GEO 101 1st Edition Lecture 9 Outline of Last Lecture 1 Weathering Sedimentary Rocks 2 External processes 3 Weathering 4 Sedimentary Rocks 5 Clastic Sedimentary Rocks Outline of Current Lecture 1 Clastic Sedimentary Rocks 2 Biochemical and Organic Rocks 3 Chemical Sedimentary Rocks 4 Sedimentary Structures 5 Bedforms 6 Bed Surface Markings 7 Depositional Environments Current Lecture Clastic Sedimentary Rocks Clastic sedimentary rocks reflect several processes o Weathering Generation of detritus via rock disintegration o Erosion Removal of sediment grains from rock o Transportation Dispersal by wind water and ice o Deposition Settling out of the transporting fluid o Lithification Transformation into solid rock Transforms loos sediment into solid rock Burial more sediment is added onto previous layers Compaction overburden weight reduces pore space Sand 10 to 20 Clay 50 80 Classified on the basis of texture and composition o Clast grain size The average diameter of clasts Range from very coarse to very fine Boulder cobble pebble sand silt and clay With increasing transport average grain size decreases o Clast composition The mineral makeup of sediments These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute May be individual minerals or rock fragments Mineral identities provide clues about The source of the sediment The environment of deposition o Angularity and sphericity indicate degree of transport Fresh detritus is usually angular and non spherical Grain roundness and sphericity increases with transport Well rounded Long transport distances Angular Negligible transport o Sorting The uniformity of grain size Well sorted Uniform grain sizes Poorly sorted Wide variety of grain sizes Sorting becomes better with distance from source o Character of cement Minerals that fill sediment pores Fluids with dissolved solids flush through port sysem Dissolved ions slowly crystallize and fill pores Cementation varies from weak to strong Common cements Quartz Calcite Hematite Clay materials Coarse Clastics Composed of gravel sized clasts o Breccia Comprised of angular fragments Angularity indicates a lack of transport processing Deposited relatively close to source o Conglomerate Comprised of rounded gravel Indicates water transport Clasts bang together forcefully in flowing water Collisions round angular corners and edges of clasts Conglomerates are deposited at a distance from the source Sandstone Clastic rock made of sand sized particles o Forms in many depositional settings o Quartz is by far the dominant mineral in sandstones o Sandstone varieties Arkose Contains abundant feldspar pink color Quartz sandstone Almost pure quartz Fine Clastics Composed of silt and clay o Silt sized sediments are lithified to form siltstone o Clay sized particles form shale o Fine clastics are deposited in quieter waters Floodplains lagoons mudflats deltas deep water basins Organic rich shales are the source of petroleum Biochemical and Organic Rocks These are sediments derived from living organisms o Biochemical Hard mineral skeletons o Organic Cells of plants algae bacteria and plankton Biochemical Limestone CaCO3 skeletal shell remains o Warm tropical shallow clear O2 rich marine water o Diverse organisms plankton corals clams snails etc o Many textural varieties Reefs Shell debris Lime mud micrete Chert Rock made of cryptocrystalline quartz o Formed from opalline silica SiO2 skeletons Diatoms Radiolarians o Oppaline silica added to bottom sediments dissolves o Silica pore fluids solidify to form chert nodules or beds Organic Rocks Made from organic carbon o Coal Altered remains of fossil vegetation Accumulates in lush tropical wetland settings Requires deposition in the absence of oxygen Can be formed in swamps due to large amounts of dead plants don t move due to lack of strong flow of water Buried under more dead plants increased pressure forms coal o Oil shale Shale with heat altered organic matter Oil attaches to grains in shale very fine grained Chemical Sedimentary Rocks Comprised of minerals precipitated from water solution Evaporites created from evaporated seawater o Evaporation triggers deposition of chemical precipitates o Examples include halite rock salt and gypsum Travertine Calcium carbonate CaCO3 precipitated from groundwater where it reaches the surface o Dissolved calcium Ca2 reacts with bicarbonate HCO3 o CO2 expelled into the air causes CaCO3 to precipitate Thermal hot springs Caves Dolostone Limestone altered by magnesium rich fluids o CaCO3 altered to dolomite CaMg CO3 2 by Mg2 rich water o Dolostone looks like limestone except It has a sugary texture and a pervasive porosity It weathers to a buff tan color Replacement chert nonbiogenic in origin o Many varieties Flint Black or gray from organic matter Jasper Red or yellow from iron oxides Petrified wood Wood grain preserved by silica Agate Concentrically layered rings Sedimentary Structures Features imparted to sediments at or near deposition o Layering o Surface features on layers o Arrangement of grains Help decipher conditions at or near time of deposition Sedimentary rocks are usually layered or stratified o Arranged in planar close to horizontal beds o Bedding is often laterally continuous for long distances o Beds are often similar in composition color and texture Bedding reflects changing conditions during deposition These can be changes in o Energy conditions and hence grain size o Disturbance by organisms Bedding may also reflect non deposition or erosion A series of beds are referred to as strata Formation Strata recognized on a regional scale Geologic maps display the distribution of formations o i e Coconino Formation Water flowing over loose sediment creates bedforms Bedforms are linked to flow velocity and sediment size o Ripples cm scale ridges and troughs indicate flow Asymmetric ripples Unidirectional flow Symmetric ripples Wave oscillation Ripples are commonly preserved in sedimentary rocks Bedforms Cross beds Created by ripple and dune migration o Sediment moves up the gentle side of ripple of dune o 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 o Form from wind blown sand in dessert or beach regions o Often preserve large internal cross laminations Turbidity Currents o Sediment