GEOL102, Chapter 5 Notes
GEOL102, Chapter 5 Notes Geol 102
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This 7 page Class Notes was uploaded by Mallory McClurg on Friday March 4, 2016. The Class Notes belongs to Geol 102 at University of Mississippi taught by Steeper, Janet in Spring 2016. Since its upload, it has received 13 views. For similar materials see Historical Geology in Geology at University of Mississippi.
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Date Created: 03/04/16
GEOL102 Chapter 5 Notes FACTORS AFFECTING SEDIMENTARY CHARACTERISTICS Time Tectonic setting Physical, chemical, & biological processes in depositional environment The way the sediment was transported – e.g. wind, water, etc. Other rocks found in the source area where the sediment came from Climate (and its effect on weathering) Chemical weathering – e.g. in rainforests Physical weathering – e.g. in deserts Post-depositional processes of lithification (cementation, compaction) TECTONICS/STRUCTURAL BEHAVIOR Tectonics – the forces controlling deformation or structural behavior of a large area of the Earth’s crust over a long period of time The Midwestern U.S. is tectonically stable New Orleans & Mexico City are sinking/subsiding New England & parts of Canada after the glacial retreat have been rising gently Parts of Oregon and the Cascade Mountains are rising actively, causing mountain and plateau formations The grain size and thickness of the sediment are affected by tectonics PRINCIPLE TECTONIC ELEMENTS OF A CONTINENT Craton – the stable interior of a continent, which has remained unchanged by mountain formations since the Precambrian Eon Shield – large area of exposed crystalline rock Platforms – ancient crystalline rocks underneath layers of sedimentary rocks Orogenic belts – mountainous belts surrounding the craton, which have been deformed by compression since Precambrian Eon DEPOSITIONAL ENVIRONMENTS The environment’s physical, chemical, biological, and geographical conditions in which sediments are deposited Types of depositional environments: Continental – on land Continental sedimentary rocks are: Intrabasinal – forming inside a basin (a natural depression on earth’s surface, usually holding water); intrabasinal sediments are chemical precipitates – mostly carbonate rocks & cal. Extrabasinal – formed outside of the basin, from the weathering of pre-existing rocks that had been transported to the environment of deposition Alluvial Fans – fan-shaped deposits at the base of mountains; mainly found in arid regions where it rarely rains…but when it does rain, it rains a lot and erosion is rapid; sediment is coarse, poorly-sorted gravel and sand GEOL102 Chapter 5 Notes Glacial regions – sediment is eroded, transported, and deposited by glaciers; till (glacial deposits) contain large volumes of unsorted mixtures of gravel, sand, boulders, and clay Eolian regions – common in desert environments where wind is the agent of sediment transport and deposition; water is the erosional agent because it will move larger amounts and larger sediment materials; dominated by sand and silt Transitional – along contact between ocean and land Transitional sedimentary rocks are: Deltas – fan-shaped accumulations of sediment, where a river meets a large standing body of water like the ocean or a lake If the land has a clean edge, usually that means the water there has turbulence If the land has a jagged edge (birds feet), the water is calmer because the sediment was deposited smoothly, rather than quickly pushed outward into the body of water Beaches & barrier islands – these occur because of shoreline deposits of marine sediment, wave energy, and marine fauna; separated by the mainland by a lagoon; may be associated with tidal flats Marine – in the ocean Marine depositional environments have four sections: Continental shelf – relatively flat with shallow water; larger grains are deposited near the shore, due to waves, tides, and currents (may find quartz sands, silt, clay, etc) Continental slope – steep slope at edge of continent between continental and oceanic crust; rapid sediment transport down the slope by currents Continental rise – at the base of continental slope where it begins to flatten out; turbidity currents deposit thick sediment (turbidites) toward the abyssal plain Abyssal plain – nearly flat & covered in fine-grained sediment PROPERTIES OF SEDIMENTARY ROCKS Color: Red/maroon/pink sedimentary rocks usually mean the rock was deposited in a well-oxygenated area – mostly found in continental sedimentary environments, but can also be transitional & marine Black/dark grey sedimentary rocks indicate the presence of organic carbon &/or iron; organic carbon rocks require an anoxic (lacking oxygen) environment – shales, for example Green/grey sedimentary rocks suggest the presence of iron, but in a reduced (not oxidized) state – Ferrous Iron (Fe ), for example Texture: NOT the way a rock feels; size, shape, sorting, & arrangement of grains in a sedimentary rock; can provide clues about the depositional environment Textural components of sedimentary rocks: GEOL102 Chapter 5 Notes Clasts – larger grains in the rock (gravel, sand, silt) Matrix – fine-grained material surrounding clasts (often clay) Cement – “glue” that holds the clasts and