basin A

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.
Further Reading
Allen, Philip A., and John R. Allen. Basin Analysis, Principles and
Applications. Oxford: Blackwell Scientific Publications, 1990.
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