COASTAL CAVES IN QUATERNARY EOLIAN CALCARENITES OF THE BAHAMAS: SEA CAVES OR BREACHED FLANK MARGIN CAVES?

Sea cave formation is the result of wave action in the littoral zone along cliffed coastlines. Wave energy exploits existing weaknesses in coastal rocks, preferentially eroding a cavity which can have highly variable morphology, depending on the initial weakness itself and further modification by secondary processes such as collapse or bioerosion. Sea cave classification is based on this initial weakness, which is usually a joint or fault in the rock. Numerous sea caves are found and documented along cliffed coastlines around the world, especially in crystalline rocks, which are strong enough to support these voids. Sea caves also form in softer rocks such as the Quaternary eolian calcarenites of the Bahamas, where this study was conducted. The Bahamas are tectonically stable, so the only structure exhibited in these young limestones is depositional bedding planes. However, sea caves as large as 30m x 15m in area footprint have been discovered and mapped. A total of 41 sea caves were mapped.

In the coastal carbonate regions of the world, a mechanism for developing large voids in the fresh-water lens near coastlines exists. Mixing dissolution creates flank margin caves, which in the rock record are useful indicators of past sea level position and paleo-lens configuration. Flank margin caves may be breached in coastal settings, and quickly modified by wave action. They have a distinct morphology characterized by cuspate and undulating ceilings and floors, bedrock pillars, speleothems, and maze-like phreatic tunnels which reflect their eogenetic, dissolutional origins. Modification by wave action may erase some of these distinct features, but their overall morphology is still distinct from a cave that has formed from wave energy alone. This study compared 14 Holocene eolianite coastal caves, in which relict flank margin caves could not exist, with 27 Late Pleistocene eolianite coastal caves, in which flank margin caves could possibly exist. The data demonstrate that a morphologic analysis of true sea caves versus breached flank margin caves in the Bahamas allows differentiation between these two types of coastal caves. The data additionally indicate that other factors, such as angle of wave attack and wave refraction, may control sea cave morphology.