Chap 6 notes
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This 4 page Class Notes was uploaded by Chris Lause on Monday February 9, 2015. The Class Notes belongs to 100 at Ohio State University taught by Dr. Anne Carey in Fall. Since its upload, it has received 35 views. For similar materials see Earth Science in Earth Science at Ohio State University.
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Date Created: 02/09/15
Chapter 6 Detrital Definition worn rock fragments and mineral grains Produced by mechanical and chemical weathering Water wind glacial transport Sortedsize Sizes Gravel conglomerates Sand sandstone Clay siltstone and shale Chemical precipitates from seawater Calcium carbonate Chemical limestone Dolomite CaMg CO32 dolostone Halite NaCL rock salt Gypsum CaSO4 ZHZO rock gypsum Iron minerals Iron stones Opal Chert Biochemical Produced directly or indirectly by living organisms Calcium carbonate CaCO3 Shellsskeletal limestone Opal SIOZ H20 Shellsbiogenic opal Organic matter Coal Sedimentary Cover Earth is covered by a thin Veneer of sediment The veneer caps igneous and metamorphic basement Sediment cover varies in thickness from 020km Thinner or missing where igneous and metamorphic rocks outcrop Thicker in sedimentary basins Sediments are the building blocks of sed rocks Sediments are diverse as are the rocks made from them Four classes Clastic made from weathered rock fragments Biochemical cemented shells of organisms Organic carbonrich remains of plants Chemical minerals that crystallize directly from water Clastic Sedimentary Rocks Clastic sedimentary rocks re ect several processes Weathering generation of detritus via rock disintegration Erosion removal of sediment grains from rock Transportation dispersal by wind water and ice Depostion settling out of the transporting uid Lithification transformation into solid rock Lithification trasnforms loose sediment into solid rock Burial more sediment is added onto previous layers Compaction overburden weight reduces porespace Sand 1020 Clay 5080 Cementation minerals grow in pores gluing sediments Classified on basis of texture and comp Clast grain size Clast composition Angularity and sphericity Sorting Character of cement These variables produce a diversity of clastic rocks Clast size the avg diameter of clasts Range from very coarse to very fine Boulder cobble pebble sand silt and clay With increasing transport avg grain size decreases Clast composition the mineral makup of sediments May be individual minerals or rock fragments Mineral identities provide clues about The source of the sediment Environment of depostion Angularity and sphericity indicate degree of transport Fresh detritus is usually angular and nonspherical Grain roundness and shpericity increases with transport Well rounded long transport distances Angular negligible transport Sorting the uniformity of grain size Well sorted uniform grain sizes Poorly sorted wide variety of grain sizes Sorting becomes bbetter with distance from source Cement minerals that fill sediment pores Fluids swith dissolved solids ush through pore system Dissolved ions slowly crystallize and fill pores Cementation varies from weak to strong Common cements Quartz Calcite Hematite Clay minerals Coarse clastics composed of gravel sized clasts Breccia composed of angular fragments Deposited relatively close to source Conglomerate composed of rounded gravel Indicates water transport Clasts bang together forcefully in owing water Collisions round off angular comers and edges of clasts Sandstone clastic rock made of sand sized particles Forms in many depositional settings Quartz is by far the dominant mineral in sandstones Sandstone contains Arkose contains abundant feldspar Quartz sandstone almost pure quartz Fine clastics composed of silt and clay Silt sized sediments are lithi ed to form siltstone Clay sized particles form shale Fine clastic materials are deposited in quieter waters Floodplains lagoon mud ats deltas deepwater basins Organic rich shales are the source of petroleum Biochemical and organic rocks These are sediments derived from living organisms Biochemical hard mineral skeletons Organic cells of plants algae bacteria and plankton Biochemical limestone CaCO3 skeletal shell remains Warm tropical shallow clear OZrich marine water Diverse organisms plankton corals clams snails etc Many textural varieties Reefs Shell debris Lime mud micrite Chert rock made of cryptocrystalline quartz Formed from opal line silica SiOZ skeletons Diatoms Radiolarians opal line silica added to bottom sedimetns dissolves Silica pore uids solidify to form chert nodules or beds Organic Rocks Made from organic carbon Coal altered remains of fossil vegetation Accumulates in lush tropical wetland settings Requires deposition in the absence of oxygen Oil shale shale with heat altered organic matter Chemical Sedimentary rocks Composed ofminerals precipitated from water solution Evaporates created from evaporated seawater Evaporation triggers deposition of chemical precipitates Examples include halite rock salt and gypsum Travertine calcium carbonate CaCO3 precipitated from groundwater when it reaches the surface Dissolved calcium Ca2 reacts with bicarbonate HCO3 C02 expelled into the air causes CaCO3 to precipitate Thermal hot springs Caves Dolostone limestone altered by Mg rich uids CaCO3 altered to dolomite CaMgCO32 by Mg2 rich water Dolostone looks like limestone except It has a sugary texture and a pervasive porosity It weathers to a buff tan color Replacement chert no biogenic in origin Many varieties Flint black or gray from organic matter Jasper red or yellow from Fe Oxides Petrified wood wood grain preserved by silica Agate concentrically layered rings Sedimentary structures Features imparted to sediments at or near deposition Layering Surface features on layers Arrangement of grains Help decipher conditions at or near time of deposition Sedimentary rocks are usually layered or stratified Arranged in planar close to horizontal beds Bedding is often laterally continuous for long distances Beds are often similar In composition color texture Bedding re ects changing conditions during deposition These can be changes in Energy conditions and hence grain size Disturbance by organisms Bedding may also re ect non deposition or erosion Sedimentary structures Series of beds is referred to as strata Formation strata recognized on a regional scale Geologic maps display the distribution of formations Ie Coconino formation Water owing over loose sediment creates bed forms bed forms are linked to ow velocity and sediment size Ripples cm scale ridges and troughs indicate ow Asymmetric ripples unidirectional ow Symmetric ripples wave oscillation Ripples are commonly preserved in sedimentary rocks Bedforms Cross beds created by ripple and dune migration Sediment moves up the gentle side of a ripple or dune Sediment piles up then slips down the steep face The slip face continually moves downstream Added sediment forms sloping cross bedded layers Dunes similar to ripples except much larger Form from wind blown sand in desert or beach regions Often preserve large internal crosslaminations Bed surface markings Occur after deposition while sediment is still soft mud crackspolygonal desiccation feature sin wet mud Indicate alternating wed and dry conditions Necessitate deposition in a terrestrial setting Scour marks troughs eroded in soft mud by current ow Fossils evidence of past life Footprints Shell impressions
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