ABSTRACT

Stuby, James Lyle. M.S., Department of Geological Sciences, Wright State University, 2000. Petrographic Analysis and Depositional History of an Open, Carbonate Lagoon: Rice Bay, San Salvador, Bahamas.

Rice Bay is an open, high-energy, sandy carbonate lagoon located on the northeastern end of San Salvador Island in the Bahamas. Macroscopic and petrographic analysis was carried out on Holocene unconsolidated sediments from surface samples and three cores from the floor of the lagoon, as well as on consolidated rock samples from Man Head Cay, a small Pleistocene island of crossbedded eolianites in the bay. The goal of this thesis is to establish a depositional history of Rice Bay.

Point count data of empregnated whole sediment from the surficial and core sediments indicate that the lagoonal sediments are comprised of 30-50% mollusk fragments, 5-30% algal fragments, ~5% foraminifera, ~5% other skeletal grains, 0-19% ooids, which decrease in abundance up the cores, 5-10% peloids, and up to 30% clasts. Several sedimentary facies were found in the cores. Most are variants of lagoonal sediments, but one facies is very likely a beach sand. This beach sand was a possible source of sand of the Holocene eolianites of North Point, which is a peninsula separating Rice Bay from Grahams Harbour to the west.

A statistical analysis showed that there is no significant correlation of any major grain type with seagrass density on the sea floor of Rice Bay.

Peat was found at the base of two of the sediment cores. The peat deposits, generated by mangroves, establish that Rice Bay was once a more restricted lagoon than it is at present. Radiometric dates were obtained from peat deposits (6300 BP). These dates confirm the Holocene nature of the lagoonal sediment, and allow the calculation of sedimentation rates in the lagoon, which range from 56 to 70 cm/1000yr.

Results from Man Head Cay indicate that it is a weakly cemented skeletal grainstone and is highly porous (26%). The grains are skeletal grains, intraclasts, and peloids. No mud, ooids, or oolitic clasts were observed.

The depositional history of Rice Bay has been controlled almost entirely by rising and falling sea level. The oldest rocks in the lagoon are those of Man Head Cay, which are late Pleistocene in age (Carew and Mylroie, 1995). These probably formed during regression following the highstand of sea level 125,000 years ago (Oxygen Isotope Stage 5e highstand), although there is some evidence of deposition during the transgressive phase. The rest of the deposits in Rice Bay are Holocene in age. As the platform was first flooded about 7000 years ago, mangroves existed in the restricted lagoon silled by Man Head Cay. As the transgression proceeded, more of the bay was flooded and production of carbonate sand accelerated so that a beach prograded over the mangroves. Much sand was blown into the dunes of North Point, which are between 6340 and 5250 years old (Colby and Boardman, 1989). Also at this time there was an ooid-forming event, indicated by the ooid content of North Point dunes (18-42%; Thompson, 1996) and the ooid content near the base of the two sediment cores (8-19%). The transgression continues to the present day and lagoonal sediments now accumulate in the bay and beachrock is forming along the present beaches. North Point continues to erode and corals are growing on the wave-cut platform adjacent to it.




Copyright 2006 James L. Stuby. All Rights Reserved.