NEW STUDIES OF PLEISTOCENE REEFS ON SAN SALVADOR ISLAND, BAHAMAS: IMPLICATIONS FOR ISLAND GEOLOGY

The oxygen isotope substage 5e (~131 to 119ka) sea level highstand, during the late Pleistocene, covered large portions of San Salvador Island to +6 m, creating an environment for carbonate reef growth and development at elevations above modern sea level. These reefs are now fossilized and subaerially exposed on the island. There are fourteen fossil reefs that have been mentioned in the literature; three more fossil reefs were located by this study. Two reefs, the Cockburn Town Reef and Sue Point reef, have been previously documented in detail due to their excellent preservation. The Grotto Beach Reef and Mosquito Reef also have received significant study. The others have received little attention, primarily because exposures are poor. The ruggedness and remoteness of the island interior coupled with the paucity of good outcrops, make it feasible that other large and well-developed fossil reefs exist there. Fossil reefs should be found within the extensive Grotto Beach Formation, which formed during the stillstand and regressive-phase of oxygen isotope substage 5e, and covers 49% (77 km2) of San Salvador.

The known fossil reef locations, and the three newly reported reefs from North Storrs Lake, Salt Pond and Crab Cay, are on or near the current island perimeter, suggesting San Salvador was an atoll-like bank during the last interglacial (substage 5e). The reefs that are the most complex and diverse are located on the west and southwest sides, indicating development in the lee of the southeastern tradewinds. The Crab Cay reef appears to have been overstepped by the regressive-phase eolianite of the Cockburn Town member of the Grotto Beach Formation that forms the cay. However, a substage 5a age (~85 ka) for this eolianite, reported by other workers based on AAR data, cannot be ruled out by field relationships. The North Storrs Lake reef is the site of a karst feature that drains hypersaline water from Storrs Lake during tidal lows, but yields marine water during tidal highs. The degree to which the reef facies has influenced this karst feature is unclear. The Salt Pond reef covers an extensive area. It occurs in a location with broad, unvegetated rock surfaces and the extent of the reef may indicate the advantages of good outcrop exposure, as opposed to the reef being exceptional in size.