Resumen de Sedimentation and porosity enhancement in a breached flank margin cave
Lee J. Florea, John E. Mylroie, Adam Price
San Salvdor Island, Bahamas, provides unique opportunities to study modern geologic processes on carbonate platforms as a result of constraints in time and space. The time span of exposed geology is limited to the middle Pleistocene through Holocene (<500 ka), and the island lies on an isolated platform (12 by 19 km). Altar Cave, formed within an oxygen isotope substage 5e eolianite (approximately 125 ka) of the Grotto Beach Formation on San Salvador, 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) facilitates a sedimentation study. Sediment profiles from trenches dug at three locations in Altar Cave show that the deposits in the cave formed as an early stage of development of a Holocene strand plain that is present today between the cave and the beach. Altar Cave was breached by Holocene coastal processes;14C dates show sand fill deposits in the cave to be Holocene (4.7 ka).14C dates, 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 prior to breaching of the cave in the form of dissolution residuum accumulating during cave development. Petrologic analysis shows that this leaching has resulted in increased bedrock porosity below the sediment profile. Also, introduced organics have contaminated the late Pleistocene bedrock with young carbon, resulting in14 C ages of 14 ka at 0.3 m in depth and 28 ka at 1.3 m in depth. The results of this study demonstrate a potential method of porosity enhancement in young carbonates by vadose leaching. Porosity-enhanced zones have implications for our understanding of recharge to fresh-water lenses on carbonate islands.
