EOGENETIC KARST: A DECADE LATER
The top of the fresh-water lens was originally presented as an speleogenetic environment as a result of vadose/phreatic fresh-water mixing, primarily based on banana hole development in the Bahamas. The geochemical evidence for dissolutional aggressivity at the top of the lens was equivocal, and banana holes have been reinterpreted as flank margin caves sequentially developing in a migrating lens margin hosted by a prograding strand plain. Despite both field and model evidence of large permeability increases in carbonate rocks throughout the fresh-water lens as touching-vug porosity, mega porosity is restricted to the lens margin in islands too small to develop conduit flow. In larger islands (and continental carbonate coasts such as the Yucatan), conduit cave and flank margin cave mega porosity co-exist as independent flow systems. Conduit flow systems are a major contributor to progradational collapse systems, which produce most subaerial caves on Bermuda and most blue holes in the Bahamas. Conduit/lens interactions remain a research frontier, especially where the conduit is perched on non-carbonates before reaching the lens.
The flank margin cave morphological pattern is the result of volumetric dissolution, hosted in eogenetic rocks by the high value (~30%) of primary porosity. In telogenetic rocks, where such 3-D dissolution occurs it is the result of the carbonate rock having conditions that mimic eogenetic rocks. In Croatia this mimicry is achieved by a paleotalus facies, in New Zealand by a high density fracture system produced by tectonics. The New Zealand example is instructive, as the telogenetic rocks involved there are Oligocene, in tectonically quiescent Florida the same age rocks are eogenetic.