CAVE MORPHOLOGY AND CARBONATE PETROGRAPHY AS INDICATORS OF LARGE FLANK MARGIN CAVE SPELEOGENESIS IN THE BAHAMAS

Several large flank margin caves from the Bahamas were investigated with respect to their size, elevation, geometric parameters, relation to topography and petrography of the enclosing carbonate eolian ridges. The hosting calcarenites are characterized by high primary porosity (up to 35%) and weak cementation of grains. Large flank margin caves form as the result of the intersection of small and medium-sized flank margin caves in the fresh-water lens margin. Once the caves become large enough, so that their dimensions are comparable with the hosting land mass, the main controlling factor in their development becomes the overlying topography. If they form under the flank of the enclosing ridge, they tend to be linear, whereas if they form under the terminus or nose of the dune, they will wrap around the eolianite, mimicking the former position of the area of maximum dissolution in the margin of the fresh-water lens. Phreatic ceilings found at over 12 m elevation can be explained by local geological and hydrological conditions. If a paleosol exists in the eolianite ridge, it will control the behavior of the fresh-water lens, and thus the morphology of the cave in terms of lateral and vertical expansion. The ramping of the water against the relatively impermeable paleosol will distort the fresh-water lens, resulting in higher passage ceilings being created landward. Flow within the lens is diverted parallel to this barrier resulting in very linear passage morphologies. Transient inflation of the fresh-water lens due to storm events can determine oscillations in the water table that would also create dissolution ceilings at elevations higher than 6 m. The diagenetic history of the host eogenetic carbonate eolianites is short but complex nonetheless. As the system evolves, a rearrangement of porosity occurs, usually leading to increased permeability due to the development of secondary touching-vug porosity. Several meteoric (vadose and phreatic) overprints can be identified in the rock samples but the succession of events and climatic setting cannot be determined, as the composition and abundance of cements varies from sample to sample and from site to site. The amount of carbonate removed is a result of the action of a very powerful dissolution mechanism upon a diagenetically immature carbonate rock.