Dc Swc Swc Swc Swc Swc Swc Swc Swc Swc Swc Swc Swc Swc Swc Swc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Dc Swc Dc Dc Dc Dc Dc Dc Dc Swc Dc Dc Swc Dc Swc Dc Dc Dc Dc Dc Dc Dc AGSO DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO AGSO DME AGSO AGSO AGSO AGSO AGSO AGSO AGSO DME AGSO AGSO DME AGSO DME AGSO AGSO AGSO AGSO AGSO AGSO AGSO 1040-1045 1050-1055 1060-1065 1070-1075 1075-1080 1085-1090 1095-1100 1105-1110 1115-1120 1125-1130 1135 1145-1150 1155-1160 1165 1175-1180 1185-1190 1195-1200 1200-1205 1210-1215 1220-1225 1230 1240-1245 1250-1255 1260 1270-75* 1280-85* 1290 1300-1305 1310-1315 1320 1330-1335 1340-1345 1350 1360-1365 1370-1375 1380 1390-1395 1400-1405 1410 1420-1425 1430-1435 1440-1445 1450-1455 1460-1465 1470 1470-1475 1480-1485 1490 1500-1505 1505.1 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Swc Dc Dc Swc Dc Dc Dc Dc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Dc Dc Dc Swc Dc Dc Swc Dc Swc AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO DME AGSO AGSO DME AGSO AGSO AGSO AGSO AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO AGSO AGSO AGSO DME AGSO AGSO DME AGSO DME 1515-1520 1525-1530 1535-1540 1540 1550-1555 1560-1565 1570 1580-1585 1590-1595 1600 1610-15 1620-25 1630 1640-45 1650-55 1655 1660-65 1670-75 1675 1685-1690 1695 1705-10 1715 1725-30 1730 1735-40 1745 1745-50 1755 1765-70 1765 1775-80 1785.1* 1785-90 1795 1795-1800 1805 1810-15 1820-25 1830-35 1837 1840-50 1850-55 1855-60 1860-65 1875-80 1876* 1885-90 1895 1905-10 Dc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Swc Dc Swc Dc Swc Dc Swc Dc Swc Dc Swc Dc* Swc Dc Swc* Dc Swc Dc Dc Dc Swc* Dc Dc Dc Dc Dc Swc Dc Swc Dc AGSO AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO DME AGSO DME AGSO DME AGSO DME AGSO DME AGSO DME AGSO DME AGSO DME AGSO DME AGSO AGSO AGSO DME AGSO AGSO AGSO AGSO AGSO DME AGSO DME AGSO 1915 1915-20 1925-30 1935-40 1940-45 1955-60 1965-70 1975-80 1985-90 1995-2000 1995 2005-2010 2015 2030-45 2035-40 2045-60 2045-50 2055-60 2065-70 2075-80 2075-90 Swc DME Dc AGSO Dc AGSO Dc AGSO Dc AGSO Dc AGSO Dc AGSO Dc AGSO Dc AGSO Dc AGSO Swc* DME Dc AGSO Swc DME Dc WOOD Dc DME Dc WOOD Dc DME Dc DME Dc DME Dc DME Dc WOOD 363 Goodwyn 7 Sample Location (m) Type Source Sample Location (m) Type Source Sample Location (m) Type Source 600 640 651 665 700 715-725 730 770 780 790 800 810 820 830 840 850 860 870 880 890 900 920 930 940 950 960 970 980 990 1000 1015 1025 1040 1060 1070 1085 1100 Dc Dc Swc Dc Dc Dc Swc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Swc Dc Dc Dc Dc Swc Dc Dc Swc Dc Dc Dc Swc AGSO AGSO DME AGSO AGSO AGSO DME AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO DME AGSO AGSO AGSO AGSO DME AGSO AGSO DME AGSO AGSO AGSO DME 1110 1120 1130 1140 1155 1165 1175 1185 1195 1205 1220 1235 1245 1260 1273 1290-1295 1300 1310 1330 1340 1370 1380 1390 1400 1410 1425 1434 1445-1450 1470 1485 1500 1510 1530 1540-1550 1560 1575-1580 Dc Dc Dc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Dc Swc Dc Swc Dc Dc Swc Swc Dc Dc Swc Dc Dc Swc Dc Swc Dc Swc Dc Dc Dc Dc Dc AGSO AGSO AGSO AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO AGSO DME AGSO DME AGSO AGSO DME DME AGSO AGSO DME AGSO AGSO DME AGSO DME AGSO DME AGSO AGSO AGSO AGSO AGSO 1600 1628 1645 1660 1675-1685 1695 1705 1740 1745-1765 1770 1780 1790 1800 1835-1845 1854 1860 1890 1900 1915 1953 1965 1980 2000 2015 2040 2050 2065 2075-2080 2107.5 2125 2140 2150 2160 2175 2185 2210 Swc Swc Dc Swc Dc Dc Dc Swc Dc Swc Dc Dc Dc Dc Swc Dc Dc Dc Dc Swc Dc Dc Dc Dc Dc Dc Dc Dc Swc Dc Dc Dc Dc Dc Dc Dc DME DME AGSO DME AGSO AGSO AGSO DME AGSO DME AGSO AGSO AGSO AGSO DME AGSO AGSO AGSO AGSO DME AGSO AGSO AGSO AGSO AGSO AGSO AGSO AGSO DME AGSO AGSO AGSO AGSO AGSO AGSO AGSO 364 Eastbrook 1 Sample Location (m) Type Source Sample Location (m) Type Source Sample Location (m) Type Source 1690 1700 1710 1720 1730 1740 1750 1760 1770 1780 1790 1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100 2110 2120 2130 2140 2150 2160 2170 2180 2190 2200 2210 2220 2230 2240 2250 2260 2270 2280 2290 2300 2310 2320 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD 2330 2340 2350 2360 2380 2390 2400 2410 2420 2430 2440 2450 2460 2470 2500 2520 2530 2540 2550 2560 2580 2590 2600 2610 2620 2630 2640 2650 2660 2670 2680 2690 Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc Dc WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD WOOD 365 Appendix 3 Checkshot surveys from well-completion reports 1111 1141 1041* KEY used acquired not used checkshots reference checkshots 366 Dampier 1 (RT = 9.1) Depth 1WTT Average (m below SL) (s) velocity (m/s) 67* 0.044 1524 600 0.2271 2644 905 0.3223 2808 1210 0.4214 2871 1484 0.5233 2836 1819 0.6406 2840 2094 0.7369 2842 2338 0.8197 2852 2643 0.9212 2869 2978 1.0107 2947 Goodwyn 2 (RT=12.5 m) Depth 1WTT Average (m below SL) (s) velocity (m/s) 133.2* 0.087 1524 780 0.3654 2134 1054.3 0.4598 2292 1276.2 0.5238 2435 1557.2 0.6099 2554 1801.06 0.6829 2636 2185.11 0.7989 2734 2547.8 0.9187 2773 Goodwyn 3 (RT = 30.2) Depth 1WTT Average (m below SL) (s) velocity (m/s) 121* 0.079 1524 815* 0.3601 2263 1041* 0.4522 2302 1261* 0.5143 2452 1457* 0.5723 2546 1651* 0.6298 2621 1891* 0.7005 2700 2068* 0.7468 2769 2284* 0.8157 2800 2428* 0.8631 2813 Goodwyn 4 (RT = 30.2 m) Depth 1WTT Average (m below SL) (s) velocity (m/s) 130.6* 0.085 1524 518 0.262 1978 792.5 0.3607 2112 1006.8 0.442 2280 1219.8* 0.5015 2432 1250 0.5141 2432 1499.9 0.5902 2542 1828.8 0.6753 2664 2111.3 0.7712 2727 2286.3 0.8262 2768 2511.8 0.9032 2780 367 Goodwyn 6 (RT = 8.0) Depth 1WTT Average (m below SL) (s) velocity (m/s) 124* 0.081 1524 781 0.3504 2229 811 0.3584 2263 815* 0.3601 2263 1021 0.4435 2302 1041* 0.4522 2302 1081 0.463 2335 1111 0.4696 2366 1141 0.4786 2384 1171 0.4856 2411 1201 0.4965 2419 1231 0.5036 2444 1261 0.5143 2452 1291 0.5234 2467 1321 0.5325 2481 1351 0.5421 2492 1381 0.551 2506 1411 0.5595 2522 1441 0.569 2532 1457* 0.5723 2546 1471 0.5777 2546 1502 0.5864 2561 1531 0.5945 2575 1561 0.603 2589 1591 0.6119 2600 1621 0.6206 2612 1651 0.6298 2621 1681 0.6388 2632 1711 0.6477 2642 1741 0.656 2654 1771 0.6651 2663 1801 0.6738 2673 1831 0.6829 2681 1861 0.692 2689 1891 0.7005 2700 1921 0.7088 2710 1951 0.717 2721 1981 0.725 2732 2011 0.7329 2744 2041 0.7405 2756 2068 0.7468 2769 Depth (m below SL) 2071 2101 2131 2161 2191 2221 2251 2281 2284* 2311 2341 2371 2401 2428* 2431 Goodwyn 6 (cont.) 1WTT Average (s) velocity (m/s) 0.7478 2769 0.7559 2779 0.7646 2787 0.7743 2791 0.7844 2793 0.7942 2796 0.8049 2797 0.8145 2800 0.8157 2800 0.8247 2802 0.8345 2805 0.8446 2807 0.854 2811 0.8631 2813 0.8642 2813 368 Goodwyn 7 (RT = 17.0) Depth 1WTT Average (m below SL) (s) velocity (m/s) 134* 0.0879 1524 693 0.3306 2096 723 0.3417 2116 813 0.3778 2152 843 0.3879 2173 873 0.3989 2188 903 0.4082 2212 933 0.4182 2231 963 0.4276 2252 993 0.4367 2274 1023 0.4455 2296 1053 0.4538 2320 1083 0.4623 2343 1113 0.4705 2366 1143 0.4791 2386 1173 0.4879 2404 1203 0.4969 2421 1233 0.5055 2439 1263 0.5146 2454 1293 0.5233 2471 1323 0.5327 2483 1353 0.5422 2495 1383 0.5515 2508 1413 0.5614 2517 1443 0.5704 2530 1473 0.5795 2542 1503 0.5883 2555 1533 0.5967 2569 1563 0.6056 2581 1593 0.6146 2592 1623 0.624 2601 Goodwyn 7 (RT = 17.0) continued... 1653 0.6334 2610 1683 0.6408 2627 1713 0.6498 2636 1743 0.6589 2689 1773 0.6671 2658 1983 0.7334 2704 2015 0.7439 2709 2043 0.7497 2725 2073 0.7577 2736 2103 0.7661 2745 2133 0.7752 2752 2163 0.7846 2757 2193 0.7954 2757 2223 0.8052 2761 2253 0.8156 2762 2283 0.8263 2763 2313 0.8364 2765 2343 0.8466 2768 2373 0.8567 2770 2403 0.8669 2772 2433 0.877 2774 2463 0.8872 2776 2493 0.8971 2779 2523 0.9069 2782 2553 0.9174 2783 2583 0.9273 2785 2613 0.9372 2788 2643 0.9455 2795 2673 0.953 2805 2703 0.9611 2812 2733 0.97 2817 2763 0.9787 2823 2793 0.9885 2825 2823 0.997 2831 2853 1.0051 2838 369 Eastbrook 1 (RT = 25.9) Depth 1WTT Average (m below SL) (s) velocity (m/s) 168.1* 0.1103 1524 174.1 0.1105 1576 254.1* 0.1555 1634 274.1 0.1677 1634 334.1* 0.1925 1736 374.1 0.2155 1736 474.1 0.2594 1827 574.1 0.3003 1912 604.1* 0.316 1912 674.1 0.3387 1990 759.1 0.369 2057 774.1 0.3746 2067 874.1 0.4106 2129 974.1 0.4462 2183 1064* 0.4735 2247 1074.1 0.4781 2247 1174.1 0.5085 2309 1274.1 0.539 2364 1374.1 0.5695 2413 1474.1 0.5993 2460 1559.1 0.6235 2500 1574.1 0.6274 2509 1674.1 0.6567 2549 1684* 0.6517 2584 1774.1 0.6866 2584 1874.1 0.7163 2616 1974.1 0.745 2650 2074.1 0.7725 2685 2174.1 0.7986 2722 2274.1 0.8233 2762 2292* 0.8298 2762 2374.1 0.8511 2790 2459.1 0.8772 2803 2474.1 0.8817 2806 2569* 0.912 2817 2574.1 0.9137 2817 2674.1 0.9472 2823 2759.1 0.9743 2832 2774.1 0.9792 2833 2859.1 1.009 2834 370 Appendix 4 Preliminary Research Report Into Neogene Sediments of the North West Shelf and Associated Ostracod Faunas AGSO Internal Report Victoria Passlow 371 INTRODUCTION Donna Cathro, who is sponsored by AGSO, is currently carrying out a PhD at the University of Texas (Cathro, 1999). Her study involves the examination of stacked clinoforms in the Neogene of the NWS region, with the aim to unravel sea level and structural history. To provide "ground-truthing", Donna initially sampled six cores located on her seismic sections. Crucial to Donna's work is the determination of palaeo-water depth. Ostracods provide an excellent tool for the determination of water-depth (Brouwers, 1988a; Passlow, 1994). Deep-water forms, which evolved in the early Tertiary, occur worldwide and are readily distinguishable from shallowerwater taxa. In some regions, ostracods have been shown to be more sensitive indicators than benthic foraminifers (e.g on the Lord Howe Rise, M. Ayress 1994). In addition, because of their mode of growth, they are excellent indicators of downslope sediment transport (Brouwers, 1988b; Passlow, 1994; 1997). Ostracods have been used extensively in biostratigraphy, particularly in the North Sea. This study provides an opportunity to examine the potential to development a new biozonation for the Tertiary, a period identified as requiring additional work (Foster, 1999). 