Editing Limestone (section)

===Grains===
[[File:Ooids, Joulter Cays, Bahamas.jpg|thumb|[[Ooid]]s from a [[beach]] on Joulter's Cay, [[The Bahamas]] ]]
[[File:Ooids Carmel Formation Jurassic.jpg|thumb|Ooids in limestone of the [[Carmel Formation]] (Middle Jurassic) of southwestern Utah.]]
[[File:CarmelOoids.jpg|thumb|Thin-section view of a Middle [[Jurassic]] limestone in southern [[Utah]], U.S. The round grains are [[ooid]]s; the largest is {{cvt|1.2|mm|in|2}} in diameter. This limestone is an oosparite.]]
Most grains in limestone are skeletal fragments of marine organisms such as [[coral]] or [[foraminifera]].{{sfn|Blatt|Middleton|Murray|1980|p=452}} These organisms secrete structures made of aragonite or calcite, and leave these structures behind when they die. Other carbonate grains composing limestones are [[ooids]], [[peloids]], and limeclasts ([[intraclasts]] and {{ill|extraclast|lt=extraclasts|ca}}).{{sfn|Blatt|Tracy|1996|pages=295–300}}

Skeletal grains have a composition reflecting the organisms that produced them and the environment in which they were produced.{{sfn|Blatt|Middleton|Murray|1980|p=449}} Low-magnesium calcite skeletal grains are typical of articulate [[brachiopod]]s, planktonic (free-floating) foraminifera, and [[coccolith]]s. High-magnesium calcite skeletal grains are typical of benthic (bottom-dwelling) foraminifera, [[echinoderm]]s, and [[coralline algae]]. Aragonite skeletal grains are typical of [[mollusc]]s, calcareous [[green algae]], [[stromatoporoid]]s, [[coral]]s, and [[tube worm]]s. The skeletal grains also reflect specific geological periods and environments. For example, coral grains are more common in high-energy environments (characterized by strong currents and turbulence) while bryozoan grains are more common in low-energy environments (characterized by quiet water).{{sfn|Boggs|2006|p=161–164}}

Ooids (sometimes called ooliths) are sand-sized grains (less than 2mm in diameter) consisting of one or more layers of calcite or aragonite around a central quartz grain or carbonate mineral fragment. These likely form by direct precipitation of calcium carbonate onto the ooid. Pisoliths are similar to ooids, but they are larger than 2&nbsp;mm in diameter and tend to be more irregular in shape. Limestone composed mostly of ooids is called an ''[[oolite]]'' or sometimes an ''oolitic limestone''. Ooids form in high-energy environments, such as the Bahama platform, and oolites typically show [[Cross-bedding|crossbedding]] and other features associated with deposition in strong currents.{{sfn|Blatt|Tracy|1996|pp=297-299}}{{sfn|Boggs|2006|pp=164-165}}

''Oncoliths'' resemble ooids but show a radial rather than layered internal structure, indicating that they were formed by algae in a normal marine environment.{{sfn|Blatt|Tracy|1996|pp=297-299}}

Peloids are structureless grains of microcrystalline carbonate likely produced by a variety of processes.<ref>{{cite journal |last1=Adachi |first1=Natsuko |last2=Ezaki |first2=Yoichi |last3=Liu |first3=Jianbo |title=The fabrics and origins of peloids immediately after the end-Permian extinction, Guizhou Province, South China |journal=Sedimentary Geology |date=February 2004 |volume=164 |issue=1–2 |pages=161–178 |doi=10.1016/j.sedgeo.2003.10.007|bibcode=2004SedG..164..161A }}</ref> Many are thought to be fecal pellets produced by marine organisms. Others may be produced by [[endolithic]] (boring) algae{{sfn|Blatt|Tracy|1996|p=298}} or other microorganisms<ref>{{cite journal |last1=Chafetz |first1=Henry S. |title=Marine Peloids: A Product of Bacterially Induced Precipitation of Calcite |journal=SEPM Journal of Sedimentary Research |date=1986 |volume= 56 |issue=6 |pages=812–817 |doi=10.1306/212F8A58-2B24-11D7-8648000102C1865D}}</ref> or through breakdown of mollusc shells.<ref>{{cite journal |last1=Samankassou |first1=Elias |last2=Tresch |first2=Jonas |last3=Strasser |first3=André |title=Origin of peloids in Early Cretaceous deposits, Dorset, South England |journal=Facies |date=26 November 2005 |volume=51 |issue=1–4 |pages=264–274 |doi=10.1007/s10347-005-0002-8|bibcode=2005Faci...51..264S |s2cid=128851366 |url=http://doc.rero.ch/record/322424/files/10347_2005_Article_2.pdf }}</ref> They are difficult to see in a limestone sample except in thin section and are less common in ancient limestones, possibly because compaction of carbonate sediments disrupts them.{{sfn|Blatt|Tracy|1996|p=298}}

Limeclasts are fragments of existing limestone or partially [[lithification|lithified]] carbonate sediments. Intraclasts are limeclasts that originate close to where they are deposited in limestone, while extraclasts come from outside the depositional area. Intraclasts include ''grapestone'', which is clusters of peloids cemented together by organic material or mineral cement. Extraclasts are uncommon, are usually accompanied by other clastic sediments, and indicate deposition in a tectonically active area or as part of a [[turbidity current]].{{sfn|Blatt|Tracy|1996|p=299–300, 304}}