Modern Bahamian Carbonate Platform
There are different geographical activities that lead to the formation of carbonate platforms. In the 1950s and 1960, the formation of carbonate platforms occurred due to various deposits that took place in the ocean beds either in the deep or shallow water. These types of deposits brought about different types of depositions and the formation of carbonate platforms. The study of different types of sediments using petrographic studies of varied types of sediments found in shallow waters carbonate environments played an important role in the facie formation of the zones of carbonate in its environment. However, the process that led to the initiation and evolution of theses habitats, especially those concerning the control of isolated platforms of carbonate remained largely unknown to human beings. Recent research studies have shown that the same facie could be responsible for the formation of modern carbonate platforms. This paper is a report about the formation of modern carbonate platforms in the Bahamas.
The case study
This report shows a case study that has revealed that there are different types of carbonate platforms formed in the course of both deep sea and shallow waters. They existence of the platforms follow a given pattern that occur as a result of activities of water in the sea beds found in the country. Such activities have lead to the formation of different types of carbonate platforms of varied sizes. This carbonate platform is considered one of the most modern carbonate depositions in recent history. It has been studied by many geologists and seem to give a proper explanation into the modern method of deposition and hence formation of carbonate platforms.
Discussion
Anselmeti et al (2000) defines a carbonate platform as a sedimentary remain which possesses landscape relief, which is made out of autochthonous deposits of calcium found in any water body that is able to sustain carbonate platform formation. Platform growth is aided by the decomposition of sea organisms whose skeleton build up the reef. They can also be made from organisms such as microbes, which encourage the production of carbonates through their metabolic activities. Researchers believe that constant metabolic activities of microbes and other microorganism are responsible for the formation of carbonate platforms (Wilson, 1975). Some distinct platforms occur when such deposit are made in shallow waters in a systematic way producing a unique pattern
In the modern day, geologist use the current analogues like the platforms and shallow water carbonates found in the Bahamas and the Florida water bodies. More specifically, the Bahamian platforms are usually considered as the most modern forms of carbonate platform (Oberlin, 2001). This is because, many geologist are for the opinion that the analysis of carbonate platforms facies has a foundation that was formed by precise petrographic interpretation of the carbonate facy belts. The geologists assert that anatomy of the platforms is precisely laid in a manner that has never been seen in any carbonate belt.
Geologist believe that the evolution of the carbon platform in the Bahamas is unique and has often been used as the point of comparison with the old types of platforms in order to help find answers to controversies that surrounds the old types of carbonate bodies (Wilson, 1975). It is possible for geologist to understand the nature and sedimentation of the platforms in the Bahamas because of the available sufficient data for their use.
Many scientists have also referred to the unique nature of the Bahamian archipelago to use it as a foundation of the differences observed between its modern sediment layout and the layout of those formed in the 1960 and backwards (Oberlin, 2001). Similarly, the models proposed for the carbonate platforms in the Bahamas are quite different from those found in those that existed in the olden days and seem to offer appropriate explanation making their studies very easy.
In order for carbonate platforms to form, they require deposits of sediments in shallow or deep sear waters that are able to form the right amount of reefs. The formations also need favorable climatic conditions, topographic relief, and adequate seismic activities so that the necessary sediments may be produced for the formation of the carbonate platforms (Schlager, 1981). Any carbonate platform cannot be formed unless all these conditions are met. Furthermore, the formation of such geological features require a topographic relief features that separates the oceans banks from the deep waters that might sweep away the already formed platforms of carbon.
The amount of topography required for the proper formation of carbonate platforms is not well known since a study of modern and past features indicate that some were formed as a results of less seismic activities while others occurred due to less or moderate amount (Schlager, 1981). A closer look at the layers of the carbonate platforms of the Bahamas reveals that they used many sedimentary deposits that took place over time based on intense topography that lead to their layout. The topography acted in making in unique especially at the bank of the little Bahamian bank.
The available geographical data concerning the foundations of the Bahamian carbonate platforms indicate a distinct thick crust ranging from 13 to 15km accompanied with intermediate wave velocity as seen from seismic refraction. The platform patter n is however not known to have originated from either oceanic or continental activities. Moreover, the available data is geographically but did not come from drilling activities. To be able to resolve the controversy of whether the carbonate platform in the Bahamas are as a result of oceanic or continental crust, two theory were formed based on its underlying pattern (Oberlin, 2001)s. The theories are the graben and the megabank hypothesis, which were compatible with the existing geographical data concerning the Bahamian carbonate platforms.