372 OBJECTIVES The study was designed to provide a preliminary examination of the available material, carried out 1 day per week over 10 weeks. The objectives were: 1. to provide an initial assessment of the quality of samples and the potential for ostracods within them to provide water depth estimates. 2. to carry out a preliminary interpretation of the ostracod faunas in a selected range of samples; 3. to assess the suitability of the ostracod material as a biostratigraphic tool; and 4. if possible, to provide advice on the utility of ostracods as palaeo-water depth indicators in the NWS region. Such data will complement waterdepth interpretations based on other micropalaeontological groups and contributes to AGSO's development of the preliminary ODP proposal: Carbonate Clinoforms on Australia's North West Shelf - A Key Link in Global Neogene Sea-Level History; (Bradshaw et al., 1998). MATERIAL The total number of samples examined is detailed below: Well Goodwyn 6 Eastbrook 1 Dampier 1 Goodwyn 4 Goodwyn 3 Goodwyn 2 Interval(s) sampled 495-2090m 1690-2690m 760-3740 ft 1580-7300 ft 2440-6920 ft 2000-7120 ft 373 Sample types 115 DC 51 SWC 96 DC 65 DC 185 DC 67 DC 168 DC No. samples 166 96 65 185 67 168 Sediment samples were processed by AGSO's Palaeontology laboratory. Details of the preparation techniques used are provided in Appendix 1. All samples were examined under the light microscope to provide information on sediment characteristics and to determine whether they were fossiliferous. Any distinctive features of the microfauna or sediments were noted, as was the degree of reworking of sediments, based on fossil preservation. Nine samples from Goodwyn 3 were picked for ostracods. With the exception of the sample at 2780-2800 ft, the whole coarse-sand fraction (>125 m) was picked. BACKGROUND TO GEOLOGY 1. Lithological descriptions Lithological descriptions are taken from data provided in Well Completion Reports, with the exception of Eastbrook 1. Data for the latter well was taken from the Stratigraphy table of PEDIN database, due to lack of availability of the Well Completion Report. A summary of the lithological data in each well is provided in Table 2. The sediments sampled are dominated by carbonates. Cemented clasts are common in a number of intervals, Due to their angular nature these most likely represent intraclasts created by post-depositional cementation. They are not typical of syn-depositional ooids or pelloids. 374 Sidewall core samples, in comparison with adjacent ditch cutting samples, typically contain a higher proportion of fine-grained sand, suggesting that ditch cuttings have been partially washed and have lost a proportion of fines. 2. Age of material Age data were taken from palaeontological studies carried out for Well Completion Reports and summary stratigraphic tables. Data for Eastbrook 1 well was sourced from the Stratigraphy table of PEDIN database, due to lack of availability of the Well Completion Report. Age data are summarised against lithological data in Table 2. In most cases palaeontological analyses were carried out on bulk sample intervals, in some cases using ditch cuttings, so the level of precision is often poor. Little interpretation of palaeoenvironment or preservation was carried out in most cases. RESULTS 1. Sediment characteristics and suitability for ostracod work The dominant lithologies present are: skeletal calcarenites or shell hash, variably dominated by pelecypod fragments, bryozoans or foraminifers; calcarenites and calcilutites with faunal content dominated by foraminifers, and little or no shell hash content; calcsiltites and calcilutites with low to moderate foraminiferal content; and marl 375 Other, less abundant lithologies include quartz sandstone, calcareous and glauconitic sandstones (greensand) and dolomites. Alteration, including fracturing of quartz grains and recrystallisation, is common in many of the intervals examined. Other samples show a degree of reworking, evident in fragmentation and abrasion of the biogenic component. Cemented clasts are a feature of many of the calcarenite and calcilutite lithologies. They vary from a low percentage to up to 90% of the sample. In some samples they appear to consist of cemented, highly reworked sediment, with highly abraded biogenic fragments. Other clasts are more crystalline and contain no recognisable biogenic material. They are typically angular in shape. The suitability of intervals for ostracod work varied considerably in the samples examined. In general, the presence of a reasonably abundant planktonic foraminiferal fauna is indicative that ostracods are likely to be present. Where the foraminifers are highly abraded, the presence of ostracods is less likely, as they are generally more prone to abrasion and dissolution than foraminifers (Passlow, 1994). The best intervals are those falling into the second and third lithologies described above. A more detailed indication of the preservation and biogenic content in the intervals examined is provided in Table 2. Details of the ostracod faunas in samples from Goodwyn 3 are discussed below. The extent of reworked and altered intervals was far greater than expected. As a result, the priorities suggested by Cathro (pers. comm.) were revised. Samples from Goodwyn 3 were selected for ostracod work based on 376 the intervals judged to be most productive. Two additional samples in heavily reworked intervals were picked to test the degree of preservation of ostracods. In terms of suitability for further micropalaeontological work, the wells were rated as follows: Goodwyn 3 Interval 5800-6820 shows strong reworking; otherwise remaining intervals are reasonably fossiliferous. Goodwyn 6 Top of core (above 1370 m) reasonably fossiliferous with ostracods present, below this depth fossil content low and/or highly abraded. Dampier 1 Interval 1890-2790 ft largely cemented clasts, the remainder of the samples is reasonably fossiliferous. Goodwyn 4 Intervals 4210-4510 ft, 6760-7000 ft and 7120-7300 ft are poorly preserved or largely unfossiliferous. Goodwyn 2 Upper interval (above 3680 ft) reasonably fossiliferous, remainder of the core is generally poorly preserved or contains low abundances. Eastbrook 1 Sampled interval is largely barren of microfossils. 2. Ostracods Notes on the ostracods picked from samples in Goodwyn 3 are provided in Appendix 2. Ostracods were identified to family or, where possible, generic level and were identified as shallow-water or deep-water in origin. The distinction between these two groups is readily made for many ostracod taxa. 377 Those from shallow-water are typically smaller and less robust in form, many are sighted, with prominent eye tubercles. Additional information collected included the number of specimens, the relative abundance of adult to juvenile specimens, the number of carapaces vs valves and the preservation of the specimens, based on evidence of dissolution of the valve surface. The nature of the ostracod population can provide information about syn- and post-depositional effects. The ratio of adults to juveniles can indicate removal of smaller forms (early juvenile stages) for example by currents, reworking or dissolution. Currents more readily transport these small, light forms. Juveniles are also more readily attacked by dissolution, being less-heavily calcified than adult carapaces. The extent of disarticulation of carapaces provides information about the conditions of sedimentation. Where the proportion of carapaces is very high, this suggests either relatively rapid sedimentation or little postdepositional reworking. The surface of the ostracod valve can provide information about the degree of dissolution, which has occurred since deposition. Ostracod valves have been shown to be more sensitive indicators of dissolution than the more commonly used foraminiferal indicators in bathyal depths (Passlow et al., 1997). Taxonomic identifications were based on comparisons with contemporaneous faunas of Victoria (Neil, 1994, 1997; Warne, 1986, 1987, 1990; Warne and Whatley, 1994; Warne and Idris, 1995), Recent material 