Carbonate mounds are important and ubiquitous components of the sedimentary rock record throughout the Phanerozoic. They have been formed by a variety of organisms since the early Paleozoic. The structure and genesis of fossil carbonate mounds are a matter of debate, basically because true modern analogues are thought to be scarce. In analogy to modern tropical coral reefs, most ancient carbonate mounds have widely been linked with ramp/shelf settings in tropical environments, although other depositional environments have been suggested (cf. review in Pratt, 1995).
Factors currently used to constrain the water depth that ancient mounds grew in include water energy, occurrence of specific organisms that perform photosynthesis (particularly calcareous algae), distinct non-biogenic components such as peloids and post-depositional features such as micritization.
In reviewing the main criteria used to delineate water depth of formation of fossil carbonate mounds, most (if not all) criteria traditionally used to assume shallow-water settings appear to be no longer substantiated in the light of current knowledge gained from modern carbonate mounds. High energy, assumed to indicate shallow-water conditions, is ongoing in water depths exceeding 1000 m. The occurrence of peloids and processes such as micritization is not diagnostic of bathymetry. Furthermore, virtually none of the calcareous algae reported from ancient mounds and assumed to indicate the photic zone have a taxonomically granted affiliation.
Over the last years, research in the deep ocean, in particular thanks to the deployment of new devices and techniques, provided a good database revealing a wide distribution of modern carbonate mounds. Reviewing the characteristics of ancient carbonate mounds, with emphasis on depositional settings, and confronting the criteria used with current knowledge from sub-recent and modern carbonate mounds, we conclude that for many ancient mounds a bathyal origin is, at least, as likely as a shallow-water origin. This conclusion is well supported by the sedimentary records left by these modern mounds that are astonishingly similar to those seen in Palaeozoic and Mesozoic carbonate mounds (Hebbeln and Samankassou, 2015).
References:
Hebbeln, D., Samankassou, E. (2015) Where did ancient carbonate mounds grow — In bathyal depths or in shallow shelf waters? Earth-Science Reviews, v. 145, pp. 56-65.
Pratt, B.R. (1995) The origin, biota and evolution of deep-water mud-mounds. IAS Spec. Publ. No 23, pp. 49-123.
Topics: Biotic sedimentary processes , Topics: The frontiers of sedimentology
