Sedimentary Basin
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Sedimentary Basin The term sedimentary basin is used to refer to any geographical feature
exhibiting subsidence and consequent infilling by sedimentation. As the sediments are
buried, they are subjected to increasing pressure and begin the process of lithification. A
depression in the crust of the Earth formed by plate tectonic activity in which sediments
accumulate. Continued deposition can cause further depression or subsidence. Sedimentary
basins, or simply basins, vary from bowl-shaped to elongated troughs. If rich hydrocarbon
source rocks occur in combination with appropriate depth and duration of burial, hydrocarbon
generation can occur within the basin. Methods of Formation It is common to categorise
sedimentary basins according to the mechanism of formation: tectonic compression (e.g.,
foreland basins, caused by lithospheric flexure), tectonic extension (e.g., back-arc basins,
caused by lithospheric stretching), and tectonic strike-slip (such as pull-apart basins). 1.
Lithospheric stretching If the lithosphere is caused to stretch horizontally, by mechanisms
such as ridge-push or trench-pull, the effect is believed to be twofold. The lower, hotter part
of the lithosphere will "flow" slowly away from the main area being stretched,
whilst the upper, cooler and more brittle crust will tend to fault (crack) and fracture. The
combined effect of these two mechanisms is for the earth\'s surface in the area of extension
to subside, creating a geographical depression which is then often infilled with water and/or
sediments. (An analogy might be a piece of rubber, which thins in the middle when
stretched.) An example of a basin caused by lithospheric stretching is the North Sea - also
an important location for significant hydrocarbon reserves. Another such feature is the Basin
and Range province which covers most of the USA state of Nevada, forming a series of horst
and graben structures. Another expression of lithospheric stretching results in the formation
of ocean basins with central ridges; The Red Sea is in fact an incipient ocean, in a plate
tectonic context. The mouth of the Red Sea is also a tectonic triple junction where the Indian
Ocean Ridge, Red Sea Rift and East African Great Rift Valley meet. This triple junction is
also the only place on the planet where seafloor crust is subaerially exposed. The reason for
this is twofold, due to a high thermal buoyancy of the junction, and a local crumpled zone of
seafloor crust acting as a dam against the Red Sea. 2. Lithospheric compression/shortening
and flexure If a load is placed on the lithosphere, it will tend to flex in the manner of an elastic
plate. The rate and degree of flexure is a function of the flexural rigidity of the lithosphere,
which is itself a function of the lithospheric mineral composition and thermal regime. The
nature of the load is varied. For instance, the Hawaiian Islands chain of volcanic edifices has
sufficient mass to cause deflection in the lithosphere. The obduction of one tectonic plate
onto another also causes a load and often results in the creation of a foreland basin, such as
the Po basin next to the Alps in Italy, the Molasse Basin next to the Alps in Germany, or the
Ebro basin next to the Pyrenees in Spain. 3. Strike-slip deformation Deformation of the
lithosphere in the plane of the earth (i.e. such that faults are vertical) occurs as a result of
horizontal differential stresses. The resulting zones of subsidence are known as strike-slip or
pull-apart basins. Basins formed through strike-slip action occur where a vertical fault plane
curves. When the curve in the fault plane moves apart, a region of transtension results,
creating a basin. Another term for a transtensional basin is a rhombochasm. A classic
rhombochasm is illustrated by the Dead Sea rift, where northward movement of the Arabian
Plate relative to the Anatolian Plate has caused a rhombochasm. The opposite effect is that
of transpression, where converging movement of a curved fault plane causes collision of the
opposing sides of the fault. An example is the San Bernardino Mountains north of Los
Angeles, which result from convergence along a curve in the San Andreas fault system. The
Northridge earthquake was caused by vertical movement along local thrust and reverse
faults bunching up against the bend in the otherwise strike-slip fault environment. Ongoing
development of sedimentary basins As more and more sediment is deposited into the basin,
the weight of all the newer sediment may cause the basin to subside further because of
isostasy. A basin can continue having sediment deposited into it, and continue to subside, for
long periods of geological time; this can result in basins many kilometres in thickness.
Geologic faults can often occur around the edge of, and within, the basin, as a result of the
ongoing slippage and subsidence. Study of sedimentary basins The study of sedimentary
basins as a specific entity in themselves is often referred to as basin modelling or
Sedimentary Basin Analysis. The need to understand the processes of basin formation and
evolution are not restricted to the purely academic. Indeed, sedimentary basins are the
location for almost all of the world\'s hydrocarbon reserves and as such are the focus of
intense commercial interest. Sedimentary basin analysis is a geologic method by which the
history of a sedimentary basin is revealed, by analyzing the sediment fill itself. Aspects of the
sediment, namely its composition, primary structures, and internal architecture, can be
synthesized into a history of the basin fill. Such a synthesis can reveal how the basin formed,
how the sediment fill was transported or precipitated, and reveal sources of the sediment fill.
From such syntheses models can be developed to explain broad basin formation
mechanisms. Examples of such basinal environments include backarc, forearc, passive
margin, epicontinental, and extentional basins. Sedimentary basin analysis is largely
conducted by two types of geologists who have slightly different goals and approaches. The
petroleum geologist, whose the ultimate goal is to determine the possible presence and
extent of hydrocarbons and hydrocarbon-bearing rocks in a basin, and the academic
geologist, who may be concerned with any or all facets of a basin\'s evolution. Petroleum
industry basin analysis is often conducted on subterrannean basins through the use of
reflection seismology and data from well logging. Academic geologists study subterranean
basins as well as those basins which have been exhumed and dissected by subsequent
tectonic events. Thus academics sometimes use petroleum industry techniques, but in many
cases they are able to study rocks at the surface. Techniques used to study surficial
sedimentary rocks include: measuring stratigraphic sections, identifying sedimentary
depositional environments and constructing a geologic map. World Map of sedimentary
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