378 (Passlow, 1994; Yassini and Jones, 1995). Studies of shallow-water ostracods from the NWS were of little use, since these have examined very shallow (mostly eulittoral) taxa (Hartmann, 1978; Yassini, Jones and Jones, 1993; Whatley, Cooke and Warne, 1996). Similarly sudies of older faunas had few taxa in common with those found in this study (Bate, 1972; Neale, 1975). Since this study is a preliminary examination only, a detailed comparison with the Victorian material has not been carried out, but would be useful as part of any further study. As would a comparison with Recent material from the region. The ostracod faunas in Goodwyn 3 show distinct change downhole. Upper core samples (samples above 2780 ft) are dominated by taxa from shallow-water depths. Trachyleberids, including sighted Bradleya spp., are indicative of water depths above the photic zone, based on comparisons with modern material from the Australasian region (Passlow, 1994; Yassini and Jones, 1995). They were probably deposited in mid to outer shelf depths. The faunas in these samples are sparse for the likely water depths. Shallow water samples could typically contain hundreds of specimens. Given that the ditch core samples appear to be low in fines, compared with sidewall core samples from similar depths, it is likely that smaller specimens have been removed, thus biasing the populations towards larger specimens. The poor preservation of much of the material indicates either a slow rate of deposition or reworking of the sediment. This is supported by the lack of early juvenile stages preserved, as they are most susceptible to removal by 379 dissolution or currents. Removal of early juveniles and smaller taxa may also reflect washing out of sediment fines in ditch cutting samples. Samples from intervals 2780-2800 and 3100-3120 ft indicate considerable reworking of the sediments, based on the very low numbers and very poor preservation of the ostracods. The sample at 5800 ft contains an abundant fauna, which includes shallow-water elements common to the upper core samples. The dominance of taxa such as small Krithe species suggests deposition on an outer shelf or upper slope, in waters deeper than the shallow water samples described above. The sample shows a strong contrast to the ostracod faunas sampled below this depth. Samples from the lowest section contain larger taxa, indicative of deeper waters, based on comparisons with Recent material (Passlow, 1994) and contemporaneous material from Victoria (Warne, 1990; Warne and Whatley, 1994; Warne and Idris, 1995). They were most likely deposited in a slope environment in water depths of several hundreds to over a thousand metres. Determination of a more exact depth is difficult, since there are no faunas of contemporaneous or more recent age available from the region for comparison. Based on modern faunas from the Otway margin (Passlow, 1994), these samples could have been deposited in up to 1500 m (mid-lower bathyal depths). These faunas are abundant, particularly in comparison with the shallowwater faunas higher in the well. Their preservation indicates that there was little post-depositional reworking, since most specimens are preserved as carapaces. 380 This would suggest fairly rapid sedimentation rates. This is reflected in the low rate of dissolution evident from valve surfaces. Many of the carapaces show post- depositional infilling and subsequent pressuring, causing fractures and, in some cases, partial disarticulation. DISCUSSION: INTERPRETATION OF PALAEOENVIRONMENT The interpretations of palaeoenvironment provided in Well Completion Reports for these wells (BOC of Australia Limited, 1969, 1972, 1973a, 1973b) generally assume a shallow shelf environment throughout the Tertiary. More recent studies (Bradshaw et al., 1988; Bradshaw et al., 1998; Apthorpe, 1988) show a more complex history, with significant sea-level and climatic variations through the Tertiary. Of these, the study by Apthorpe (1988) provides the most detailed analysis of the time frame covered by this study. Palaeoenvironmental interpretations, based on ostracods and lithological data, are fitted into the context of her framework. Apthorpe (1988) has identified a series of four depositional cycles, spanning Paleocene to Recent. Her study is based on wells from the offshore Canning Basin and the northern Carnarvon Basin, including some of the wells examined in this study. The timing of the cycles is controlled by sea-level fluctuations, the relative positions of wells on the shelf or slope and is overprinted by tectonic effects. 1. Palaeo-Water Depth: Evidence From Ostracods The deepest samples examined (DC 6340-6360 ft, DC 6360-6380 ft, DC 6760-6780 ft) fall within Apthorpe's Cycle 3A. In Goodwyn 3, this cycle is 381 very thick and appears to span the Oligocene (5938-6800 ft) through to at least the Aquitanian section (?4978-5588 ft) and possibly higher. The base of the cycle is marked by the Oligocene to Upper Eocene unconformity. In Goodwyn 3, zones P.20 to P.18 are missing (Wright, 1973). The top of the section is not clearly identifiable in the well, due to lack of dating precision and the lack of identification of unconformities. It is likely that the top 3A/base 3B boundary and cycle 3B occur within the interval identified as "Middle Miocene to Burdigalian"1 in age (4520-4910 ft). Apthorpe suggests a maximum water depth of hundreds of metres at the base of the sequence. She interprets the prograding wedge associated with this sequence in seismic profiles as the edge of the Late Oligocene shelf. Evidence from the ostracods in Goodwyn 3 indicates that the base of this cycle was deposited on the slope in bathyal water depths. The transition to shallower, shelf depths occurred some time in the Late Oligocene to early Miocene (zone N3/N4 interval) and is evident from the fauna in sample DC 5800-5820 ft. 2. Palaeo-Water Depth: Evidence from Biogenic Content and Lithology A comparison of biogenic composition and lithologies in cores during Cycle 3 provides further evidence of deep-water deposition to the west and a gradient in water depths from east to west at the base of the cycle. The most easterly of the wells, Dampier 1 shows evidence of sub-aerial exposure during the Oligocene lowstand. A strongly radioactive residual weathering horizon was identified in the Well Completion Report at 2998-3205 1 The "Middle Miocene to Burdigalian" age interval is used in the sense of BOCAL. 382 ft. This is interpreted as a residual soil resulting from the U. Eocene to Oligocene hiatus. (BOC of Australia Ltd, 1969). The interval was not observed in samples examined in this study. The overlying unit, dated as Aquitanian, is typical of shallow-water sediments with bryozoans, pelecypod shell fragments, larger benthic foraminifers typical of shallow depths (Lloyd in B.O.C. of Australia Ltd, 1972) and rare corals. In Goodwyn 3, the units overlying the hiatus, dated as Oligocene and N3/N4 zone equivalents, contain predominantly planktonic foraminifers. Rare shell fragments can be attributed to downslope transport processes. The biogenic content, including the ostracod fauna, indicates bathyal water depths. The transition to shallower depths (outer shelf/upper slope) occurred within the N3/N4 interval. Distinct shallow-water benthic foraminifers are not recorded until the overlying section, dated as Aquitanian (Wright, 1973). The lowstand unconformity in Goodwyn 6 is overlain by a unit of N2 zone age, the fauna of which is dominated by planktonics. The overlying N3 age unit contains planktonics and abundant benthics. A number of these taxa are known from modern deep-water slope sediments (eg Passlow et al., 1997; Nees et al., 1999). The overlying N4 and N5 sections are again dominated by planktonics. Other biogenic indicators of shallow water, such as bryozoans and pelecypods, are absent from these intervals. Distinct shallow-water larger benthic foraminifers are not present until the overlying Early Miocene interval, which in this well probably represents the top of Cycle 3A. 383 Post-unconformity deposition in Goodwyn 4, which span zones N2/N3 up to N5, is dominated by planktonics, with little evidence of shallow water biota. The lithology in this section is dominantly marl. Zones P20 and P19/18 are present in Goodwyn 2, with no clear break identified between these zones and older sediments. The Oligocene through to Aquitanian section contains diverse planktonic foraminiferal faunas and in many cases diverse benthic faunas also. The latter suggest deep-water conditions. Shallow-water benthics were identified from fragments in ditch cuttings in the Aquitanian. Otherwise, the lithology and biota both indicate deep-water deposition. Shallow benthics, including larger forms, were not abundant until the overlying Burdigalian interval. The poor age control in Eastbrook 1 makes it is difficult to correlate with other wells. However, the sparse faunal content and the dominance of marl and claystone lithologies suggest deposition in deep water, probably near the level of the carbonate compensation depth. In summary, Eastbrook 1 and Goodwyn 2 probably remained in deep water through the Eocene/Oligocene lowstand. In wells to the east (Goodwyn 4, Goodwyn 3, Goodwyn 6) the hiatus is followed by deposition in bathyal depths. Only Dampier 1 shows evidence of a shallow-water lithology typical of a shelf environment. The available evidence suggests that shallowing did not occur until zones N3/N4 and was progressive. No attempt has been made to correlate other intervals sampled, because of restrictions on time and the limitations of much of the currently available age 384 control. As a general observation, the extent of facies clearly identifiable as shallow-water in origin is much less than was expected. Except for Dampier 1, "shell hash" lithologies, dominated by shallow-water taxa, such as bryozoans are limited to upper Miocene and younger intervals. This suggests that widespread shallowing of the region covered by this study did not occur until a late stage. 