Bahamian Flank Margin Caves as Hypogene Caves

Abstract

Fieldwork in the Bahamian Archipelago in the 1970s and 1980s identified a new cave type, the flank margin cave, as macroscopic dissolutional voids developed in the margin of a freshwater lens, under the flank of the enclosing landmass. These voids are produced by three conditions that exist at the lens margin: mixing dissolution, organic decay horizons, and the increase in freshwater flow rate. The water flow enters flank margin caves as diffuse flow and exits as diffuse flow, a flow regime that produces dissolutional sculpture lacking turbulent flow features, such as asymmetric scallops. The caves are tied to sea level, which controls the freshwater lens position, and as such are excellent indicators of past sea-level position. The caves form without entrances and become accessible only after subaerial erosion has breached their ceilings or walls. Flank margin caves initiate as individual globular dissolutional voids that then intersect as the voids enlarge, increasing cave size in a sudden stepwise manner. As cave development is restricted to the lens margin, the largest flank margin caves acquire a linear shape as voids interconnect parallel to the lens margin. Flank margin caves are hypogene caves based on their diffuse, slow-flow regimes, because the dissolutional aggressiveness is generated below the surface by mixing, and the ascending marine water following the base of the lens to the site of dissolution at the lens margin. Because these caves form rapidly at shallow depths, there is a debate as to their hypogene classification, but they meet all criteria for hypogene speleogenesis.