Geo Chapter 5

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Sedimentary Basins

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sites of deposition, in the Mississippi delta and Gulf of Mexico
- sediments pile up on top of one another, layer after layer, and be buried to depths where they may become filled w valuable oil and gas

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Weathering

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the general process by which rocks are broken down at Earth’s surface to produce sediment particles

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Physical weathering

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takes place when solid rock is fragmented by mechanical processes that do not change its chemical composition

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Chemical weathering

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occurs when the minerals in a rock are chemically altered or dissolved

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- erosion

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mobilizes the particles produced by weathering most commonly by rainwater running downhill

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- Transportation

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occurs when currents of wind and water and the moving ice of glaciers transport particles to new locations – sediment sinks – downhill or downstream

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- Deposition (sedimentation)

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occurs when sedimentary particles settle out as winds die down, water currents slow, or glacier edges melt.

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- Burial

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occurs as layers of sediment accumulate in sedimentary basins and older, previously deposited sediments are compacted and progressively buried deep within the basin.

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- Diagenesis

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the physical and chemical changes – including pressure, heat, and chemical reactions – by which sediments buried within sedimentary basins are lithified, or converted into sedimentary rocks

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Clastic particles

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formed by physical and chemical weathering of preexisting rocks and are transports and deposited as clastic sediments. Clastic particles range in size from boulders and pebbles to particles of sand, silt, and clay. Also vary in shape

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Siliclastic sediments

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the weathering of rocks composed largely of silicate minerals

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Chemical sediments

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form at or near their place of deposition, usually from seawater

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Biological sediments

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form near their place of deposition, but are the result of mineral precipitation within organisms as they grow

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Bioclastic sediments

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shallow-water sediments consist predominantly of two calcium carbonate minerals – calcite and aragonite – in variable proportions.

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Sorting

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segregating sediments by size

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Salinity

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the total amount of dissolved substances in a given volume of water
- Ocean’s salinity remains constant

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Subsidence

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a broad area of the crust sinks (subdues) relative to the surrounding crust
- Forms depressions

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Rift basins

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deep, narrow, and long, with successions of sedimentary rocks and extrusive and intrusive igneous rocks

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Thermal subsidence basins

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cooling leads to an increase in the density of lithosphere, which in turn leads to its subsidence below sea level, where sediments can accumulate. Because cooling of the lithosphere is the main process ccreating these basins, they are called thermal subsidence basins. Sediments are suuplied from erosion of the adjacent land and fill the basin to sea level along the edge of the continent, thus creating the continental shelf.

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Flexural basin

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produced by the weight of the overriding plate that causes the underlying plate to bend of flex down

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Sedimentary environment

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a geographic location characterized by a particular combination of climate conditions and physical, chemical, and biological processes

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Siliciclastic sedimentary environments

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dominated by siliclastic sediments. Include continental alluvial (stream), desert, lake, and environments & shoreline too

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Chemical and biological sedimentary environments

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characterized principally by chemical and biological precipitation

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Sedimentary structures

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include all kinds of features formed at the time of deposition

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Bedding (stratification)

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occurs when layers of differen grain sizes or compositions are deposited on top of one another

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Cross-bedding

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consists of sets of bedded material deposited by wind or water and inclined at angles as large as 35° from the horizontal

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Graded bedding

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most abundant in continental slope and deep-sea sediments deposited by dense muddy currents called turbidity currents, which hug the bottom topography of the ocean as they move downhill

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Ripples

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very small dunes of sand or silt whose long dimension is at right angles to the current

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Biotubation

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burrows and tunnels excavated by clams, worms, and many other marine organisms that live on the bottom of the sea. These organisms burrow through muds and sands

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Bedding sequences

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built of interbedded and vertically stacked layers of sandstone, shale, and other sedimentary rock types.

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Graded bedding

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most abundant in continental slope and deep-sea sediments deposited by dense, muddy currents called turbidity current, which hug the bottom topography of the ocean as they move downhill.

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Ripples

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are very small dunes of sand or silt whose long dimension is at right angles to the current. Form low, narrow ridges. Most only a centimeter or two high, separated by wider troughs.

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Bioturbation

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the process where clams, worms, and other marine organisms burrow and make tunnels through in structures and leave ruminants.

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Bedding sequences

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built of interbedded and vertically stacked layers of sandstone, shale, and other sedimentary rock types. May consist of cross-bedded sandstone, overlain by bioturbated siltstone, overlain in turn by ripped sandstone.

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Diagenesis

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the many physical and chemical changes that continue until the sediment or sedimentary rosk is either exposed to weathering or metamorphosed by heat and pressure.

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Canemtaion

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a chemical change where buried sediments are also continuously bathed in groundwater full of dissolved minerals, which can precipitate in the pores

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Porosity

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the percentage of a rock’s volume consisting of open pores between grains.

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Compactation

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a decrease in the volume and porosity of a sediment.

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Lithification

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the hardening of soft sediment into rock

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Gravel

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the coarsest siliciclastic sediment, consisting of particles larger than 2 mm in diameter and including pebbles, cobbles, and boulders.

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Conglomerates

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lithified equivalents of gravel

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Sand

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medium-sized particles that are between .062 - 2mm in size

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Sandstone

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the lithified equivalent of