The term
sedimentary basin is used to refer to any geographical feature exhibiting
subsidenceSubsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...
and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing
pressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
and begin the process of lithification. A number of basins formed in extensional settings can undergo
inversionIn structural geology inversion or basin inversion relates to the relative uplift of a sedimentary basin or similar structure as a result of crustal shortening. This normally excludes uplift developed in the footwalls of later extensional faults, or uplift caused by mantle plumes...
which has accounted for a number of the economically viable oil reserves on earth which were formerly basins.
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 basins250px|thumb|[[Cami Lake]] in [[Tierra del Fuego]] develops on a [[Patagonian Ice Sheet|glacially]] excavated pull apart basin along the [[Magallanes-Fagnano Fault]], hence its elongated form...
).
Lithospheric stretching
If the
lithosphereThe lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's 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
crustIn geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
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 SeaIn the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
- also an important location for significant
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
reserves. Another such feature is the
Basin and RangeThe Basin and Range Province is a vast physiographic region defined by a unique topographic expression. Basin and Range topography is characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins...
province which covers most of the USA state of Nevada, forming a series of
horst and
grabenIn geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
structures.
Another expression of lithospheric stretching results in the formation of ocean basins with central ridges; The
Red SeaThe Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez...
is in fact an incipient ocean, in a plate tectonic context. The mouth of the Red Sea is also a tectonic
triple junctionA triple junction is the point where the boundaries of three tectonic plates meet. At the triple junction a boundary will be one of 3 types - a ridge, trench or transform fault - and triple junctions can be described according to the types of plate margin that meet at them...
where the Indian Ocean Ridge, Red Sea Rift and
East African RiftThe East African Rift is an active continental rift zone in eastern Africa that appears to be a developing divergent tectonic plate boundary. It is part of the larger Great Rift Valley. The rift is a narrow zone in which the African Plate is in the process of splitting into two new tectonic plates...
meet. This is the only place on the planet where such a triple junction in oceanic crust is exposed
subaerialThe term subaerial is mainly used in geology to describe events or structures that are located at the Earth's surface...
ly . 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.
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 magnitude of the flexure is a function of the imposed load and the
flexural rigidity of the lithosphere, and the wavelength of flexure is a function of flexural rigidity alone. Flexural rigidity is in itself, a function of the lithospheric mineral composition, thermal regime, and effective elastic thickness. The nature of the load is varied. For instance, the
Hawaiian IslandsThe Hawaiian Islands are an archipelago of eight major islands, several atolls, numerous smaller islets, and undersea seamounts in the North Pacific Ocean, extending some 1,500 miles from the island of Hawaii in the south to northernmost Kure Atoll...
chain of volcanic edifices has sufficient mass to cause deflection in the lithosphere.
The
obductionObduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary. It can occur during an orogeny, or mountain-building episode....
of one tectonic plate onto another also causes a load and often results in the creation of a
foreland basinA foreland basin is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure...
, 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
PyreneesThe Pyrenees is a range of mountains in southwest Europe that forms a natural border between France and Spain...
in Spain.
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 near horizontal maximum and minimum principal stresses. The resulting zones of subsidence are known as strike-slip or
pull apart basin250px|thumb|[[Cami Lake]] in [[Tierra del Fuego]] develops on a [[Patagonian Ice Sheet|glacially]] excavated pull apart basin along the [[Magallanes-Fagnano Fault]], hence its elongated form...
s.
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 SeaThe Dead Sea , also called the Salt Sea, is a salt lake bordering Jordan to the east and Israel and the West Bank to the west. Its surface and shores are below sea level, the lowest elevation on the Earth's surface. The Dead Sea is deep, the deepest hypersaline lake in the world...
rift, where northward movement of the
Arabian PlateThe Arabian Plate is one of three tectonic plates which have been moving northward over millions of years and colliding with the Eurasian Plate...
relative to the
Anatolian PlateThe Anatolian Plate is a continental tectonic plate consisting primarily of the country of Turkey.The easterly side is a boundary with the Arabian Plate, the East Anatolian Fault, a left lateral transform fault....
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 MountainsThe San Bernardino Mountains are a short transverse mountain range north and east of San Bernardino in Southern California in the United States. The mountains run for approximately 60 miles east-west on the southern edge of the Mojave Desert in southwestern San Bernardino County, north of the...
north of
Los AngelesLos Ángeles is the capital of the province of Biobío, in the commune of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123,445 inhabitants...
, which result from convergence along a curve in the
San Andreas faultThe San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...
system. The
Northridge earthquakeThe Northridge earthquake was a massive earthquake that occurred on January 17, 1994, at 04:31 Pacific Standard Time in Reseda, a neighborhood in the city of Los Angeles, California, lasting for about 10–20 seconds...
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
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
isostasyIsostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
. 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 modellingBasin modelling is the term broadly applied to a group of geological disciplines that can be used to analyse the formation and evolution of sedimentary basins, often but not exclusively to aid evaluation of potential hydrocarbon reserves....
or
Sedimentary Basin AnalysisSedimentary 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...
. 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
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
reserves and as such are the focus of intense commercial interest.
See also
- Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
- Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
- Structural basin
- Drainage basin
A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...
- Endorheic basin
- Sink (geography)
A geographic sink is a depression within an endorheic basin where water collects with no visible outlet. Instead of discharging, the collected water is lost due to evaporation and/or penetration...