Editing Sedimentary rock (section)

== Sedimentary basins ==
{{main|Sedimentary basin}}
[[File:Volcanic Arc System SVG en.svg|thumb|upright=1.65|[[Plate tectonics]] diagram showing convergence of an oceanic plate and a continental plate. Note the [[back-arc basin]], [[forearc basin]], and [[oceanic basin]]. ]]

Places where large-scale sedimentation takes place are called [[sedimentary basin]]s. The amount of sediment that can be deposited in a basin depends on the depth of the basin, the so-called ''[[Accommodation (geology)|accommodation space]]''. The depth, shape and size of a basin depend on [[tectonics]], movements within the Earth's [[lithosphere]]. Where the lithosphere moves upward ([[tectonic uplift]]), land eventually rises above sea level and the area becomes a source for new sediment as [[erosion]] removes material. Where the lithosphere moves downward ([[tectonic subsidence]]), a basin forms and sediments are deposited.

A type of basin formed by the moving apart of two pieces of a continent is called a [[rift basin]]. Rift basins are elongated, narrow and deep basins. Due to divergent movement, the lithosphere is [[Deformation (mechanics)|stretched]] and thinned, so that the hot [[asthenosphere]] rises and heats the overlying rift basin. Apart from continental sediments, rift basins normally also have part of their infill consisting of [[volcanic rock|volcanic deposits]]. When the basin grows due to continued stretching of the lithosphere, the [[rift]] grows and the sea can enter, forming marine deposits.

When a piece of lithosphere that was heated and stretched cools again, its [[density]] rises, causing [[isostasy|isostatic]] subsidence. If this subsidence continues long enough, the basin is called a [[sag basin]]. Examples of sag basins are the regions along [[passive margin|passive]] [[continental margin]]s, but sag basins can also be found in the interior of continents. In sag basins, the extra weight of the newly deposited sediments is enough to keep the subsidence going in a [[virtuous circle and vicious circle|vicious circle]]. The total thickness of the sedimentary infill in a sag basin can thus exceed 10&nbsp;km.

A third type of basin exists along [[convergent boundary|convergent plate boundaries]] – places where one [[tectonic plate]] moves under another into the asthenosphere. The [[subduction|subducting]] plate bends and forms a [[fore-arc basin]] in front of the overriding plate – an elongated, deep asymmetric basin. Fore-arc basins are filled with deep marine deposits and thick sequences of turbidites. Such infill is called [[flysch]]. When the convergent movement of the two plates results in [[continental collision]], the basin becomes shallower and develops into a [[foreland basin]]. At the same time, tectonic uplift forms a [[mountain belt]] in the overriding plate, from which large amounts of material are eroded and transported to the basin. Such erosional material of a growing mountain chain is called [[molasse]] and has either a shallow marine or a continental facies.

At the same time, the growing weight of the mountain belt can cause isostatic subsidence in the area of the overriding plate on the other side to the mountain belt. The basin type resulting from this subsidence is called a [[back-arc basin]] and is usually filled by shallow marine deposits and molasse.<ref>For an overview of sedimentary basin types, see {{harvnb|Press|Siever|Grotzinger|Jordan|2003|pp=187–189}}; {{harvnb|Einsele|2000|pp=3–9}}.</ref>

[[File:Blue lias cliffs at Lyme Regis.jpg|thumb|Cyclic alternation of [[Competence (geology)|competent]] and less competent beds in the [[Blue Lias]] at [[Lyme Regis]], southern England]]

=== Influence of astronomical cycles ===
In many cases facies changes and other lithological features in sequences of sedimentary rock have a cyclic nature. This cyclic nature was caused by cyclic changes in sediment supply and the sedimentary environment. Most of these cyclic changes are caused by [[astronomy|astronomic]] cycles. Short astronomic cycles can be the difference between the tides or the [[spring tide]] every two weeks. On a larger time-scale, cyclic changes in climate and sea level are caused by [[Milankovitch cycles]]: cyclic changes in the orientation and/or position of the Earth's rotational axis and orbit around the Sun. There are a number of Milankovitch cycles known, lasting between 10,000 and 200,000 years.<ref>For a short explanation of Milankovitch cycles, see {{harvnb|Tarbuck|Lutgens|1999|pp=322–323}}; {{harvnb|Reading|1996|pp=14–15}}.</ref>

Relatively small changes in the orientation of the Earth's axis or length of the seasons can be a major influence on the Earth's climate. An example are the [[Quaternary glaciation|ice ages of the past 2.6 million years]] (the [[Quaternary]] [[Geologic time scale|period]]), which are assumed to have been caused by astronomic cycles.{{sfn|Stanley|1999|p=536}}{{sfn|Andersen|Borns|1994|pp=29=32}} Climate change can influence the global sea level (and thus the amount of accommodation space in sedimentary basins) and sediment supply from a certain region. Eventually, small changes in astronomic parameters can cause large changes in sedimentary environment and sedimentation.