Graben hypothesis asserts that the modern platform of the Bahamas resembles a feature that resulted from a block faulted landforms of the Jurassic rifts phase. In this phase, the remnants of the graben form the intra-platform seaways, which build up in stages to form the entire carbonate platform. This also indicates that the platforms are the sites where the initial platforms occurred first, the new ones just form on top of the old one leading to a well developed carbonate feature along the Bahamian coast (Anselmeti et al, 2000). The megabank theory on the other hand proposes that the carbonate platforms in the Bahamas were once a part of a much larger but very extensive carbonate platforms in shallow waters under which inherited rift were buried and later formed during earlier stages of platform evolutions. The initial platform is what is referred to as the megabank, and according to this theory, it disintegrated during the mid cretaceous stages. Only smaller platforms were recovered later and formed the foundations of the modern carbonate platforms found along the Bahamas. The megabank theory does not seem to provide an explanation into the process that lead to the irregular patterns of the modern types of the platforms in the region.
Geologist referred to drilling techniques to be able to explain the irregular patterns of the modern Bahamian carbonate platforms. However, the drilling results did not provide any conclusive results for their question. Instead, they obtained an answer that showed that there were large platforms that existed in the banks of the Bahamas, whose integration might have lead to the irregular forms of the modern carbonate platforms (Anselmeti et al, 2000). The existence of the larger platforms in the Bahamas plus those found at the Blake plate region at the end of the early cretaceous, which served to support the megabank theory. However, the lower cretaceous deep waters in the north east Providence Channel provides the evidence of a Jurassic (old) platforms that re-entered the point. Further data that shows the existence of more mega platforms were also available at this point. These discoveries strengthened the theory of the megabank and made the existence of the modern carbonate platforms at the Bahamas more understandable compared to its Jurassic counterparts.
Modern data gathered from seismic reflection profiles found across the banks of the great Bahamas, for the first time revealed a structure of a given portion of the part of the bank and aimed at settling the controversy over the existence of the modern carbonate platforms of the Bahamas bank. The seismic profile further proves that the modern platform patterns did not occur due to old or mid cretaceous topography, instead they have undergone constant modification through pro-gradation and destructive processes that have shaped them they way they are (Oberlin & Ginsburge, 1998).
A look at the profiles of the Bahamian carbonate platforms reveals that their existence was largely controlled by tectonic activities, which controlled the formation of the original pattern, then pro-gradation welded or joined them together to form the modern platforms. Subsequently, seismic stratigraphy techniques further explains that repeated activities of tectonic segmentation and coalitions due to deposition constantly re-modified the pattern (Oberlin & Ginsburge, 1998). Further, the fact that repeated segmentation lead to the modification of the platforms indicates that tectonic activities are very fundamental to the formation of the modern carbonate platforms of the Bahamas, by forming very stable platforms.
Geologists state that a productive bank has the healing power, which if the ability to fill up the faults and burry the tectonic lines by the infilling of the bank margins and the seaways. The Bahamas is considered as one of those productive banks has the healing power to fill its banks and sea way. In the case of the great Bahamas, the elaborate pro-gradation activities advanced the modern banks in the western side by approximately 27 kilometers in a basin ward motion over its original fault. This could be the reason why many of its banks are not visible along seismic lines the seaway of the Bahamas (Oberlin & Ginsburge, 1998). It made these banks to disappear by depositing more carbonic materials that covered up the sear ways making them invisible.
Structure and growth patterns of the platforms in the Bahamas
Even though the pro-gradation of carbonate shelves or the attached carbonate platform were noted to have developed throughout the pharezonic layers, geological research show that the growth pattern for individual platform is believed to be vertical. Geological comparison of the sleet slope of the Bahamas, with the atolls of old platforms also showed similar growth patterns. Similarly, many researchers have stated that the Bahamian carbon platforms as a huge coral atolls (Oberlin & Ginsburge, 1998). They believed that the carbonate platforms of these regions were formed as a result of massive accumulation atolls that formed along its banks. Early submarine cementation of other features along the banks could also have lead to the formation of the platforms.
Geologist stated that the isolated carbonate platforms along the Bahamas as buckets held together by competent layers of reefs. The Bahamian banks formed as a result of depositions of sediments that were deposited on the coast of the banks. Similarly, research shows that an isolated platform has the ability to propagate when the deposited sediments are heavy than those found along the vertical growth rate of the same features along the banks (Oberlin & Ginsburge, 1998). Proper formations of carbonate platforms also require an adequate energy source. Sources of energy could b wind or water required to transport the sediments off the banks. The speed and rate at which the sediments are transported off the banks depends on factors such as duration, magnitude and direction of the physical source of energy causing deposition of sediments. Types of the bank margin also have effects on the transportation rates and speed of the sediments.
