The main geological, oceanographic and physical factors that cause the drowning of carbonate platforms?
The destruction of carbonate platforms are mostly linked to the short term processes. The study of climatic changes, induced due to the human interactions, would be important for the saving these valuable assets from drowning and decline. However, beyond these factors there are long term processes constantly weakening the growth of the reefs.
The abundance of the drowned reefs in tectonically dormant north eastern Australian margin has been attributed to the rates of sea level changes and the margin subsidence. It has been found that the long term growth rates of the reefs are outpaced by the margin subsidence.
The ‘Carbonate platform’ is in general used to describe a shallow marine sedimentary relief formed by calcareous deposits, along the continental margins. The development of the carbonate platforms are induced by the sessile marine lives which build up reefs with their skeletal deposits or the micro-organisms which cause the sedimentation of carbonates through metabolism. The carbonate platforms may be few to several hundred kilometres thick and wide.
The carbonate platforms are found in interesting topographical geometries which range between the two major ones. On one side there are low and gradually sloping ramps or distally steep ramps and platforms with flat tops which have steep margins. The later can be classified as rimmed and non rimmed shelves. Understanding the growth of the microbes and the water turbulence can explain the variety of structures of carbonate platforms.
The development or formation of the carbonate platforms requires typical environment conditions. They have been found in tropical as well as the temperate regions. In the tropical belt they have been found between north and south latitudes, as the region where the tropical platform building microbes are found require warm temperatures ( ), shallow clear water are available. In the temperate belts they are available in the region above north and below south latitudes. These regions have cooler temperatures and they inhabit in the water with low silicates. Thus, they are only found in places where the requisite temperature, abundance of light and clear water are found.
The carbonate ramps are found in the tropical as well as the temperate belts where the size and type of grain vary with the relative distance from the shore. They are subdivided into two different types depending upon their slopes. The homoclinal ramps belong to the category of ramps with slow and gradual slopes (less than gradients), with no steep margins or slope breaks. These ramps are believed to be formed by mud producing organisms and there by developing fine grain gradual ramps. The other type of ramps is called as distally steep ramps which are marked with slope breaks and steep margins. The gradient of distally steep ramps is significantly slower than then the flat topped platforms (FTP). They are marked with non uniform slope breaks and coarse grained deposits.
Rimmed carbonate shelves are the flat topped carbonate platforms which are characterised with flat tops and are shallow (0-30 m). They have found as narrow deposits with a distinct and sharp slope breaks where the gradient can be greater than .near the shore line. The shallow water frame building organisms which leave the skeletal deposits, may lead to the formation of reefs, which may resist the flow of water into the back reef area leading to the formation of lagoons. These are only found in the tropical regions as the reef forming biota requires warm climates and light.
Figure 1: The Morphology of carbonate platforms. Figure shows the different platforms form the structural view.
The rimmed margin shelves are characterised with shallow (less than 10 m depth) and high energy wave resistant build ups formed by frame building biota and sedimentation. These structures are compared with bucket fill model with the rim made of hard skeletal organic material while the inside deposits being soft and made of sand. Even though the composition of the rim and the interior are of different compositions, the growth of both of them is dependent on the potential of rim for growth. Another model of development of rimmed shelves suggests that they are built by the progradation of reef and filling material seaward form the interior rather than from the sea platform lagoon ward transportation of materials. It has been suggested that this model of platform development would be applicable to the growth of other platforms.
In case of the non-rimmed carbonate shelves there is no break of slope as found in the previous one. This leads to the little protection of the platforms from the waves. These platforms consist of gravel size deposits and the micro-organisms hardly need light or warm temperatures. Thus they are found in both the temperate and the tropical regions.
The carbonate platforms are home to the rich reservoirs of hydrocarbons and thus their evolution and formation are of special interests for research. The different carbonate platforms have different geometry, facie, porosity and permeability which results into the differences in the distribution of the reservoirs. The benthic skeletal production of carbonate components is of high importance to the carbonate platforms. However, the open sea or the shallow water production also forms a significant part. The presence of light for the photosynthesis in autotrophic, mixotrophic or heterotrophic organisms is important as the production of carbonates happens mostly in the water at a depth of less than 10 m, below those water levels the production decreases significantly due to unavailability of light.
