North Central GSA Meeting
Champaign, IL, April 22-23, 1999

Petrographic Analysis and Depositional History of an Open Carbonate Lagoon: Rice Bay, San Salvador, Bahamas

James L. Stuby and Cindy K. Carney, Department of Geological Sciences, Wright State University, 3640 Col. Glenn Hwy., Dayton, OH 45435
Mark R. Boardman, Geology Department, Miami University, Oxford, OH 45056

Rice Bay is an open, windward, sandy, carbonate lagoon, approximately 1 km x 1 km in area, located on the northeast corner of San Salvador Island in the Bahamas. It is separated from other lagoons and the open Atlantic Ocean by North Point, Cut Cay, and Man Head Cay, all consisting of lithified eolianites. There are beaches on the mainland and on Man Head Cay. The subtidal surface of Rice Bay consists of a mosaic of subenvironments: barren carbonate sand and mud, Thalassia and Syringodium meadows, areas of calcareous green and red algae, subtidal beachrock, and patch reefs.

Forty-six intertidal and subtidal sediment samples were collected from the sea floor of Rice Bay along a 450 m long transect between the two beaches. This "Megatransect" passes through all of the subenvironments except a patch reef. Three sediment cores were taken from Rice Bay: two close to Rice Bay beach and one from near the center of the Megatransect. Sediment thicknesses in the cores ranged from 3.58 m to 4.51 m. Two of the cores contained peat deposits, which will be C-14 dated. Eight supratidal rock samples were collected from the southwestern side side of Man Head Cay.

Preliminary investigations indicate that the distribution of grain types in the Megatransect correlate to subenvironments. Seven facies have been recognized in the sediment cores. All but the peat deposits seem to be variants of lagoon deposits. The peat represents deposition of organic matter (mangroves) in the intertidal zone. Deposition of the unconsolidated sediments in Rice Bay probably began about 4000 years ago as the current marine transgression flooded the platform of San Salvador, and we expect this date to be confirmed by C-14 dates from the peat deposits. Grain-type distributions from the Megatransect will be used to predict the depositional nature of facies within the sediment cores.