, Auditorium
Epigenic continental carbonate aquifers are usually telogenetic and have long been known to self modify through time, developing a hierarchy of permeability ending in conduit flow. Island carbonate aquifers are commonly eogenetic with substantial primary porosity that reorganizes into touching-vug permeability. If allogenic catchment is available, or island size exceeds ~40 km (an area to perimeter phenomena), conduit flow can initiate. The successful use of the freshwater lens as a potable water source requires procedures different from inland continental locations.
As touching vug permeability increases, the freshwater lens thins, moving the halocline or mixing zone upward in the island section. Changes in meteoric flux also control lens thickness. At the same time, increasing void development causes the margin of the lens to step backward into the island, and void development continues. Base level can change quickly due to tectonics or glacialeustasy, and the island perimeter may display specific horizons of flank margin cave development that correlate with static lens positions.
The top and base of the freshwater lens are density interfaces that collect organics. The lens base commonly becomes anoxic, and oceanic sulfate is biomediated to sulfide. Sulfate is a dolomite precipitation poison, its removal at the lens base results in a preferential location for dolomitization. The movement of the lens by internal processes and outside forces can lead to an interleaving of limestone and dolomite horizons.
John E. Mylroie and Joan R. Mylroie
Department of Geosciences
Mississippi State University