DEFINITION OF BASIN

A depression in the surface of the Earth or other

celestial body. There are many types of basins, including

depressed areas with no outlet or with no outlet for deep levels

(such as lakes, oceans, seas, and tidal basins), and areas of

extreme land subsidence (such as volcanic calderas or sinkholes).

In contrast, drainage basins include the total land area

that contributes water to a stream. Drainage (river or stream)

basins are geographic areas defined by surface slopes and

stream networks where all the surface water that falls in the

drainage basin flows into that stream system or its tributaries.

Groundwater basins are areas where all the groundwater

is contained in one system, or flows toward the same

surface water basin outlet. Impact basins are circular depressions

excavated instantaneously during the impact of a comet

or asteroid with the Earth or other planetary surface.

Areas of prolonged subsidence and sediment accumulation

are known as sedimentary basins, even though they may

not presently be topographically depressed. There are several

types of sedimentary basins, classified by their shape and

relationships to bordering mountain belts or uplifted areas.

Foreland basins are elongate areas on the stable continent

sides of orogenic belts, characterized by a gradually deepening,

generally wedge-shaped basin, filled by clastic and lesser

amounts of carbonate and marine sedimentary deposits. The

sediments are coarser grained and of more proximal varieties

toward the mountain front, from where they were derived.

Foreland basins may be several hundred feet to about 9–12

miles (100s of meters to 15–20 km) deep and filled entirely

by sedimentary rocks, and they are therefore good sites for

hydrocarbon exploration. Many foreland basins have been

overridden by the orogenic belts from where they were

derived, producing a foreland fold-thrust belt, and parts of

the basin incorporated into the orogen. Many foreland basins

show a vertical profile from a basal continental shelf type of

assemblage, upward to a graywacke/shale flysch sequence,

into an upper conglomerate/sandstone molasse sequence.

Rift basins are elongate depressions in the Earth’s surface

where the entire thickness of the lithosphere has ruptured in

extension. They are typically bounded by normal faults along

their long sides, and display rapid lateral variation in sedimentary

facies and thicknesses. Rock types deposited in the

rift basins include marginal conglomerate, fanglomerate, and

alluvial fans, grading basinward into sandstone, shale, and

lake evaporite deposits. Volcanic rocks may be intercalated

with the sedimentary deposits of rifts and in many cases

include a bimodal suite of basalts and rhyolites, some with

alkaline chemical characteristics.

Several other less common types of sedimentary basins

form in different tectonic settings. For instance, pull-apart rift

basins and small foreland basins may form along bends in

strike-slip fault systems, and many varieties of rift and foreland

basins form in different convergent margin and divergent

margin tectonic settings.

See also CONVERGENT PLATE MARGIN PROCESSES; DIVERGENT

OR EXTENSIONAL BOUNDARIES; TRANSFORM PLATE

MARGIN PROCESSES; DRAINAGE BASIN; FORELAND BASIN;

OCEAN BASIN; RIFT; PLATE TECTONICS.

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