6/13/2011 1 Weathering, Sediments & Sedimentary Rocks Erosion and Sediments ?Transportation ?Deposition ? Sedimentary Rock ? Sedimentary Structures: Sediments ?Clastic sediments are preexisting rock fragments. ?Chemical sediments are dissolved minerals that recrystallize during evaporation. ?Biologic processes can form both clastic and chemical sediments. Transportation Water, wind and glaciers (ice) are all responsible for transporting sediment. How much sediment and the size of sediment carried depends on strength of the current. For example, wind carries only small sizes like fine sand, silt and clay. Strong water currents can transport gravel and coarse sands for long distances. 6/13/2011 2 Sediment can be modified during transportation. Water segregates sediment according to its size, this is called sorting. Sediment is also worn down by abrasion resulting in rounder grains. Both sorting and roundness of sediment provide information about how long and how far the sediment has been transported. Transportation Deposition When current velocity is sufficiently low, water can no longer move sediment. The sediment is said to be deposited. Two important things happen. ? Sediment is deposited in horizontal layers. ?New (Younger) sediment is deposited onto older sediment. These processes are important when studying past environments preserved in the rock record. Sedimentary Processes ?Once produced, sediment is transported and deposited by erosion into areas called sedimentary environments. ?Here are some examples: - Rivers & Streams - Desert - Lakes - Continental shelf - Delta - Deep ocean - Beach - Carbonate Reef 6/13/2011 3 07.01.a1 07.01.a6 07.02.a7 6/13/2011 4 07.01.a7 07.01.a9 07.02.a3 6/13/2011 5 07.02.a9 07.05.a6 When sediment becomes buried over time? ?Compaction: air, water and space is reduced. ?Cementation: new minerals precipitate in the empty (pore) spaces between the grains and bind the grains together. Lithification 07.05.a4 Silt and Clay ? Settle out of water very slowly compared to larger grain ? They are platy minerals thus stack in layers as shown ? This can explain why shale is fissle and breaks into thin layers 6/13/2011 6 Sediment Clastic (Detrital) Sediment Clastic sediment range in size: Gravel > 2 mm Sand 1/16-2 mm Silt 1/256-1/16 mm Clay < 1/256 mm 07.04.a2-4 Various sizes of sediment Classification of Sedimentary Rocks Based on composition and texture Clastic Gravel Sand Silt Clay Rock Conglomerate Breccia Quartz sandstone Arkose Siltstone Mudstone Shale Claystone *(if fissle) 6/13/2011 7 Produced by evaporation and chemical precipitation Composition Rock Calcite Limestone Dolomite Dolostone SiO2 Chert/Flint Evaporates Gypsum Rock Gypsum Halite Rock Salt Chemical Sedimentary Rocks Tend to have a crystalline texture Organic Sedimentary Rocks Texture Composition Rock Clastic calcium carbonate Chalk calcite and fossils Fossiliferous Limestones decomposed Coal plant material. Sedimentary Structures ? bedding: horizontal and cross bedding ?mud cracks ? ripple marks ? fossils 6/13/2011 8 07.07.b7 07.07.b10 Sedimentary Structures: Mud cracks 6/13/2011 9 Sedimentary Structures: Ripple marks Fossils Sue Clement-Smith Sediments and Sedimentary Rock
Want to see the other 9 page(s) in L6_Weathering_Sediments_&_...pdf?JOIN TODAY FOR FREE!