Editing Limestone (section)

== Occurrence ==
About 20% to 25% of sedimentary rock is carbonate rock,{{sfn|Boggs|2006|p=159}} and most of this is limestone.{{sfn|Blatt|Tracy|1996|pp=295–300}}{{sfn|Boggs|2006|p=159}} Limestone is found in sedimentary sequences as old as 2.7 billion years.{{sfn|Blatt|Middleton|Murray|1980|p=445}} However, the compositions of carbonate rocks show an uneven distribution in time in the geologic record. About 95% of modern carbonates are composed of high-magnesium calcite and aragonite.{{sfn|Blatt|Middleton|Murray|1980|p=448}} The aragonite needles in carbonate mud are converted to low-magnesium calcite within a few million years, as this is the most stable form of calcium carbonate.{{sfn|Blatt|Tracy|1996|p=300}} Ancient carbonate formations of the [[Precambrian]] and [[Paleozoic]] contain abundant dolomite, but limestone dominates the carbonate beds of the [[Mesozoic]] and [[Cenozoic]]. Modern dolomite is quite rare. There is evidence that, while the modern ocean favors precipitation of aragonite, the oceans of the Paleozoic and middle to late Cenozoic favored precipitation of calcite. This may indicate a lower Mg/Ca ratio in the ocean water of those times.{{sfn|Boggs|2006|p=159-161}} This magnesium depletion may be a consequence of more rapid [[sea floor spreading]], which removes magnesium from ocean water. The modern ocean and the ocean of the Mesozoic have been described as "aragonite seas".{{sfn|Boggs|2006|p=176-177}}

Most limestone was formed in shallow marine environments, such as [[continental shelves]] or [[Carbonate platform|platforms]]. Such environments form only about 5% of the ocean basins, but limestone is rarely preserved in continental slope and deep sea environments. The best environments for deposition are warm waters, which have both a high organic productivity and increased saturation of calcium carbonate due to lower concentrations of dissolved carbon dioxide. Modern limestone deposits are almost always in areas with very little silica-rich sedimentation, reflected in the relative purity of most limestones. Reef organisms are destroyed by muddy, brackish river water, and carbonate grains are ground down by much harder silicate grains.{{sfn|Blatt|Middleton|Murray|1980|p=446, 733}} Unlike clastic sedimentary rock, limestone is produced almost entirely from sediments originating at or near the place of deposition.{{sfn|Blatt|Middleton|Murray|1980|p=468-470}}
[[File:El Cap GUMO.jpg|thumb|[[El Capitan (Texas)|El Capitan]], an ancient limestone reef in Texas]]
Limestone formations tend to show abrupt changes in thickness. Large moundlike features in a limestone formation are interpreted as ancient [[reef]]s, which when they appear in the geologic record are called ''bioherms''. Many are rich in fossils, but most lack any connected organic framework like that seen in modern reefs. The fossil remains are present as separate fragments embedded in ample mud matrix. Much of the sedimentation shows indications of occurring in the intertidal or supratidal zones, suggesting sediments rapidly fill available [[Accommodation (geology)|accommodation space]] in the shelf or platform.{{sfn|Blatt|Middleton|Murray|1980|p=446-447}} Deposition is also favored on the seaward margin of shelves and platforms, where there is upwelling deep ocean water rich in nutrients that increase organic productivity. Reefs are common here, but when lacking, ooid shoals are found instead. Finer sediments are deposited close to shore.{{sfn|Blatt|Tracy|1996|p=306-307}}

The lack of deep sea limestones is due in part to rapid [[subduction]] of oceanic crust, but is more a result of dissolution of calcium carbonate at depth. The solubility of calcium carbonate increases with pressure and even more with higher concentrations of carbon dioxide, which is produced by decaying organic matter settling into the deep ocean that is not removed by [[photosynthesis]] in the dark depths. As a result, there is a fairly sharp transition from water saturated with calcium carbonate to water unsaturated with calcium carbonate, the ''[[lysocline]]'', which occurs at the ''[[calcite compensation depth]]'' of {{convert|4000|to|7000|m|ft|abbr=in}}. Below this depth, foraminifera tests and other skeletal particles rapidly dissolve, and the sediments of the ocean floor abruptly transition from carbonate ooze rich in foraminifera and coccolith remains (''[[Globigerina]]'' ooze) to silicic mud lacking carbonates.{{sfn|Blatt|Middleton|Murray|1980|p=474-479}}
[[File:Mønsted kalkgruber exposure fused 2014-07-18.jpg|thumb|[[Mønsted]] is the largest limestone [[Mining|mine]] in the world.]]
In rare cases, [[turbidite]]s or other silica-rich sediments bury and preserve benthic (deep ocean) carbonate deposits. Ancient benthic limestones are microcrystalline and are identified by their tectonic setting. Fossils typically are foraminifera and coccoliths. No pre-Jurassic benthic limestones are known, probably because carbonate-shelled plankton had not yet evolved.{{sfn|Blatt|Tracy|1996|p=308-309}}