Once the sediment has been transported off the bank, they must be deposited on the adjacent slopes so that they can contribute to the lateral growth of the carbonate platform. The Bahamian platform was also formed in this way, in its case, newly deposited sediments were piled on the larger of the existing mega platforms, which increased their sizes. It led to the rugged patterns of the platforms because the existing mega platforms were not lying in a uniform patter. The relationship between lateral and vertical growth rated determines the patterns of the carbonate platforms forms along the Bahamian bank (Oberlin & Ginsburge, 1998). The relationship in this ratio also determines the type of morphology that the carbonate platforms will assume. Factors such as the initial morphology of the basin, fluctuations in the sea levels, rate of sediment production and the direction of physical energy influence the ratio needed for the formation of the carbonate platforms.
Foundations of the Bahamian carbonate platforms
Five different characteristics were proposed to be associated with the carbonate platforms found in the Bahamian bank.
- They are atoll like platforms, which have the characteristics of vertical growth, which allows small lateral migrations along the banks. The features forms following the bucket theory and of has a cemented floor that edge that surrounds the carbonate platforms deposited on the interior sides.
- The platforms of asymmetric nature has sides that grow laterally while others prograde. They are caused by un-directorial energy that allows for quick transportation of the deposited sediment.
- The symmetric carbonate platforms formed along the Bahamas has a clinophone propagation that originate from two directions and work in two directive seismic profile that are asymmetric along the banks
- The double staged platforms contain two dominating types of units. The first one is the vertically growing than the second in the laterally developing platform. In this case, the filling of the seaway by the deposited platforms creates the type of each platform. This architecture is typical of the carbonate found in the Bahamas where siliclastic sediments deposition took place in two different forms (Anselmeti et al, 2000).
- The composite platforms of the Bahamas comprises of two platforms are welded together and formed one massive platform at the bank. Similarly, the composite platform represents the full grown stage of an isolated platform that has lived for long.
The diagrams below shows the stages
Carbonate factories
Research shows that under favorable circumstances, carbonate platform may form vertically to produce enough sediment. The carbonate factory of the Bahamas bank produces sediments at a high rate than the rate at which it is accumulated (Anselmeti et al, 2000). The production rate of the carbonate sediments is higher than that of any known geological process.
Sediment export at the Bahamas bank
Research shows that many geologists have documented sediment exportation activities from shallow to deep waters by making direct observation of these activities as they took place in the Bahamas bank (Anselmeti et al, 2000). They were recorded to enable other geologists and researchers to ascertain the method by which sediments were deposited in the banks of the Bahamas. The occurrence of such activities is evident in the modern banks such as the Bahamas. The geological evidence further states that the fine grains of sediments can be transferred from a distance of 120km before they are deposited to form the carbonate platforms.
Oberlin (1991) also states that the ability to recognize large peril-platforms of sediment drift measuring up to 600kn along the Bahamas bank along the banks of the old platforms occurred as a result of this activity. Studies of modern carbonate platforms along the Bahamian banks indicate both high sediment depositions at platform formation along the banks. Further research shows that the rate at which sediments are deposited and the speed at which carbonate platforms are formed are even higher during glacial activities (Oberlin, 1991). They also noted a situation where sediments are stored in the shelf of the reef during high sea level activities and are deposited during low sea level activities.
Conclusion
Studies show that many geologists consider the carbonate platforms at the banks of the great Bahamas as the current indications of modern platforms. Carbonate platform are formed due to sediment deposition along the banks. The rate at which the sediments are deposited at the banks depends on factors such as sediment transportation, water activities, and energy of the water at banks. The carbonate platforms at the Bahamas have a vertical or lateral growth that also takes place in two distinct directions. Further studies indicate that the formation of the carbonate platforms at the Bahamian banks occurred due to processes such as the megabank theory of the graben theories. The main technique in these two theories is the depositions of the sediments along the banks.
Isolated platforms
Vertically formed carbonate platform
Banks where sediments are deposited to form carbonate platform
The great and mini Bahamas bank
References
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Schlager W. (2005). Carbonate sedimentology and sequence stratigraphy. SEPM, Tulsa, Oklahoma.
Wilson J. L., (1975). Carbonate Facies in Geologic History. New York, NY: Springer-Verlag, 471 pp.
Bosellini A., 1984, Progradation geometries of carbonate platforms: examples from the Triassic of the Dolomites, northern Italy. “Sedimentology”, Vol. 31, pp. 1-24