HOLOCENE SEDIMENTATION AND BEDROCK ALTERATION IN A BREACHED FLANK MARGIN CAVE

San Salvador Island, Bahamas, provides unique opportunities to study modern sedimentation processes on carbonate platforms. The time span of exposed geology is limited to the middle Pleistocene through Holocene (< 500 ka). Altar Cave, formed within an oxygen isotope substage 5e eolianite (approximately 125 ka) of the Grotto Beach Formation, is a classic example of a flank margin cave that has been exposed during hillslope retreat. The nature of Altar Cave (restricted entrance, simplistic morphology, and easy access) makes a sedimentation study easier to perform. Sediment profiles from trenches dug at three locations in Altar Cave show that deposition in the cave occurred during the early stage of development of a Holocene strand plain that is present today between the cave and the beach. ¹⁴C dates show sand fill deposits in the cave to be Holocene (4.7 ka). Dates from sediment and bedrock from the back of the cave, plus XRD and geochemical analyses show the surficial sediment to be recent (0.6 ka), and that leaching has altered the bedrock floor of the cave. Petrologic study of the floor rock has provided evidence of autogenic sedimentation in the form of dissolution residuum accumulating during cave development. Petrologic analysis shows that leaching has increased bedrock porosity below the soil profile. Also, introduced organics have contaminated the late Pleistocene bedrock with young carbon, resulting in ¹⁴C ages of 14 ka at 0.3 m in depth and 28 ka at 1.3 m in depth. Results of this study demonstrate a potential method of porosity enhancement in young carbonates. Porosity-enhanced zones have implications for our understanding of recharge to fresh-water lenses on carbonate islands.