CHARACTERIZATION OF BEACHROCK FORMATION PROCESSES AS AN INDICATOR OF PAST SEA LEVEL, SAN SALVADOR, THE BAHAMAS

The island of San Salvador (SSI), Bahamas located on the easternmost edge of the Bahamian carbonate platform contains numerous examples of well-exposed beachrock. These exposures are present both on the leeward and windward sides of the island. Beachrock forms in the intertidal zone beneath the surface of overlying unconsolidated sediment. Beachrock always dips slightly seaward. The formation process requires meteoric and marine water, calcium carbonate (CaCO₃), degassing of carbon dioxide (CO₂), and is highly influenced by microbial activity. Because beachrock undergoes an especially rapid cementation process and only forms in the intertidal zone, it is a useful indicator of past sea levels. Not only does beachrock help reconstruct past sea levels, it may also be used to predict coastline morphology. Once beachrock that formed underground is exposed on the surface (through erosion or a storm event), the dynamics of the beach environment are greatly affected. Sand dune migration, variety of marine fauna and flora, grain size and sorting of unconsolidated sediment, and coastal progradation may all be altered by the presence of exposed beachrock. The degree to which such changes may occur depends on the extent of the cementation process prior to surface exposure. In hand specimen, collected beachrock samples are coarse-grained, poorly sorted, bioclastic calcarenites. Because tectonic uplift does not play a significant role in the structure of the Bahamian Islands, changes in the stratigraphic position of the intertidal deposited and cemented beachrock can only be attributed to the rise and fall of past sea levels. Thin section analysis reveals two cement types present in sampled beachrock. Original menisci shaped magnesium calcite cement occurs at grain contact along with a secondary acicular aragonite cement radiating from grains. At the northwest point of SSI, beachrock with embedded Strombus gigas harvested by native peoples ca. 1000 years ago have been identified by Blick (2004) about 1-2 m above present sea level. Brecciated and recemented beachrock near present sea level also contains 18^th century artifacts. These data suggest that small-scale sea level changes may be recorded on SSI.