matrix together Silica (quartz, SiO 2 Calcite (CaCO ) 3 Iron Oxide Other minerals Grain sizes Fine-grained – usually means it was deposited in calm water Coarse-grained – requires turbulent water to be transported Wentworth Scale: how sediments are categorized by size: Gravel: >2mm Sand: (1/16) to 2mm Silt: (1/256) to (1/16)mm Clay: < (1/256)mm Sorting: distribution of grain sizes Well-sorted – all grains are same size; high-porosity & permeability if not tightly cemented; create an environment more suitable for aquifers/oil deposits/natural gas reservoirs Poorly-sorted – mixture of grain sizes (like sand & clay); rapid deposition of sediment without sorting by currents; great for roads because it is so densely compacted, and doesn’t leave room for water to seep through pores; e.g. alluvial fan deposits and glacial till The size(s) of sedimentary grains in a rock can help determine the depositional environment Fine grain sizes (like silt & clay) can be winnowed out by turbulent water, leaving just sands on the beach Shape: grain shape is described in terms of roundness of gran edges & sphericity (equal spherical dimensions Rounding results from abrasion against other particles and grain impact during the transport; very well rounded sand grains suggest that sand may have been recycled from older sandstones SEDIMENTARY STRUCTURE Sedimentary structures form during (or shortly after) the deposition of sediments, and before lithification; some structures are created by water and wind, while others are created after deposition, like footprints, worm trails, and mud cracks Structures can provide information about the environmental conditions in which it was deposited; e.g. some form in high energy moving water, while others form in quiet, calm water. Stratification – layering/bedding; visible because of differences in color, texture, composition to the adjacent beds Graded Bedding – when a turbidity current deposits sediment; it settles in layers - larger, coarse-grained on bottom, finer on top GEOL102 Chapter 5 Notes Ripple marks – movement of sediments by water/wind to form a wavelike surface Symmetrical ripple marks – produces by waves or oscillating water Asymmetrical ripple marks – form in unidirectional currents (like in streams/rivers); have a steep slope on the downstream side, and gentle slope on the upstream side; can be used to determine ancient current directions Mud cracks – polygonal pattern of cracks where mud’s surface has dried; can be broken up later by water movement Which way is up? Rocks can be overturned by tectonic forces, but observing normal sedimentary structures will tell us how they were deposited. These are the structures to consider, when trying to find which way is up: graded beds, cross beds, mud cracks, symmetrical ripples, scour marks, stromatolites, burrows, tracks, etc SANDS & SANDSTONES Sandstones are generally classified on the composition of their grains; three materials are generally considered, which help us determine weathering and transport of the sand grains Quartz grains Feldspar grains Rock fragment grains Intense weathering and long transport tend to destroy the feldspars and ferromagnesian because they’re less stable, and produce a sandstone dominated by quartz (considered mature sandstones) Sandstones with abundant feldspars and ferromagnesian minerals indicate relatively little weathering and transport (considered immature sandstones) Types of sandstones: Quartz sandstone – dominated by quartz grains Arkose – contain 25% ormore feldspar, with quartz Greywacke – contains about 30% of dark, fine grained matrix (clay, silt, chlorite, micas) along with quartz, feldspar, and rock fragments Lithic sandstone – dominated by quartz, muscovite, chert, and rock fragments with matrix less than 15%; feldspars are scarce Sandstone environmental interpretation – each type of sandstone implies something about its depositional history and environment Quartz sandstone – long time in depositional basin; location: tectonically stable, shallow water environments (ripple marks and cross bedding) Arkose – short time in depositional basin, because feldspar usually weathers quickly; implies rapid erosion, arid climate, tectonic activity, and steep slopes Greywacke – implies tectonically active source area and depositional basin with rapid erosion GEOL102 Chapter 5 Notes Lithic sandstone – deltaic, coastal plains; near-shore marine environments and swamps/marshes CARBONATE ROCKS & SEDIMENTS Carbonate rocks are chemical/biochemical in origin; they form inside the basin of deposition, so they are considered intrabasinal; can be found in southern Florida, Bahama Banks, etc There are two minerals present in carbonate rocks: Limestone (Calcite and Aragonite) Dolostone (Dolomite) Depositional and environmental conditions where carbonate rocks are formed: Usually form in a shallow, marine environment; depth and temperature of clear water, tropical climate, and sunlight are characteristics of most carbonate environments Some form in lakes, caves, and hot springs Most limestones are the direct or indirect result of biologic activity May contain shells or the remains of other marine organisms Photosynthesis by microscopic marine plants and algae removes CO 2rom seawater, leading to calcium carbonate precipitation