Editing Sedimentary rock (section)

==Classification based on origin==

[[File:Uluṟu (Ayers Rock), Sunset.jpg|thumb| [[Uluru]] (Ayers Rock) is a large [[sandstone]] formation in [[Northern Territory]], [[Australia]].]]

Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical (biogenic) sedimentary rocks, chemical sedimentary rocks, and a fourth category for "other" sedimentary rocks formed by impacts, [[volcanism]], and other minor processes.

===Clastic sedimentary rocks===
{{main|Clastic rock}}
[[File:GLMsed.jpg|thumb|upright=1.2|[[Claystone]] deposited in [[Glacial Lake Missoula]], [[Montana]], [[United States]]. Note the very fine and flat bedding, common for deposits coming from [[lake]] beds further away from the source of sediment.]]

Clastic sedimentary rocks are composed of rock fragments (''clasts'') that have been cemented together. The clasts are commonly individual grains of [[quartz]], [[feldspar]], [[clay minerals]], or [[mica]]. However, any type of mineral may be present. Clasts may also be ''lithic fragments'' composed of more than one mineral.

Clastic sedimentary rocks are subdivided according to the dominant particle size. Most geologists use the [[Particle size (grain size)#Wentworth|Udden-Wentworth]] grain size scale and divide unconsolidated sediment into three fractions: [[gravel]] (>2&nbsp;mm diameter), [[sand]] (1/16 to 2&nbsp;mm diameter), and [[mud]] (<1/16&nbsp;mm diameter). Mud is further divided into [[silt]] (1/16 to 1/256&nbsp;mm diameter) and [[clay]] (<1/256&nbsp;mm diameter). The classification of clastic sedimentary rocks parallels this scheme; [[Conglomerate (geology)|conglomerates]] and [[breccia]]s are made mostly of gravel, [[sandstone]]s are made mostly of [[sand]], and [[mudrock]]s are made mostly of mud. This tripartite subdivision is mirrored by the broad categories of [[rudite]]s, [[arenite]]s, and [[lutite]]s, respectively, in older literature.

The subdivision of these three broad categories is based on differences in clast shape (conglomerates and breccias), composition (sandstones), or grain size or texture (mudrocks).

====Conglomerates and breccias====
{{main|Conglomerate (geology)}}
Breccias are dominantly composed of angular gravel in a [[Matrix (geology)|groundmass (matrix)]],<ref name=dkp/> while conglomerates are dominantly composed rounded gravel.

====Sandstones====
[[File:Sedimentgesteine auf Malta.JPG|thumb|upright=1.1|Sedimentary rock with sandstone in [[Malta]], southern Europe]]
[[File:Lower antelope 3 md.jpg|thumb|upright=1.2|[[Lower Antelope Canyon]] was carved out of the surrounding [[sandstone]] by both mechanical weathering and chemical weathering. Wind, sand, and water from [[flash flood]]ing are the primary weathering agents.]]
{{main|Sandstone}}

Sandstone classification schemes vary widely, but most geologists have adopted the Dott scheme,{{sfn|Dott|1964}} which uses the relative abundance of quartz, feldspar, and lithic framework grains and the abundance of a muddy matrix between the larger grains.

:''Composition of framework grains''
:The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance.

::* Quartz sandstones have >90% quartz grains
::* Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains
::* Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grains

:''Abundance of muddy matrix material between sand grains''
::When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud (silt and/or clay sized particle).
::* "Clean" sandstones with open pore space (that may later be filled with matrix material) are called arenites.
::* Muddy sandstones with abundant (>10%) muddy matrix are called wackes.

Six sandstone names are possible using the descriptors for grain composition (quartz-, feldspathic-, and lithic-) and the amount of matrix (wacke or arenite). For example, a quartz arenite would be composed of mostly (>90%) quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.

Although the Dott classification scheme{{sfn|Dott|1964}} is widely used by sedimentologists, common names like [[greywacke]], [[arkose]], and quartz sandstone are still widely used by non-specialists and in popular literature.

====Mudrocks====
{{main|Mudrock}}
Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles. These relatively fine-grained particles are commonly transported by [[turbulence|turbulent flow]] in water or air, and deposited as the flow calms and the particles settle out of [[Suspension (chemistry)|suspension]].

Most authors presently use the term "mudrock" to refer to all rocks composed dominantly of mud.{{sfn|Blatt|Middleton|Murray|1980|p=782}}{{sfn|Prothero|Schwab|2004}}{{sfn|Boggs|2006}}{{sfn|Stow|2005}} Mudrocks can be divided into siltstones, composed dominantly of silt-sized particles; mudstones with subequal mixture of silt- and clay-sized particles; and claystones, composed mostly of clay-sized particles.{{sfn|Blatt|Middleton|Murray|1980|p=782}}{{sfn|Prothero|Schwab|2004}} Most authors use "[[shale]]" as a term for a [[Fissility (geology)|fissile]] mudrock (regardless of grain size) although some older literature uses the term "shale" as a synonym for mudrock.

===Biochemical sedimentary rocks===
[[File:OilShaleEstonia.jpg|thumb|Outcrop of [[Ordovician]] [[oil shale]] ([[kukersite]]), northern Estonia]]
Biochemical sedimentary rocks are created when organisms use materials dissolved in air or water to build their tissue. Examples include:
* Most types of [[limestone]] are formed from the calcareous skeletons of organisms such as [[coral]]s, [[mollusk]]s, and [[foraminifera]].
* [[Coal]], formed from vegetation that has removed [[carbon]] from the atmosphere and combined it with other elements to build their tissue, this vegetation gets compressed by overlying sediments and undergoes chemical transformation.<ref name=dkp/>
* Deposits of [[chert]] formed from the accumulation of siliceous skeletons of microscopic organisms such as [[Radiolarian|radiolaria]] and [[diatom]]s.

===Chemical sedimentary rocks===
Chemical sedimentary rock forms when mineral constituents in [[Solution (chemistry)|solution]] become [[supersaturation|supersaturated]] and inorganically [[precipitate]]. Common chemical sedimentary rocks include [[Oolite|oolitic limestone]] and rocks composed of [[evaporite]] minerals, such as [[halite]] (rock salt), [[sylvite]], [[baryte]] and [[gypsum]].

===Other sedimentary rocks===
This fourth miscellaneous category includes volcanic [[tuff]] and [[Breccia#Volcanic|volcanic breccias]] formed by deposition and later cementation of lava fragments erupted by volcanoes, and [[Breccia#Impact|impact breccias]] formed after [[impact events]].

<gallery mode="packed" heights="180px">
File:PIA17603 Erosion by Scarp Retreat in Gale Crater, Annotated Version.jpg|Sedimentary rocks on Mars, investigated by NASA's [[Curiosity (rover)|Curiosity Mars rover]]
File:Chalus road - ninara 02.jpg|Steeply dipping sedimentary rock strata along the [[Road 59 (Iran)|Chalous Road]] in northern Iran
File:Entropy in Nature.jpg|Stratified remains of [[Papakolea Beach|Puʻu Mahana cinder cone]]
</gallery>