Carbonate platform
A carbonate platform is a marine sedimentary rocks deposited body that dominates its surroundings and from deposited locally calcification is.
The growth of the platform caused by sessile organisms whose skeletons build a reef, or by the precipitation of lime from the sea water, which is mainly caused by the metabolism of microbes.
Spectacular Examples of carbonate platforms today are the Bahama Banks (several kilometers thick ), the Yucatán Peninsula, the Great Barrier Reef and the atolls of the Maldives. The Florida peninsula is a carbonate platform, but it is no longer covered by water. All these reefs are restricted to tropical latitudes. Today's reefs are built primarily of stony corals, in the geological past, other organisms in the formation of carbonate platforms were involved, such as tabulata and Rugosa, other cnidarians or Archaeocyaths.
- 3.1 reef
- 3.2 Inner Lagoon
- 3.3 reef slope
Karbonatsedimentation
The mineralogical composition of carbonate platforms is either kalzitisch or aragonitisch. Sea water is supersaturated with carbonate (CaCO3 ), so that under suitable conditions the precipitation of CaCO3 is possible. Carbonate precipitation is thermodynamically favored at high temperature and low pressure. Meeting the necessary conditions for the precipitation together, so come by the meshing of the formation factors of production Kalksediment in motion that can generate enormous amounts of Karbonatablagerungen ( "carbonate factory" ). Three types of carbonate precipitation are possible: biologically controlled, biologically induced and abiotic.
Biologically controlled carbonate precipitation is the utilization of dissolved carbonate in sea water during the construction of skeletons about by corals. Biologically induced carbonate precipitation takes place outside of organisms, so that the carbonate is not directly produced by the organisms, but the precipitation is caused by the activity of their metabolism. Abiotic precipitation is little or not affected by biological processes.
Carbonate platforms can not occur everywhere, but only where the living conditions of Riffbildner are given. Limiting factors include sunlight, water temperature and light transmittance of sea water. So carbonate precipitation is widely used as on the Atlantic coast of South America, just at the mouth of large rivers such as the Amazon of which put out a large suspended load into the sea, it does not take place.
Classification
The above described three types of carbonate precipitation determine different platform geometries. These can be classified according to the involved organisms, their sedimentary environment and the type of carbonate precipitation.
Tropical platforms
In these carbonate platforms that carbonate precipitation is biologically controlled mainly by autotrophic organisms such as corals, green algae, foraminifera and mollusks. Tropical carbonate platforms occur only in about 20 ° C warm and well a backlit water that has a high oxygen content and low in nutrients and suspended solids. Such conditions are found today only between 30 ° North and 30 ° South on both sides of the equator. The tropical type is today the most common, but is also often in the fossil record.
Platforms cooler water
As the name suggests, these carbonate platforms occur in cooler water and higher latitudes than tropical platforms. Carbonate precipitation is biologically controlled by heterotrophic organisms, sometimes in conjunction with photoautotrophic organisms such as red algae. The sea water in which these platforms emerge, is characterized by a higher percentage of nutrients than in the tropical platforms.
Mud Mounds
These platforms are characterized by abiotic and biologically induced carbonate precipitation. They grow in nutrient- rich water, which has a low oxygen content. The characterizing them Mud Mounds are mainly fossil from the Paleozoic and Mesozoic known.
Geometry of carbonate platforms
Several factors affect the geometry of a carbonate platform, including the surface shape of its subsoil, synsedimentary tectonics, ocean currents and winds like the Passat. However, the most important factor is the nature of the platform itself platforms cooler water generally result in the formation of ramp-like structures ( Karbonatrampen ), tropical platforms are usually surrounded by steep slopes and Mud Mounds form mountain-like body with soft shapes. The most indented geometry have the tropical platforms. They can be divided into three main areas: reef, lagoon and reef slope.
Reef
The reef is part of the carbonate platform, which is created by the growth of sedentary organisms essentially. Today's reefs are built from hermatypical ( living in symbiosis with zooxanthellae ) organisms. The reef is the " spine " of the carbonate platform and separates the lagoon from the reef slope. The survival of the platform is the existence of the reef depends, as only this as a rigid structure provides protection against waves and wind. Reefs as isolated and largely completed around structures occur (example: Maldives), or as lying in front of the continent and epicontinental connected thereto Riffbauten, such as the reefs of Belize and the Florida Keys. From a geological point of view, at the Riffgesteinen to massive Bound Stones, ie bound by the coral limestones.
Interior lagoon
The inner lagoon is behind ( landward ) the reef lying part of the carbonate platform. The water depth is low and the protection of the reef, the water is generally calm. The sediments of the lagoon to exist alongside the prevailing Karbonatschlamm from Rifffragmenten and hard parts of organisms. Near land, a proportion of clastic sediments added. In some lagoons, such as in the Florida Bay, green algae produce large amounts of Karbonatschlamm. The limestones formed here have to be addressed by the Dunham classification as mudstones to Grain Stones, depending on the kinetic energy of the water in the lagoon.
Reef slope
The reef slope is the outer part of tropical platforms, it connects to the reef to the ocean basins. Here a large part of the transported through various processes of the lagoon and the reef sediment is deposited, including through the breakaway already solidified limestone during storm events. For this reason you find yourself in the area of coarser sediments Riffabhangs than in the lagoon and reef. According to Dunham, the rocks of the Riffhangs can be described as Rudstones or Grain Stones. Due to the deposition on a slope sediments form characteristic arched or tabular layers that are inclined already in the deposition in relation to the surrounding layers ( Klinoformen ).
Carbonate platforms in the geologic tradition
Carbonate platforms are already known from rocks of the Precambrian, where they were formed by the growth of stromatolites. In the Cambrian Archaeocyaths were the Hauptbildner of carbonate platforms, the sponges similar Metazoa. In Paleozoic brachiopods as Richthofenida and the group of Stromatoporoidea came as Riffbildner it. Corals were important to the tabulata in the Silurian and Devonian Rugosa for the formation of carbonate platforms, stony corals only from the Carnian in the Upper Triassic. During the Cretaceous reefs were formed by rudists ( an extinct order of mussels).
Carbonate platforms with wide distribution can be found for example in Devon and Carboniferous of the resin, the Rhenish Slate Mountains and in the south of England and Ireland. In many places the structure in the lagoon, reef and reef slope can still prove, for example in the Attendorner dump in the city of Attendorn.
One of the best examples of a carbonate platform can be found in the Dolomites. In this region of the Alps were formed in the Triassic in the sea of Tethys numerous, still well preserved atolls, such as in Sella, Gardenaccia, Sasso Lungo or Latemar. The Karbonatablagerung took place on a constantly sinking underground for a long time. Continued subsidence led to the accumulation of sediments with a significant thickness. Similar sediments also form the Northern Limestone Alps, also from the Triassic.
In French, Swiss and South German Jura and the Jura powerful sediments were deposited on a carbonate platform as well as on the Swabian and Franconian Alb, here are the carbonate sediments from the Jurassic.