FORESHORE TO DUNE DEPOSITIONAL SEQUENCES OF MIDDLE TO LATE HOLOCENE DEPOSITS OF THE BAHAMAS: IMPLICATIONS OF RAPID PROGRADATION AND INTERPRETATION OF SEA-LEVEL HISTORY

Grainstones of the Hanna Bay and North Point Members, middle to late Holocene Rice Bay Formation, crop out along the periphery of most Bahamian islands, indicating the onset of prolific allochem production on Bahamian platforms with rising sea level. Deposits are of appreciable volume and height to influence the geomorphology and ecology of an island through creation of lakes and restricted lagoons. The younger Hanna Bay Member contains a sequence of foreshore to dune deposits representing extensive dune progradation over beach deposits during a time of decelerated sea-level rise (SLR). Sedimentologic, stratigraphic, and paleontologic evidence from several islands including San Salvador, Eleuthera, and Exuma shows a consistent pattern of paleoenvironmental change. Hanna Bay sequences often begin with foreshore deposits that sit as much as 2 meters above current mean SL. This facies, however, represents diachronous, ephemeral regional highstands due to local tidal perturbations, rather than eustatic processes. Consequently, this foreshore facies does not support the occurrence of middle to late Holocene eustatic SL highs. Foreshore deposits are more commonly preserved on the lee of a platform, where sediments are less likely to be reworked and eroded by subsequent wave action. Though ephemeral highstands may be numerous through time, their preservation is presumed rare. Backshore and foredune deposits prograde seaward over older sediments. Dune strand plains are common and are found on virtually every Bahamian island. These plains, when found in leeward settings, have dunes with low amplitudes and short wavelengths. Alternatively windward regions have larger amplitude dunes that are wave-eroded to form cliffs. Dune progradation was at times rapid. Vertical structures, possibly of multiple origin but presumed mostly to be the trunks of silver thatch palms, a coastal coppice tree common to this xeric habitat, are commonly preserved as external molds in both members of the Rice Bay Fm. Palm frond molds also occur and can be common. Wind-formed bedding plane features (e.g., winnow flutes; deflected wind lineations) are evident at one locality, indicating trunks were present at time of deposition. Palm trunk molds are more commonly found on windward coasts, where rates of eolian deposition were greatest.