OBSERVATIONS BEARING ON EVOLUTION OF BARFIELD BAY, A LARGE PLEISTOCENE DUNE-RIMED DEPRESSION IN SOUTHWEST FLORIDA

The unusual height and continuity of the relict dune rim of elliptical Barfield Bay have motivated efforts to understand its geomorphology. Previous investigators proposed that it is: 1) the sand rim of an outlying Carolina Bay and 2) a parabolic dune. We are studying vibracores up to 6 m long to better understand the evolution of the dune and bay. Radiocarbon dates are pending.

Our cores show that Holocene sealevel rise submerged the base of the dune at Horr's Island a minimum of 4 m and the Pleistocene floor of Barfield Bay a minimum of 6.5 m. As the bay filled with sediment, foraminifers changed from miliolid- to Ammonia-predominance facies indicating that bay water changed from hypersaline to hyposaline. One core sample with miliolid predominance facies occurs in an Anomalocardia-dominated shell bed having a few, tabular, euhedral gypsum crystals similar to those found much more numerously in mudflats of hypersaline Laguna Madre, South Texas. Evidently, Barfield Bay was cut off from freshwater dilution for a while after it was flooded by rising sealevel, allowing evaporation to raise salinity in the restricted bay. The mangrove-lined tidal channel that now connects Barfield Bay to the seasonally dilute estuary on the east probably did not yet exist. This supports prior speculation that Horr's Island is connected to Marco Island by a low place in the dune, later breached by rising sea level. Thus, a single partially submerged sand dune ridge probably surrounds Barfield Bay, interrupted only on the southwest at the 0.5 km wide channel that connects Barfield Bay to Caxambas Pass. It remains to be determined whether the dune rim was breached at this location also.

Thus far, our data neither exclude nor confirm hypotheses offered to explain Barfield Bay. Nothing has yet been found to support interpretation of the dune as a Pleistocene aeolian lake shore deposit, the hypothesis that now dominates interpretation of Carolina Bay sand rims. We have not yet found sand-size extraterrestrial indicators such as those used recently to argue for impact origin of Carolina Bays. Barfield Bay resembles three of the dune-rimed depressions in west central Florida that have been interpreted as Pleistocene parabolic dunes. However, these three do not appear to be simple parabolic dunes and they may have been initiated or modified by karst processes.