3. Interpretation of Lithologies: Comparisons with Modern Sediment Facies The shallow-water faunas present are more typical of a cool-water carbonate environment than of a tropical environment. The dominant taxa are pelecypods, bryozoans and foraminifera (variably planktonic or benthic). Corals and coralline algae, typical of tropical "chlorozoan" associations (Lees & Buller, 1972) are very rare and indicate that these organisms were either not abundant or localised. The other typical component of tropical carbonates is non-skeletal carbonate grains, such as ooliths, pellets and aggregates. These are similarly absent from the sediments studied here. Clasts present in samples examined in this study are angular, suggesting that they are either relict material, or related to syn- or post-depositional cementation. Intraclasts are a feature of many lithologies, both shallow and deep. Intraclasts and hardgrounds are features of the modern Carnarvon ramp, where intraclasts were found to include both modern and relict material (James et al., 1999). Although early Well Completion Report interpretations (BOC of Australia Limited, 1969, 1972, 1973a, 1973b) suggest that the Tertiary was a 385 period of tropical carbonate deposition, later interpretations (eg. Apthorpe, 1988) indicate that the environment was temperate to sub-tropical in nature. This is consistent with palaeolatitude reconstructions for the period (Feary et al., 1994). It is also consistent with the nature and style of sedimentation observed in this study. Intervals where the foraminiferal fauna is dominated by larger benthics suggest warmer, shallow conditions (Chaproniere, 1975;), although not necessarily tropical (Apthorpe, 1988). James et al. (1999) have observed extant species in waters with annual temperature ranges of 17-22 C. Reworking of the biogenic fraction is a common feature, both in the skeletal "shell hash" and the foraminiferal-dominated lithologies. Similar reworked skeletal sands are found on the mid-ramp on the modern Carnarvon margin (James et al., 1999). Reworking of sediments is also associated with off-shelf transport of cool-water sediments on the southern margin (Passlow, 1997). Interpretation of the depositional environment needs to take this possibility into account. This is where the benthic biota can aid in identifying material sourced from shallower depths and the in-situ component. Substantial reworking of sediments is associated with transgressions in cool-water carbonate environments (Boreen and James, 1993). Although no attempt has been made to correlate the timing of these reworked facies in this preliminary study, it would be useful to do, particularly once there is a better biostratigraphic framework available. 386 IMPLICATIONS FOR SEDIMENT GEOMETRY Cool-water depositional and progradational patterns have been well documented from southern margin examples (James and von der Borch, 1991; Boreen and James, 1993; Passlow, 1997). Typically the nature of sediment generation results in development of a ramp with a series of extensive prograding clinoforms (James and von der Borch, 1991). In contrast to tropical carbonate models, sea-level rise is not marked by rapid aggradation. Abrasion and sweeping of the platform by swells limits accumulation on the shelf and cementation-inhabiting factors, such as stable calcite mineralogy and cool water, make the sediments more prone to reworking (Boreen and James, 1993). These features are consistent with the extent and degree of reworking observed in these samples. Furthermore, they suggest that the reworked sediments and the pattern of prograding clinoforms represent deposition on the upper slope, beyond the shelf margin. This interpretation is consistent with the palaeo-depth interpretations based on ostracods. Apthorpe (1988) was concerned evidence of deep-water taxa at the base of Cycle 3was inconsistent with the global sea-level signal. Recent revision of accommodation space, based on backstripping and a revised foraminiferal interpretation, in wells from this region allows a deeper water model (Kaiko, A.R. and Tait, A.M., 2000). The sudden transition to shallow shelf edge depths in the Upper Miocene is associated with the onset of foresets observed on seismic. 387 CONCLUSIONS Evidence from lithologies, biogenic composition and ostracods in Goodwyn 3 suggests that during Apthorpe's Cycle 3 (Late Oligocene to Middle Miocene) wells from west to east show a transition from deep water (abyssal) to slope, with only Dampier 1 showing evidence of shallow-water deposition. A period of substantial sediment reworking occurred, probably early within Apthorpe's Cycle 4, based on evidence from ostracods in Goodwyn 3 and the biogenic component in other wells. This degree of reworking is characteristic of cool-water carbonate margins. Preliminary interpretation of the lithologies, including the biogenic content of shallow-water deposits and the widespread extent of reworking, suggests that this system behaved as a cool-water carbonate system, not as a tropical model. Many of the ostracod taxa identified are long-lived and are unlikely to provide a well-detailed biozonation. Because preservation is poor, observation of internal features required to make species-level determinations would not be possible for much of the material. The change in water depth also makes the material unsuitable for biozonation. The wells examined in this study are inappropriate for providing a good biostratigraphic framework for the region. The extent of reworking is such that continuity is poor. The bulk of samples are ditch cuttings, with 388 few sidewall core samples available. Hence, the reliability of many of the samples is low. Wells more appropriate to biostratigraphic work should be sourced from deeper water areas, which are less likely to have been substantially reworked. However, revision of the foraminiferal biostratigraphy in the wells will undoubtably be of great use in the interpretation of palaeo-water depth for the Cathro study. 389 References Apthorpe, M. 1988. Cainozoic Depositional History of the North West Shelf. In: P.G. & R.R. Purcell (Eds). The North West Shelf. Petroleum Exploration Society of Australia Limited. Pp 56-84. Ayress, M.A., 1994. Cainozoic palaeoceanographic and subsidence history of the eastern margin of the Tasman Basin based on Ostracoda. In G. Van Der Lingen, K. Swanson & R.J. Muir (Eds). Evolution of the Tasman Sea Basin. A.A. Balkema, Rotterdam. Pp:139-157. Bate, R.H., 1972. Upper Cretaceous Ostracoda from the Carnarvon Basin, Western Australia. Special Papers in Palaeontology, 10; 85 pp. BOC of Australia Limited, 1969 (unpublished). Dampier No. 1 Well Completion Report WA-28-P. BOC of Australia Limited, 1972 (unpublished). Goodwyn No. 2 Well Completion Report WA-28-P. BOC of Australia Limited, 1973a (unpublished). Goodwyn No. 3 Well Completion Report WA-28-P. BOC of Australia Limited, 1973b (unpublished). Goodwyn No. 4 Well Completion Report WA-28-P. Bradshaw, J., Sayers, J., Bradshaw, M., Kneale, R., Ford, C., Spencer and Lisk, M., 1998. In: P.G. & R.R. Purcell (Eds). The Sedimentary Basins of Western Australia 2. Petroleum Exploration Society of Australia Limited. Pp 95-121. Bradshaw, M.T., Yeates, A.N., Beynon, R.M., Brakel, A.T., Langford, R.P., Totterdell, J.T. and Yeung, M., 1988. Palaeogeographic Evolution of the North West Shelf Region. In P.G. & R.R. Purcell (Eds). The North West Shelf. Petroleum Exploration Society of Australia Limited. Pp 20-54. Bradshaw, M., Karner, G., Kaldi, J., Newman, S., Collins, L.B., 1998. A Preliminary Proposal for ODP Drilling: Carbonate Clinoforms on Australia's North West Shelf - a Key Link in Global Neogene Sea-Level History. Australian Geological Survey Record 1998/29. 