Carbonate platform

A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonous calcareous deposits (Wilson, 1975). Platform growth is mediated by sessile organisms whose skeletons build up the 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, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American 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 under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform,[1] the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes.[2] Today’s reefs are built mainly by 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 mineralogic composition of carbonate platforms may be either calcitic or aragonitic. Seawater is oversaturated in carbonate, so under certain conditions CaCO3 precipitation is possible. Carbonate precipitation is 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 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 corals, green algae, foraminifers and molluscs. 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 nutrients and low in oxygen. Mud-mound factories are known only from the fossil record, especially Palaeozoic 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 ramps, tropical factories usually form high-relief rimmed platforms and mud-mounds, 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 boundstones.

Generalized cross-section of a typical carbonate platform.

Internal lagoon

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.

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 algae produce great volumes of carbonate mud. Rocks here are mudstones to grainstones 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 rudstones or grainstones. Sediments have a characteristic geometry: they are organized in clinoforms. Clinoforms are beds that have a sigmoidal or tabular shape, but are always deposited with a primary inclination.

Carbonate platforms in the geological record

The Cimon del Latemar (Trento province, Dolomites, northern Italy) represents 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 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. The middle Liassic "bahamian type" carbonate platform of Morocco (Septfontaine, 1985) is characterised by the accumulation of autocyclic regressive cycles and spectacular supratidal (top of sequences) deposits and vadose diagenetic features with dinosaur tracks. The Tunisian coastal "chotts" and their cyclic muddy deposits represent a good recent equivalent (Davaud & Septfontaine, 1995). Such cycles were also observed on the mesozoic Arabic platform, Oman and Abu Dhabi (Septfontaine & De Matos, 1998) with the same microfauna of foraminifera in an almost identical biostratigraphic succession.

High Atlas middle Liassic carbonate platform of Morocco with first order autocyclic regressive cycles
Détails de cycles émersifs métriques dans deux coupes du Lias moyen distantes de 230km; à noter les niveaux de tempêtes et tsunami (?) riches en foraminifères déplacés. Maroc.
Virtual metric "shallowing upward sequence" observed all along (more than 10,000 km) the south Tethyan margin during middle Liassic times. The (micro)fossils are identical till Oman and beyond.

In the Cretaceous period there were platforms built by bivalvia (rudists).

Gallery

See also

Footnotes

  1. ↑ Geologic Map of Florida
  2. ↑ http://reefgis.reefbase.org/mapper.asp

References

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