Bahamas Natural History

The Bahamas are low, carbonate islands that rest on two large bank systems, with water depths of less than 10 m. The Little Bahama Bank is in the northern Bahamas; the Great Bahama Bank extends from central to southwestern Bahamas. The remaining islands are isolated small platforms beginning at 24°N latitude in the eastern Bahamas and extending to Navidad Bank just north of Haiti. These shallow seas give the Bahamas its name, from the Spanish "Baja Mar." The bank systems are separated from each other by a deep water basin, with depths up to 4,000 m.

The origin of the Bahamas is still controversial. The archipelago may be the result of plate tectonics activity 200 million years ago, forming as a horst graben (Mullins and Lynts, 1977), or it may be the remnant of a much larger platform (Meyerhoff and Hatten, 1974). It is generally accepted that these banks originated at a latitude with warm, shallow waters that encouraged the growth of a variety of marine organisms whose skeletal remains were deposited as sediments. The weight of these sediments caused subsidence as deposition continued, developing carbonate deposits which, at present, reach a thickness of more than 5.4 km (Meyerhoff and Hatten, 1974). The islands are all less than 61 m above sea level; the highest elevation on San Salvador is 37.5 m, atop one of the ridges on the east side of Flamingo Pond (Shaklee, 1994).

The Bahama Platform became exposed during sea-level lowstands as a result of four major glacial advances during the Pleistocene. Winnowed eolianite dunes formed during these periods and became lithified after flooding, thus forming the islands. During the interglacial stages, weather altered the landscape into karst formations of caves, sink holes, and solution pits. These conduits honeycomb the islands. Because of the great porosity of the limestone, water from rainfall and runoff is rapidly delivered underground through these conduits, resulting in a scarcity of freshwater rivers and streams in the Bahamas.

During the last ice advance, sea level was 91.5 m lower than today, exposing the banks. This allowed plants and animals to arrive from the Greater Antilles, across the short gap from Cuba or Hispaniola. Some plants also arrived via birds or as flotsam, as is the case with mangroves. Thus, the majority of life forms are Caribbean in origin rather than North American. Vegetation on San Salvador is limited by poor soil, full sun, and periods of drought. Mangrove swamps line the inland brackish lakes and are also found in protected basins such as Pigeon Creek and French Bay where the average height of the trees is 4 m. Even in the mangrove flats vegetation is sparse. Plants grow out of cracks in the rocks or in pits where soil has collected and the average tree height is less than 0.3 m.

Taken from Introduction to the geology of the Bahamas and San Salvador Island with an overflight guide, by H. Allen Curran

When weather conditions permit, an excellent overview of the Great Bahama Bank and its associated features can be obtained during the flight to San Salvador (Fig. 1). The island is located about 620 km (385 mi.) ESE of Ft. Lauderdale; flight time should be 2.5 to 3 hours. On some trips, a short stop for refueling and/or Bahamas immigration may be made at Rock Sound Airport on Eleuthera. The following is a brief guide to points of geographic/geologic interest located along the flight route.

1. Waters of the narrow continental shelf (width about 7 km) east of Ft. Lauderdale quickly give way to the Straits of Florida channel, with its dark-colored water and a depth of about 800 m. The channel is about 85 km wide.
2. The Bimini Islands, about 92 km east of Ft. Lauderdale, likely will be visible on the starboard side of the aircraft. The islands lie at the NW edge of the Great Bahama Bank. The northern part of the bank between the Biminis and the Berry Islands is largely an area of relict to intermittently active sand sheets composed of skeletal grains, ooids, lithified pellets, and composite grains (Hine, 1983a, b). Larage relict sand waves lie to the north of the Biminis, and westward offbank sediment transport occurs immediately to the north of the islands.
3. Chub Cay of the Berry Islands chain lies about 147 km east of the Biminis. This small island is developed and has an airstrip and marina. Just prior to reaching Chub Cay, large and active oolitic sand shoals can be seen to port, on the leeward side of the Berry Islands. To starboard, the Joulters ooid shoal, described by Harris (1983), is located off the north end of Andros Island.
4. East of the Berry Islands, waters quickly deepen into the Northeast Providence Channel (4000 m depth), which extends south to form the Tounge of the Ocean. The channel is about 40 km wide.
5. After crossing the channel, New Providence Island and the city of Nassau, the capital of the Bahamas, can be seen to starboard. The island forms the NW corner of the Exuma-Eleuthera bank. We will be over the bank for a distance of about 100 km. The western part of the bank is covered by inactive to intermittently active sand sheets similar to those crossed earlier. This region of generally stable bottom conditions gives way to an area of active oolitic shoals with varied bedofrm on the eastern part of the bank at the head of Exuma Sound (Hine, 1983a, b). A shallow, protected lagoon with diverse bedforms can be seen off the western coast of Eleuthera.
6. After passing close to or crossing Eleuthera, the narrow, windward shelf of Eleuthera will be visible on the Atlantic side of the island. Breaking waves and brownish areas mark the occurence of patch and bank/barrier coral reefs. Similar features can be seen on the northern and eastern shelf of Cat Island, the next island to be encountered, about 58 km SE of Eleuthera.
7. The distance between the northern end of Cat Island and San Salvador is about 135 km. Here open waters of the western North Atlantic Ocean are crossed (water depth about 4600 m), and floating masses of the brown alga Sargassum can commonly be seen.
8. San Salvador Island lies on a small, isolated bank at the eastern edge of the Bahamas. On the approach to the island from the west, on will see that the narrow shelf drops precipitously to deep water. The interior of the island is occupied by numerous shallow, hypersaline lakes separated by ridges composed of carbonate eolianite. The Club Med complex can be seen to port, and the village of Cockburn Town is to starboard, south of the runway.

Geologic Setting of San Salvador Island

San Salvador is a small island, about 11 km wide by 21 km long, and it is bordered by a narrow shelf with an abrupt shelf-edge break leading to a very steep slope (along the west side of the island, this slope is known to divers as "The Wall"). The topography of the island is dominated by arcuate ridges interpreted as representing successive stages of carbonate eolian accretion. Shallow hypersaline lakes occupy the low inter-dune ridge areas. The island's shoreline is characterized by cliffed headlands of eroded eolianite; fine to medium grained carbonate skeletal sands form beaches between headlands, and Holocene beachrock is common.

Shallowing-upward facies sequence of Bahamian carbonate rocks

References:
Curran, H.A., 1985, Introduction to the geology of the Bahamas and San Salvador Island with an overflight guide, in H.A. Curran (ed.), Pleistocene and Holocene carbonate environments on San Salvador Island, Bahamas. Boulder: Geological Society of America Field Trip Guidebook 2:1010.
Gerace, Donald T., Ostrander, Gary K., and Smith, Garriet W., Environment and development in coastal regions and in small islands, in CARICOMP – Caribbean coral reef, seagrass and mangrove sites, Coastal region and small island papers 3, UNESCO, Paris, 347p.