Carbonate platform
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A '''carbonate platform''' is a [[Sedimentary rock|sedimentary]] body which possesses [[topographic]] relief, and is composed of autochthonous calcareous deposits (Wilson, 1975). Platform growth is mediated by [[sessile]] organisms whose [[skeleton]]s build up the [[coral reef|reef]] or by organisms (usually [[microbes]]) which induce [[carbonate]] precipitation through their [[metabolism]]. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: [[light]], [[water]] [[temperature]] and [[Transparency (optics)|transparency]]. For example, carbonate sedimentation along the [[Atlantic]] [[South America]]n coasts takes place everywhere but at the mouth of the [[Amazon River]], because of the intense [[turbidity]] of the water there (Carannante et al., 1988). Spectacular examples of present-day carbonate platforms are the [[Bahama Banks]] (which are roughly 8 [[kilometer]]s thick), the [[Yucatan Peninsula]], the [[Great Barrier Reef]] and the [[Maldive]] [[atoll]]s.[http://www.tamug.edu/cavebiology/Bahamas/BahamaIntro.html] Even the [[Florida]] Peninsula is a carbonate platform, albeit one no longer submerged.<ref>[http://geology.about.com/library/bl/maps/blfloridamap.htm?terms=geologic+map+of+Florida Geologic Map of Florida<!-- Bot generated title -->]</ref> All these [[reef]]s are confined to [[tropical]] latitudes.<ref>http://reefgis.reefbase.org/mapper.asp</ref> Today’s reefs are built mainly by [[scleractinia|scleractinian corals]], but in the distant past other organisms, like [[archaeocyatha]] (during the [[Cambrian]]) or extinct [[cnidaria]], ([[tabulata]] and [[rugosa]]) were important reef builders.
==Carbonate sedimentation==
The [[mineral|mineralogic]] composition of carbonate platforms may be either [[calcite|calcitic]] or [[aragonite|aragonitic]]. [[Seawater]] is oversaturated in carbonate, so under certain conditions CaCO<sub>3</sub> precipitation is possible. Carbonate precipitation is [[Thermodynamics|thermodynamically]] favoured at high temperature and low [[pressure]]. Three types of carbonate precipitation are possible: ''biotically controlled'', ''biotically induced'' and ''abiotic''. Carbonate precipitation is biotically controlled when organisms (such as corals) are present that exploit carbonate dissolved in seawater to build their calcitic or aragonitic skeletons. Thus they may develop hard reef structures. Biotically induced precipitation takes place outside the cell of the organism, thus carbonate is not directly produced by organisms, but precipitates because of their metabolism. Abiotic precipitation involves little or no [[biological]] influence.
==Classification==
The three types of precipitation described above determine different platform geometries. According to these characteristics a classification was created, consisting of three types of carbonate factories. A carbonate factory is the ensemble of the [[Sedimentary depositional environment|sedimentary environment]], the intervening organisms and the precipitation processes that lead to the formation of a carbonate platform (Schlager, 2005).
===Tropical factory===
In these platforms precipitation is biotically controlled, mostly by [[autotrophic]] organisms. Organisms that build this kind of platform are mostly [[coral]]s, [[green algae]], [[foraminifer]]s and [[mollusc]]s. These platforms are found only in warm (more than 20°C) and sunlit waters, high in oxygen and low in nutrients. This means that they are found between 30° north and 30° south of the [[equator]]. This type of factory is the most widespread today, and is often found fossilised.
===Cool-water factory===
As the name suggests, this type of platform extends its domain to cooler waters and higher latitudes than tropical factories. Precipitation is biotically controlled by heterotrophic organisms, sometimes in association with photo-autotrophic organisms such as [[red algae]]. The sea-waters of these platforms are characterised by a higher amount of nutrients than in tropical factories.
===Mud-mound factory===
These platforms are characterised by abiotic precipitation and biotically induced precipitation. They grow in waters high in [[nutrient]]s and low in [[oxygen]]. Mud-mound factories are known only from the [[fossil record]], especially [[Paleozoic]] and [[Mesozoic]].
==Geometry of carbonate platforms==
Several factors influence the geometry of a carbonate platform, including inherited topography, synsedimentary [[tectonics]], exposition to currents and [[trade winds]]. However, the one most important factor is perhaps the type of carbonate factory. Thus, cool-water factories tend to generate [[Carbonate ramp|ramps]], tropical factories usually form high-relief rimmed platforms and [[mud-mound]]s, mound-shaped high relief bodies without a clear zone of dip break. The best known platform geometry, however, is that of modern tropical factories. Tropical carbonate platforms can be subdivided into three principal sedimentary environments: reef, internal lagoon and slope.
