Carbonate platform
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.
[6] 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 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.
[6] 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.
[6] 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).
[10] 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.
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.
[15] The Aalenian Iberian Platform includes Volcanic intrusions that led to ephemeral islands, like in the El Pedregal Formation.
[16] 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).
[17] 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.
[18] In the geologic record of a drowned carbonate platform, neritic deposits change rapidly into deep-marine sediments.
[18] Since drowning of carbonate platforms requires exceptional rise in the relative sea level, only limited number of processes can cause it.
According to Schlager,[18] only anomalously quick rise of relative sea level or benthic growth reduction caused by deteriorating changes in the environment could explain the drowning of platforms.
For instance, regional downfaulting, submarine volcanism or glacioeustasy could be the reason for rapid rise in relative sea level, whereas for example changes in oceanic salinity might cause the environment to become deteriorative for the carbonate producers.
It is believed to be drowned by rapid sea level rise caused by deglaciation and subsidence of the platform, which enabled coralline algal-foraminiferal nodules and halimeda limestones to cover the coral reefs.
[18][7] For example, guyots located in the Pacific Basin between Hawaiian and Mariana Islands are believed to be transported to low southern latitudes (0-10°S) where equatorial upwelling occurred.
[7] High amounts of nutrients and higher productivity caused decrease in water transparency and increase in bio-eroders populations, which reduced carbonate accumulation and eventually led to drowning[further explanation needed].
[21] Highstand shedding is pronounced on tropical carbonate platforms because of the combined effect of sediment production and diagenesis.
[6] The effect of increased highstand production is enhanced by the rapid lithification of carbonate during lowstands, because the exposed platform top is karstified rather than eroded, and does not export sediment.
[6] Slope shedding is a process typical of microbial platforms, in which the carbonate production is nearly independent from sea level oscillations.
The carbonate factory, composed of microbial communities precipitating microbialites, is insensitive to light and can extend from the platform break down the slope to hundreds of meters in depth.
