402-pp9 - UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary...

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Unformatted text preview: UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary Petrology Lecture 9: Walther's Law Last Time Sedimentary Facies 1. What are facies 2. Defining characteristics of facies 3. Facies and depositional environments Sedimentary Facies There are literally dozens of different depositional environments that cover every imaginable marine and non-marine situation. And in most cases, there are multiple varieties of each basic environment of deposition. For example, rivers come in at least 3 "flavors": 1) Meandering 2) Braided 3) Anastimosing Sedimentary Facies Why do sedimentologists collect all this data in the first place? Ultimately, it is in order to interpret the environment of deposition of the sediment/sedimentary rocks. http://earthsci.org/mineral/rockmin/sed/clastics.gif http://earthsci.org/mineral/rockmin/sed/clastics.gif Sedimentary Facies Defining characteristics for facies include: Grain size Lithology Mineralogy Paleontology Sedimentary and biogenic structures Palaeocurrent orientations Obvious sedimentary trends Bedding (Bed thickness) Lamination Bedding contacts Lateral variations Anything else that you can think of Sedimentary Facies Facies can, and will, repeat vertically through a sedimentary sequence, but may vary in character as a result of environmental and/or evolutionary change through time e.g., fossil content. offshore (10m) shoreface (5m) swash zone (0m) shoreface (5m) Sedimentary Facies Facies may also change laterally through a deposit as a result of changing environments with distance at the same time. Then they determine which depositional environment best fits this variety of facies. This is called: Facies Modeling http://geology.isu.edu/Digital_Geology_Idaho/Module2/Belt_SedStruct_sml.jpg http://geology.isu.edu/Digital_Geology_Idaho/Module2/Belt_SedStruct_sml.jpg Facies Modeling Facies modeling is best done back at your lab/office where you can think about your data. And drink a few beers to help you think about your data. Prograding Open Beach Today's Agenda 1. Walther's Law 2. Sequence stratigraphy 3. Markov Chain Analysis Walther's Law Walther's Law Named after Johannes Walther (18601937), a German geologist, who in 1894, noted a fundamental relationship between the vertical and lateral distribution of facies. Walther's Law Sedimentary environments that started out side-by-side will end up overlapping one another over time due to transgressions and regressions. Walther's Law Sedimentary environments that started out side-by-side will end up overlapping one another over time due to transgressions and regressions. The result is a vertical sequence of beds. The vertical sequence of facies mirrors the original lateral distribution of sedimentary environments. Walther's Law Sedimentary environments that started out side-by-side will end up overlapping one another over time due to transgressions and regressions. Walther's Law But... Walther's Law can only apply to sections without unconformities. Tan and white layers of Mesozoic Era Period Pio Nono Formation in Georgia's Coastal Plain Province. http://itc.gsw.edu/faculty/daskren/fallline.htm http://itc.gsw.edu/faculty/daskren/fallline.htm Walther's Law And... Walther's Law can only apply to a section without subdividing diachronous boundaries [e.g., transgressive surfaces (TS), maximum flooding surfaces (mfs) etc.] Sequence Stratigraphy First utilized by the petroleum industry to interpret depositional surfaces on seismic sections. Now used by all geologists to explain vertical and lateral changes in sediment rock distribution. http://strata.geol.sc.edu/exerices/seismic/07SeqNo_LST_TST_HST.jpg http://strata.geol.sc.edu/exerices/seismic/07SeqNo_LST_TST_HST.jpg Sequence Stratigraphy First utilized by the petroleum industry to interpret depositional surfaces on seismic sections. Now used by all geologists to explain vertical and lateral changes in sediment rock distribution. http://strata.geol.sc.edu/exerices/seismic/07SeqNo_LST_TST_HST.jpg http://strata.geol.sc.edu/exerices/seismic/07SeqNo_LST_TST_HST.jpg The 3 controls are: 1) sea