The carbonate producing biota is divided into three major sub groups depending on the light conditions required by them to grow. The first group belong to the autotrophs and mixotrophs which groups which require abundance of light and therefore occupy the shallow water with penetration of light greater than 0.1%. These are known as the euphotic biota. The next group belongs to those autotrophs and mixotrophs which require lesser light penetration (0.001% to 0.1%) and less temperature and are known as Oligophotic biota. The last group belongs to the groups which are photo independent and are found in waters with lesser light penetration (less than 0.001%). They are mostly dependent on the availability of food in the region for their growth. There are other factors like salinity of water, availability of phytoplankton, water movement, the resistance posed by the platforms to the water waves.
Figure 2: Distribution of biota along the carbonate platforms
Based on the type of biota producing carbonate deposits, there has been proposed different production medium or factories. The tropical carbonate producing, shallow water euphotic, biota produces the rimmed shelves. The heterotrophic cool water skeleton producing biota produce the non-rimmed carbonate shelves. The light independent biota consists of mud producing microorganisms which develop the ramp carbonate platforms. The carbonate producing factories are developed through the intrinsic factors like presence of micro organisms and the factors leading to their growth (light availability, temperature, water turbulence, salinity, availability of food etc.) and extrinsic factors like the climate and water conditions. Once developed the carbonate platforms are affected by the wind and water activity like waves, tides, ocean currents etc. Thus it is the hydraulic regime which controls the growth and transport of the material between centre and margins of carbonate platforms. The relative rise or fall of the sea level and the rate and style of the subsidence determine the development of carbonate platforms. Suppose the relative sea level subsides the carbonate platform above water would start to degrade and erode.
The carbonate platform digenesis and geometry of the facie are dependent on the composition of the sediment and biota which cause the precipitation of carbonate skeletal. Along with these factors it is also influenced on platform scale by long term factors like platform subsidence and drowning.
Figure 3: Carbonate production profiles with T: Tropical factory, M=mud factory and C: temperate factory
The relative change in the sea level with respect to the carbonate platforms is one of the factors controlling evolution. This is because the production of the carbonates is dependent on light and temperature. The maximum production occurs in the level up to 10 m and decreases thereafter.
Throughout the globe there have been changes in the sea level, which is the result of the eustatic changes, sedimentation process and the tectonic movements has happened cyclically throughout he periods. The lifecycle of carbonate platforms range from five to several tens of million years, which is same as the third order sea level cycles. The response of carbonate platforms relative to the sea level changes has been outlined by Moore through the evolution period. The response of the carbonate platforms with relative sea level changes occurs in three different phases after exposure. The first phase is the ‘start up’ phase where the carbonate sedimentation lags the relative sea level rise. The next phase is called ‘catch up’ phase when the carbonate sedimentation builds faster than the sea level rise. The third and last phase is the ‘keep up’ phase where the carbonate sedimentation closely matches the relative sea level changes and the top or ‘crest’ of the sedimentation is close to the sea level.
During the first phase ‘start up’ there is high level growth of the carbonate platform leading to the development of ramp. The high energy currents moves from the shoreline o the centre and here are no developed margins to the ramps which are bathymetrically controlled. The carbonate platform accumulates during the catch up phase when the rise of sea level slows down leading to the growth of vertical stack. The ramp develops into rimmed shelves due to the high production of carbonate. In the next phase the carbonate platform growth is limited along the vertical stack, while the lateral development or evolution of the ramp may happen.
The platforms drowned below the sea levels are not morphologically different from the platforms discussed in the previous sections. They comprise of the similar structures like drowned ramps or rimmed shelves. They undergo through the period of evolution and a possibly change in the sea level or the tectonic movement which might have resulted into platform subsidence. The Pacific Ocean and the Indian Ocean are common with the examples of the isolated drowned platforms and atolls, subsided, following the ceased volcanic activity, below the reach of light. This leads to the cessation of the growth of carbonate forming biota and hence the platform growth.
Figure 4: The Carbonate platform shaping up due to the relative sea-level changes and the response of the carbonate producing factories
The Blake plateau which was drowned during the Holocene and Cretaceous period is currently more than 2000 m deep under sea level. The platform is rich in pelagic, ferrous and manganese deposits which are characteristic of cretaceous period. The Holocene St Croix Island are beginning to drown are formed with coral reefs (W. H. Adey 1976). The Pourtales Terrace (East of Florida), characteristic of Pleistocene period, is about 150 m to 300 m deep. It is rich in algal and pelagic growth.
