View source for Oolite

{{Distinguish|Oncolite}}
{{short description|Sedimentary rock formed from ooids}}
{{for|the video game|Oolite (video game)}}
{{for|the oogenus|Oolithes}}
[[File:Ooids, Joulter Cays, Bahamas.jpg|thumb|Modern ooids from a beach on [[Joulter Cays]], The Bahamas, with 0.5 mm scale]]
[[File:OoidSurface01.jpg|thumb|right|Ooids on the surface of a limestone; Carmel Formation (Middle Jurassic) of southern Utah]]
[[File:CarmelOoids.jpg|thumb|right|Thin-section of calcitic ooids from an oolite within the [[Carmel Formation]] (Middle [[Jurassic]]) of southern Utah]]

'''Oolite''' or '''oölite''' ({{ety|grc|''[[wikt:ᾠόν|ᾠόν]]'' (ōión)|egg stone}})<ref>{{Cite book|last=|first=|url=https://books.google.com/books?id=9IsAEAAAQBAJ&pg=PA77|title=A to Z of Rocks, Minerals and Gems|date=2020|publisher=Quarto Publishing Group UK|isbn=978-0-7112-5684-2|language=en}}</ref> is a [[sedimentary rock]] formed from [[ooid]]s, spherical grains composed of concentric layers.<ref>{{Cite web|title=Oolite|url=https://geokansas.ku.edu/oolite|access-date=October 8, 2021|website=[[Kansas Geological Survey]]}}</ref> Strictly, oolites consist of ooids of diameter 0.25–2 millimetres; [[rock (geology)|rocks]] composed of ooids larger than 2&nbsp;mm are called [[pisolite]]s. The term ''oolith'' can refer to oolite or individual ooids.

==Composition==
Ooids are most commonly composed of [[calcium carbonate]] ([[calcite]] or [[aragonite]]), but can be composed of phosphate, clays, [[chert]], [[dolomite (mineral)|dolomite]] or [[iron]] minerals, including [[hematite]]. Dolomitic and chert ooids are most likely the result of the replacement of the original texture in limestone. Oolitic hematite occurs at [[Red Mountain, Birmingham, Alabama|Red Mountain]] near [[Birmingham, Alabama]], along with oolitic limestone.

They are usually formed in warm, supersaturated, shallow, highly agitated marine water intertidal environments, though some are formed in inland lakes. The mechanism of formation starts with a small fragment of sediment acting as a 'seed', such as a piece of a shell. Strong intertidal currents wash the 'seeds' around on the seabed, where they accumulate layers of chemically precipitated calcite from the supersaturated water. The oolites are commonly found in large [[current bedding]] structures that resemble sand dunes. The size of the oolites reflect the time that they were exposed to the water before they were covered with later sediment.

Oolites are often used in the home aquarium industry because their small grain size (0.2 to 1.22&nbsp;mm) is ideal for shallow static beds and bottom covering of up to 1" in depth. Also known as "oolitic" sand, the sugar-sized round grains of this sand pass easily through the gills of [[Goby|gobies]] and other sand-sifting organisms. This unusually smooth sand promotes the growth of bacteria, which are important biofilters in home aquaria. Because of its extremely small grain size, oolitic sand has a lot of surface area, which promotes high bacterial growth.

==Occurrence==
Some exemplar oolitic [[limestone]] was formed in [[England]] during the [[Jurassic]] period, and forms the [[Cotswold Hills]], the [[Isle of Portland]] with its famous [[Portland Stone]],<ref name=Atkinson1992>{{cite book|last1=Atkinson|first1=Richard|last2=Atkinson|first2=Frances|series=The Observer's Book of|title=Rocks & Minerals|orig-year=1979|year=1992|publisher=Bloomsbury|location=London|pages=161–162|quote=Oolitic limestones … are among the most popular building stones and include Portland stone and Bath stone|isbn=1-85471-044-3}}</ref> and part of the [[North York Moors]]. A particular type, [[Bath Stone]], gives the buildings of the [[World Heritage City]] of [[Bath, Somerset|Bath]] their distinctive appearance. [[Carboniferous]] [[Hunts Bay Oolite]] lies under much of south [[Wales]].

