We had a terrific group of people in our recent course in the Bahamas
The Bahaman Archipelago consists of two carbonate banks which were formed by a chain of carbonate platforms (Curran 1). Having an understanding of the geology of the area leads to a better understanding of why the islands are shaped the way they are. The geology leads to an explanation of the placement of the islands along the carbonate banks, along with an explanation of both the tilt and the composition of the rocks. Also, with a greater understanding of the islands, models that focus on salinity levels and sedimentation patterns can be built and analyzed (Curran 2). Overall, knowing the geology of the area allows scientists to understand what has happened in the evolution of the islands and predict the future of the islands. A better understanding of how the islands are created also lend to a better understanding of the coral reefs because scientists know how the islands where made and can observe the roles that the coral reefs played in this evolution (Curran 3).
The Bahamas was originally formed by the rifting of Pangea, the super-continent, which resulted in the opening of North Atlantic basin. The rifting of Pangea was accompanied by volcanic activity due to the nature of the colliding North American and Caribbean plates. The collisions, commonly subduction zones where one plate is pulled under the other, formed the lower layer, which is commonly referred to as the basement rocks upon which the Bahamian islands now reside. Evidence of the volcanic activity is found in the “tilted fault blocks of Jurassic volcaniclastics” which are commonly found in the Florida Straits area (Vacher 95). In the southern region of the Bahamas, the basement rocks are oceanic crust, showing that the area was not a transitional region during the opening of the North Atlantic basin.
The Bahamas are referred to as carbonate islands, which is due to the formation of carbonate banks. This “megabank” formed in the Late Jurassic and is evidence of an absence of deep water at the time of formation due to the type of rock formed. Carbonates are more likely to form in shallower waters, thus the formation of two major carbonate banks in the Bahamas shows that there was an absence of deep water (Marshak). There is also evidence of faulting which is shown in the tilting of the Bahaman Banks. This tilting is due to the subduction of the North American plate under the Caribbean plate, in the vicinity of Cuba. The angle of tilting, which is “left-lateral wrench faulting” is in the direction of the subduction, supporting Cuban vicinity as the location of subduction (Vacher 96). This faulting occurred because as the North American plate subducted under the Caribbean plate, not all the rock layers moved as one continuous unit. The Bahaman islands remained in the same location, thus the rocks had to fault, or break, in order for the North American plate to continue subduction and the islands to remain in their current location.
Subsurface stratigraphy is the study of underlying rock layers, especially the distribution, deposition, and age of sedimentary rocks (Marshak). Studying the subsurface stratigraphy gives an idea of what types of environments that the Bahamas evolved in over time. These underlying rock layers are studied through various methods including seismic refraction, seismic reflection, magnetics, and shallow and deep drilling (Vacher 96).
Using deep drilling it was determined that the Upper Jurassic carbonates are approximately 5 km down. Above these carbonates are Lower Cretaceous dolostone, limestone, and evaporites, which are sedimentary deposits that result from the evaporation of seawater. On the margins of the banks there is a known transition from “Pliocene skeletal and reefal facies to Quaternary oolites and eolianites” (Vacher 96). This is believed to have occurred with the onset of the glaciation of the northern hemisphere. Another important observation is the Pleistocene-Holocene sediments are not uniform showing that the deposition situation was not constant throughout the entire Bahaman Archipelago, but through the exposed coral reefs and flank margin caves the entire archipelago behaved similarly for the past 300 kiloyears (Vacher 96).
There are five types of Holocene sediments; peloidal, oolitic, coralgal, grapestone and aggregate, and mud, found in the Bahamas are indications of shallow waters at the time of deposition (Ginsburg 18). These sediments were deposited onto of a regional unconformity that “is the karsted top of the cemented Pleistocene deposits that developed during the subaerial exposure of the last glacial lowstand… approximately 120,000 years ago” (Ginsburg 19). In these deposits, early reef development has been found. The reefs developed in the early Holocene when sea level was well below the platform top and eventually were covered by peloidal sands from the platform.
Using the depositional nature of the Holocene sediments as a guide for the Pliocene – Pleistocene carbonate rocks, the islands were determined to have formed on the windward or ocean facing margins of the carbonate banks. Along the Pleistocene sea-facing bank margins, there is limestone evidence of coral reefs that defended the islands. There is also evidence of varying water levels due to the Lucayan Formation. The Lucayan Formation is nonskeletal packstones and grainstones, and formed in a shallow water (less than 10m), semirestricted environment. The Lucayan Formation is not found on the Pleistocene reef side, showing that the water was deeper there during the Pliocene. The interior of the bank has skeletal rich limestone that is pre-Lucayan which indicates water deeper than 10 m with open circulation (Ginsburg 19).
