2 Controls on sedimentary rock features Tectonic settingPhysical, chemical, and biological processes in the depositional environmentMethod of sediment transportRocks in source area from which sediment is derivedClimate (and its effect on weathering)Post-depositional processes of lithification (cementation, compaction)
3 Tectonic Setting Tectonics: The forces controlling deformation or structural behavior of a large area of the Earth's crust over a long period of time.Tectonic Settings influence:Size of clastic particlesThickness of depositRate of erosion or subsidence
4 Continental Tectonic Regimes Craton - stable interior of a continent; undisturbed by mountain-building events since the PrecambrianShields- large areas of exposed crystalline rocks)Platforms- like shields but covered by flat-lying or gently warped sedimentary rocksOrogenic belts - elongate regions bordering the craton which have been deformed by compression since the Precambrian
5 Figure 3-1 (p. 62) The craton and orogenic belts of North America.
6 Environments of Deposition All of the physical, chemical, biologic and geographic conditions under which sediments are deposited.Sediments formed from the weathering of pre-existing rocks outside the basin, and transported to the environment of depositionOrSediments form inside the basin; includes chemical precipitates, most carbonate rocks, and coal.
8 Figure 3-4 (p. 65) Deep-sea fan built of land-derived sediment emerging from the lower part of a submarine canyon. Such fans occur in association with large rivers, such as the Amazon, Congo, Ganges, and Indus. (Vertical exaggeration 200:1.)
10 Deltas Deltas fan-shaped accumulations of sediment Mississippi DeltaNiger DeltaDeltasfan-shaped accumulations of sedimentriver flows into a standing body of water, such as a lake or seasediments are dropped, forming this progradational feature.
11 Barrier IslandsBeaches and Barrier Islands are shoreline deposits exposed to wave energy and dominated by sand with a marine fauna.Lagoons are bodies of water on the landward side of barrier islands.Tidal flats are low-lying plains near lagoons. Marshy
17 Color of Sedimentary Rocks Clues about depositional environment:Black and dark gray coloration in sedimentary rocks generally indicates the presence of organic carbon and ironReddish coloration in sedimentary rocks indicates the presence of oxidized ironGreen and gray coloration in sedimentary rocks indicates the presence of reduced iron
21 Interpretation of Clastic Sedimentary Rocks The texture of a sedimentary rock can provide clues to the depositional environment.Fine-grained= quiet waterLarge grains= higher energy (velocity) deposition
22 Grain SizeSedimentary grains are categorized according to size using the Wentworth Scale.Wentworth Scale for sedimentary grain size:GRAVEL (>2mm)SANDSILTCLAY (<1/256mm)
23 SortingSorting refers to the distribution of grain sizes in a rock.POOR!GOOD!
24 SortingIn general, windblown sediments are better sorted than wave-washed sediments.Well-sorted sandsHave higher porosity and permeability than poorly-sorted sands (if not tightly cemented),May be good reservoirs for petroleum and natural gas.Poor sorting is the result of rapid deposition of sediment without sorting by currents.Examples:alluvial fan depositsglacial tillites.
25 Grain ShapeGrain shape is described in terms of rounding of grain edges and sphericity (equal dimensions).Rounding results from abrasion and grain impact during transport.
26 Figure 3-13 (p. 72) Shape of sediment particles Figure (p. 72) Shape of sediment particles. (A) An angular particle (all edges sharp). (B) A rounded grain that has little sphericity. (C) A well-rounded, highly spherical grain. Roundness refers to the smoothing of edges and corners, whereas sphericity measures the degree of approach of a particle to a sphere.
