Are reefs and mud mounds really so different?


Rachel Wood


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Sedimentary Geology

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Although both ‘ecologic reefs’ and mud mounds are demonstrably rigid, framework reefs, they are still considered to be distinct in terms of their dominant processes of formation and preferred environmental settings. This distinction has rested largely upon the assumption that ecologic reefs are dominated by skeletal metazoans growing in shallow waters, in contrast to the complex autochthonous micrite-supported cavity systems that characterise deep-water mud mounds, now considered to represent either organomineralic deposits (where carbonate precipitation has taken place in association with nonliving organic substrates to form ‘automicrite’) or various types of microbialite (where carbonate forms as a direct result of the physiological activity or decay of benthic microorganisms). Yet, such autochthonous micrite is increasingly recognised as an important component of many ancient shallow ecologic reefs as well as some modern coral reefs, and indeed may contribute locally up to 80% of the reef rock. These observations raise doubts as to the validity of current fabric-based definitions used to distinguish between mud mounds and ecologic reefs. Whether the autochthonous micrite in mud mounds proves to be dominated by either automicrite or microbialite, both require particular environmental conditions for their formation. Automicrites form where surplus organic matter from metazoans has degraded to release quantities of acidic amino acids with a significant ability to bind Ca2+ , and microbialite formation also often requires either unusual marine chemistries or ecological conditions. Such conditions might include changes in terrigenous influx, ground water seepage, local anoxia, and increases in the pH of interstitial reef waters or in nutrient concentration.

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