Editing Limestone (section)

====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}}