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Sedimentary Environments Help

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By — McGraw-Hill Professional
Updated on Sep 1, 2011

Sedimentary Environments

Sedimentary environments are places where sediments collect and sedimentary rocks form. They can be grouped into three main areas: terrestrial (land), marine, and transitional (border) environments.

1. Terrestrial sedimentary environments (land)

(a) Rivers, streams, and ponds

(b) Lakes

(c) Swamps

(d) Deserts

(e) Glacial environment

2. Transitional environments (border areas between the land and marine environments)

(a) Beach and barrier islands

(b) Delta

(c) Lagoons

(d) Estuaries

3. Marine environments

(a) Continental shelf

(b) Continental slope and rise (deep sea fans)

(c) Abyssal plain

(d) Reefs

Terrestrial Sedimentary Environments

The first of the sedimentary environments is the best known since most people have visited streams, rivers, or lakes at one time or another. A good amount of clastic fragments are deposited into sedimentary layers within terrestrial sedimentary environments . This happens through the action of water current or blowing wind. Depending on the way the sediments are laid down, different layering patterns are seen.

Transitional Environments

An in-between or transitional sedimentary environment is found where major sources of water currents meet the ocean. In delta areas, there is a rich mixture of sediments that arrive from all along the route of the current. The mouth of the Mississippi River delta near New Orleans, Louisiana (United States) deposits many tons of silt into the Gulf of Mexico in a wide fan of sand and mud that can be seen from space.

Beach environments have a lot of wave and tidal energy that moves particles constantly. This back-and-forth grinding movement polishes and sorts them according to size. Fine sediments are washed away to settle further out in the tidal flats, where the wave action is less.

Marine Environments

Marine environments include fine sediments that settle to the ocean bottom as the remains of marine organisms and plants.

The finest marine sediments are found far from the continental margins. Pelagic (from the Greek word pelagos , meaning sea) sediments are so tiny as to be found suspended in salt water most of the time. Think of the super fine dust that is always settling out of the air onto furniture at home. You can’t even see it unless a ray of sunlight makes it visible.

Pelagic sediments are made up of the calcium-containing shells of microorganisms such as foraminiferans , radiolarians , and diatoms . These microorganisms live near the ocean’s surface and when they die, their shells sink, decay, and become part of the fine-grained mucky ooze on the ocean’s bottom. Pelagic sediments, dispersed all through the oceans, settle out and form layers of fine sediment onto deep ocean plains.

Unique marine sediment is created by chemical precipitation in seawater. Precipitates of manganese oxides and hydroxides form golf ball to basketball size lumpy nodules strewn around the ocean floor.

Weathering

Water, wind, and ice all work together to breakdown solid rocks into small rocky particles and fragments. These bits of rock are swept away by rain into streams. Gradually these particles get deposited at the bottom of stream beds or in the ocean. As more and more sediment builds up, it gets crushed together and compacted into solid rock.

Weathering wears away existing rocks and produces lots of small rock bits.

With every tick of the clock, day after day, rock surfaces are worn away by wind and rain. Small bits of dirt, sand, mud, and clay are slowly ground away and washed into streams, rivers, lakes, and oceans. After these tiny bits of sand and rock settle at the bottom, they become sediment.

Water minerals and microscopic or tiny organisms also get mixed with the dirt and sand to form sediment. Over time, more and more sediment piles up on top of what was there before. After millions of years the sediment builds up into deep layers. The heavy weight and extreme pressure from the constantly added sediment turns ocean sediment at the bottom into sedimentary rock. The oldest ocean sedimentary rocks are thought to be around 600 million years old.

These oldest sedimentary rocks were formed long ago, but since then, they have been crushed, heated, and transformed into what is known as metamorphic rock. We will see how metamorphic rock is different from igneous or sedimentary rock.

Practice problems of this concept can be found at: Sedimentary Rock Practice Test

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