THE GEOLOGY OF ELEUTHERA ISLAND, BAHAMAS: REINTERPRETATION OF MEGABOULDER AND OXYGEN ISOTOPE SUBSTAGE 5A DEPOSITS

Eleuthera Island, Bahamas has outstanding outcrops important to understanding Bahamian geology. The island contains two exceptional exposures interpreted by Hearty (2000, QSR) to represent (1) megaboulders deposited by tsunami or super-storm activity; and (2) a demonstrable contact between carbonate units deposited during oxygen isotope substages 5a and 5e. The evidence Hearty used to support the boulder hypothesis is: (1) the dips of the bedding within the eolianite boulders is variable and sometimes steeper than eolian deposition allows; (2) the boulders rest on top of a pre-existing paleosol surface; and (3) amino acid racemization [AAR] data indicate the boulders are older than the units they rest upon. Our field investigation in the spring of 2002 indicates that the purported megaboulders do not exhibit the widely variable dips reported. Rather, the dips are consistent with eolian deposition, and are consistent with the dips of the surrounding and underlying eolianites. The "boulders" also have well-developed phreatic cave systems in their bases, which contradicts the boulder hypothesis. We regard the "boulders" to be residual karst towers, which explains the presence of the caves. This reinterpretation indicates that AAR whole rock data from eolianites are not a reliable age indicator. The purported oxygen isotope substage 5a/5e contact cited by Hearty occurs between overlying eolianites and underlying intertidal calcarenites. Hearty identified a reddish rubble layer that appears to separate the two units as a paleosol. Hearty interpreted whole-rock AAR data from the eolianite to represent deposition during the time of substage 5a. We found that the red rubble unit is not laterally extensive, and is only associated with dissolution conduits developed in a swale along the eolianite/intertidal calcarenite contact. Elsewhere in the nearby vicinity the eolianite rests directly on the intertidal calcarenite with no intervening paleosol. We regard the hypothesized paleosol to be a cave sediment deposit. We believe that the outcrops in question represent a regressive-phase substage 5e eolianite overlying substage 5e intertidal calcarenites. This reinterpretation further indicates that AAR whole rock data from eolianites are not a reliable age indicator, especially at the resolution applied here.