12: LECTURE Sequence Stratigraphy
A branch of stratigraphy that subdivides the rock record into a succession of depositional sequences as regional and interregional correlative units.
Conformable succession of genetically-related strata, bounded at the top and base by: Unconformities (surfaces of erosion) or their Correlative conformities (surfaces lacking erosion) Sequence Boundaries represent: Basinward shifts in depositional facies Incis Surfaces of erosion and/or noned V alley Rive deposition F r de posi ill: t Surfaces across which there may be S.B. a significant biostratigraphic break C
ont i nent a depo l Shelf sit
Depositional Sequence
Class reading from Boggs, Principles of Sedimentology and Stratigraphy Chapter 13: p.451 - 462.
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Headland versus Shoreline Attached SequenceBoundaries
Open water
Numerical model of T. Sun et al., WWR, v. 38, n.8, 10.1029/2001WR000284, 2002
Headland-Attached Sequence Boundary MAXIMUM EROSION AT UPSTREAM POSITION IN SYSTEM. Can be produced through 1) uplift in the basin interior or basin margin, or 2) increase in water discharge associated with river channel.
Open water
Shoreline-Attached Sequence Boundary Produced by fall in elevation of sea level.
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How does a shoreline respond to a change in relative sea-level? The answer depends on whether the depositional system is carrying a sufficient amount of sediment to build land at the rate that space is being created by the sum of sea-level change and basin subsidence (sum = relative sea-level change). Relevant Ratio:
<q s >
< H + > L
shoreline
area created = area filled
L
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Transgression: landward migration of the shoreline Regression: seaward migration of the shoreline Progradation: seaward growth of entire alluvial system Retrogradation: landward retreat of the entire alluvial system.
1 2 34 5
Formation of Incised Valley
Time 3
.5 .3 .2
.4
.1
Cross section: white = non-marine deposition Black = marine deposition
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Connecting Sequence Boundaries, Flooding Surfaces & Incised Valleys
Floodplain
Shoreline
(Van Wagoner et al., 1990)
B
Maximum Flooding Surfaces Basinwide drowning Maximum shoreward extent of the deepest water facies Significant biostratigraphic boundaries
Control of sea level on coastal stratigraphy
in Sea Level 22 16 thousand years ago
[SEPM Special Pub 79, 2004]
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Topographic maps of buried, 20,000 year-old surface showing valleys cut into marine deposits at lowstand in sea level.
Lowstand deltas deposits at the downstream ends of these valleys, at the edge of the continental shelf. Delta A is connected to the Colorado River Delta B & C to the Brazos & Trinity & Sabine rivers.
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Seismic imaging of TX deltas during last lowstand in sea level (Deltas A & B)
Notice that these deltas a feeding sediment into the deep ocean.
[SEPM Special Pub 79, 2004]
What is the response of the coastal system to rising sea-level?
Notice progressive reduction in rate of sea-level rise.
1. Filling of valleys cut during lower base level (Mississippi River example to the left). 2. Flooding of coastline (see next page).
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Coastline today: 1. Notice that the Sabine Shoreline of 5500 BP is now Sabine Bank, 10 m below mean sea level. 2. Notice that there is no topographic expression of the ~ 20,000 BP valleys. They are now filled and buried by sediment.
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Upstream Limit to the Control of Sea-level Change on Sedimentation Patterns.
CBA-3 = Mid Holocene CBA-2 = Early Holocene ELA = ~ 20,000 years BP, last glacial maximum
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Sea-level Rise (mm)
C
B
Recent Progradation of the Galveston Island Bolivar Peninsula segment of TX shoreline. Notice the recent of regression the shoreline tied to a low rate of eustatic sea-level rise. Sediment delivered by rivers to the coastal zone exceeds the space being created by sea-level rise and sedimentation is pushing the shoreline seaward.
A
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Recent progradation of the shoreline during condition of slow sealevel rise. The shoreline is prograding because the TX rivers are providing enough sediment to more than fill the space created by sea-level rise and basin subsidence.
Galveston Island at San Luis Pass
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Hierarchy of Stratal Units in Sequence Stratigraphy
Van Wagoner, J. C., Mitchum, R. M., Campion, K. M., Rahmanian, V. D., Siliciclastic sequence stratigraphy in well logs, cores, and outcrops; concepts for high-resolution correlation of time and facies, Methods in Exploration Series, 7, p. 55, 1990.
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Progradation of the distributary channel and its mouth bar into the open sea produces a coarsening and thickening upward sequence. The abandonment of the channel causes the sequence to be overlain (usually abruptly) by mudstones.
Shallowing upward parasequence
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Dots on maps mark a hypothetical position for the stratigraphic section drawn to the right. Parasequence a relatively conformable, genetically related succession of beds bounded by marine flooding surfaces or their correlative surfaces.
Shallowing-upward Parasequences
T5
T4-T6 = Parasequence Boundaries
(Van Wagoner et al., 1990)
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Parasequence-stacking patterns in parasequence sets; cross-section and well-log expression.
(Van Wagoner et al., 1990)
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Aggradational and Progradational Parasequence Sets
Pletmos Basin, offshore South Africa [Cretaceous (Barremian)]
~500m
Brown, L. Frank, Jr., Benson, James M., Brink, Gerhard J., Doherty, Sheila, Jollands, Andy, Jungslager, Eric H. A., Keenan, J. H. G., Muntingh, Andre, van Wyk, Nicolaas J. S., Sequence stratigraphy in offshore South African divergent basins; an atlas on exploration for Cretaceous lowstand traps by Soekor (Pty) Ltd, AAPG Studies in Geology, 41, p. 184, 1995.
Lab parasequences
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Retrogradational Parasequence Set
31m
1.6km
(Van Wagoner et al., 1990)
Subsurface Example: Almond Sandstone, Washakie Basin, WY
Picture 10
LAB
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Questions You Should be Able to Answer
1. What is sequence stratigraphy? 2. What is a depositional sequence? 3. What are key properties of sequence boundaries? 4. What are the possible causes of a headland-attached sequence boundary? 5. What is the cause of a shoreline-attached sequence boundary? 6. What is base level? 7. What is eustatic sea level? 8. What is relative sea level? 9. How does sediment delivered to the coastal zone influence change in shoreline position? 10. What is transgression, regression, progradation, and retrogradation? 11. What is a maximum flooding surface? 12. What is an incised valley? 13. When was the last glacial maximum and how is it connected to eustatic sea level? 14. What was the elevation of sea level at its last lowstand, ~ 20,000 years ago? 15. What is a lowstand delta? 16. What are the rates of eustatic sea-level rise during the past 10,000 years? 17. How far upstream from the present-day coastline are patterns of erosion and sedimentation associated with sea-level change observed on the Colorado River? 18. Is Galveston Island prograding or retrograding and for how long has this process been occurring? 19. How can shorelines prograde under conditions of relative sea-level rise?
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Questions You Should be Able to Answer
20. What is parasequence? 21. What is an aggradational, progradational, and retrogradational parasequence set? 23. Why do parasequences commonly consist of shallowing-upward sedimentary deposits?
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