The Miami Terrace which was drowned during the Miocene period, evolved with the growth of the basin during the early Miocene period. The terrace was exposed due to the fall of the sea level during the late Miocene period which was subsequently drowned later. It is endowed with pelagic and phosphorite sediments.
The vertical growth of the carbonate platform is approximated to be 100 cm per thousand years which exceed far from the changes in relative and eustatic sea level changes. The platforms drowned as the result of the sea level changes present as a paradox in itself as the rate of sea level rise in the second or third order of eustatic changes is less in magnitude compared with the healthy rise in level of carbonate platforms. According to Schlager, the eustatic changes in the sea level or subsidence could outpace the vertical growth of carbonate accumulation only in case of sudden change in extrinsic factors like the change in the fault lines can lead to the sudden subsidence of the platform. The sudden influx of water due to melting of glaciers can also lead to the change in sea level. Other factors like the change in the salinity, temperature, water turbulence, depletion of the nutrients, rise of silicates in water, change in the photic levels of sea water can also lead to the sudden decrease or death of the underwater organisms. It has been pointed out there may not be a singular factor responsible for the drowning of the carbonate platforms but there may be a combination of two more factors. Schlager has pointed out that to contain the growth of reef forming biological growth, the factors should appear in brief periods and should repeat themselves at regular intervals, once in period less than a million years.
Even though there may be a multiplicity of reasons for drowning of the platforms, the major role is played by the interactions between the change of relative or eustatic sea level and growth of the carbonate factory. The different rates of sea level changes have a different level of impact and responses of the carbonate factory change accordingly. The carbonate platform may shift basin ward (prograde), stack on vertically (aggrade), step back (retrograde) or step down. These responses can be traced back to the changes in eustatic or relative sea level changes. The responses are of importance to predict the changes in the platform.
Figure 5: The phases of evolution and drowning of the carbonate factories
The carbonate platforms which have drowned in the recent past are results of the sea level rise exceeding the potential growth of the platform. However, it has not been enough to stop the production of the carbonate factory. These carbonate platforms are above the sea level along the rimmed side but are under water towards the shoreline (lagoon wards).The platforms subsidence which have been caused by the movement of the tectonic plates are more susceptible to drowning of the entire platform. These subsided platforms may recover with the cycles of rise and fall of the relative sea level. The carbonate ramp platform of Barbados is currently around 100 m below the sea level. The Mayotte drowned platform is mostly drowned while only a part of it, drowned in the Holocene period, has the continued growth of reefs.
The carbonate platforms may respond to the relative rise in the sea level, in case the rise is no rapid with respect to the growth of reef, by retreating or back stepping. The retreating is rare while the back stepping can be seen in the rimmed shelves. The back stepping can be advantageous to the reef growth, as the platform recedes to the shallower water. This reduces the platform width basin wards while the high growth potential of the carbonate factory is resumes equilibrium with respect to the rise in the sea level. Another advantage of the back stepped platform is the reduced destructive forces of high energy sea waves. The energy of the high energy waves is dissipated with friction with the bottom towards the platform or the shoreline.
The back stepping saves the platform from the subsidence towards the higher topography. This again saves the platform from subsiding along the fault lines. The back stepping creates a step like structure of the platform.
Figure 6: Incipiently drowned carbonate platforms
The back stepping of the rimmed carbonate platform happens faster than the lagoon. In case the rim drowns under the sea level there are lesser chances to recover the platform. The back stepped platform would lead to higher growth of the reef in the equilibrium period, while the fall of the sea level in the ensuing period may lead to basin ward growth of the platform. The cycle of back stepping and basin ward growth leads to development of step like morphology.
As the platform evolves through growth and drowning, it interacts with the environment which leaves a mark on the topography. During the growth phase the platform may prograde or aggrade accumulating the sediments along the basin or vertical stack. The platform may subside during the phase of eustatic change of the sea level leading to the retrograde and back stepping of the platform. The platform may get exposed to the air due to the relative fall of the sea level and undergo through erosion, decay of reef and diagenesis. Each of evolution passes leaves its own signature. These signatures are helpful in tracking the history of evolution and reservoir of resources. These tools include:
1. Taphonomic signatures: These are associated with the analysis of the history of the organic accumulation in the platforms, including the skeletal material. These are mainly related to the understanding of Taphonomic and diagenesis of the reef material under the sea level.
2. Diagenesis Signatures: It involves with the study of the physical and chemical nature of the deposits.
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