- Slides: 60
Sediments and Sedimentary Rocks n Rock Cycle
Definitions n Sediments: Solid fragmental material transported and n Lithification: The conversion of a newly deposited n Diagenesis: All the changes (chemical, physical, or deposited by wind, water, or ice, chemically precipitated from solution, secreted by organism, and that forms in layers in loose unconsolidated form. sediment into a solid rock involving processes such as cementation, compaction, and crystallization. biological) undergone by a sediment after its initial deposition, exclusive of weathering and metamorphism. Definitions from Dictionary of Geologic Terms
Diagenesis n n Compaction: occurs when the weight of the overlying sediments pushes down on sediments; compaction is accompanied by the expulsion of water from these sediments Cementation: Occurs when solution “crystallizes” around sediment n Common elements in sedimentary rocks n n Silica (Si. O 2) Calcite (Ca. CO 3) Iron (Fe 2 O 3) Recrystallization (pressure solution): Occurs when sediments are exposed to very high pressures; as a result, the melting points of the constituent minerals are lowered and liquification occurs.
Environmental Clues in Sedimentary Rocks n Fossils Salt Water - Corals, Echinoderms n Fresh Water - Insects, Amphibians n Terrestrial - Leaves, Land Animals n n Color And Chemistry Red Beds - Often Terrestrial n Black Shale - Oxygen Poor, Often Deep Water n Evaporites – Arid Climates n
Bedding or Stratification n n Almost Always Present in Sedimentary Rocks Originally Horizontal Tilting by Earth Forces Later Variations in Conditions of Deposition Size of Beds (Thickness) n Usually 1 -100 Cm n Can Range From Microscopic to 50 m
Sedimentary Rocks Clastic Rocks n Made of Fragmentary Material n Deposited by n Water (Most Common) n Wind n Glacial Action n Gravity Biochemical Sedimentary Rocks n Evaporation n Precipitation n Biogenic Sediments
Clastic Rocks Classified by: n Grain Size n Grain Composition n Texture
Sediment Sizes and Clastic Rock Types Rock Type Sediment Grain Size Shale Clay less than 0. 001 mm Siltstone Silt . 001 -0. 1 mm Sandstone Sand . 01 -1 mm Conglomerate Gravel 1 mm + Sedimentary rocks made of silt- and claysized particles are collectively called mudrocks, and are the most abundant sedimentary rocks.
Range of Clast Size in Sediments Coarse Medium Fine Boulder > 256 mm Boulder >28 mm 64 mm < Cobble < 256 mm 26 mm < Cobble < 28 mm 2 mm < Pebble < 64 mm 21 mm < Pebble < 26 mm 1/16 mm < Sand < 2 mm 2 -4 mm < Sand < 21 mm 1/256 mm < Silt < 1/16 mm 2 -8 mm < Silt < 2 -4 mm Mud < 1/256 mm Mud < 2 -8 mm
Chemical Sediments Evaporites -Water Soluble n Halite n Gypsum n Calcite Precipitates Gypsum n Limestone n Iron Formations Alteration After Deposition n Dolomite Biogenic Sediments n Limestone - Shells, Reefs, Etc. Organic Remains n Coal n Petroleum
Coal Seams, Utah
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Sediments and Sedimentary Rocks n Sedimentary Structures (cont’d) n Ripple marks - small ridges formed on the surface sediment layer by moving wind or water n Wave generated ripple marks are symmetric n Current generated ripple marks are asymmetric
Sediments and Sedimentary Rocks n Sedimentary Structures (cont’d) n Fossils - traces of plants or animals preserved in sedimentary rock; includes replacement by calcite or silica, voids spaces or molds reflecting the former shape, filling in the molds to form casts, flat carbon imprints. footprints, trails, and burrows