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Organic carbonate buildups from where conditions are favorable for calcareous organisms to flourish and to secrete enough calcium carbonate to build up the substrate locally. Advantages from buildup include inducing better water-circulation patterns and providing firm substrate for organisms not suited to live elsewhere. Perhaps most importantly, buildup involves simply production of enough sediment for the substrate to remain continually in the optimum zone for proliferation of the organisms.
R. J. Dunham's distinction between "ecologic reefs" in which organisms provide rigid framework and bind sediment, and "geologic reefs" in which the restricted area of thickened carbonate is due to localized organic proliferation without necessity of framework or sediment binding, resolves much of the nomenclatural controversy concerning organic carbonate buildups. Perhaps distinction also can be made between geologic
"reefs," which are elongate and differentiate facies on either side, and "mounds" which are merely bumps on the sea floor. Ecologic reefs can be either geologic reefs (barrier and fringing) or mounds (patch reefs); a bank is a geologic reef with no organic framework.
All calcareous organisms are capable of providing sediment to buildups. The more important modern contributors are algae, foraminifers, corals, and mollusks; in the Paleozoic they include pelmatozoans, bryozoans, and brachiopods. Organisms providing framework in large-scale ecologic reefs today are mainly hexacorals, but formerly have included rudistids, stromatoporoids, and perhaps tetracorals. Sediment binding on the same scale is provided mainly by red algae today and has involved blue-green algae, stromatoporoids, and Problematica in the past. Builders of small-scale "mounds" that also are ecologic reefs, include red algae, foraminifers, sponges, corals, bryozoans, brachiopods, polychaete worms, oysters, and sessile gastropods. In many of these mounds, the same organism served as rame and binder; in others, blue-green algae, red algae, Problematica, or bryozoans were binders. Some modern carbonate mounds are not organic in origin, but are merely hydrodynamic accumulations of sediment; perhaps some ancient carbonate mud mounds have a similar inorganic origin.
Formation of organic carbonate buildups results from any combination of environmental factors that causes localized organic proliferation. Favorable oxygenation, water circulation, and nutrient replenishment are necessary for all organic buildups; other factors may have different optima for different organisms, and exclusively invertebrate buildups can form at any depth. Buildups containing algae, however, are restricted to the photic zone, thus are more predictable as to initiation and maintenance. Algal buildups tend to start on better-lit topographic highs, and with bottom subsidence, grow upward where the algae remain in optimum photic conditions. Invertebrate buildups, however, form where other factors are optimal, which may or may not be on highs.
Initiation of a geologic reef involving algae requires simply a bottom slope upon which algae proliferate only above a certain depth. The interval on the slope within which subsidence is equally compensated by algal sedimentation eventually becomes steeper and forms a "reef front" as algal and associated sedimentation keeps the entire shallower side near the surface, whereas the deeper side on which algae are inhibited receives progressively less autochthonous sediment and eventually depends primarily on allochthonous material from any source.
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