THE ORIGIN OF HOLOCENE STRANDPLAINS OF THE BAHAMAS: INFLUENCE OF TROPICAL STORMS AND CLIMATE CHANGE

Late Holocene strandplains are common on the Mid-Atlantic and Gulf coastal plains of North America and on many adjacent tropical islands. In the Bahamas, beach-ridge sets develop seaward of older eolian ridges and have added considerable habitable area to the archipelago. We investigate the origin of these strandplains and the influence climate and sea-level rise (SLR) have had on their development. The surficial and subsurface geology of 6 strandplains on Little and Great Exuma, and Eleuthera was studied using sedimentologic and stratigraphic methods, ground-penetrating radar (GPR) imaging, and radiocarbon geochronology.

Bahamian strandplains consist of up to 20 semi-lithified beach ridges that are catenary or zetaform in plan view, and anchored to Quaternary headlands at one or both ends. GPR radargrams confirm that ridges are founded on erosional scarps within foreshore or backshore sands. Outcrops of these basal sediments reveal sedimentologic and paleontologic evidence for a beach origin. In a regime of net progradation, scarps produced by moderate storms cause trapping and aggradation of eolian sediments along the length of the contemporary shoreline. Consequently, most beach ridges are capped by dunes that can appreciably increase their heights. Heights of exposed foreshore limestones and of radargram contacts between beach and eolian sediments are within +2 m relative to mean sea level. Because normal tidal variability and storm surges can result in deposition at these elevations, a eustatic sea-level highstand is not necessitated. Whole-rock radiocarbon dates for foreshore limestones and for eolian ridges span 4100-700 cal yBP, with most dates clustering between 1200-700 yBP. This interval coincides with a <3000 yBP interval of increased storminess, established through paleotempestology, and terminating within the Medieval Warm Period. We hypothesize that the more storm-conducive climate, coupled with modest SLR rates and plentiful carbonate sediment, predisposed the archipelago to strandplain formation. Ephemeral highstands during times of frequent storm-wave constructional conditions resulted in deposition of elevated beach sequences. These beaches were subsequently scarped during erosional phases that later evolved into foredune-capped ridges of the strandplain.