CLAYS & SHALES The term “clay” has two different meanings in geology: 1.) a grain size, and 2.) a layered silicate mineral which sticks together when wet, and hardens when dry Clay minerals are complex hydrous aluminosilicates with atoms arranged in layers or sheets. Types of clay minerals: Kaolinites – weathering product of feldspars Smectites – may contain magnesium, calcium, and/or sodium ions; they swell when they are wet Illites – these clays contain potassium; major mineral component of ancient shales Deposition of clays – due to its fine-grain size, the clay will remain suspended in high energy waters; it will settle in quiet waters given enough time, and for that reason they are more common in deep water marine deposits where they are sheltered from waves and currents Claystone – fine-grained (less than 1/256mm) rock composed of clay, minerals, mica, and quartz grains; grains are too small to see with naked eye or a hand lens; feels smooth to the touch (not gritty); not fissile – it breaks irregularly Shale – very fine-grained rock composed of silt, clay, and mud; shale is fissile – it splits readily into thin, flat layers LITHOSTRATIGRAPHIC UNIT A body of sedimentary, extrusive igneous, and metamorphic rock distinguished on the basis of lithologic characteristics (texture, color, composition, etc) and stratigraphic position (position in the rock sequence); smallest lithostratigraphic rock unit is the bed. GEOL102 Chapter 5 Notes FORMATIONS Distinct/different from rock units above and below it Composed of a single rock type or characteristic set of rock types Traceable from exposure to exposure & of sufficient thickness to be mappable Named for some geographic locality where they are particularly well- exposed Chattanooga Shale Red Mountain Formation Dakota Sandstone Organization of lithostratigraphic units from largest to smallest Super groups > groups (set of similar/related formations) > formations > members (subdivisions w/in formations) FACIES AND SEA LEVEL CHANGES The characteristics of a particular rock unit, which can be used to interpret depositional environment Each depositional environment grades laterally into other depositional environments Transgression – sea level rise; produces fining-upward (finer grained facies overlying coarser-grained facies); sometimes called an onlap sequence Causes of transgressions: Melting of continental glaciers Displacement of ocean water by undersea volcanism Localized sinking/subsidence of the land in the coastal area Regression – sea level drop; produces coarsening-upward (coarser-grained facies overlying finer-grained facies; sometimes called an offlap sequence Causes of regressions: Buildup of continental glaciers Localized uplift in the land in coastal areas CORRELATION Lithostratigraphic correlation – matching up rock units on the basis of their lithology (composition, texture, color, etc) and stratigraphic position in the rock sequence Biostratigraphic correlation – matching up of rock units on the basis of the fossils they contain Chronostratigraphic correlation – matching up of rock units on the basis of age equivalence, as determined by radioactive dating methods or fossils TYPES OF CONTACT BETWEEN ROCK UNITS Conformable – indicate that no significant time gap or break in deposition has occurred Abrupt contact – bedding planes resulting from sudden minor changes in depositional conditions Gradational contact – more gradual changes in depositional conditions than abrupt contact GEOL102 Chapter 5 Notes Unconformable (unconformities) – surfaces which represent a gap in the geological time record, due to erosion or nondeposition; this time gap can vary greatly, ranging from millions of years to hundreds of millions of years Types of unconformities: Angular – an erosional surface which cuts folded or tilted strata Nonconformities – an erosional surface which cuts igneous or metamorphic rocks Disconformities – an irregular erosional surface which cuts flat-lying sedimentary rocks DEPICTING THE PAST Various ways in which the distribution of rocks can be depicted: Geologic columns – columnar sections show the vertical succession of rock units at a given location’ used in correlation and in the construction of cross sections Stratigraphic cross-sections – tie together several geologic columns from different locations; the purpose is to show how rock units change in thickness, lithology, and fossil content across a given area Cross section – vertical view of the interior of the earth; like cutting a piece of cake! Structural cross-sections – show the timing of tilting, folding, and faulting of rock units; the datum is a level line, parallel to sea level; tops and bottoms of the units are plotted according to their true elevations; folds an faults are depicted clearly Geologic maps – geologic maps show the distribution of various layers and types of rocks in an area; map symbols indicate structural features (folds/faults/etc) and formation names Paleogeographic maps – interpretive maps which depict the geography of an area at some point in the past Isopach maps – show the thickness of formations or other units in an area Lithofacies maps – show distribution of lithofacies that existed at a given time over an area
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