402-pp11 - UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary...

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Unformatted text preview: UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary Petrology Lecture 11: Petrology of Mature Siliciclastic Sed. Rocks Last Time 1. Walther's Law 2. Sequence stratigraphy 3. Markov Chain Analysis 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. 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. 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 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 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 Randomness can be determined through mathematical relationships 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 Today's Agenda 1. Recap major types of sediment & sedimentary rock 2. Recap important grain parameters 3. Cement versus matrix 4. Mature sediment/sedimentary rocks Petrography Game Plan Tuesday Lecture: petrographic parameters Thursday Lecture: environments of deposition Tues/Thurs. Labs: representative samples (thin-sections and hand specimens) Major Sedimentary Rock Types* * Examined in GY 402 Major Sedimentary Rock Types Siliciclastic Major Sedimentary Rock Types Siliciclastic Volcaniclastic Major Sedimentary Rock Types Siliciclastic Volcaniclastic Carbonates (evaporites/chemical, non-skeletal, skeletal) Siliciclastic Sedimentary Rocks Mature versus immature varieties... ... are best distinguished via QFR ternary plots Siliciclastic Sedimentary Rocks Q Q = quartz F = feldspars R = lithic fragments (includes chert) F R Source: Blatt, H., Middleton, G and Murray, R., 1980: Origin of Sedimentary Rocks. Prentice Hill, 782 p. Blatt, Rocks. Siliciclastic Sedimentary Rocks Q "Mature" rocks (mineralogically stable) Enriched in quartz and clay minerals F R Source: Blatt, H., Middleton, G and Murray, R., 1980: Origin of Sedimentary Rocks. Prentice Hill, 782 p. Blatt, Rocks. Siliciclastic Sedimentary Rocks Q "Immature" rocks (mineralogically unstable) Enriched in feldspars and unstable rock fragments F R Source: Blatt, H., Middleton, G and Murray, R., 1980: Origin of Sedimentary Rocks. Prentice Hill, 782 p. Blatt, Rocks. Important Parameters Grain rounding Important Parameters Grain rounding Immature Increasing transport distance Mature Important Parameters Grain sorting Important Parameters Grain sorting Low energy increasing energy of deposition High energy Important Parameters Grain size Important Parameters Grain size High energy Decreasing energy of deposition Low energy Mature Siliciclastic Petrography PPL 250 :m XN 250 :m Mature Siliciclastic Petrography Quartz-rich (quartz arenites) XN 250 :m Mature Siliciclastic Petrography Quartz-rich (quartz arenites) Generally well rounded grains XN 250 :m Mature Siliciclastic Petrography Quartz-rich (quartz arenites) Generally well rounded grains Poorly sorted to well sorted XN 250 :m Mature Siliciclastic Petrography Quartz-rich (quartz arenites) Generally well rounded grains Poorly sorted to well sorted Gravel to clay sized grains XN 250 :m Mature Siliciclastic Petrography An important question.... XN 250 :m Mature Siliciclastic Petrography An important question.... ...What holds the rock together? XN 250 :m Matrix versus Cement Matrix versus Cement Matrix: fine-grained* material deposited simultaneously with larger particles. Generally appears as darker-coloured detritus between grains * This is a relative term. Matrix is material that is finer than the dominant particle size comprising the sand/gravel sediment fraction. Matrix versus Cement Matrix: fine-grained* material deposited simultaneously with larger particles. Generally appears as darker-coloured detritus between grains Cement: a chemical precipitate between grains formed from pore-water long after deposition. Matrix versus Cement Matrix Heterogeneous Chemically impure Drapes over grains Predates cements Generally dark in color XN PPL 100 m Matrix versus Cement Cement Homogeneous Chemically pure Lines pores Specific fabrics (acicular, drusy, overgrowths etc.) Multiphased Zoned 50 m PPL Hematite cement Quartz cement Matrix versus Cement Matrix Heterogeneous Chemically impure Drapes over grains* Predates cements Generally dark in color* Cement Homogeneous Chemically pure Lines pores* Specific fabrics Multiphased Zoned * Can be confusing Hand specimens Mature siliciclastic sandstones (quartz arenite) Liesengang banding, clay and iron oxide cement Quartz and chalcedony cement 4 cm Limonite cement Thin-section Photomicrographs ppl xn 750 :m monocrystalline quartz Thin-section Photomicrographs ppl cement xn 750 :m polycrystalline quartz Thin-section Photomicrographs xn cement Chert xn 1250 :m sedimentary rock fragment Thin-section Photomicrographs Weakly cemented (friable) ppl porosity quartz xn 500 :m Thin-section Photomicrographs Strongly cemented ppl quartz xn 250 :m Thin-section Photomicrographs Glauconite cement ppl glauconite xn 250 :m Thin-section Photomicrographs Chalcedony cement ppl chalcedony xn 500 :m Thin-section Photomicrographs Chalcedony cement ppl chalcedony xn 500 :m Thin-section Photomicrographs Quartz (overgrowth) cement ppl xn Quartz overgrowth cement 125 :m Upcoming Stuff Homework 1) Embedded writing assignments in labs (they are redo-able; the rest of the labs are NOT) 2) Grain size project due Friday This Week's Lab Mature sedimentary rocks (Due Thursday) This Thursday: 1) Beaches and Barrier Islands ...
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This note was uploaded on 02/05/2012 for the course GLY 402 taught by Professor Staff during the Fall '11 term at S. Alabama.

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