Carbonate platform

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, by definition, involves little or no biological influence. ==Classification==

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 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. 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. 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 (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 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 (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. ==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 (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 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. M-type carbonate platforms M-type carbonate platforms are characterized by an inner platform, an outer platform, an upper slope made by microbial boundstone, and a lower slope often made by breccia. The slope may be steeper than the angle of repose of gravels, with an inclination that may attain 50°. In the M-type carbonate platforms the carbonate production mostly occurs on the upper slope and in the outer part of the inner platform. at Jebel Akenzoud (Dadès Gorges, Central High Atlas Basin, Morocco). (B) Close-up view of the left-hand side of the panel. Lateral accretion within the ooidal shoal in the uppermost part of the infilling unit can be seen (white arrows). (C) Stratigraphic correlation. (D) Close up view on the right-hand side shoulder of the infilling unit. (E) Close up on the erosion surface. ==Carbonate platforms in the geological record==

record a well preserved carbonate platform, including it´s sedimentary cycles and facies. 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. 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). Some of the best examples of carbonate platforms are in the Dolomites, deposited during the Triassic. This region of the Southern Alps contains many well preserved isolated carbonate platforms, including the Sella, Gardenaccia, Sassolungo and Latemar. The middle Liassic "bahamian type" carbonate platform was notoriously widespread in the western Tethys Ocean, including the Rotzo Formation on Italy, the Aganane Formation & the Calcaires du Bou Dahar of Morocco (Septfontaine, 1985) is characterised by the accumulation of autocyclic regressive cycles, spectacular supratidal deposits and vadose diagenetic features with dinosaur tracks. In the same area the overlying Tafraout Group (Toarcian-Aalenian) records a massive Siliclastic-Carbonate Platform. Other coeval records of platforms are seen in the Adriatic Platform, in areas such as the Dinaric Alps (The Seoca Lithiotis Limestone of Montenegro), Rotzo or the Podpeč Limestone of Slovenia. Others regional records include examples like the Coimbra Formation (Sinemurian) in the Lusitanian Basin. The Aalenian Iberian Platform includes Volcanic intrusions that led to ephemeral islands, like in the El Pedregal Formation. The Cretaceous (Campanian) Calcare di Aurisina in Italy, records a series of islands adjacent to a shallow carbonate sea in a platform built by bivalvia (rudists). 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. platform of Morocco with first order autocyclic regressive cycles metre-scale peritidal sedimentary cycles in two outcrops. The two outcrops are 230 km apart. Storm beds and possibly tsunamites include abundant reworked foraminifera. This image is an example of the continuity of peritidal cycles in a carbonate platform environment. == Sequence stratigraphy of carbonate platforms ==

With respect to the sequence stratigraphy of siliciclastic systems, carbonate platforms present some peculiarities, which are related to the fact that carbonate sediment is precipitated directly on the platform, mostly with the intervention of living organisms, instead of being only transported and deposited. In the geologic record of a drowned carbonate platform, neritic deposits change rapidly into deep-marine sediments. Typically hardgrounds with ferromanganese oxides, phosphate or glauconite crusts lie in between of neritic and deep-marine sediments. Plate movements carrying carbonate platforms to latitudes unfavourable for carbonate production are also suggested to be one of the possible reasons for drowning. Highstand shedding Highstand shedding is a process in which a carbonate platform produces and sheds most of the sediments into the adjacent basin during highstands of sea level. This process has been observed on all rimmed carbonate platforms in the Quaternary, such as the Great Bahama Bank. Flat topped, rimmed platforms with steep slopes show more pronounced highstand shedding than platforms with gentle slopes and cool water carbonate systems. Highstand shedding is pronounced on tropical carbonate platforms because of the combined effect of sediment production and diagenesis. Examples of margins that may be affected of slope shedding that are characterized by various contributions of microbial carbonate growth to the upper slope and margin, are: • the Canning Basin in Australia • the Guilin platform in the southern China • the Permian of the US Permian Basin • the middle Triassic carbonate platforms of the Dolomites. == Gallery ==

File:Cycle émersif Maroc.jpg|"Shallowing upward" cycle in the Aganane Formation of the high Atlas (Morocco). Algal dolomitized laminations on top. File:Cycle émersif Oman.jpg|"Shallowing upward" cycles in the lagoonal Lias of the Musandam Peninsula. (N-Oman). File:Cycle émersif Musandam.jpg|"Shallowing upward" liassic cycles arranged in decametric sequences, Musandam Peninsula, (N-Oman). File:Cycle émersif Laghdar.jpg|"Shallowing upward" cycle in the Middle Jurassic (Saghtan form.) of the jbel Laghdar Range (Oman). File:Desiccation Rnim.jpg|Desiccation figures on top of a regressive sequence; Aganane Formation, High Atlas, Morocco. File:Calcretes et ammonites.jpg|Ammonites and belemnites washed over a supratidal surface (calcretes and "teepees"); Aganane Formation of the High Atlas, Morocco. File:Brèche de tempête.jpg|Hurricane breccia cemented (early diagenesis) at the surface of a bed, top of a regressive, metric, sequence. Aganane Formation, High Atlas. File:Pisolithes vadoses.jpg|Vadose ferrugenous pisolites (soil) and coastal (tempestite) sediment with birdseyes in an outer platform environment. Aerial diagenesis. Aganane Formation, High Atlas, Morocco. File:Keystone vugs meniscus.jpg|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. File:Calcretes remaniés dolomie.jpg|Reworked calcretes concretions from the supratidal environment in a marine (dolomitised) sediment displaced by hurricanes on the inner platform flat. Top of emersive sequence. Aganane Formation, High Atlas, Morocco. File:Ciment stalactite vadose.jpg|Stalactitic cement in sediment from the supratidal zone, vadose environment, top of "shallowing upward" sequence. Aganane Formation, High Atlas. Thin section. L = 0.3 mm. File:Traces dino géant.jpg|Giant dinosaur tracks (sauropod) on top of a regressive sequence, Aganane Formation, High Atlas, Morocco. File:Calcretes et birdseyes.jpg|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. Aganane Formation, High Atlas, Morocco. File:Séquences Lias Todhra.jpg|Autocyclic filling (metric to hectometric) sequences in the Middle Liassic lagoon, South (Todhra) of the Aganane Formation, High Atlas, Morocco. File:Teepee supratidal flat.jpg|"Teepee" structure, due to increasing sediment volume by dolomitisation on the inner platform supratidal flat. Top of emersive cycle. Middle Lias, Aganane Formation, High Atlas. File:Carottes chott tunisien.jpg|Quaternary to recent equivalent of a "shallowing upward sequence", cores in a Tunisian "chott", intertidal laminations in yellow. File:Tunisie teepee.jpg|Recent "teepee" structures in a Tunisian salt lagoon, "chott". File:Tunisie carottes Zarzis.jpg|Recent equivalents of "shallowing upward sequences", cores in a Tunisian salt lagoon, "chott". File:Tunisie gypse.jpg|Top of a regressive sequence with algal laminations (yellow) and crystallised gypsum, salt lagoon "chott", Tunisia. File:Tunisie tempête.jpg|Eolian bioclastic (calcareous algae and porcellaneous foraminifera) sand dune on Tunisian shore. ==See also==