Limestones also form in freshwater environments.<ref>{{Cite journal|last1=Roeser|first1=Patricia|last2=Franz|first2=Sven O.|last3=Litt|first3=Thomas|date=1 December 2016|title=Aragonite and calcite preservation in sediments from Lake Iznik related to bottom lake oxygenation and water column depth|journal=Sedimentology|language=en|volume=63|issue=7|pages=2253–2277|doi=10.1111/sed.12306|s2cid=133211098 |issn=1365-3091}}</ref> These limestones are not unlike marine limestone, but have a lower diversity of organisms and a greater fraction of silica and clay minerals characteristic of [[marl]]s. The [[Green River Formation]] is an example of a prominent freshwater sedimentary formation containing numerous limestone beds.{{sfn|Blatt|Middleton|Murray|1980|p=480-482}} Freshwater limestone is typically micritic. Fossils of [[charophyte]] (stonewort), a form of freshwater green algae, are characteristic of these environments, where the charophytes produce and trap carbonates.{{sfn|Blatt|Tracy|1996|p=309-310}}

Limestones may also form in [[evaporite]] [[Sedimentary depositional environment|depositional environments]].<ref>{{cite journal |last1=Trewin |first1=N. H. |last2=Davidson |first2=R. G. |year=1999 |title=Lake-level changes, sedimentation and faunas in a Middle Devonian basin-margin fish bed |journal=[[Journal of the Geological Society]] |volume=156 |issue=3 |pages=535–548 |doi=10.1144/gsjgs.156.3.0535 |bibcode=1999JGSoc.156..535T |s2cid=131241083 }}</ref><ref>{{cite web|url=http://www.glossary.oilfield.slb.com/Display.cfm?Term=evaporite|url-status=dead|website=Oilfield Glossary|title=Term 'evaporite'|archive-url=https://web.archive.org/web/20120131020924/http://www.glossary.oilfield.slb.com/Display.cfm?Term=evaporite |archive-date=31 January 2012 |access-date=25 November 2011}}</ref> Calcite is one of the first minerals to precipitate in marine evaporites.{{sfn|Boggs|2006|p=662}}

===Limestone and living organisms===
[[File:Lembongan-penida-snorkeling.jpg|thumb|Coral reef at [[Nusa Lembongan]], Bali, Indonesia]]
Most limestone is formed by the activities of living organisms near reefs, but the organisms responsible for reef formation have changed over geologic time. For example, ''[[stromatolites]]'' are mound-shaped structures in ancient limestones, interpreted as colonies of [[cyanobacteria]] that accumulated carbonate sediments, but stromatolites are rare in younger limestones.{{sfn|Blatt|Middleton|Murray|1980|pp=446, 471-474}} Organisms precipitate limestone both directly as part of their skeletons, and indirectly by removing carbon dioxide from the water by photosynthesis and thereby decreasing the solubility of calcium carbonate.{{sfn|Blatt|Tracy|1996|p=309-310}}

Limestone shows the same range of [[sedimentary structures]] found in other sedimentary rocks. However, finer structures, such as [[Lamination (geology)|lamination]], are often destroyed by the burrowing activities of organisms ([[bioturbation]]). Fine lamination is characteristic of limestone formed in [[playa lake]]s, which lack the burrowing organisms.{{sfn|Blatt|Middleton|Murray|1980|pp=446-471}} Limestones also show distinctive features such as ''geopetal structures'', which form when curved shells settle to the bottom with the concave face downwards. This traps a void space that can later be filled by sparite. Geologists use geopetal structures to determine which direction was up at the time of deposition, which is not always obvious with highly deformed limestone formations.{{sfn|Blatt|Tracy|1996|p=304}}

