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UA GEO 101 - Sedimentary Rocks
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GEO 101 1st Edition Lecture 9 Outline of Last Lecture 1. Weathering & Sedimentary Rocks 2. External processes3. Weathering4. 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 Structures5. Bedforms6. Bed-Surface Markings7. Depositional EnvironmentsCurrent LectureClastic Sedimentary Rocks- Clastic sedimentary rocks reflect several processeso Weathering – Generation of detritus via rock disintegrationo Erosion – Removal of sediment grains from rocko Transportation – Dispersal by wind, water, and iceo Deposition – Settling out of the transporting fluido 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 compositiono 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 decreaseso Clast composition – The mineral makeup of sedimentsThese 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 depositiono 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 transporto Sorting – The uniformity of grain size Well-sorted – Uniform grain sizes Poorly sorted – Wide variety of grain sizes Sorting becomes better with distance from sourceo 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 clastso Breccia – Comprised of angular fragments Angularity indicates a lack of transport processing Deposited relatively close to sourceo 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 particleso Forms in many depositional settingso Quartz is, by far, the dominant mineral in sandstoneso Sandstone varieties: Arkose – Contains abundant feldspar (pink color) Quartz sandstone – Almost pure quartz- Fine Clastics – Composed of silt and clayo Silt-sized sediments are lithified to form siltstoneo Clay-sized particles form shaleo Fine clastics are deposited in quieter waters Floodplains, lagoons, mudflats, deltas, deep-water basins Organic-rich shales are the source of petroleumBiochemical and Organic Rocks- These are sediments derived from living organismso Biochemical – Hard mineral skeletonso Organic – Cells of plants, algae, bacteria and plankton- Biochemical Limestone – CaCO3 skeletal (shell) remainso Warm, tropical, shallow clear, O2 – rich, marine watero Diverse organisms (plankton, corals, clams, snails, etc.)o Many textural varieties Reefs Shell debris Lime mud (micrete)- Chert – Rock made of cryptocrystalline quartzo Formed from opalline silica (SiO2) skeletons Diatoms Radiolarianso Oppaline silica added to bottom sediments dissolveso Silica pore fluids solidify to form chert nodules or beds- Organic Rocks – Made from organic carbono 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 dueto 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 seawatero Evaporation triggers deposition of chemical precipitateso Examples include halite (rock salt) and gypsum- Travertine – Calcium carbonate (CaCO3) precipitated from groundwater where it reaches the surfaceo 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 fluidso CaCO3 altered to dolomite CaMg(CO3)2 by Mg2+ -rich watero 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 origino 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 ringsSedimentary Structures- Features imparted to sediments at or near depositiono Layeringo Surface features on layerso 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 distanceso 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 sizeo 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 formationso i.e. Coconino Formation - Water flowing over loose sediment creates bedforms- Bedforms are linked to flow velocity and sediment sizeo Ripples, cm-scale ridges, and troughs, indicate flow Asymmetric ripples – Unidirectional flow Symmetric ripples – Wave oscillation Ripples are commonly preserved in sedimentary rocksBedforms-


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UA GEO 101 - Sedimentary Rocks

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