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Basin

basins sedimentary drainage structural

Within the broad field of geology, the term basin can be used to represent a number of features. These include topographic or drainage basins, structural basins, and sedimentary basins. In some cases, a single basin can include aspects of more than one of these types of basins.

A topographic, or drainage, basin is a sloping or depressed area from which runoff collects and flows into a channel, stream, or lake. The flow of any river or stream is collected within that area of land comprising its particular drainage basin. These basins can vary greatly in size from millions of square miles to a few square feet. The Amazon River's drainage basin covers more than 2.7 million sq mi (3.22 million km). A drainage divide lies at the top of a topographically elevated ridge that separates each drainage basin from the neighboring basins. The divide encircles the entire basin. Closed drainage basins have no outlet. Any water entering one of these cannot exit by overland flow. Death Valley is a closed basin and the floor of the valley is covered with salts that have been precipitated from water that was later lost to evaporation.

An area in which the rocks dip or tilt toward the center of the structure is known as a structural basin. A structural basin is analogous to a set of progressively smaller bowls that fit neatly within each other. Each of the bowls represents a different layer of rock, all symmetrically arranged around a common center. The rocks that make up such a basin have been displaced downward at the center of the feature, leaving the flanking rocks with a relatively equal dip in all directions. Most structural basins are from a few miles to hundreds of miles in width. In some cases, the rocks that form the outer limbs of the structural basin might be removed by erosion, leaving the center of the basin intact and higher than the outside of the basin. Such a feature would be a topographic high but have the internal structure of a basin. The entire lower peninsula of Michigan is an example of this type of structural basin.

A sedimentary basin is a depressed area in the earth's crust in which sediments tend to accumulate over a long period of time. Sedimentary basins form when the crust of the earth subsides, or becomes depressed, as a result of changes within the earth resulting from tectonic events or other internal changes, or downward warping as sediments are loaded onto the crust. Sedimentary basins can be shaped like bowls or elongate troughs, or have irregular shapes. They can be tens to hundreds of miles in diameter and can contain layers of sediment as thick as 49,000 ft (15,000 m). Much of our interest in these basins results from the fact that some sedimentary basins contain economically significant accumulations of petroleum. There are approximately 600 sedimentary basins on Earth, of which about 150 contain petroleum.

Sedimentary basins often form beneath large deltas. Deltas form in areas where rivers drain continents and deposit sediment. The load of sediment can cause the crust to sag or warp, forming a basin. The Gulf of Mexico is an area of tremendous sediment deposition by the Mississippi River and also contains significant petroleum reserves.

See also Fold; Tectonics.

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