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Elevated Reefs and Related Pleistocene Stratigraphy of the Southern Exuma Islands, Bahamas

DILL, ROBERT F., University of South Carolina and Caribbean Marine Research Center, San Diego, CA, ROBERT B. HALLEY, EUGENE A. SHINN, and JACK L. KINDINGER, U.S. Geological Survey, St. Petersburg, FL, and DANIEL R. MUHS, U.S. Geological Survey, Denver, CO

At least seven elevated coral reefs are exposed on the lee side of an inner chain of low islands on the eastern margin of Great Bahama Bank in the Southern Exumas. Starting at Fowl Cay in the north, they extend at least to Pigeon Cay in the south, a distance of about 44 nautical mi (81 km). These reefs formed around preexisting Pleistocene core-islands and hardgrounds during a transgressive higher than present stand of sea level, prior to the Holocene. As sea level dropped, the reefs were karsted and capped by caliche crusts, a unique paleosol breccia containing land snails (Cerithidea. sp.) and a now lithified dune system with well-developed rhizomorphs. Shallow sea caves eroded into steep, clifflike notches are often located on ridges shore-ward of the reef, with roof heights of up o 5 m above the top of the elevated reefs. The reefs form a 5- to 10-m wide terrace, approximately 1 m above present sea level. Shoreline exposures often exceed 300 m (1000 ft). Most exposures have a base thicket of interwoven branches of Acropora cervicornus approximately 70-cm thick. The badly corroded branches are surrounded by a cemented matrix of ooid sands, marine cement, and coralline debris. The upper 30 cm is formed by a much more diverse reef community of broken shells and corals (Montastrea, Diploria Agaricia, Porites, and Acropora). Crustose coralline algae and colonial foraminifera (Homotrema) form a cementing crust around many corals. These reefs and their relationship to paleosols within subaerial dune deposits provide stratigraphic markers that play an important role in i terpreting the development of Pleistocene deposits of the Great Bahama Bank.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)