UNIVERSITY OF SOUTH ALABAMA GY 402: Sedimentary Petrology Lecture 10: Petrology of Mature Siliciclastic Sed. Rocks Instructor: Dr. Douglas W. Haywick Last Time (on line) 1. Walther’s Law 2. Sequence stratigraphy 3. Markov Chain Analysis Walther’s Law Named after Johannes Walther (1860- 1937), 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 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 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 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 Walther’s Law (Markov Chain Analysis) SS A B C D E F G SS +.48 -.11 -.08 -.08 -.04 -.08 -.09 A -.13 +.20 +.07 -.08 +.04 -.08 -.02 B +.05 -.24 +.01 +.08 +.04 +.00 +.06 C -.24 -0.0 +.35 +.08 -.03 -.07 -.09 D +.11 -.27 -.21 -.13 -.03 +.60 -.08 E -.22 -.26 -.20 +.38 -.06 +.44 -.08 F +.45 -.27 -.20 -.13 -.06 -.03 +.25 G +.77 -.27 -.21 -.13 -.06 -.03 -.07 Positive transitions occur in nature, high positive transitions dominate. Walther’s Law (Markov Chain Analysis) 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 (Markov Chain Analysis) 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 (Markov Chain Analysis) Next calculate transition probabilities for a random sequence nj Rij = 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 (Markov Chain Analysis) 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 Online Lecture (student recap): 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. 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. 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. Important Parameters Grain rounding Important Parameters Grain rounding Immature Mature Increasing transport distance Important Parameters Grain sorting Important Parameters Grain sorting Low energy High energy increasing energy of deposition Important Parameters Grain size Important Parameters Grain size High energy Low energy Decreasing energy of deposition 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 XN Predates cements Generally dark in color PPL 100 µm Matrix versus Cement Cement Homogeneous Chemically pure Lines pores Specific fabrics (acicular, drusy, overgrowths etc.) Multiphased Zoned 50 µm PPL Hematite cement Quartz cement Matrix versus Cement Matrix Cement Heterogeneous Homogeneous Chemically impure Chemically pure Drapes over grains* Lines pores* Predates cements Specific fabrics Generally dark in color* 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 xn 750 m cement polycrystalline quartz Thin-section Photomicrographs xn xn 1250 m cement Chert sedimentary rock fragment Thin-section Photomicrographs Weakly cemented (friable) ppl xn 500 m porosity quartz Thin-section Photomicrographs Strongly cemented ppl xn 250 m quartz Thin-section Photomicrographs Glauconite cement ppl xn 250 m glauconite Thin-section Photomicrographs Chalcedony cement ppl xn 500 m chalcedony Thin-section Photomicrographs Chalcedony cement ppl xn 500 m chalcedony Thin-section Photomicrographs Quartz (overgrowth) cement ppl xn 125 m Quartz overgrowth cement Upcoming Stuff Homework Write 3 due Today Today’s Lab Activity 3: James Sed Section/Grain Size Online Lecture 9: Walter’s Law Next Week…. BUSY!: 1) Online paper Activity 4 and Midterm exam 2) Midterm exam issued Tuesday due Tuesday February 14th 3) Thin-section analysis in labs 4) Embedded writing assignments begin in labs (they are redo-able; the rest of the labs are NOT) GY 402: Sedimentary Petrology Lecture 10: Mature Siliciclastic Sedimentary Rocks Instructor: Dr. Doug Haywick [email protected] This is a free open access lecture, but not for commercial purposed. For personal use only. .