Carbonate platform


A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms 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. 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. 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, 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. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha or extinct cnidaria were important reef builders.

Carbonate precipitation from seawater

What makes carbonate platform environments different from other depositional environments is that carbonate is a product of precipitation, rather than being a sediment transported from elsewhere, as for sand or gravel. This implies for example that carbonate platforms may grow far from the coastlines of continents, as for the Pacific atolls.
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 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, by definition, involves little or no biological influence.

Classification

The three types of precipitation cluster into three "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. The differences between three factories is the dominant precipitation pathway and skeletal associations. In contrast, a carbonate platform is a geological structure of parautochotonous carbonate sediments and carbonate rocks, having a morphological relief.

Platforms produced by the "tropical factory"

In these carbonate factories, precipitation is biotically controlled, mostly by autotrophic organisms. Organisms that build this kind of platforms are today mostly corals and green algae, that need sunlight for photosynthesis and thus live in the euphotic zone. Tropical carbonate factories are only present today in warm and sunlit waters of the tropical-subtropical belt, and they have high carbonate production rates but only in a narrow depth window. The depositional profile of a Tropical factory is called "rimmed" and includes three main parts: a lagoon, a reef and a slope. In the reef, the framework produced by large-sized skeletons, as those of corals, and by encrusting organisms resists wave action and forms a rigid build up that may develop up to sea-level. The presence of a rim produces restricted circulation in the back reef area and a lagoon may develop in which carbonate mud is often produced. When reef accretion reaches the point that the foot of the reef is below wave base, a slope develops: the sediments of the slope derive from the erosion of the margin by waves, storms and gravitational collapses. This process accumulates coral debris in clinoforms. The maximum angle that a slope can achieve is the settlement angle of gravel.

Platforms produced by the "cool-water factory"

In these carbonate factories, precipitation is biotically controlled by heterotrophic organisms, sometimes in association with photo-autotrophic organisms such as red algae. The typical skeletal association includes foraminifers, red algae and molluscs. Despite being autotrophic, red algae are mostly associated to heterotrophic carbonate producers, and need less light than green algae. The range of occurrence of cool-water factories extends from the limit of the tropical factory up to polar latitudes, but they could also occur at low latitudes in the thermocline below the warm surface waters or in upwelling areas. This type of factories has a low potential of carbonate production, is largely independent from sunlight availability, and can sustain a higher amount of nutrients than tropical factories. Carbonate platforms built by the "cool-water factory" show two types of geometry or depositional profile, i.e., the homoclinal ramp or the distally-steepened ramp. In both geometries there are three parts: the inner ramp above the fair weather wave base, the middle ramp, above the storm wave base, the outer ramp, below the storm wave base. In distally steepened ramps, a distal step is formed between the middle and outer ramp, by the in situ accumulation of gravel-sized carbonate grains

Platforms produced by the "mud-mound factory"

These factories are characterised by abiotic precipitation and biotically induced precipitation. The typical environmental settings where "mud-mound factories" are found in the Phanerozoic are dysphotic or aphotic, nutrient-rich waters that are low in oxygen but not anoxic. These conditions often prevail in the thermocline, for example at intermediate water depths below the ocean's mixed layer.
The most important component of these platforms is fine-grained carbonate that precipitates in situ by a complex interplay of biotic and abiotic reactions with microbes and decaying organic tissue.
Mud-mound factories do not produce a skeletal association, but they have specific facies and microfacies, for example stromatolites, that are laminated microbialites, and thrombolites, that are microbialites characterized by clotted peloidal fabric at the microscopic scale and by dendroid fabric at the hand-sample scale.
The geometry of these platforms is mound-shaped, where all the mound is productive, including the slopes.

Geometry of carbonate platforms

Several factors influence the geometry of a carbonate platform, including inherited topography, synsedimentary tectonics, exposure to currents and trade winds. Two main types of carbonate platforms are distinguished on the base of their geographic setting: isolated or epicontinental. However, the one most important factor influencing geometries is perhaps the type of carbonate factory. Depending on the dominant carbonate factory, we can distinguish three types of carbonate platforms: T-type carbonate platforms, C-type carbonate platforms, M-type carbonate platforms. Each of them has its own typical geometry.
Image:CARBONATIC PLATFORM EN.PNG|500px|thumb|Generalized cross-section of a typical carbonate platform.

T-type carbonate platforms

The depositional profile of T-type carbonate platforms can be subdivided into several sedimentary environments.
The carbonate hinterland is the most landward environment, composed by weathered carbonate rocks. The evaporitic tidal flat is a typical low-energy environment.
Image:FangoCarb1.jpg|thumb|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, and so it is a low-energy sedimentary environment. Sediments are composed by reef fragments, hard parts of organisms and, if the platform is epicontinental, also by a terrigenous contribution. In some lagoons, green algae produce great volumes of carbonate mud. Rocks here are mudstones to grainstones, depending on the energy of the environment.
The reef is the rigid structure of carbonate platforms and is located between the internal lagoon and the slope, in the platform margin, in which the framework produced by large-sized skeletons, as those of corals, and by encrusting organisms will resist wave action and form a rigid build up that may develop up to sea-level. 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. The reef is created by essentially in-place, sessile organisms. Today's reefs are mostly built by hermatypic corals. Geologically speaking, reef rocks can be classified as massive boundstones.
The slope is the outer part of the platform, connecting the reef with the basin. This depositional environment 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, with an inclination depending on the grain size of sediments, and that could attain the settlement angle of gravel at most. The slope contains coarser sediments than the reef and lagoon. These rocks are generally rudstones or grainstones.
The periplatform basin is the outermost part of the t-type carbonate platform, and carbonate sedimentation is there dominated by density-cascating processes.
The presence of a rim damps the action of waves in the back reef area and a lagoon may develop in which carbonate mud is often produced. When reef accretion reaches the point that the foot of the reef is below wave base, a slope develops: the sediments of the slope derive from the erosion of the margin by waves, storms and gravitational collapses. This process accumulates coral debris in clinoforms. Clinoforms are beds that have a sigmoidal or tabular shape, but are always deposited with a primary inclination.
The size of a T-type carbonate platform, from the hinterland to the foot of the slope, can be of tens of kilometers.