Editing Limestone (section)

===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}}