20pp. 390 Brouwers, E.M., 1988a. Paleobathymetry on the continental shelf based on examples using ostracodes from the Gulf of Alaska. In Ostracoda in the Earth Sciences, De Deckker,P., Colin, J.P. & Peypouquet, J.P. eds, Elsevier, Amsterdam, pp.55-76. Brouwers, E.M., 1988b. Sediment transport detected from the analysis of ostracod population structure: an example from the Alaskan continental shelf. In Ostracoda in the Earth Sciences, De Deckker,P., Colin, J.P. & Peypouquet, J.P. eds, Elsevier, Amsterdam, pp. 231-244. Cathro, D.L., 1999. Three-dimensional stratal development of a (?mixed) carbonate clastic sedimentary regime, Northern Carnarvon Basin, North West Shelf, Australia. Ph.D. Proposal. The University of Texas at Austin. Collins, L.B., Zhong Rong Zhu and Wyrwoll, K.-H., 1998. In: P.G. & R.R. Purcell (Eds). The Sedimentary Basins of Western Australia 2. Petroleum Exploration Society of Australia Limited. Pp 647-663. Feary, D.A., James, N.P. and McGowran, B., 1994. Cenozoic cool-water carbonates of the Great Australian Bight: reading the record of Southern Ocean evolution, sealevel, paleoclimate and biogenic production. Revised ODP proposal December 1994. AGSO Record 1994/62. 92pp. Foster, C.B.1999. Building Research Links in Australasian Palaeontology, AGSO. Hartmann, G., 1978. Die Ostracoden der Ordnung Podocopina G.W. M ller, 1894 der tropisch-subtropisch Westk ste Australiens (zwischen Derby im Norden und Perth im S den). In: G. Hartman and G. HartmannSchr der: Zur Kenntnis des Eulitorals der australischen K sten unter besonderer Ber cksichtingung der polychaeten und Ostracoden. Teil 1., Mitt. Hamburg. Zool. Mus. Inst., 75: 64-219. Heath, R., 1982. Micropalaeontological Report on the W.O.P. et al. Goodwyn No. 6 Well, Western Australia. (unpublished). Included in Woodside Offshore Petroleum, 1986 (unpublished). Kaiko, A.R. and Tait, A.M., 2000. Post-rift tectonic subsidence and palaeowater depths in the northern Carnarvon Basin; Western Australia. Abstract of Talk in PESA News, 47: 19. 391 Neale, J.W., 1975. The ostracod fauna from the Santonian Chalk (Upper Cretaceous) of Gingin, Western Australia. Special Papers in Palaeontology, 16; 81 pp. Neil, J.V., 1994. Miocene Ostracoda of the Trachlyberididae and Hemicytheridae from the Muddy Creek area, south-western Victoria. Memoirs of the Museum of Victoria 54: 1-49. Neil, J.V., 1997. A Late Palaeocene ostracode fauna form the Pebbel Point formation, south-west Victoria. Proceedings of the Royal Society of Victoria. 109(2): 167-197. Passlow, V. 1994. Late Quaternary History of the Southern Ocean Offshore Southeastern Australia, Based on Deep-Sea Ostracoda. Ph.D. Thesis, Australian National Univ. Passlow, V., 1997. Slope sedimentation and shelf to basin sediment transfer: a cool-water carbonate example from the Otway margin, southeastern Australia. In: N.P. James and J.A.D. Clarke (Editors). Cool-Water Carbonates. SEPM Special Publication No. 56, Tulsa. Passlow, V, Wang Pinxian and Chivas, A.R., 1997. Late Quaternary palaeoceanography near Tasmania, southern Australia. Palaeogeography, Palaeoclimatology, Palaeoecology, 131: 433-463. Warne, M.T., 1986. Paranesidea and Papillatabairdia (Crustacea, Ostracoda) from the Miocene of the Port Phillip and Western Port Basins, Victoria, Australia. Proceedings of the Royal Society of Victoria 98(1): 41-48. Warne, M.T., 1987. Lithostratigraphical associations of the ostracode fauna in the marine Neogene of the Port Phillip and Western Port Basins, Victoria, southeastern Australia. In: K.G. McKenzie (Ed.) Shallow Tethys 2. A.A. Balkema, Rotterdam. Warne, M.T., 1990. Bythocypridae (Ostracoda) from the Miocene of the Port Phillip and Western Port Basins, Victoria. Proceedings of the Royal Society of Victoria 102(2): 105-115. Warne, M.T., 1990. Polycopidae (Ostracoda) from the Late Tertiary of the Port Phillip and Western Port Basins, Victoria. Proceedings of the Royal Society of Victoria 102(1): 59-66. 392 Warne, M.T. and Idris, A.A., 1995. Palaeoenvironmental significance of Miocene ostracod preservation in Barracouta-1 well, Gippsland Basin, SE Australia. Memoirs of the Australasian Association of Palaeontologists, 18: 199-202. Warne, M.T. and Whatley, R.C., 1994. Palaeo-oceanographic significance of Miocene deep-sea Ostracoda from the Kingfish 8 well, Gippsland Basin, southeastern Australia. Whatley, R., Cooke, P.C.B. and Warne, M.T., 1996. The Ostracoda from Lee Point on Shoal Bay, Northern Australia: Part 2 Podcopina (Bairdiacea and Cypridacea). Revista Espanola de Micropaleontologia 28(1): 5-41. Whatley, R. and Downing, S., 1983. Middle Miocene Ostracoda from Victoria, Australia. Revista Espanola de Micropaleontologia 15(3): 347-407. Wright, C.A., undated. Micropalaeontological Report on the B.O.C. et al. Goodwyn No. 2 Well Western Australia. Report contained in BOC of Australia Limited, 1972 (unpublished). Woodside Offshore Petroleum, 1986 (unpublished). Goodwyn 6 Well Completion Report Volume 2. Yassini, I. and Jones, B.G., 1995. Recent Foraminifera and Ostracoda from Estuarine and Shelf Environments on the Southeastern Coast of Australia. University of Wollongong Press. 484 pp. Yassini, I., Jones, B.G.,and Jones, M.R., 1993. Ostracods from the Gulf of Carpentaria, northeastern Australia. Senckenbergiana lethaea, 73(2): 375-406. 393 Table 2a. Dampier 1: Summary of lithology, age and sediment information Age and lithological data are sourced from BOC of Australia Ltd, 1969 (unpubl.) and observations of samples. Depth intervals are determined from samples examined and may vary from those associated with lithological boundaries in the well data. Depth interval (ft) 760-870 Age ?PlioceneQuaternary Lithology Skeletal calcarenite, with varying amounts of calcilutite, especially below 900 ft; minor marl and quartz. Skeletal calcarenite with varying amounts of calcilutite and minor sand. Calcareous clasts abundant in intervals. Quartz sandstone, interbedded with calcarenite. Cemented clasts abundant in calcareous intervals. Calcarenite, locally sandy and tending towards calcisiltite and silty marl. Cemented clasts present. Biogenic content Dominantly foraminifers, bryozoans, shell fragments (lamellibranchs/pelecypods), echinoid spines, other gastropod fragments As above. Preservation Fair. 900-1230 Miocene Fair to poor. 12601350 13801440 15301680 Miocene Miocene Foraminifers predominate, content sparse; shell fragments present. Foraminifers sparse. Fair. Fair to poor. Lower-Mid Miocene Chalky calcilutite interbedded with Foraminifers rare to low; shell thin calcarenite bands grading locally fragments present. to calcisiltite. Preservation poor. Evidence of reworking, including fragmentation and abrasion. 394 Depth interval (ft) 17102260 23003180 32003460 3550 Age Lower-Mid Miocene Lower Miocene Lower Eocene Palaeocene Lower Eocene Palaeocene Lower Eocene Palaeocene Lithology Extensively recrystallised and cemented calcilutite and dolomite. Calcarenite, calcisiltite locally, thin beds of dolomite. Cemented intervals. Calcilutite. Cemented clasts present. Biogenic content Biogenic fragments very rare. Preservation Preservation very poor, highly altered. Poor, abraded. Forams throughout, including larger benthics; bryozoans, shell fragments, very rare ?coral fragments. Foraminifers low. Poor Calcareous sandstone, glauconitic Foraminifers present. Reworked. 35703710 Calcareous siltstone grading to calcareous claystone; with interbedded marl and glauconitic sandstone. Glauconitic sandstone dominates in samples studied. Moderately abundant planktonic foraminifers, benthic foraminifers and ostracods. Fair to good. 395 Table 2b. Eastbrook 1: Summary of lithology, age and sediment information Age and lithological data are sourced from AGSO's PEDIN database and observations of samples. Depth intervals are determined from samples examined and may vary from those associated with lithological boundaries in the database. Depth interval (ft) 16902450 24602690 Age Tertiary Tertiary/Late Cretaceous Lithology Calcisiltite, calcilutite and marl. Carbonate-cemented clasts dominate in intervals. Marl, ?claystone and calcilutite. Biogenic content Foraminifers and shell fragments, varying from largely absent to low abundance. Largely barren. Preservation Poor, altered and recrystallised. 396 Table 2c. Goodwyn 2: Summary of lithology, age and sediment information Age and lithological data are sourced from BOC of Australia Ltd, 1972 (unpubl.) and observations of samples. Depth intervals are determined from samples examined and may vary from those associated with lithological boundaries in the well data. Depth interval (ft) 20003410 Age ?Late Miocene Lithology Biogenic content Preservation Fair to good; foraminifers more abraded below 3050 ft. 34103650 ?Late Miocene 36805020 Early. Miocene Burdigalian Calcisiltite interbedded with Shelly fragments (pelecypod), calcarenite, calcirudite and rare corals, bioclasts, highcalcilutite. Cemented clasts common. moderate foraminifers (predominantly benthic; planktonic more abundant in fine-sand fraction). Larger foraminifers present. Foraminifers much less abundant below 3050 ft. Dolomite with interbedded dolomitic Low-moderately high shell calcarenite. content (pelecypod) and bioclasts, rare corals. Foraminifers low to rare (planktonic and benthic). Dominantly calcarenite with Foraminifers low to rare. interbedded calcilutite, marl, calcirudite and calcisiltite. Cementation in calcareous intervals. Moderately abraded. Foraminifers highly abraded from 4340 ft. 397 Depth interval (ft) 50406810 Age Burdigalian Oligocene Lithology Dominantly calcilutite with interbedded marl and calcisiltite. Biogenic content Foraminifers (planktonic and benthic) abundance varies from low to moderate; higher below 5650 ft. Preservation Moderately abraded. 