Recent evolution of a Bahamian ooid shoal: Effects of Hurricane Andrew

Hurricane Andrew, a category 4 hurricane having wind velocities of ≈240 km/hr, passed north of Joulters Cays, Bahamas, in a westerly direction on August 23, 1992. We documented three sedimentary facies in a 2.7 km 2 study area dominated by mobile ooid sands before the hurricane, using aerial photographs, surface observations, and shallow coring. The shoal crest at this locality had aggrading and northward-prograding (parallel to depositional strike) washover bars composed of cross-bedded, well-sorted ooid sands. Burrowed, poorly sorted ooid sands were present seaward of the washover bars, whereas poorly sorted ooids and mud occupied a stabilized area bankward of the actively migrating shoal and local areas between washover bars on the crest of the shoal. The shoal was crosscut by tidal channels, and older washover bars were being dissected by tidal currents. Although Hurricane Andrew profoundly changed surface features within the study area, its effects will probably be only partly preserved. The hurricane eroded washover bars and transported sediment seaward, leaving a nearly flat shoal crest overlain by a laterally continuous, decimeter-thick lens of well-sorted ooid sand that thins seaward and bankward. Post-hurricane tidal currents deposited a centimeter-thick discontinuous layer of carbonate mud over this lens of well-sorted ooid sand and transported ooids seaward off the shoal. The well-sorted ooid sand layer will most likely be reworked when an actively migrating shoal crest is reestablished, although some of this storm deposit may be preserved on the shoal crest where the ooid sand layer was deposited in areas of normally less agitated conditions. Ooids may also be preserved in finer grained sediments seaward of the shoal, as suggested by previous studies. Mud deposits on the shoal crest may be preserved where buried beneath reestablished washover bars, although some of this mud will be removed by reworking during diurnal tides.