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Sedimentary Rock Classification

Sedimentary Rock Classification. Sedimentology 340. Classification of sedimentary rocks is based first on the two main types of rocks: I. Siliciclastic Sedimentary Rocks (from clasts, i.e.detritus) II. Chemical & Biochemical Rocks (from precipitates).

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Sedimentary Rock Classification

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  1. Sedimentary Rock Classification Sedimentology 340

  2. Classification of sedimentary rocks is based first on the two main types of rocks: I. Siliciclastic Sedimentary Rocks (from clasts, i.e.detritus) II. Chemical & Biochemical Rocks (from precipitates) I. Siliciclastic Sedimentary Rocks are classified according to dominant grain size of the clasts: A. Conglomerates (gravel size, > 2mm) B. Sandstones (sand size, 2mm – 63 microns) C. Mudstones / Shales (63 microns >silt & clay size)

  3. Conglomerates Sandstones Mudstones

  4. A. Conglomerates are also classified by shape: 1. Conglomerates (rounded clasts) 2. Breccias (angular clasts) by grain composition of framework grains: 1. oligomict – all one composition (e.g. quartz) 2. polymict – mixed lithologies by mode of origin: Epiclastic (weathering of rocks) Volcanoclastic (volcanic eruptions) Cataclastic (in faults) Meterorite Impact by sorting (% matrix) a. clast –supported conglomerate b. matrix-supported conglomerate

  5. A. Conglomerates are also classified by shape: 1. Conglomerates (rounded clasts) 2. Breccias (angular clasts) conglomerate breccia

  6. Grain compoistion Oligomictic vs. Polymictic Oligomictic = >90% resistant grains; Polymictic = unstable frags dominate Quartz pebble oligomictic Rock cobble polymictic

  7. by mode of origin: Epiclastic (normal weathering of rocks) Volcanoclastic (volcanic eruptions) Cataclastic (in faults) Meterorite Impact tuff Fault zone shear Produces tectites

  8. by Sorting Matrix-supported 15 - 50% matrix Grain-supported <15% matrix

  9. Diamictite - matrix-supported, poorly sorted sediment (usually glacial in origin)

  10. Sandstones Framework grains Q = Quartz F = Feldspar L = Lithic (rock fragment) Compositional Approach Like Fig 5.5

  11. Sandstone A 3-D classification can be created by adding matrix to framework grains Arenites – 0-15% Wackes – 15-75 % Mudstones-75-100%

  12. Mudstones / Shale Most abundant sedimentary rocks (55-60%) Shale – has fissility (called flaggy, slabby, platy) Argillite – slightly metamorphosed Slate - metamorphosed Poorly understood weather easily mineralogy hard to determine fine-grained- hard to see grains easily deformed, absorb water Grain Size 0%- 1/3 clay = siltstone 1/3-2/3 clay = mudstone 2/3-100% clay =claystone -color (red (Fe2O3) black (organic matter), gray, brown, white) bulk chemical composition (calcareous, phosphatic, micaceous, quartzose)

  13. ORIGIN AND OCCURRENCE -Low energy environments lakes loess (wind blown dust) river flood plains lagoons estuaries sea

  14. COMPOSITION -Average Shale 30% quartz 10% feldspar 50% clay minerals 10% cement -clay mineralogy – garbage-can minerals because they contain a wide variety of cations and anions

  15. Clay minerals are phyllosilicates (sheet slicates) T-sheets and O-sheets T-sheets – tightly bonded silicate tetrahedron O-sheets (OH) and Ca, Al, Mg cations

  16. 1:1 = T O Clay Minerals kaolinite 1:1 (Al) illite 2:1 (K) smectite 2:1 (Mg and other cations) glauconite 2:1 (Fe) chlorite 2:1 (Mg, Al, Fe) 2:1 = T O T T = tetrahedral O = Octohedral

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