UNEXPECTED KARST FEATURES IN THE BAHAMAS: ABACO'S HIGH LEVEL CAVES AND CONE KARST

Abaco Island, Bahamas, exhibits two sets of karst features that are unusual and unexpected in that setting. First, caves at the south end of the island exist in a steep-sided ridge at elevations from 10 to 22 m above msl. Current models of flank margin cave formation in the fresh water – salt water mixing zone require that cave genesis occur at elevations between 0 and 6 m above msl, in agreement with OIS 5e ~125,000 years ago. Given the tectonic stability of the Bahamas, a sea level highstand up to ~20 m above msl would be required to develop the observed caves. Such a highstand has been proposed for OIS 11, but the data have been controversial, especially in the Bahamas. Field work indicates that the caves are small in dimension and lack phreatic dissolutional surfaces. It is most likely the caves formed during OIS 5e, when the hillside was cliffed by that sea-level highstand, and the exposed soft interior of the eolianite ridge was eroded by wind, to form tafone-like features.

Second, Abaco also has landforms that bear a striking resemblance to tropical cone karst, features not known from other Bahamian islands. These hills are symmetrical in shape, range in height from 3.5 to 22 meters, and are formed from the erosional dissection of eolianite ridges. One slope of the hill follows the dip of the foreset beds while the other slopes are formed by truncated beds and talus slopes. The absence of surface streams on Abaco implies that meteoric subaerial dissolution is the dominant process. One element of meteoric dissolution is pit cave formation, which causes slope failure on the periphery of the hills. The land surface is further mobilized by forest fire-induced exfoliation of the eolianites and disruption of the bedrock surface by vegetation. As eolian ridges initiate with high relief, rock loosened by fire, vegetative and epikarst processes mass-wastes downslope as talus to create the relative symmetry of the hills. Abaco is the only Bahamian Island that has both the high eolian relief and climate with a large positive water budget necessary for the observed erosion.