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Sedimentary Rock http://soest.hawaii.edu/coasts/cgg_main.html Sedimentary Rocks are the product of sediment deposition, diagenesis, and lithification Oceans rise and fall, lakes come and go, streams run and disappear, deserts become forests and forest become swamps…all these changes will be recorded in sedimentary rocks Sedimentary rock contains sediment grains, cement holding them together, and empty space called “pores” To understand Earth history, a geologist seeks to interpret the “Environment of Deposition” of a sedimentary rock…this reveals something about changes occurring on Earth’s surface Eolian Environment – grains of uniform size “well sorted” Coastal Environment – grains coarser and of mixed size Stream Environment – grains much coarser and “poorly sorted” Glacial Environment – grains very coarse with no sorting, but usually rounded Landslide Environment – grains very coarse with no sorting, with no rounding Marsh/Mudflat Environment – grains very fine Evaporite environment – chemical sediments Reef environment – biological sediments Deep sea environment?? – Plankton sediment and clays from land Sedimentary Structures…. Cross-beds – former dunes Sedimentary rocks tend to form layers or strata…each layer records a depositional event Ripple marks modern Mud cracks ancient Sediment becomes sedimentary rock…the texture and composition of the sediment determine the type of sedimentary rock Two types of sedimentary rock – Clastic Sedimentary Rock – made of pieces of broken crust Conglomerate Biochemical Sedimentary Rock – made of precipitated minerals Rock Salt Clastic Sedimentary Rock Biochemical Sedimentary Rock Conglomerate/Breccia – gravel texture Limestone – CaCO3 composition Sandstone – sand texture Chert – SiO2 composition Shale – silt/clay texture Coal – Carbon composition How does coal form? Pressure forms coal Coal strip mine… Environments of deposition Sedimentary texture Sediment composition Global Climate history Clastic – Biochemical sedimentary rox Breccia/Conglomerate Sandstone Shale Limestone Chert Coal How does coal form? Sedimentary structures But why was there an ice age? Ice stores O16 so that oceans are O16 depleted in an ice age + O18/O16 - Periodicity of 100,000 yrs CaCO3 Layer by layer sampling of plankton reveals oxygen isotope record 41 kyrs 21 kyrs 100 kyrs These were made during an ice age!