The Sedimentary Archives CHAPTER 3

Controls on sedimentary rock features Tectonic setting Physical, chemical, and biological processes in the depositional environment Method of sediment transport Rocks in source area from which sediment is derived Climate (and its effect on weathering) Post-depositional processes of lithification (cementation, compaction)

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 particles Thickness of deposit Rate of erosion or subsidence

Continental Tectonic Regimes Craton - stable interior of a continent; undisturbed by mountain-building events since the Precambrian Shields- large areas of exposed crystalline rocks) Platforms- like shields but covered by flat-lying or gently warped sedimentary rocks Orogenic belts - elongate regions bordering the craton which have been deformed by compression since the Precambrian

Figure 3-1 (p. 62) The craton and orogenic belts of North America.

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 deposition Or Sediments form inside the basin; includes chemical precipitates, most carbonate rocks, and coal.

Marine Environments Continental shelf (shallow) Continental slope Continental rise Abyssal plain (deep)

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.)

Transitional Environments Transitional environments = shoreline deposits Examples include: Deltas Beaches and Barrier Islands Lagoons Tidal flats

Deltas Deltas fan-shaped accumulations of sediment Mississippi Delta Niger Delta Deltas fan-shaped accumulations of sediment river flows into a standing body of water, such as a lake or sea sediments are dropped, forming this progradational feature.

Barrier Islands Beaches 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

Barrier Island System Subenvironments:

The Outer Banks Dauphin Island

Continental Environments Continental environments are those environments which are present on the continents. Examples: Fluvial (River) Alluvial fans Lakes (lacustrine) Glacial Eolian (wind)

Alluvial Fan Fluvial Lacustrine

Eolian Glacier

Color of Sedimentary Rocks Clues about depositional environment: Black and dark gray coloration in sedimentary rocks generally indicates the presence of organic carbon and iron Reddish coloration in sedimentary rocks indicates the presence of oxidized iron Green and gray coloration in sedimentary rocks indicates the presence of reduced iron

Rock Colors Dolomite Red Siltstone Gray Evaporite

Size and Sorting of Clasts Texture refers to the size, shape, sorting, and arrangement of grains in a sedimentary rock. Three textural components in clastic rocks: Clasts Matrix Cement

Clasts and matrix Clasts Matrix

Interpretation of Clastic Sedimentary Rocks The texture of a sedimentary rock can provide clues to the depositional environment. Fine-grained= quiet water Large grains= higher energy (velocity) deposition

Grain Size Sedimentary grains are categorized according to size using the Wentworth Scale. Wentworth Scale for sedimentary grain size: GRAVEL (>2mm) SAND SILT CLAY (<1/256mm)

Sorting Sorting refers to the distribution of grain sizes in a rock. POOR! GOOD!

Sorting In general, windblown sediments are better sorted than wave-washed sediments. Well-sorted sands Have 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 deposits glacial tillites.

Grain Shape Grain shape is described in terms of rounding of grain edges and sphericity (equal dimensions). Rounding results from abrasion and grain impact during transport.

Figure 3-13 (p. 72) Shape of sediment particles Figure 3-13 (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.

Sedimentary Structures Sedimentary structures are visible at the scale of an outcrop (LARGE!) that formed at the time of deposition or shortly thereafter (before lithification) Evidence of processes operating

Bedding Sedimentary rocks generally have bedding or stratification Individual layers less than 1 cm thick are laminations common in mudrocks Beds are thicker than 1 cm common in rocks with coarser grains

Graded Bedding Graded bedding is common in turbidity current deposits Some beds show an upward gradual decrease in grain size, known as graded bedding Graded bedding is common in turbidity current deposits

Graded Bedding

Cross-Bedding Cross-bedding forms when layers come to rest at an angle to the surface Cross-beds result from transport by either water or wind http://www.wwnorton.com/earth/egeo/flash/5_1.swf

Cross Beds

Ripple Marks Small-scale alternating ridges and troughs are known as ripple marks and are common in sandstone Current ripple marks form in response to water or wind currents flowing in one direction and have asymmetric profiles Wave-formed ripple marks result from the oscillation of waves tend to be symmetrical

Current Ripple Marks Ripples with an asymmetrical shape Internally cross- bedded Flow upper right to lower left

Wave-Formed Ripples As the waves wash back and forth, symmetrical ripples form Produced by wave (shallow)

Mud Cracks When clay-rich sediments dry, they shrink and crack into polygonal patterns fractures called mud cracks Mud cracks require wetting and drying to form,

Ancient Mud Cracks

Geopetal Structures Which way is up?? Sedimentary structures can be used to determine "up direction". graded beds cross beds mudcracks Flute marks symmetrical (but not asymmetrical) ripples stromatolites burrows tracks,

Interpretating Sands in Clastic Rocks Quartz-rich (mature; quartz sand) Feldspar-rich (immature; arkosic sand) Rock Fragment-rich (immature; lithic sand) Clay-rich (immature; greywacke)

Figure 3-23 (p. 77) Four categories of sandstone as seen in thin section under the microscope. Diameter of field is about 4 mm.

Figure 3-24 (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.

Figure 3-26 (p. 79) Geologic environment in which arkose may be deposited.

Figure 3-28 (p. 80) Tectonic setting in which graywacke is deposited Figure 3-28 (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.

Figure 3-29 (p. 80) Deltaic environment in which lithic sandstones may be deposited.

Interpretation of Carbonates Main Processes Chemical direct precipitates (carbonate mud) Biochemical: organic contribution (shells, etc.)

Characteristics of most marine carbonate environments Warm water Tropical 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

Some limestones may be the accumulation of shells Shell fragments (coquina) Microscopic Foraminifera (chalk) Fossiliferous limestone

Dolomite CaMg(CO3)2 Rock and mineral Original (forming today) is rare Many older rocks have altered (dolomitized) over time

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)

Unconformities Unconformities 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 years The rock record is incomplete!

The origin of an unconformity Deposition began 12 million years ago (MYA), continuing until 4 MYA For 1 million years erosion occurred removing 2 MY of rocks ** Total of 3 million year hiatus** The last column is the actual stratigraphic record with this unconformity

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 another A nonconformity is an erosional surface cut into metamorphic or intrusive igneous rocks and covered by sedimentary rocks An angular unconformity is an erosional surface on tilted or folded strataover which younger rocks were deposited

Figure 3-48 (p. 92) Four types of erosional unconformities Figure 3-48 (p. 92) Four types of erosional unconformities. (A) Angular unconformity. (B) Nonconformity. (C) Disconformity.