Carbonate platform
editClassification
editThe 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 [1]. The differences between three factory is the dominant precipitation pathway and skeletal associations [1].
In contrast, a carbonate platform is a geological structure of parautochotonous carbonate sediments and carbonate rocks, having a morphological relief [1].
Platforms produced by the "tropical factory"
editIn 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 high light conditions and live in the euphotic zone (i.e., shallow water environments in which sunlight penetrates easily). Tropical carbonate factory 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 [1]. 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 will resist wave action and form a rigid build up that may develop up to sea-level[2]. The presence of a rim produces restrict 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°) [2][1].
Platforms produced by the "cool-water factory"
editIn 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[3]. 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 [3]
Platforms produced by the "mud-mound factory"
editThese 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[1].
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[1].
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. [1]
Note
edit- ^ a b c d e f g h Schlager, Wolfgang (2005). Carbonate sedimentology and sequence stratigraphy. SEPM Concepts in Sedimentology and Paleontology. ISBN 1565761162.
- ^ a b Pomar L., 2001 - Types of carbonate platforms: a genetic approach. Basin Research,.
- ^ a b "Carbonate factories: A conundrum in sedimentary geology". Earth-Science Reviews. 87 (3–4): 134–169. 2008-03-01. doi:10.1016/j.earscirev.2007.12.002. ISSN 0012-8252.