The [[Miami Rock Ridge]] of southeastern Florida, the islands of the Lower [[Florida Keys]], and much of the [[Everglades]], are underlain by [[Miami Oolite]].<ref>{{Cite web|url=https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=FLPSm%3B0|title=Miami Limestone|website=USGS|access-date=May 7, 2019}}</ref> This limestone was formed by deposition when shallow seas covered the area between [[Glacial period|periods of glaciation]]. The material consolidated and eroded during later exposure above the ocean surface.

One of the world's largest freshwater lakebed oolites is the Shoofly Oolite,<ref>{{Cite web|url = http://www.blm.gov/id/st/en/environmental_education/science-research/ACEC/Shoofly_Oolite.html|title = Mud Flat Oolite|date = 2015-05-28|website = www.blm.gov|access-date = 2016-03-04|archive-url = https://web.archive.org/web/20160306043038/http://www.blm.gov/id/st/en/environmental_education/science-research/ACEC/Shoofly_Oolite.html|archive-date = 2016-03-06|url-status = dead}}</ref> a section of the Glenns Ferry Formation on southwestern [[Idaho]]'s [[Snake River Plain]]. 10 million years ago, the Plain formed the bed of [[Lake Idaho]]. Wave action in the lake washed sediments back and forth in the shallows on the southwestern shore, forming [[ooids]] and depositing them on steeper benches near the shore in 2- to 40-foot thicknesses.  When the lake drained (2 to 4 million years ago), the oolite was left behind, along with [[siltstone]], volcanic [[tuff]]s and alluvium from adjacent mountain slopes.  The other sediments eroded away, while the more resistant oolite weathered into hummocks, small arches and other natural "sculptures". The Shoofly Oolite lies on public land west of [[Bruneau, Idaho]] managed by the [[Bureau of Land Management]] (BLM). The physical and chemical properties of the Shoofly Oolite are the setting for a suite of rare plants, which the BLM protects through land use management and on-site interpretation.

This type of limestone is also found in [[Indiana]] in the [[United States]]. The town of [[Oolitic, Indiana]], was founded for the trade in limestone and bears its name. Quarries in Oolitic, [[Bedford, Indiana|Bedford]], and [[Bloomington, Indiana|Bloomington]] contributed the materials for such U.S. landmarks as the [[Empire State Building]] in [[New York City|New York]] and [[the Pentagon]] in [[Arlington, Virginia]].  Many of the buildings on the [[Indiana University Bloomington|Indiana University]] campus in Bloomington are built with native oolitic limestone material, and the [[Soldiers' and Sailors' Monument (Indianapolis)|Soldiers' and Sailors' Monument]] in downtown [[Indianapolis]], Indiana, is built mainly of grey oolitic limestone.

Oolites also appear in the [[Conococheague Formation|Conococheague limestone]], of [[Cambrian]] age, in the [[Great Appalachian Valley]] in [[Pennsylvania]], [[Maryland]], [[West Virginia]], and [[Virginia]].

Rogenstein is a term describing a specific type of oolite in which the cementing matter is [[argillaceous]].

==See also==
*{{annotated link|Geologic time scale}}
*{{annotated link|Pearl}} – also formed from concentric layers of calcium carbonate
*{{annotated link|Oolitic aragonite sand}}

==References==
{{reflist}}

==External links==
* {{Commons category-inline|Oolite}}
* {{Wikibooks-inline|Historical Geology|Ooids and oolite}}
{{Rock type}}
{{Authority control}}

[[Category:Sedimentary rocks]]
[[Category:Limestone]]
[[Category:Petrology]]