BAHAMIAN STRATIGRAPHY: ESTIMATING CARBONATE ROCK AGES WITH MINERALOGY AND DIAGENESIS ON SAN SALVADOR, BAHAMAS

The timing and duration of sea-level highstands as interpreted from Bahamian rocks are in dispute because it is difficult to assign ages to partially altered carbonate rocks, especially those older than ~125 kyr. Eolian packstones and grainstones are poor candidates for isotope or amino-acid racemization age estimates and correlating formations by paleosols is not successful. However, carbonate mineralogy provides relative stratigraphic ages to test and refine published models of late Pleistocene deposition and stratigraphy (Carew and Mylroie, 1995; Hearty and Kindler, 1995; Titus, 1986).

Vacher (1990) proposes that the alteration of aragonite to calcite can be used to determine the relative age of carbonate rocks. The rate of alteration varies depending on the duration of rock interaction with freshwater as the water table changes with rainfall and fluctuating sea-level. Vacher's data suggest that the percentage of aragonite decreases by about 50% with each extended exposure to meteoric diagenesis. Thus, each sea-level highstand that perches the freshwater lens might reduce the percentage of aragonite by 50%. Average aragonite concentration of the Cockburn Town member is 65%, consistent with a 125 kyr time of formation (one sea-level highstand). Dunes that were formed during isotope stage 1 have undergone less alteration and consequently have a higher concentration of aragonite (74% mean). However, these Holocene rocks consist of more initial calcite than the Cockburn Town corals so the aragonite concentration since diagenesis is inherently lower. Moreover, the mineralogy suggests that diagenesis is rapid during early stages of dune lithification. The Dixon Hill member has been assigned ages between 85,000 and 125,000 years old by Vacher and Hearty (1989) and Carew and Mylroie (1995), respectively. However, the mineralogy (4-13% aragonite) suggests that Dixon Hill has undergone significant alteration and is much older than 125 kyr. The formation at Owl’s Hole consists of two units separated by a paleosol. Aragonite concentration for the upper unit is 56% and the lower unit consists of 40% aragonite. The mineralogy at Owl’s Hole represent dune formation during two separate highstands of sea level.