LATE HOLOCENE SEA-LEVEL FLUCTUATION AND COASTAL PROGRADATION IN THE BAHAMAS AS RECORDED IN THE HANNA BAY MEMBER LIMESTONES OF THE SOUTHERN EXUMA ISLANDS

Hanna Bay Member limestones are extensive on peripheries of the southern Exuma Islands (Little Exuma, Great Exuma, and Stocking Islands) and provide a record of late Holocene carbonate deposition and sea-level effects on progradation and distribution of coastal environments. Three lithofacies occur and represent foreshore, backshore, and dune environments. Progradational dune strand plains, composed of eolian rather than beach ridges, are common. Ridges lithify quickly by freshwater vadose cementation, which increases preservation potential. Dune forms are well preserved, and exhibit original lee and stoss surfaces, sedimentary structures, and vegetative and faunal trace fossils. Foreshore and backshore deposits sit either stratigraphically below or outboard of eolian sands, indicating that dune facies commonly prograded over beach deposits or that beach deposits were involved in strand plain formation. Outcrops may exhibit multiple phases of foreshore deposition and erosion. Occasionally within strand plains, wide limestone pavements remain between adjacent ridges and are interpreted as preserved backshores. Cross cutting relationships of dune ridge sets, paleo-cliff faces, and boulder breccias indicate periods of erosion and set-back punctuating times of progradation. Foreshore limestones are found at elevations up to 2 m above modern sea level. Elevation of such intertidal deposits has been cited as evidence for late Holocene sea-level highstands. However, similar to the foreshore facies of Hanna Bay limestones on San Salvador and Eleuthera, sedimentologic, petrographic, and fossil evidence is more consistent with an ephemeral, non-eustatic sea-level high caused by localized coastal phenomena.

Trace fossils are common in dune and backshore/upper foreshore facies. Psilonichnus upsilon, the distinctive burrow made by the ghost crab Ocypode quadrata, is common in the beach facies, including specimens formed by juveniles. Insect burrows, particularly stellate burrows attributed to the activity of halictid bees, are distinctive, large, and commonly dense in dune facies. Piperock occurs within dune facies at some localities; although piperock may have multiple origins, at least one instance of piperock can be attributed to the eolian burial of silver thatch palm tree trunks.