DEPOSITIONAL ENVIRONMENTAL INTERPRETATION OF THE MIDDLE TO LATE HOLOCENE HANNA BAY LIMESTONE, SAN SALVADOR BAHAMAS: IS THERE EVIDENCE FOR A SEA-LEVEL HIGHSTAND?

The Hanna Bay Member (belonging to the middle to late Holocene Rice Bay Fm.) crops out along the periphery of San Salvador Island, Bahamas and has been interpreted as a nearshore deposit. Outcrops currently rest as much as 2 m above current mean sea level, suggesting the existence of a sea-level highstand. This hypothesis was tested through study of the sedimentology, petrology, and ichnology of the limestones. Numerous outcrops were investigated with two extraordinary exposures studied in greater detail. Additionally the sedimentology of modern beaches on San Salvador was characterized for comparison. Whole rock AMS radiocarbon dating of the two sections yields different ages (Cal BP 3260 – 4070 for type section; Cal BP 780 – 1230 for Grotto Beach). Ages taken at different heights within the same section are stratigraphically disordered. Variability and disordering within a section may be explained by the confounding effects of averaging the age of various allochems and cements within the same rock. Nonetheless, the difference in age-range between the two sections suggests they represent diachronous intervals. Sedimentary structures within the Hanna Bay Member are consistent with a foreshore origin and include fenestrae, rill marks, and swash marks. Limestones are composed of planar thin beds that dip seaward between 4-8 degrees, also consistent with a foreshore interpretation. Grain size distributions and allochem types are comparable to those seen in modern fore- and backshore settings. Mean grain size of the beds varies between medium to very coarse sand, indicating these nearshore deposits were not exclusively deposited during storm events. The trace fossils and vegemorphs present, however, are consistent with a backshore to dune environment. Marine cements are conspicuously absent; blocky isopachous and meniscus cements are common, supporting a non-marine, freshwater cementation. We propose these sediments were deposited in a foreshore setting during a short-duration highest tide interval and then quickly restored to a backshore position once the tide subsided. This interpretation is not consistent with a sea-level highstand at least for these Hanna Bay Member rocks. A mechanism is still needed to explain a short-lived interval of high tides without storm conditions.