27 Sedimentary Structures Sedimentary structures arevisible at the scale of an outcrop (LARGE!)that formed at the time of deposition or shortly thereafter (before lithification)Evidence of processes operating
28 Bedding Sedimentary rocks generally have bedding or stratification Individual layers less than 1 cm thick are laminationscommon in mudrocksBeds are thicker than 1 cmcommon in rocks with coarser grains
29 Graded Bedding Graded bedding is common in turbidity current deposits Some beds show an upward gradual decreasein grain size, known as graded beddingGraded bedding is common in turbidity current deposits
33 Ripple MarksSmall-scale alternating ridges and troughs are known as ripple marks and are common in sandstoneCurrent ripple marksform in response to water or wind currentsflowing in one directionand have asymmetric profilesWave-formed ripple marksresult from the oscillation of wavestend to be symmetrical
34 Current Ripple Marks Ripples with an asymmetrical shape Internally cross- beddedFlow upper right to lower left
35 Wave-Formed RipplesAs the waves wash back and forth, symmetrical ripples formProduced by wave (shallow)
36 Mud CracksWhen clay-rich sediments dry, they shrink and crack into polygonal patterns fractures called mud cracksMud cracks require wetting and drying to form,
38 Geopetal Structures Which way is up?? Sedimentary structures can be used to determine "up direction".graded bedscross bedsmudcracksFlute markssymmetrical (but not asymmetrical) ripplesstromatolitesburrowstracks,
40 Figure (p. 77) Four categories of sandstone as seen in thin section under the microscope. Diameter of field is about 4 mm.
41 Figure (p. 78) Idealized geologic conditions under which quartz sandstone may be deposited. There is little tectonic movement in this environment. Water depth is shallow, and the basin subsides very slowly.
42 Figure 3-26 (p. 79) Geologic environment in which arkose may be deposited.
43 Figure 3-28 (p. 80) Tectonic setting in which graywacke is deposited Figure (p. 80) Tectonic setting in which graywacke is deposited. Frequently graywackes are transported by masses of water highly charged with suspended sediment. Because of the suspended matter, the mass is denser than surrounding water and moves along the sloping sea floor or down submarine canyons as a turbidity current. Graywacke sediment characteristically accumulates in deep-sea fans at the base of the continental slope.
44 Figure 3-29 (p. 80) Deltaic environment in which lithic sandstones may be deposited.
45 Interpretation of Carbonates Main ProcessesChemical direct precipitates (carbonate mud)Biochemical: organic contribution (shells, etc.)
46 Characteristics of most marine carbonate environments Warm waterTropical climate (30 ° N - 30 ° S of equator)Shallow water (less than 200 m deep)Clear water (low to no terrigenous input)Sunlight required for photosynthesis by algae
47 Some limestones may be the accumulation of shells Shell fragments (coquina)MicroscopicForaminifera (chalk)Fossiliferous limestone
48 Dolomite CaMg(CO3)2 Rock and mineral Original (forming today) is rare Many older rocks have altered (dolomitized) over time
49 Interpretation of Shales Shale - very fine-grained rock composed of clay, mud, and silt.Types:Quartz-rich shales (quartz sandstones)Feldspar-rich shales (arkoses)Chlorite-rich shales (greywackes)Mica-rich shales (greywackes)
50 UnconformitiesUnconformities in sequences of strata represent times of nondeposition and/or erosion that encompass long periods of geologic time,perhaps millions or tens of millions of yearsThe rock record is incomplete!
51 The origin of an unconformity Deposition began 12 million years ago (MYA), continuing until 4 MYAFor 1 million years erosion occurredremoving 2 MY of rocks** Total of 3 million year hiatus**The last column is the actual stratigraphic record with this unconformity
52 Types of Unconformities Three types of surfaces can be unconformities:A disconformity is a surface separating younger from older rocks, both of which are parallel to one anotherA nonconformity is an erosional surface cut into metamorphic or intrusive igneous rocks and covered by sedimentary rocksAn angular unconformity is an erosional surface on tilted or folded strataover which younger rocks were deposited
53 Figure 3-48 (p. 92) Four types of erosional unconformities Figure (p. 92) Four types of erosional unconformities. (A) Angular unconformity. (B) Nonconformity. (C) Disconformity.
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