level position, 2) sediment input, 3) accommodation space. Sequence Stratigraphy Changes in sea level and sediment supply produce changes in the "stacking of sedimentary packages". Here sediment supply keeps up with sea level changes Sequence Stratigraphy Here sediment supply exceeds sea level change. Sequence Stratigraphy Here sea level changes exceeds sediment supply. Sequence Stratigraphy Ultimately, sedimentary "packages" are bounded by specific surfaces (reflectors in the seismic lines), and this is where/when Walther's Law breaks down Walther's Law But even with these limitations, Walther's Law offers powerful predictive capabilities, especially for basic facies modeling. Consider the adjacent sequence... Walther's Law SS: Scoured contact Walther's Law SS: Scoured contact A: laminated red shale Walther's Law SS: Scoured contact B: rippled siltstone A: laminated red shale Walther's Law C: Cross-stratified c-quartz arenite SS: Scoured contact B: rippled siltstone A: laminated red shale Walther's Law D: Parallel laminated f-quartz arenite C: Cross-stratified c-quartz arenite SS: Scoured contact B: rippled siltstone A: laminated red shale Walther's Law E: Trough cross bedded vc-quartz arenite D: Parallel laminated f-quartz arenite C: Cross-stratified c-quartz arenite SS: Scoured contact B: rippled siltstone A: laminated red shale Walther's Law F: massive c-quartz arenite E: Trough cross bedded vc-quartz arenite D: Parallel laminated f-quartz arenite C: Cross-stratified c-quartz arenite SS: Scoured contact B: rippled siltstone A: laminated red shale Walther's Law G: rippled c to vc-quartz arenite F: massive c-quartz arenite E: Trough cross bedded vc-quartz arenite D: Parallel laminated f-quartz arenite C: Cross-stratified c-quartz arenite SS: Scoured contact B: rippled siltstone A: laminated red shale Walther's Law Recall that facies repeat in a sedimentary sequence. If you determine the number and variety of transitions, you can start to understand the means by which facies shifted during deposition. Walther's Law Below are the observed facies transitions for the outcrop(s) in question. The problem is that you really don't know which are random. All data in these slides from Walker (1979) Walther's Law Enter statistics (Markov Chain Analysis). Add up the number of transitions from each facies to every other facies and put in a 9 x 9 matrix (one row/column per facies) Walther's Law SS SS A B C D E F G 2 5 1 1 2 4 1 2 4 A 12 B 2 6 C 1 3 2 D E F G 1 2 1 2 1 1 1 1 2 1 Walther's Law SS SS A B C D E F G .667 1.00 .333 .500 .154 .308 .077 .286 .571 A .800 B .133 .462 C .067 .231 .154 D E F G .077 .154 .143 .667 .500 .077 .077 .077 .154 .333 Then calculate the observed transition probabilities (each row =1.00). Walther's Law Next calculate transition probabilities for a random sequence Rij = nj N-ni Where Rij is the random probability of transition from facies i to j, ni and nj are the number of occurences of facies i and j and N is the total number of occurrences of all facies Walther's Law SS SS A B C D E F G .280 .259 .237 .222 .215 .222 .226 .315 .288 .270 .262 .270 .274 .220 .206 .200 .206 .210 .127 .123 .127 .129 .062 .063 .065 .032 .032 .065 A .320 B .245 .260 C .151 .160 .148 D .075 .080 .074 .068 E .038 .004 .037 .034 .032 F .075 .080 .074 .068 .063 .062 G .094 .100 .093 .085 .079 .077 .079 Now it's a matter of simple math. Observed random transitions Walther's Law SS SS A B C D E F G -.13 +.05 -.24 +.11 -.22 +.45 +.77 -.24 -0.0 -.27 -.26 -.27 -.27 +.35 -.21 -.20 -.20 -.21 -.13 +.38 -.13 -.13 -.06 -.06 -.06 -.03 -.03 -.07 A +.48 B -.11 +.20 C -.08 +.07 +.01 D -.08 -.08 +.08 +.08 E -.04 +.04 +.04 -.03 -.03 F -.08 -.08 +.00 -.07 +.60 +.44 G -.09 -.02 +.06 -.09 -.08 -.08 +.25 Positive transitions occur in nature, high positive transitions dominate. Walther's Law Walther's Law Upcoming Stuff Homework 1) Finish practice sedimentary section exercise (due Thursday) 2) Grain Size Results: Due Thursday 3) Grain Size Introduction/Methods-redo: Due Friday Today's Lab 1) Bogus Sedimentary section 4) St Stephens Quarry composite section; exchange data! Thursday: No lecture. Flume activity (time to play!) ...
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