QUATERNARY GLACIOEUSTATIC SEA-LEVEL HIGHSTANDS FROM THE BAHAMAS: THE KARST SIGNATURE

The Bahamian Archipelago, because of the young age of its surficial rocks, 100% carbonate geology, and tectonic passivity, has long been used as a test of Quaternary glacioeustatic sea-level highstand position and chronology. With the exception of Mayaguana Island, which displays slight vertical rotation to the south exposing much older units, fossil coral reef U/Th dates from the archipelago all fall within the MIS 5e time window, ~120 ka. It has been argued that older corals formed either below modern sea-level elevation (MIS 7), or have been taken below modern elevation by isostatic subsidence of 1 to 2 m per 100 ka (MIS 9, 11, etc.). Flank margin cave position, on average at 0 to 6 m elevation, and a scattering of stalagmite dates less than 120 ka, supports a MIS 5e origin interpretation for the caves. In contrast, some caves, primarily in the southern Bahamas, display phreatic dissolution at elevations up to 17 m above sea level, and a stalagmite from Conch Bar Cave on the Caicos Platform was recently dated by U/Th to 266 ka. These observations suggest a lack of subsidence, or perhaps mild uplift, in the southern Bahamas. If so, where are the pre-MIS 5e fossil corals and related subtidal deposits? An often overlooked aspect is the karst denudation rate. Work on Guam in the western Pacific discovered 5 m of landscape denudation of a MIS 5e reef, extrapolating that rate of 5 m per 100 ka to the Bahamas (and accounting for climatic differences) suggests that those older reefs are entirely denuded, and that the remaining sea-level signature is preserved only in remnant flank margin caves, formed within the eolian high ground topography, which allowed survival to the present.