Modern Depositional Systems
The evidence from the subsurface statigraphy shows that the water level fluctuated constantly over time and events around the world, like glaciation affected the Bahamas even though the islands were not directly a part of the glaciation. Using this knowledge, the modern day lithofacies, rock records of any sedimentary environment, are used as models for interpretation of ancient carbonates (Vacher 97). As supported by the previously discussed Holocene sediments it was shown that the present is the key to the past which respect to geological terms and ideology. There is a wide variation in the accumulation, depositional style, and sediment type, all which is affected by the islands’ varying orientations to currents and winds (Vacher 98). An example of this is stromatolites which range from very large subtidal ones to small costal and subtidal ones to one found in hypersaline lakes, where “salinity fluctuates in response to rainfall” (Curran 118). Stromatolites generally occur where rapid currents or hypersaline prevents grazing by macrofauna or where there is rapid cementation (Curran 118).
Shaping of the landscapes
The landscapes of the islands can be attributed to the accumulation of carbonate sediments which are deposited by currents, waves, and winds. Along with accumulation, erosion is another major factor in the shaping of the landscape because all the rocks are subjected to erosion by currents, waves, and winds. On the islands there are two major landforms which dominate. There are eolianite ridges which rise 30m above sea level and low lands which are composed of marine and terrestrial deposits. On the interior of the islands, the land is typically below sea level and thus contains marine to hypersaline lakes.
Since the islands consist mainly of limestone for the first 5m, there is a large possibility for the formation of caves. Limestone is a very porous material and thus is easily eroded by rainfall and runoff from the surface. The rate at which the surface water is carried underground causes there to be few freshwater rivers on the islands (Robinson). The water weathers the limestone, forming a large variety of karst formations including caves, sink holes, and solution pits. Since the majority of the islands are limestone, there are many channels for carrying the water underground, providing the opportunity for the formation of karsts throughout the islands (Robinson).
Another type of cave that can be formed is a flank margin cave, which is found the outer edges of the islands. The flank margin caves are formed by dissolution caused by the mixing of fresh and salt water (Curran 251). Dissolution caused by the combination of fresh and salt water is a relatively quick process, having dissolution rates as high as 1 m3/yr. Since these caves are created on the margins of the islands, the flank margin caves are indications of sea levels throughout history (Curran 253).
Coastal effects, deposition, and erosion
Erosion caused by the sea water has had a major role in the shaping of the islands. The water has slowly eroded the eolianite ridges that are along the coastline. Numerous reentrants have been created, which are the eroded remnants of flank margin caves (Vacher 102). There are also tidal channels and creeks that penetrate the shoreline, resulting in tidal delta deposits. In reference to the Bahamas a creek is a restricted marine embayment (Vacher 103). Also, the deposition and erosion rates are relatively equal because while each occurs daily, there is not any significant change over the past 300 kilo-years (Vacher 108).
Carbonate dunes are formed during the transgressive phase, where the rising sea level is causing subtidal sedimentation which is transported to beaches and turned into dunes. Carbonate dunes form close to their beach sources and consist of sand-sized fragments of mollusks, limestone, and eolianites (Kennett 312). Due to the dunes’ location close to the shore, they undergo rapid cementation due to sea spray and meteoric precipitation (Vacher 111).
The stillstand phase occurs when the sea level remains relatively stable allowing carbonate sedimentation to remain high and the coral reef growth catches up with the sea level. During the stillstand phase much of the marine record is deposited on the islands because there is less factors working against the deposition like high erosion rates. Due to the stillstand phase, lagoons fill and reefs grow high enough to become barriers to transportation. On the islands, beaches develop, heavily vegetated coastal dunes develop, and inter-dune depressions may contain lakes. Currently, majority of the Bahaman islands are in the stillstand phase, before the regressive phase occurs which is accompanied by high erosion rates (Vacher 113-118).
Curran, H. Allen and Brian White, ed. Terrestrial and Shallow Marine Geology of the Bahamas and Bermuda. Northampton: Geological Society of America, 1995.
Ginsburg, Robert N., ed. Subsurface Geology of a Prograding Carbonate Platform Margin, Great Bahama Bank: Results of the Bahamas Drilling Project. Tulsa: Society for Sedimentary Geology, 2001.
Kennett, James. Marine Geology. Englewood Cliffs: Prentice - Hall, 1982.
Lidz, Barbara, and Peter R. Rose. Diagnostic Foraminiferal Assemblage of Shallow – Water Modern Environments: South Florida and the Bahamas. Miami: University of Miami, 1977.
Marshak, Stephen, and Gautam Mitra. Basic Methods of Structural Geology. Englewood Cliffs: Prentice - Hall 1982.
Multer, H. Gray. Field Guide to Some Carbonate Rock Environments: Florida Keys and Western Bahamas. Dubuque: Kendall/Hunt Publishing Company, 1977.
Robinson, Matthew C., and Vincent Voegeli. Gerace Research Center. 2002. Gerace Research Center. March 30th, 2006.
Vacher, H.L., and T.M. Quinn, eds. Geology and Hydrogeology of Carbonate Islands. New York: Elsevier, 1997.
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