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.
| Contents |
Methods of Formation
It is common to categorise sedimentary basins according to the perceived method of formation, creating such groups as foreland basins (caused by lithospheric flexure), back-arc basins (caused by lithospheric stretching) and pull-apart basins (caused by strike-slip deformation of the lithosphere). Mechanisms for formation of sedimentary basins are numerous and often fundamentally linked to the concepts of plate tectonics.
Lithospheric stretching
If the lithosphere is caused to extend, 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 away from the locus of extension, whilst the upper, cooler and more brittle crust will tend to fault 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 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 stuctures.
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.
Lithospheric 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 foreland basins, such as the Po basin in Italy and the Spanish basins of the Pyrenees.
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.
LEVANT.gif
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.
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.