68507120 U-M Eocene Calcarenite with interbedded calcisiltite; marl below 7060 ft. Foraminifers very low to absent, especially in marly intervals; predominantly planktonic. Possible caving of planktonic foraminifers identified (Wright, undated in BOC of Australia Ltd, 1972). Moderately abraded. 398 Table 2d. Goodwyn 3: Summary of lithology, age and sediment information Age and lithological data are sourced from BOC of Australia Ltd, 1973a (unpubl.) and observations of samples. Depth intervals are determined from samples examined and may vary from those associated with lithological boundaries in the well data. Depth interval (ft) 24402960 Age Pliocene Lithology Biogenic content Shell fragments, foraminifers, ostracods, trace coral fragments. Preservation Fair. Calcarenite and calcisiltite interbedded with sandstone. Calcareous intervals partially cemented. Sandstone interbedded with calcarenite. Calcilutite and calcisiltite, in part grading to calcarenite and marl. Cemented clasts present. 29803320 58006820 PlioceneMid. Miocene Oligocene Foraminifers rare to low abundance. Low to moderate foraminifers; rare shell fragments, ostracods. Fair. Evidence of reworking and high levels of abrasion above 6340 ft; less abraded below this depth.. Evidence of abrasion. 68406900 Late. Miocene ?Mid Eocene Calcilutite and calcisiltite, in part grading to calcarenite and marl. Cemented clasts present. Foraminifers rare. 399 Table 2e. Goodwyn 4: Summary of lithology, age and sediment information Age and lithological data are sourced from BOC of Australia Ltd, 1973 (unpubl.) and observations of samples. Depth intervals are determined from samples examined and may vary from those associated with lithological boundaries in the well data. Depth interval (ft) 15803670 37006730 67606970 70007300 Age Pliocene Mid. MioceneOligocene Oligocene Mid.-Upper Eocene Lithology Dominantly Calcarenite, interbedded with calcilutite and sandstone; minor calcisiltite and calcirudite Dominantly calcilutite interbedded with calcarenite and calcisiltite. Minor cementation. Dominantly marl with minor calcilutite. Partially cemented. Dominantly marl with minor calcilutite. Biogenic content Predominantly foraminifers (planktonic and benthic); shell fragments; trace ?pteropods. Foraminifers low to rare; trace ?coral and shell fragments. Trace foraminifers. Trace shell fragments; foraminifers very rare. Preservation Highly abraded; evidence of reworking. Foraminifers abraded and fragmented. Fragmented and abraded. Fragmented and abraded. 400 Table 2f. Goodwyn 6: Summary of lithology, age and sediment information Age and lithological data are sourced from Woodside Offshore Petroleum, 1986 (unpubl.) and observations of samples. Depth intervals are determined from samples examined and may vary from those associated with lithological boundaries in the well data. Depth interval (m) 495-820 Age U. PlioceneM. Miocene Miocene Miocene Miocene Miocene Lithology Calcarenite with minor calcisiltite and sandstone. Sandstone; upper section dominantly calcareous, quartzose below 870m. Calcarenite grading to calcisiltite. Cemented clasts abundant. Sandstone with thinly interbedded calcarenite. Minor cementation. Dolomite, recrystallised. Biogenic content Variable from nil to moderate. Main components: bryozoans, planktonic foraminifers, shell fragments; minor ostracods. Low to moderate. Dominantly shell fragments; foraminifers low to rare. Low to moderate. Preservation Fair. 830-910 920-970.1 980-1040 10501115 Low; foraminifers (predominantly planktonic ) with rare ostracods. Low to rare foraminifers and Highly abraded. bryozoans. Content decreases towards base of section. Evidence of reworking, including fragmentation. Reworked and cemented. Fair-poor. 401 11251460 Miocene Calcarenite and calcisiltite. Minor cementation. Depth interval (m) 14702059 Age MioceneEarly Oligocene Lithology Calcilutite and calcisiltite, minor cementation. Variable. Dominantly shell fragments (shell hash in some samples) foraminifers, variably dominated by planktonics or benthics, including larger benthics (Heath, 1982).) and bryozoans; trace echinoid spines and ?coral fragments. Biogenic content Foraminifers rare, mainly present in fine-sand fraction; variably dominated by planktonics or benthics, including larger benthics (Heath, 1982). Foraminifers rare, mainly present in fine-sand fraction; variably dominated by planktonics or benthics, including larger benthics (Heath, 1982). Highly to very highly abraded and reworked. Preservation Highly abraded. 20652075 Mid. Eocene Calcilutite and calcisiltite. Highly abraded and recrystallised. 402 APPENDIX A: FORAM PROCESSING TECHNIQUE STANDARD FORAM PROCESSING Sub-sample samples if required Label up beakers Add sample to beakers, leave small amount in original bag for reference if there is enough sample If sample dry continue to next step, if sample is moist dry out in oven at 55 degree Celsius In a fume cupboard, add 100mls of warm water then add 10mls of Hydrogen Peroxide 100vol ~30% w/w. (Note keep squeeze bottle with water ready to slow any violent reactions) Leave sample in solution for approximately 1 hour or until no reaction is visible Sieve residue with 63 micron sieve and any other which is requested Dry samples on hot plate Transfer samples to clearly labelled vials or bags Clean equipment and laboratory after completion! 403 APPENDIX B: OSTRACOD DATA GOODWYN 3 OSTRACOD SAMPLES SAMPLE: DC 2440-2460 FT NO. SPECIMENS: 14, COMPRISING 5 Carapaces 7 Valves (disarticulated) 2 Fragments PRESERVATION: Valve surfaces pitted and frosted. The bythocyprid carapace is infilled (probably by glauconitic material) and possibly reworked from older material. Adult:juvenile ratio: Ratio difficult to determine due to lack of internal detail. Most specimens are late juvenile or adult, with few early instars present. Notes on taxa: Trachyleberids (sighted) Ponticocythereis sp. aff P. quadriserialis Actinocythereis sp. (fragment) Bairdiids (Paranesidea spp./Neonesidea spp.?) Bythocyprid Palaeo-environmental interpretation: The fauna present suggests a shallow environment, to mid -shelf depths. Preservation indicates a degree of dissolution has occurred. At shallow water depths, this suggests either a slow rate of deposition or reworking of the sediment. This is supported by the lack of early juvenile stages preserved, as they are most susceptible to removal by dissolution or currents. Removal of juveniles may alternately be due to removal of the fine-sand fraction in ditch cutting samples. Overall numbers are very low for a sample from shallow water depths. 404 Goodwyn 3 Ostracod Samples SAMPLE: DC 2480-2500 FT NO. SPECIMENS: 11 3 Carapaces 5 Valves 3 Fragments PRESERVATION: Valve surfaces pitted and frosted. Adult:juvenile ratio: Ratio difficult to determine due to lack of internal detail. Most specimens are late juvenile or adult, with few early instars present. Notes on taxa: Fauna is dominated by the following three taxa. Cytherella sp. Bairdiids (Paranesidea spp./Neonesidea spp.?) Bythocyprid (probably same species as in sample 2440-2460 ft.) Palaeo-environmental interpretation: The fauna present suggests a shallow environment, to mid -shelf depths. Preservation indicates a degree of dissolution has occurred. At shallow water depths, this suggests either a slow rate of deposition or reworking of the sediment. This is supported by the lack of early juvenile stages preserved, as they are most susceptible to removal by dissolution or currents. Removal of juveniles may alternately be due to removal of the fine-sand fraction in ditch cutting samples. Overall numbers are very low for a sample from shallow water depths. 405 Goodwyn 3 Ostracod Samples SAMPLE: DC 2540-2560 FT NO. SPECIMENS: 19 5 Carapaces 11 Valves 3 Fragments 3 glauconitic infillings ?probably ostracods PRESERVATION: Valve surfaces pitted and frosted. Adult:juvenile ratio: Juveniles dominate, with many early juvenile stages present. Notes on taxa: Actinocythereis sp. Bradleya sp. (sighted) Ponticocythereis sp. aff P. quadriserialis Cytherella sp. Bythocyprid sp. ?Bairdiid fragment Trachyleberid juvenile remainder indeterminate Palaeo-environmental interpretation: The fauna present suggests a shallow environment, mid- to outer-shelf depths. The presence of a greater number of small, early instars (juvenile stages) indicates either that the sediment is finer-grained or that the sediment is better preserved (fines not removed). However, numbers are very low compared with abundances, which could be expected from a shallow-water sample. 