The [[cyanobacterium]] ''Hyella balani'' can bore through limestone; as can the [[green alga]] ''Eugamantia sacculata'' and the [[fungus]] ''Ostracolaba implexa''.<ref>{{cite book|url=https://books.google.com/books?id=GerdDmwMTLkC&pg=PA178|title=Geomicrobiology|edition=5th|first1=Henry Lutz |last1=Ehrlich|first2=Dianne K. |last2=Newman|year=2009|pages=181–182|publisher=CRC Press |isbn=978-0-8493-7907-9|url-status=live|archive-url=https://web.archive.org/web/20160510231918/https://books.google.com/books?id=GerdDmwMTLkC&pg=PA178|archive-date=10 May 2016}}</ref>

====Micritic mud mounds====
Micricitic mud mounds are subcircular domes of micritic calcite that lacks internal structure. Modern examples are up to several hundred meters thick and a kilometer across, and have steep slopes (with slope angles of around 50 degrees). They may be composed of peloids swept together by currents and stabilized by ''[[Thalassia (plant)|Thalassia]]'' grass or [[mangroves]]. Bryozoa may also contribute to mound formation by helping to trap sediments.{{sfn|Blatt|Tracy|1996|p=307}}

Mud mounds are found throughout the geologic record, and prior to the [[early Ordovician]], they were the dominant reef type in both deep and shallow water. These mud mounds likely are microbial in origin. Following the appearance of frame-building reef organisms, mud mounds were restricted mainly to deeper water.<ref>{{cite journal |last1=Pratt |first1=Brian R. |title=The origin, biota, and evolution of deep-water mud-mounds |journal=Spec. Publs Int. Ass. Sediment. |date=1995 |volume=23 |pages=49–123 |isbn=1-4443-0412-7 |url=https://books.google.com/books?id=4QCWd-BiJ04C&q=micritic+mud+mounds&pg=PA49 |access-date=4 February 2021}}</ref>

====Organic reefs====
Organic reefs form at low latitudes in shallow water, not more than a few meters deep. They are complex, diverse structures found throughout the fossil record. The frame-building organisms responsible for organic reef formation are characteristic of different geologic time periods: [[Archaeocyathid]]s appeared in the [[early Cambrian]]; these gave way to sponges by the [[late Cambrian]]; later successions included stromatoporoids, corals, algae, bryozoa, and [[rudist]]s (a form of bivalve mollusc).{{sfn|Blatt|Tracy|1996|pp=307-308}}<ref>{{cite journal |last1=Riding |first1=Robert |title=Structure and composition of organic reefs and carbonate mud mounds: concepts and categories |journal=Earth-Science Reviews |date=July 2002 |volume=58 |issue=1–2 |pages=163–231 |doi=10.1016/S0012-8252(01)00089-7|bibcode=2002ESRv...58..163R }}</ref><ref>{{cite book |last1=Wood |first1=Rachel |title=Reef evolution |date=1999 |publisher=Oxford University Press |location=Oxford |isbn=0-19-857784-2|url=https://books.google.com/books?id=H_ah6Hzib4AC&q=reef+organisms+by+geologic+period&pg=PA3 |access-date=5 February 2021}}</ref> The extent of organic reefs has varied over geologic time, and they were likely most extensive in the middle Devonian, when they covered an area estimated at {{convert|5000000|km2|sqmi|abbr=on}}. This is roughly ten times the extent of modern reefs. The Devonian reefs were constructed largely by stromatoporoids and [[Tabulata|tabulate corals]], which were devastated by the [[late Devonian extinction]].<ref>{{cite book |last1=McGhee |first1=George R. |title=When the invasion of land failed : the legacy of the Devonian extinctions |date=2013 |publisher=Columbia University Press |location=New York |isbn=978-0-231-16057-5 |page=101}}</ref>

Organic reefs typically have a complex internal structure. Whole body fossils are usually abundant, but ooids and interclasts are rare within the reef. The core of a reef is typically massive and unbedded, and is surrounded by a [[Scree|talus]] that is greater in volume than the core. The talus contains abundant intraclasts and is usually either ''floatstone'', with 10% or more of grains over 2mm in size embedded in abundant matrix, or ''rudstone'', which is mostly large grains with sparse matrix. The talus grades to planktonic fine-grained carbonate mud, then noncarbonate mud away from the reef.{{sfn|Blatt|Tracy|1996|pp=307-308}}