===Reef===
The reef is that part of a carbonate platform created by essentially in-place, sessile organisms. Today’s reefs are built by [[hermatypic]] organisms. The reef is the rigid structure of carbonate platforms and is located between the internal [[lagoon]] and the slope. Survival of the platform depends on the existence of the reef, because only this part of the platform can build a rigid, wave-resistant structure. Two reef types are distinguished: isolated (as Maldives atolls) or epicontinental (as the [[Belize]] reefs or the [[Florida Keys]]). Geologically speaking, reef rocks can be classified as massive [[Boundstone (geology)|boundstone]]s.
===Internal Lagoon===
[[Image:FangoCarb1.jpg|350 px|thumb|right|An example of carbonate mud sedimentation in the internal part of the Florida Bay lagoon. The presence of young mangroves is important to entrap the carbonate mud. Florida bay, summer 2000.]]
The internal lagoon, as the name suggests, is the part of platform behind the reef. It is characterised by shallow and calm waters. Sediments are composed of reef fragments and hard parts of organisms. If the reef is epicontinental there can be also a terrigenous contribution. In some lagoons (e.g., the [[Florida Bay]]) [[green alga]]e produce great volumes of carbonate mud.
Rocks here are [[mudstone]]s to [[grainstone]]s depending on the energy of the environment.
===Slope===
The slope is the outer part of the platform, connecting the reef with the basin. This area acts as sink for excess carbonate sediment: most of the sediment produced in the lagoon and reef is transported by various processes and accumulates in the slope. Therefore, the slope contains coarser sediments than the reef and lagoon; these rocks are generally [[rudstone]]s or [[grainstone]]s. Sediments have a characteristic geometry: they are organised in clinoforms. Clinoforms are [[bed (geology)|beds]] that have a sigmoidal or tabular shape, but are always deposited with a primary inclination.
==Carbonate platforms in the geological record==
[[Image:Bigcimon.jpg|350 px|thumb|right| The Cimon del Latemar (Trento province, Dolomites, northern Italy) rappresents the internal lagoon of a fossil carbonate platform.
Continuous sedimentation took place in an environment as the one described in the image of the Florida Bay and, given a strong [[subsidence]], led to the formation of a sedimentary series that therefore acquired considerable thickness.]]
Sedimentary sequences show carbonate platforms as old as the [[Precambrian]], when they were formed by [[stromatolite|stromatolitic]] sequences. In the [[Cambrian]] carbonate platforms were built by archaeocyatha, [[metazoa]] similar to [[porifera]]. During [[Paleozoic]] [[brachiopod]] (richtofenida) and [[stromatoporoidea]] reefs were erected. At the middle of the Paleozoic era corals became important platforms builders, first with tabulata (from the [[Silurian]]) and then with rugosa (from the [[Devonian]]). Scleractinia become important reef builders beginning only in the [[Carnian]] (upper [[Triassic]]). One of the best examples of a carbonate platform is the [[Dolomites]], deposited during the Triassic. This region of the Southern [[Alps]] contains many well preserved “atolls”, including the [[Sella]], [[Gardenaccia]], [[Sassolungo]] and [[Latemar]], the latter being a backstepping type platform. In the [[Cretaceous]] period there were platforms built by [[bivalvia]] ([[rudists]]).
==See also==
*[[Bahama Banks]]
==Footnotes:==
<references/>
==References:==
*Carannante G., Esteban M., Milliman J. D., Simone L., 1988, Carbonate lithofacies as paleolatitude indicators: problems and limitations. [http://www.elsevier.com/wps/find/journaldescription.cws_home/503361/description#description Sedimentary Geology], v. 60, pp. 333-346.
*Schlager W., 2005, Carbonate sedimentology and sequence stratigraphy. SEPM, Tulsa, Oklahoma, 200 pp. ISBN 1-56576-116-2
*Wilson J. L., 1975, Carbonate Facies in Geologic History, Springer-Verlag, 471 pp. ISBN 3-540-07236-5
*Bosellini A., 1984, Progradation geometries of carbonate platforms: examples from the Triassic of the Dolomites, northern Italy. “Sedimentology”, v.31:1-24
*[http://www.tamug.edu/cavebiology/Bahamas/BahamaIntro.html "Bahamas Introduction" (Accessed 3/8/06)]
*[http://geology.about.com/library/bl/maps/blfloridamap.htm?terms=geologic+map+of+Florida About.com: Geologic Map of Florida (Accessed 3/8/06)]
*Pinet P. R., 1996, Invitation to Oceanography. St. Paul: West Publishing Company, ISBN 0-314-06339-0
[[Category: Oceanography]]
[[Category: Geology]]