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Chapter 8 – FROM SEDIMENT INTO SEDIMENTARY ROCK Sediments and Sedimentation • Deposition – Laying down of sediment • Separated into 3 broad categories – Clastic (Detrital) – Chemical – Biogenic (Biochemical) Sediments and Sedimentation • Clastic sediment – Fragmented rock & mineral debris from weathering & erosion – Described by particle shape, angularity, & size – Volcani-clastic sediments • Pyroclasts – Distinguished by size (bombs, lapilli, ash) Sediments and Sedimentation • Chemical sediment – Precipitation of minerals dissolved in lake, river, or sea water • Limestone – Shallow sea water evaporation causes dissolved salts to precipitate Sediments and Sedimentation • Biogenic sediment – Primarily composed of plant or animal remains • Shells, bones, teeth • Wood, roots, leaves Sedimentary Rocks • Lithification – Processes where loose sediment transforms into sedimentary rock • Bedding – Layers of strata in sediment/sedimentary rock • Bedding surface – Top or bottom of rock bed • Graded bed – Bed with large clasts at bottom, fining toward top • Cross bedding – Beds are inclined relative to layer they occur in Lithification Processes • Compaction – Reduced pore space due to weight of sediments above • Cementation – Dissolved substances precipitate out & act as glue • Recrystallization – Form new crystalline mineral grains from old ones • Diagenesis – Low temperature/pressure changes Types of Sedimentary Rocks • Clastic sedimentary rock – Conglomerate or breccia • Large fragments in fine grained matrix – Sandstones • Medium grained (sand sized) • Arkose or Quartz Sandstone – Mudstones • Very fine grained rock • Same composition as shale but lacks fissility – Shale • Very fine grained fissile rock • Fissile = breaks into layers Particle Sizes in Clastic Sedimentary Rocks Diameter (mm) Particle Type More than 256 Boulder 64 to 256 Cobble 4 to 64 Pebble 2 to 4 Granule 1/16 to 2 Sand 1/256 to 1/16 Silt Less than 1/256 Clay Sediment Name Sedimentary Rock Gravel Conglomerate or Breccia Sand Sandstone Mud Siltstone, Shale or Mudstone Types of Sedimentary Rocks • Chemical sedimentary rocks – Classified mainly by composition – Evaporite • Formed by lake or sea water evaporation • Rock salt; rock gypsum – Limestone & related • Oolitic • Dolostone – Chert Types of Sedimentary Rocks • Biogenic sedimentary rocks – Limestone • Mostly made of calcite • Coquina • Chalk – Peat • Formed by accumulation & compaction of plant remains – Coal • Formed by lithification of plant-rich sediment • Bituminous coal Depositional Environments • Interpreting environmental clues – Patterns made by air & water that moved over sediments – Ripple marks – Fossils – Mud cracks Depositional Environments on Land • Streams – Alluvial fan • Forms where stream is not constrained by valley walls • Fan-shaped • Lakes – Delta • Forms where stream enters standing body of water • Usually triangle-shaped – Playa • Seasonal lake Depositional Environments on Land • Glacier – Till • Mixed particles sizes & shapes • Wind – Eolian sediment • Sediments carried & deposited by wind – Loess • Wind-blown silt & clay sediment Depositional environments in & near the ocean • Delta • Estutary – Semi-enclosed body of coastal water – Fresh water mixes with sea water • Beaches – Mixed material; quartz common • Shelves – Silt or sand sediment; numerous fossils • Carbonate platforms and reefs – Reef: wave-resistant structure built of marine invertebrate skeletons Depositional environments in & near the ocean • Turbidites & turbidity currents – Turbulent, gravity driven flow of sediment & water – Moves sediment from continental shelf to deep sea • Seafloor – Nutrient-rich • Calcareous ooze • Siliceous ooze Depositional environments in and near the ocean • Sedimentary facies – Changes in the character of sediment from one environment to another How Plate Tectonics Affect Sedimentation • Divergent plate boundaries – Rift valleys • Convergent plate boundaries – Collisional type – Subduction type • Back-arc basin • Accretionary wedges