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 (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. 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 (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders.'Shallowing upward' cycle in the middle Lias of the high Atlas (Morocco). Algal dolomitized laminations on top.'Shallowing upward' cycles in the lagoonal Lias of the Musandam Peninsula. (N-Oman).'Shallowing upward' liassic cycles arranged in decametric sequences, Musandam Peninsula, (N-Oman).'Shallowing upward' cycle in the Middle Jurassic (Saghtan form.) of the jbel Laghdar Range (Oman).Desiccation figures on top of a regressive sequence; Middle liassic, High Atlas, Morocco.Ammonites and belemnites washed over a supratidal surface (calcretes and 'teepees'); Middle Liassic of the High Atlas, Morocco.Hurricane breccia cemented (early diagenesis) at the surface of a bed, top of a regressive, metric, sequence. Middle Lias, High Atlas.Vadose ferrugenous pisolites (soil) and coastal (tempestite) sediment with birdseyes in an outer platform environment. Aerial diagenesis. Middle liassic, High Atlas, Morocco.Meniscus and point contact cement in a marine grainstone with displaced foraminifera (by tide and hurricanes) on the supratidal flat of the middle liassic platform of Morocco. Top of emersive cycle. Middle Atlas.Reworked calcretes concretions from the supratidal environment in a marine (dolomitised) sediment displaced by hurricanes on the inner platform flat. Top of emersive sequence. High Atlas, Morocco.Stalactitic cement in sediment from the supratidal zone, vadose environment, top of 'shallowing upward' sequence. Middle Liassic, High Atlas. Thin section. L = 0.3 mm.Giant dinosaur tracks (sauropod) on top of a regressive sequence, Middle Liassic, High Atlas, Morocco.Vadose stalactitic cement filling an horizontal cavity in a marine coastal sediment, outer platform. Birdseyes in the allodapic (tidal or tempestite) grainstone point to an aerial diagenesis. High Atlas, Morocco.Autocyclic filling (metric to hectometric) sequences in the Middle Liassic lagoon, South (Todhra) of the High Atlas, Morocco.'Teepee' structure, due to increasing sediment volume by dolomitisation on the inner platform supratidal flat. Top of emersive cycle. Middle Lias, High Atlas.Quaternary to recent equivalent of a 'shallowing upward sequence', cores in a Tunisian 'chott', intertidal laminations in yellow.Recent 'teepee' structures in a Tunisian salt lagoon, 'chott'.Recent equivalents of 'shallowing upward sequences', cores in a Tunisian salt lagoon, 'chott'.Top of a regressive sequence with algal laminations (yellow) and crystallised gypsum, salt lagoon 'chott', Tunisia.Eolian bioclastic (calcareous algae and porcellaneous foraminifera) sand dune on Tunisian shore. 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 (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. 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 (during the Cambrian) or extinct cnidaria (tabulata and rugosa) were important reef builders. 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 (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. The three types of precipitation (abiotic, biotically induced and biotically controlled) 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 factory 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. 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 (i.e., shallow water environments in which sunlight penetrates easily). 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 (30-34°). 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 (at about 30◦) 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 These factories are characterised by abiotic precipitation and biotically induced precipitation. The typical enivronmental 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 (automicrite) 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. Several factors influence the geometry of a carbonate platform, including inherited topography, synsedimentary tectonics, exposition to currents and trade winds. Two main types of carbonate platforms are distinguished on the base of their geographic setting: isolated (as Maldives atolls) or epicontinental (as the Belize reefs or the Florida Keys). 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 (produced by 'tropical factories'), C-type carbonate platforms (produced by 'cool-water factories'), M-type carbonate platforms ('produced by mud-mound factories'). Each of them has its own typical geometry. The depositional profile of T-type carbonate platforms can be subdivided into several sedimentary environments.