406 Goodwyn 3 Ostracod Samples SAMPLE: DC2780-2800 NO. SPECIMENS: 3 2 Carapaces 0 Valves 1 Fragment PRESERVATION: One carapace highly recrystallised with abraded valve surface. Second carapace fractured. Adult:juvenile ratio: Specimens probably all juveniles. Notes on taxa: Identification is very difficult due to poor preservation. One carapace ornamented; fragment and second carapace indeterminate. Palaeo-environmental interpretation: The sediment has probably been reworked. Preservation of the fauna has been affected also by diagenetic alteration, causing fracturing and recrystallisation. The low numbers present suggest that the fauna has been reduced by sediment reworking and possibly also by diagenesis. 407 Goodwyn 3 Ostracod Samples SAMPLE: DC 3100-3120 FT NO. SPECIMENS: 1 1 Carapace 0 Valves 0 Fragments PRESERVATION: Poor, abraded. Adult:juvenile ratio: Small juvenile carapace. Notes on taxa: Specimen indeterminate due to poor preservation. Palaeo-environmental interpretation: The sediment has probably been reworked, resulting in poor preservation and reduction of the fauna. Diagenetic effects may also have reduced the number of specimens preserved. 408 Goodwyn 3 Ostracod Samples SAMPLE:DC 5800-5820 FT NO. SPECIMENS: 91 90 Carapaces 0 Valves 1 Fragment PRESERVATION: Valve surfaces slightly frosted. Carapaces are typically infilled by postdepositional cement; cracking is common and occurred post-infilling. Adult:juvenile ratio: Late juveniles and adults dominate the fauna, although some earlier juvenile stages have been preserved. Notes on taxa: Bairdiids Krithe spp. Parakrithella sp. Parakrithe spp. Bythocyprids Polycope sp. Propontocypris sp. Callistocythere spp. Leptocytherid Xestoleberids Remainder indeterminate The fauna is dominated by the Krithe group (Krithe, Parakrithe and Parakrithella spp.) and xestoleberids, which together comprise approximately 50% of the population. Palaeo-water depth interpretation: The fauna contains shallow-water, sighted taxa and taxa, such as Krithe spp, which typically occur in somewhat deeper water. However, the overall aspect of the fauna is not typical of a deep-sea fauna. The Krithe specimens are smaller than typical deep-sea taxa and, together with the overall species composition, suggest that the sample was probably deposited within the photic zone, in outer shelf to upper slope depths. The dominance of the Krithe group indicates the water depth was greater than for the samples from 2440-2540 ft, described above. 409 Goodwyn 3 Ostracod Samples SAMPLE:6340-6360 FT NO. SPECIMENS: 55 54 Carapaces 1 Valve 0 Fragments PRESERVATION: Valve surfaces are generally in reasonable condition, with slight frosting or pitting. Carapaces have been infilled with post-depositional cement and subsequently cracked, fractured or partially disarticulated. Rare chalky specimens are present. Adult:juvenile ratio: Fauna consists of mainly late juvenile or adult stages, with a reasonable proportion of earlier juveniles. Notes on taxa: Krithe spp. Bairdiids, predominantly Abyssobaridia spp. Argilloecia sp. Bythocypris spp. Bradleya sp. (deep-sea form) Macrocypris sp. Cytherella spp. Xestoleberids (typical deep-sea forms) Parakrithe sp. Krithe spp. and Abyssobaridia spp. dominate the fauna (~50%). Palaeo-water depth interpretation: The fauna is typical of a deep-sea assemblage, probably from bathyal depths. The very high proportion of valves suggests that there was minimal disturbance of the sediment. The relatively high proportion of juvenile specimens present supports this interpretation. Deformation, such as cracking and fracturing of specimens, obviously occurred after infilling by cement. 410 Goodwyn 3 Ostracod Samples SAMPLE: DC 6360-6380 FT NO. SPECIMENS: 79 77 Carapaces 0 Valves 2 Fragments PRESERVATION: Valve surface preservation is fair, with some frosting or pitting. Rare chalky specimens are present. Post-depositional infilling by cement and subsequent cracking and fracturing is common. Adult:juvenile ratio: Fauna consists of mainly late juvenile or adult stages, with a reasonable proportion of earlier juveniles. Notes on taxa: Krithe spp. Bairdiids, mainly Abyssobairdia sp/p. Polycope sp. Macrocypris sp. Cytherelloidea sp. Cytherella sp. Australoecia spp. Argilloecia spp. Xestoleberids (typical deep-sea forms) Bythocyprid sp. Propontocypris sp. Bairdiids (mainly Abyssobairdia spp.), Argilloecia spp., Australoecia spp. and xestoleberids are most abundant, comprising about 40% of the population. Palaeo-environmental interpretation: The fauna is typical of a deep-sea assemblage, probably from bathyal depths. The very high proportion of valves suggests that there was minimal disturbance of the sediment. The relatively high proportion of juvenile specimens present supports this interpretation. Deformation, such as cracking and fracturing of specimens, obviously occurred after infilling by cement. 411 Goodwyn 3 Ostracod Samples SAMPLE: DC 6760-6780 FT NO. SPECIMENS: 69 64 Carapaces 5 Valves 0 Fragments PRESERVATION: Valve surface preservation is fair, with some frosting or pitting. Post-depositional infilling by cement and subsequent cracking and fracturing has occurred in a proportion of the specimens. Adult:juvenile ratio: Fauna consists of mainly late juvenile or adult stages, with a reasonable proportion of earlier juveniles. Notes on taxa: Krithe spp. Bairdiid spp., predominantly Abyssobairdia sp. Cytherella sp. Argilloecia spp. Australoecia spp. Cytheropteron sp. (deep-sea form) Cytherelloidea sp. Cytherella sp. Bairdiid spp. (predominantly Abyssobairdia sp.), Argilloecia spp. Australoecia spp. and Krithe spp. are most abundant, forming about 40% of the population. Palaeo-environmental interpretation: The fauna is typical of a deep-sea assemblage, probably from bathyal depths. The very high proportion of valves suggests that there was minimal disturbance of the sediment. The relatively high proportion of juvenile specimens present supports this interpretation. Deformation, such as cracking and fracturing of specimens, obviously occurred after infilling by cement. 412 Appendix 5 Synthetic Seismograms 423 240 0 DT GR 40 150 Scale (s) (m) Reflectivity Synthetic seismogram Seismic sequences seafloor 500 0.5 DLS_top MM2 DLS5 MM1 DLS4 EMM1 DLS2 OM1 DLS3 OL1 1000 DLS1 1.0 1500 2000 1.5 Dampier 1 414 240 0 DT GR 40 150 Scale (s) (m) Reflectivity Synthetic seismogram Seismic sequences 0.5 500 1000 DLS_top MM2 1.0 DLS5 MM1 1500 EMM1 DLS3 DLS4 OM1 1.5 2000 OL1 DLS2 DLS1 2500 2.0 Goodwyn 2 415 240 0 DT GR 40 150 Scale (s) (m) Reflectivity Synthetic seismogram Seismic sequences 0.5 500 DLS_top 1000 MM2 1.0 DLS5 MM1 DLS4 1500 EMM1 DLS3 OM1 DLS2 OL1 2000 1.5 DLS1 2500 2.0 Goodwyn 3 416 240 0 DT GR 40 150 Scale (s) (m) Reflectivity Synthetic seismogram Seismic sequences 0.5 500 DLS_top 1000 MM2 1.0 DLS5 MM1 DLS4 1500 EMM1 DLS3 OM1 2000 OL1 DLS2 DLS1 1.5 2500 Goodwyn 4 417 240 0 DT GR 40 150 Scale (s) (m) 0.5 Reflectivity Synthetic seismogram Seismic sequences 500 DLS_top 1000 MM2 1.0 MM1 DLS5 DLS4 1500 EMM1 DLS3 casing shoe OM1 2000 1.5 DLS2 OL1 DLS1 2500 2.0 Goodwyn 6 418 240 0 DT GR 40 150 Scale (s) (m) Reflectivity Synthetic seismogram Seismic sequences 0.5 500 DLS_top 1000 MM2 1.0 MM1 1500 DLS4 EMM1 DLS3 DLS5 casing shoe OM1 1.5 2000 OL1 DLS2 DLS1 2500 Goodwyn 7 419 240 0 DT GR 40 150 Scale (s) (m) Reflectivity Synthetic seismogram Seismic sequences 0.5 500 DLS_top 1000 MM2 1.0 MM1 1500 DLS4 EMM1 DLS3 DLS5 casing shoe OM1 1.5 2000 OL1 DLS2 DLS1 2500 base TERTIARY SANTONIAN 2.0 ALBIAN TITHONIAN 3000 3423 Goodwyn 7 tied to 101r_09 420 240 0 DT GR 40 150 Scale (s) (m) 0.5 500 Reflectivity Synthetic seismogram Seismic sequences 1000 DLS_top 1.0 1500 MM2 2000 1.5 MM1 DLS4 EMM1 OL1 DLS2 DLS1 DLS5 2500 2.0 Eastbrook 1 421 Appendix 6 Porosity vs. depth curves 422 Goodwyn 2 Porosity (shale-corrected sonic) 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Raw Data Carbonate Quartz sand Recrystallized (dolomite) 200 Fitted Curves y=0.5343e-0.0006x, R2=0.7386 y=-0.0002x + 0.4895, R2=0.74 y=0.6e-0.0004 Depth (m sub-seafloor) 600 400 1000 800 DLS_top DLS5 1200 DLS4 1400 DLS3 1600 marl increasing 1800 2000 DLS2 DLS1 423 Goodwyn 3 0 0 Porosity (shale-corrected sonic) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.1 Raw Data Carbonate Quartz sand Recrystallized (dolomite) Fitted Curves y=0.5323e-0.0006x, R2=0.8698 y=-0.0002x + 0.4916, R2=0.8847 y=0.6e-0.0004 500 DLS_top Depth (m-sub-seafloor) 1000 DLS5 DLS4 1500 DLS3 DLS2 DLS1 2000 424 Goodwyn 4 Porosity (shale-corrected sonic) 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Raw Data Carbonate Quartz sand Recrystallized (dolomite) 200 Fitted Curves y=0.5016e-0.0006x, R2=0.6902 y=-0.0002x + 0.4382, R2=0.7642 y=0.6e-0.0004 Depth (m sub-seafloor) 600 400 1000 800 DLS_top DLS5 1200 DLS4 1400 DLS3 1800 1600 2000 DLS2 DLS1 425 Goodwyn 6 Porosity (shale-corrected sonic) 0 0 0.2 0.4 0.6 0.8 1 Raw Data Carbonate Quartz sand Recrystallized (dolomite) 200 Fitted Curves y=0.637e-0.0007x, R2=0.7422 y=-0.0002x + 0.5195, R2=0.8022 y=0.6e-0.0004 600 400 Depth (m sub-seafloor) DLS_top 1000 800 DLS5 1200 DLS4 DLS3 1800 1600 1400 DLS2 DLS1 2000 426 Goodwyn 7 Porosity (shale-corrected sonic) 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Raw Data Carbonate Quartz sand Recrystallized (dolomite) 200 Fitted Curves y=0.6099e-0.0007x, R2=0.7409 y=-0.0002x + 0.5204, R2=0.7993 y=0.6e-0.0004 Depth (m sub-seafloor) 600 400 DLS_top 800 1000 DLS5 1200 DLS4 1400 DLS3 1800 1600 DLS2 2000 DLS1 427 Eastbrook 1 0 0 0.1 Porosity (shale-corrected sonic) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Raw Data Carbonate Quartz sand Recrystallized (dolomite) Fitted Curves y=0.52e-0.0008x, R2=0.704 y=-0.0002x + 0.4329, R2=0.6535 y=0.6e-0.0004 Depth (m sub-seafloor) 500 DLS_top 1000 severe hole washouts 1500 DLS5 DLS4 2000 DLS2 DLS1 428 Bibliography Adams, E.W., and W. 