7-2 sedimentology

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Sedimentary rocks Clastic (siliciclastic) rocks (80-85% of the stratigraphic record) Carbonate sediments and rocks (10-15% of the stratigraphic record) Organic (carbonaceous) sediments and rocks Evaporites Volcaniclastic sediments and rocks

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7-2 sedimentology

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  • Sedimentary rocks

    Clastic (siliciclastic) rocks (80-85% of the stratigraphic record) Carbonate sediments and rocks (10-15% of the stratigraphic

    record) Organic (carbonaceous) sediments and rocks Evaporites Volcaniclastic sediments and rocks

  • Unconsolidated clastic sediments

    Particles (or clasts) are the basic elements of any sediment Clastic (terrigenous clastic or siliciclastic) sediments (80-85% of the

    stratigraphic record) consist of particles derived from pre-existing rocks, as opposed to non-clastic sediments

    Texture Grain (particle) size Grain shape Clast/matrix relationships Fabric

    Lithology is the general characterization of a sediment or a sedimentary rock (e.g., coarse sand, mudstone)

  • Unconsolidated clastic sediments

    The Udden-Wentworth grain-size scale classifies particles of siliciclastic sediment from 4096 millimeters (boulders) in size down to 0.00006 millimeters (clay). It is based on factors of two:

    = -log2 (mm)

    Mud (4 ) Clay (8 ) Silt (463 m; 48 )

    Sand (632000 m; -14 ) Gravel (>2000 m;

  • Unconsolidated clastic sediments

    Grain shape Roundness (well rounded to very angular) Sphericity (high or low)

    Clast/matrix proportion

    The matrix is the relatively fine-grained material that lies between the relatively coarse-grained clasts Clast-supported sediments (clasts are in direct contact) Matrix-supported sediments (clasts are entirely surrounded by matrix)

    Fabric

    Preferential orientation of particles in a sediment or tendency of a rock to break in specific directions

  • Unconsolidated clastic sediments

  • Unconsolidated clastic sediments

    Clay minerals

    Clay minerals are phyllosilicates with layered crystal structures Kandite group (two layers): kaolinite Smectite group (three layers): montmorillonite, illite, chlorite

    Key physical and chemical characteristics of clay minerals Platy shape (easy to keep in suspension, very slow settling rates) Strong cohesion due to electrostatic charge (relatively difficult to erode)

  • Sedimentary rocks

    Clastic (siliciclastic) rocks

    Sandstones (20-25% of the stratigraphic record) can be subdivided according to the Pettijohn classification, based on texture and composition (relative proportions of quartz, feldspar, and lithic fragments) Quartz arenite: quartz-dominated Arkosic arenite: feldspar-dominated Lithic arenite: dominance of lithic fragments Wacke: significantly matrix-supported (>15% mud)

    Quartz wacke Greywacke (feldspathic or lithic wacke)

  • Unconsolidated clastic sediments

    Sediment maturity (degree of change compared to original bedrock: provides evidence on the history of a sediment) Textural (mud content, sorting, grain shape) Mineralogical (proportion of stable or resistant minerals)

  • Sedimentary rocks

    Clastic (siliciclastic) rocks

    Mudstones (60% of the stratigraphic record) are also known as mudrocks or shales and commonly exhibit a distinct fissility Claystone Siltstone

    Conglomerates are consolidated gravels; breccias are conglomerates with

    dominantly angular clasts Clast-supported conglomerates Matrix-supported conglomerates

  • Sedimentary rocks

    Carbonate sediments and rocks

    Principal minerals: calcite, aragonite (unstable), and dolomite (diagenetic) Principal rocks: limestone (>50% CaCO3) and dolomite (dolostone)

    (CaMg(CO3)2) Formation of carbonate sediments and rocks occurs by means of two

    main processes: Biomineralization of CaCO3 by organisms Direct chemical precipitation

  • Sedimentary rocks

    Carbonate sediments and rocks

    Biogenic carbonate formation occurs by a wide range of organisms (e.g., molluscs, corals, forams, algae, bacteria, and many others) Most organisms initially form unconsolidated carbonate sediments Coral reefs and microbial mats (e.g., stromatolites) are examples of more solid

    carbonate structures Chemical precipitation produces non-skeletal carbonate grains of various

    sizes (e.g., ooids, pisoids, micrite)

  • Sedimentary rocks

    Carbonate sediments and rocks

    Carbonate sand usually consists either of (fragmented) skeletal remains or non-skeletal grains

    Carbonate mud (micrite) is commonly the product either of chemical precipitation or algal/bacterial activity

    Dunham classification of carbonate rocks: Texturally-based subdivision (cf. clastics). Most diffuse: mudstone,

    wackestone, packstone, grainstone, rudstone Organically bound framework during formation: boundstone

  • Sedimentary rocks

    Organic (carbonaceous) sediments and rocks

    Coal consists primarily of solid organic matter; the remainder is known as ash

    Carbonaceous shales have a lower proportion of solid organic matter Oil shales (may be formed in anaerobic lake and marine environments)

    contain organic matter that can be driven off as liquid or gas by heating

  • Sedimentary rocks

    Evaporites

    Dissolved salts precipitate out of sea water due to concentration (brine formation) during evaporation (1 km of sea water --> 12 m of evaporites)

    Evaporites commonly lithify into consolidated rocks upon formation Least soluble compounds precipitate first:

    CaCO3 (calcium carbonate) CaSO4 (calcium sulphate: gypsum or anhydrite) NaCl (halite: rock salt) Other, less stable (highly soluble) chlorides

  • Sedimentary rocks

    Volcaniclastic sediments and rocks

    Lava (cooled magma flows) produces volcaniclastic sediment upon weathering

    Sedimentary rocksDiapositiva numero 2Unconsolidated clastic sedimentsDiapositiva numero 4Unconsolidated clastic sedimentsDiapositiva numero 6Diapositiva numero 7Unconsolidated clastic sedimentsDiapositiva numero 9Unconsolidated clastic sedimentsUnconsolidated clastic sedimentsDiapositiva numero 12Sedimentary rocksDiapositiva numero 14Diapositiva numero 15Diapositiva numero 16Unconsolidated clastic sedimentsSedimentary rocksDiapositiva numero 19Diapositiva numero 20Sedimentary rocksDiapositiva numero 22Diapositiva numero 23Sedimentary rocksDiapositiva numero 25Diapositiva numero 26Diapositiva numero 27Diapositiva numero 28Diapositiva numero 29Diapositiva numero 30Diapositiva numero 31Sedimentary rocksDiapositiva numero 33Sedimentary rocksDiapositiva numero 35Diapositiva numero 36Diapositiva numero 37Sedimentary rocksDiapositiva numero 39Diapositiva numero 40Diapositiva numero 41Diapositiva numero 42Diapositiva numero 43Sedimentary rocks