Sedimentary basin

Sedimentary basins are large, regional-scale hollows in the Earth's crust where the ground has sunk over time, allowing thick layers of sediment to pile up and eventually turn into a massive, three-dimensional body of sedimentary rock. This happens when long-term sinking creates a low area that can hold accumulating sediments. Over millions to hundreds of millions of years, sediment—mostly eroded material carried by water and moved by gravity—fills the depression. As more sediment buries the older layers, increasing pressure triggers compaction and lithification, turning the loose material into solid rock.

These basins form when the Earth's lithosphere deforms, usually due to plate tectonic activity. The sinking that creates a basin can result from the crust being thinned, being weighed down by sediment, tectonic forces, or volcanic material, or from changes in the thickness or density of the lithosphere nearby. Once a basin starts forming, the weight of the accumulating sediment adds extra pressure on the crust below, making the basin sink even more through isostasy, which amplifies the basin's development. The preserved rock record of a sedimentary basin is a large, continuous, three-dimensional stack of sedimentary rocks from a specific geologic time period, called a stratigraphic succession.

Geologists still call it a sedimentary basin even if it is no longer a depression on the landscape, as seen with the Williston Basin, Molasse basin, and Magallanes Basin. How well a basin is preserved depends on its tectonic setting. Intracratonic basins, found on stable continental interiors, are likely to survive.

Thermal subsidence

In contrast, basins on oceanic crust are often destroyed by subduction. Continental margins that form when continents rift apart, like those around the Atlantic, can last hundreds of millions of years but may only be partly preserved when those oceans close during continental collisions. Sedimentary basins are hugely important economically. Nearly all the world's natural gas and petroleum, and all of its coal, are found in sedimentary rock.

Many metal ores also occur in sedimentary rocks from specific environments. Scientifically, these basins are valuable because their sedimentary fill records Earth's history during the time they were actively receiving sediment. Over six hundred sedimentary basins have been identified worldwide. They range in area from just tens of square kilometers to well over a million, and their sedimentary fill can be from one to nearly twenty kilometers thick.

Classification

About a dozen common types of sedimentary basins are recognized, and several classification schemes exist, but none is considered the standard. Most schemes are based on one or more of these criteria: plate tectonic setting (proximity to divergent, convergent, or transform boundaries and the forces active during sedimentation); the nature of the underlying crust (continental versus oceanic, which have different mechanical properties and densities); the geodynamics of basin formation (the mechanical and thermal forces causing subsidence); and petroleum or economic potential (characteristics that influence the likelihood of oil or gas accumulations). While no single classification is widely adopted, several basin types are commonly accepted as distinct.

Widely recognized types

Over its entire lifespan, a single basin can go through multiple phases and evolve from one type to another—for example, a rift process completing to form a passive margin. In that case, the sedimentary rocks from the rift phase are overlain by those from the passive margin phase. Hybrid basins, where a single regional basin results from processes characteristic of multiple types, are also possible. Sedimentary basins form from regional subsidence of the lithosphere, mostly due to a few geodynamic processes.

Lithospheric stretching

If the lithosphere is stretched horizontally—by rifting (associated with divergent plate boundaries) or by ridge-push or trench-pull (associated with convergent boundaries)—the effect is twofold. The lower, hotter part of the lithosphere slowly flows away from the stretched area, while the upper, cooler, more brittle crust faults and fractures. Together, these mechanisms cause the surface in the stretched area to sink, creating a depression that is often filled with water and/or sediments. (An analogy is a piece of rubber that thins in the middle when stretched.) An example of a basin caused by lithospheric stretching is the North Sea, which is also important for its hydrocarbon reserves.