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Zoback, M.L., 1992, First- and second-order patterns of stress in the lithosphere: The world stress map project: Journal of Geophysical Research, v. 97, p. 11703-11728. 454 Vita Donna Louise Cathro was born on July 13, 1968 in Adelaide South Australia. The second of three children born to Shirley May and Donald Lloyd Cathro, she was raised in Adelaide. After graduation from Seymour College, Donna attended the University of Adelaide where she earned a Bachelor's degree in Geology and Geophysics in 1988. In 1989, she started and completed an Honours Degree from the National Centre for Petroleum Geology and Geophysics at the University of Adelaide. Her Honour's work on the sedimentology and geochemistry of an Ordovician-Silurian halite saltern in the Canning Basin, Western Australia was published in Sedimentology (Cathro et al., 1992). Following graduation, Amdel Core Services employed Donna as a sedimentary petrologist. In 1992, she joined Geoscience Australia (formerly Australian Geological Survey Organisation) as a seismic data processor in the Petroleum and Marine Division (PMD). In 1994, Donna worked briefly in the Airborne Division before returning to PMD as a member of the North West Shelf Group, working in the Carnarvon and Browse basins. This position included seismic acquisition cruises on the R.V. Rig Seismic. Donna enrolled in the doctoral program at the University of Texas at Austin in fall 1997, and pursued her research at the Institute for Geophysics. On graduation, she will return to Geoscience Australia. Permanent address: 10 The Ridgeway, Belair, SA, 5052 AUSTRALIA This dissertation was typed by the author. 455 Supplemental Data Oversized plates and tables requiring access to a 36" plotter 456 Dampier 1 SPUD - 1968 Depth 0 (m) 6 GR (API) 150 WD 76.2 m KB 9.1 m TD 4141 m Seismic Ties 1.7 Lithology Biostratigraphy (Blow, 1969; Berggren et al., 1995) rb (gm/cm3) CALIPER (in) 36 2.95 240 DT (ms/ft) 40 Depth (m) seafloor 110 m seafloor (0-5.32) .................. .................. 302 m (13.8-16.4) DLS_top (16.4-20.52) DLS5 (16.4-20.52) DLS4 .................. .................. DLS3 DLS2 (20.52-23.8) DLS1 (49-65) 1039 m LITHOLOGY calcarenite calcisitite calcilutite dolomite marl sandstone calcite claystone 1800 m casing shoe Plate 1. Composite well Log, Dampier 1 Goodwyn 2 SPUD - 1972 GR (API) 150 Depth 0 (m) 6 CALIPER (in) 30 WD 133.2 m KB 12.5 m TD 3750 m 1.7 Lithology Biostratigraphy (Blow, 1969; Berggren et al., 1995) Samples Paleobathymetry Seismic (Swc) (DC) Ties rb (g/cm3) 2.95240 DT (ms/ft) 40 Depth (m) seafloor 179 m seafloor NO RETURNS G.Moss, UTIG personal , communication, 2000, all other dates from WCR 444 m 905-969 m N15 or younger (<11.4) DLS_top 996 m N16 (8.3-10.9) (10.9) 5.5 5.3 1118-1316 m N15-N10 (10.4-14.8) 5.2 5.1 1287 m DLS5 4.3 1408 m N11-N8 (12.7-16.4) 4.1 DLS4 3.2 1563 m N8 (15.1-16.4) 3.1 1679-1682 m N7-N5 (16.4-21.5) 1716 m N5 (18.8-21.5) DLS3 2.5 1798 m N4 (21.5-23.8) 1816 m N4-P22 (21.5-27.1) 1871.5 m N4 (21.5-23.8) 1898 m N4-P22 (21.5-27.1) 2.4 1940-2080 m long ranging early Oligocene to early Miocene species 2.3 2.2 2.1 DLS2 2124 m P22-?P18 (23.8-33.8) 2154 m P22-?P18 (23.8-33.8) 2164-2173 m P18 (32-33.8) 1 2 3 4 5 6 7 8 DLS1 LITHOLOGY calcarenite calcisitite calcilutite dolomite marl sandstone calcite claystone PALEOBATHYMETRY 1 Transitional Marine (0 m) 5 Upper Bathyal 2 Inner Neritic (0-20 m) 6 Middle Bathyal 3 Middle Neritic (20-100 m) 7 Lower Bathyal 4 Outer Neritic (100-200 m) 8 Abyssal Confidence good poor med 1800 m (200-500 m) (500-1000 m) (1000 - 2000 m) (>2000 m) casing shoe Plate 2. Composite Well Log, Goodwyn 2 Goodwyn 3 SPUD - 1972 Depth 0 (m) 6 GR (API) 150 WD 120.7 m KB 30.2 m TD 3657.6 m 240 Lithology Biostratigraphy (Blow, 1969; Berggren et al., 1995) CALIPER (in) 36 Seismic Ties DT (ms/ft) 40 Depth (m) seafloor 176 m seafloor No rb or NPHI 462 m DLS_top DLS5 1360 m DLS4 DLS3 DLS2 DLS1 LITHOLOGY calcarenite calcisitite calcilutite dolomite marl sandstone calcite claystone 1800 m casing shoe Plate 3. Well Composite Log, Goodwyn 3 Goodwyn 4 SPUD - 1973 Depth 6 (m) 6 0 GR (API) CALIPER (in) BIT SIZE (in) SP (MV) -80 150 30 30 20 WD 129.8 m KB 30.2 m TD 3632.3 m 0.45 NPHI Lithology Biostratigraphy (Blow, 1969; Berggren et al., 1995) Samples Paleobathymetry Seismic (Swc) (DC) Ties (PU)-0.15240 DT (ms/ft) 40 Depth (m) seafloor 185 m seafloor G.Moss, UTIG personal , communication, 2000, all other dates from WCR 464 m 966 m N14 (11.4-11.8) DLS_top 1048 m N15-N14 (10.9-11.8) 5.3 5.2 5.1 1292 m N9 (14.8-15.1) DLS5 4.3 1347 m N8 (15.1-16.4) 1373 m 4.1 1469 m N6-N5 (17.3-21.5) 1493.5 m ?N4b (21.5-23.2) DLS4 3.2 3.1 1630 m N5-P22 (18.8-27.1) DLS3 2.5 1731 m N9 or younger (<15.1) 1767 m N4 (21.5-23.8) 1786.1 m N4-P22 (21.5-27.1) 2.4 2.3 1895.9 m P22 (23.8-27.1) 1959 m P22-P19 (23.8-32) 2.2 2.1 DLS2 DLS1 2133 m P17 (33.8-34) 1 2 3 4 5 6 7 8 LITHOLOGY calcarenite calcisitite calcilutite dolomite marl sandstone calcite claystone PALEOBATHYMETRY 1 Transitional Marine (0 m) 5 Upper Bathyal 2 Inner Neritic (0-20 m) 6 Middle Bathyal 3 Middle Neritic (20-100 m) 7 Lower Bathyal 4 Outer Neritic (100-200 m) 8 Abyssal Confidence good poor med 1800 m (200-500 m) (500-1000 m) (1000 - 2000 m) (>2000 m) casing shoe Plate 4. Composite Well Log, Goodwyn 4 Eastbrook 1 SPUD - 1998 Depth6 (m) 6 0 GR (API) CALIPER (in) BIT SIZE (in) SP (MV) -80 150 36 36 20 WD 168.1 m KB 25.9 m TD 3476 m 1.7 rb (g/cm ) 2.95 0.45 NPHI (PU)-0.15240 PEF (b/e) 10 0 3 Lithology Samples Paleobathymetry Seismic (Blow, 1969; Ties Berggren et al., 1995) (Swc) (DC) Biostratigraphy DT (ms/ft) 40 Depth (m) seafloor 226 m seafloor G.Moss, UTIG personal , communication, 2000, all other dates from WCR NO RETURNS DLS_top 5.5 5.4 5.3 1673 m 5.2 5.1 DLS5 4.3 DLS4 3.1 2240 m Eocene-Oligocene boundary DLS2 DLS1 1 2 3 4 5 6 7 8 LITHOLOGY calcarenite calcisitite calcilutite dolomite marl sandstone calcite claystone PALEOBATHYMETRY 1 Transitional Marine (0 m) 5 Upper Bathyal 2 Inner Neritic (0-20 m) 6 Middle Bathyal 3 Middle Neritic (20-100 m) 7 Lower Bathyal 4 Outer Neritic (100-200 m) 8 Abyssal Confidence good poor med 1800 m (200-500 m) (500-1000 m) (1000 - 2000 m) (>2000 m) casing shoe Plate 5. Composite Well Log, Eastbrook 1 Goodwyn 6 SPUD - 1981 GR (API) 150 Depth 0 (m) 6 CALIPER (in) 30 WD 124.0 m KB 8.0 m TD 4664 m 1.7 rb (g/cm ) 2.95 0.45 NPHI (PU)-0.15240 PEF (b/e) 10 0 3 Lithology Samples Paleobathymetry Seismic (Blow, 1969; (Swc) (DC) Ties Berggren et al., 1995) Biostratigraphy DT (ms/ft) 40 Depth (m) seafloor 155 m seafloor NO RETURNS G.Moss, UTIG personal , communication, 2000, all other dates from WCR 484 m 810 m indeterminate DLS_top 885 m N17-N8 (5.6-16.4) 5.3 1050 m N9 or older (>14.8) 5.2 5.1 1230 m mid Miocene (11.2-16.4) 1235 m indeterminate DLS5 4.3 4.1 DLS4 3.2 3.1 DLS3 1570 m early Miocene (16.4-23.8) 2.5 2.3 2.2 1715 m N5-N4 (18.8-23.8) 1800 m 2.1 1940 m N4 (21.5-23.8) DLS2 1980 m late Oligocene (23.8-28.5) DLS1 2090 m late Eocene (33.7-37) 1 2 3 4 5 6 7 8 LITHOLOGY calcarenite calcisitite calcilutite dolomite marl sandstone calcite claystone PALEOBATHYMETRY 1 Transitional Marine (0 m) 5 Upper Bathyal 2 Inner Neritic (0-20 m) 6 Middle Bathyal 3 Middle Neritic (20-100 m) 7 Lower Bathyal 4 Outer Neritic (100-200 m) 8 Abyssal Confidence good poor med 1800 m (200-500 m) (500-1000 m) (1000 - 2000 m) (>2000 m) casing shoe Plate 6. Composite Well Log, Goodwyn 6 Goodwyn 7 SPUD - 1985 GR (API) 150 Depth 0 (m) 6 CALIPER (in) 36 WD 134.0 m KB 17.0 m TD 3446 m 1.7 Lithology Biostratigraphy (Blow, 1969; Berggren et al., 1995) Samples Paleobathymetry Seismic (Swc) (DC) Ties rb (g/cm3) 2.95 240 DT (ms/ft) 40 Depth (m) seafloor 174 m seafloor NO RETURNS G.Moss, UTIG personal , communication, 2000, all other dates from WCR 443 m DLS_top 5.5 5.3 5.2 5.1 DLS5 4.3 4.1 DLS4 3.1 DLS3 2.5 2.4 1803 m 2.3 2.2 2.1 DLS2 DLS1 1 2 3 4 5 6 7 8 BTERT SANTONIAN ALBIAN TITHONIAN UC 2924 m LITHOLOGY calcarenite calcisitite calcilutite dolomite marl sandstone calcite claystone PALEOBATHYMETRY 1 Transitional Marine (0 m) 5 Upper Bathyal 2 Inner Neritic (0-20 m) 6 Middle Bathyal 3 Middle Neritic (20-100 m) 7 Lower Bathyal 4 Outer Neritic (100-200 m) 8 Abyssal Confidence good poor med 1800 m (200-500 m) (500-1000 m) (1000 - 2000 m) (>2000 m) casing shoe Plate 7. Composite Well Log, Goodwyn 7

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