CARBONATE DISSOLUTION IN A DYNAMIC FRESH-WATER LENS: LENS MARGIN MIGRATION AND THE PRODUCTION OF BANANA HOLES AS SYNGENETIC CAVES, SAN SALVADOR ISLAND, BAHAMAS

Carbonate island aquifers are one of the most unique and dynamic karst aquifers. In lagoon or shallow shelf settings, carbonate sediments build up rapidly and are accreted to the shoreline forming a strandplain. As these strandplains build seaward the island’s fresh-water lens will occupy the new land. The fresh-water lens margin is a unique environment where mixing salt and fresh waters, organic loading, and focus of flow produce aggressive dissolution that produces flank margin caves on carbonate islands around the world. Advancing strandplains and their associated fresh-water lens form flank margin caves which are abandoned as the strandplain continues to build seaward and the lens margin follows. This sequence of dissolution and abandonment leaves behind rows and clusters of relatively small voids with larger voids forming where the strandplain deposition paused the longest. Such voids are syndepositional caves. The islands of the Bahamas provide a unique setting to study this theory as strandplains are a common feature of Quaternary glacioeustasy. San Salvador Island, Bahamas is ringed by a +4m late Pleistocene terrace which is pock-marked by small collapse features known as banana holes because of their importance to local agriculture. Due to their smallish size and location in heavy bush not much is known about these features. Past researchers have noted they show phreatic dissolution features, are ovoid in plan, and are typically wider than they are deep. Their model proposes vadose/phreatic fresh-water mixing at the top of the lens to dissolve the voids in an interior setting. Preliminary investigation for this study, using a high precision Trimble^™ GPS in the Line Hole area of San Salvador has shown what appears to be a linear and clustered trend to the banana holes. Investigation of the banana hole wall rock shows fenestral porosity as well as herringbone cross beds which indicate intertidal or shallow sub-tidal deposits overlain by backbeach dunes, typical of strandplains. Evidence from geospatial and petrographic indicators seem to support banana holes being the preliminary flank margin voids as syngenetic caves, left behind by the fresh-water lens in an advancing strandplain during the last interglacial (MIS 5e).