Sedimentology and sedimentary processes

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Sedimentology and Sedimentary Processes Virginia T McLemore Sedimentology • "The scientific study of sedimentary rocks and of the processes by which they were formed; the description, classification, origin and interpretation of sediments" (Glossary of Geology, AGI, 1974) • Study of modern sediments such as sand, mud (silt),and clay • Understanding the processes that deposit them • Studies of ancient sedimentary rocks http://www.geo.umass.edu/faculty/cooke/geo101/GeologicTime.htm http://www.bbc.co.uk/schools/gcsebitesize/chemistry/changestoearthandatmosphere/0rocksrev5.shtml http://csmres.jmu.edu/geollab/fichter/SedRx/SimpModl.html Sedimentary rock types • Clastic rocks – particles derived from the weathering and erosion of precursor rocks and consist primarily of fragmental material – classified by grain size and composition • Carbonates – precipitated by a variety of organic and inorganic processes • Evaporites – evaporation of water at the Earth's surface • Chemical sedimentary rocks (chert, jasperiod) Principles • Uniformitarianism, which states that the sediments within ancient sedimentary rocks were deposited in the same way as sediments which are being deposited at the Earth's surface today • Principle of superposition Sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top • Principle of original horizontality sediments are deposited at their angle of repose which, for most types of sediment, is essentially horizontal Principles • Principle of lateral continuity states that layers of sediment initially extend laterally in all directions unless obstructed by a physical object or topography • Principle of cross-cutting relationships states that whatever cuts across or intrudes into the layers of strata is younger than the layers of strata http://en.wikipedia.org/wiki/Image:Taum_Sauk_precambrian-cambrian_unconformity.jpg http://www.uoregon.edu/~millerm/reefs.html Deep Ocean • • • • • • • The deep oceans contain a variety of sediment types Adjacent to the continents, a considerable amount of sediment is transported from the continental margins by turbidity currents As the current moves across the deep-ocean floor its velocity gradually decreases, and sediment carried in suspension settles out The resulting deposit is a widespread layer of sediment in which the size of grains grade from coarse at the base to fine at the top Such deep-sea deposits are characterized by sequences of graded beds of these "turbidites" Distant to the continents, dust transported by eolian processes may accumulate as muds In sediment-starved parts of oceans away from the continents, siliceous ooze formed of the tests of microorganisms called radiolaria accumulate These sediments form the radiolarian cherts of the rock record Facies and depositional environments • The facies concept refers to the sum of characteristics of a sedimentary unit, commonly at a fairly small (cm-m) scale • • • • • • Lithology Grain size Sedimentary structures Color Composition Biogenic content • Lithofacies (physical and chemical characteristics) • Biofacies (macrofossil content) • Ichnofacies (trace fossils) Facies and depositional environments • Facies analysis is the interpretation of strata in terms of depositional environments (or depositional systems), commonly based on a wide variety of observations • Facies associations constitute several facies that occur in combination, and typically represent one depositional environment (note that very few individual facies are diagnostic for one specific setting!) • Facies successions (or facies sequences) are facies associations with a characteristic vertical order • Walther’s Law (1894) states that two different facies found superimposed on one another and not separated by an unconformity, must have been deposited adjacent to each other at a given point in time http://www.gpc.edu/~pgore/geology/historical_lab/environmentchart.htm [...]... rounded depressions caused by swirling currents and eddies • Mud cracks are formed by evaporation on mudflats or in shallow lakes Sedimentary environments Alluvial Fans • Alluvial fans are sedimentary deposits that typically form at the margins of a dry basin • They typically contain coarse boulders and gravels and are poorly sorted • Fine-grained sand and silt may be deposited near the margin of the... pushes, rolls, and slides them along • Saltation is more complex and usually affects sand-sized particles Here, the particles are sucked up by eddies into the flow, travel with the flow for a while, and then fall back to the bottom Sediment structures • Asymmetrical ripples are ripples that have a gentle slope upstream and a steep slope downstream • Cross-bedding is inclined bedding and commonly forms... commonly in shallow lakes • These lakes may periodically dry, and evaporite deposits may result http://www.uoregon.edu/~millerm/fan.html Eolian • • Wind is an effective sorting agent and will selectively transport sand Gravel is left behind and dust-sized particles are lifted high into the atmosphere and transported great distances • Windblown sand forms dunes that are characterized by well-sorted grains... the roughness of the surface of the sedimentary grain • Sorting refers to the range of particle sizes in a sediment or sedimentary rock • Matrix is the fine-grained material (usually clays or silt) that is deposited originally with the coarser-grained material http://www.eos.ubc.ca/courses/eosc221/sed/sili/silpet.html Classification by GRAIN SIZE • Gravel > 2mm • Sand >1/16 mm < 2 mm • Mud
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