Diffuse Versus Conduit Flow in Coastal Karst Aquifers: The Consequences of Island Area and Perimeter Relationships

The majority of limestone islands are made of eogenetic carbonate rock, with intrinsic high porosity and permeability. The freshwater lenses of small islands are dominated by diffuse flow regimes as the island perimeter is everywhere close to the meteoric catchment of the island interior. This flow regime produces flank margin caves at the lens margin, where dissolution is enhanced by mixing corrosion, superposition of organic decay horizons and higher flow velocities as the lens thins. The lens interior develops touching-vug flow systems that result in enhanced permeability and lens thinning over time. As islands become larger, the area (meteoric catchment) goes up by the square, but the island perimeter (discharge zone) goes up linearly; diffuse flow becomes inefficient; conduit flow develops to produce traditional epigenic cave systems that discharge the freshwater lens by specific turbulent flow routes, which in turn are fed by diffuse flow in the island interior. Locally, diffuse flow to the island perimeter continues in coastal proximal areas between major conduit flow routes to produce flank margin caves. The Bahamian Archipelago represents a case history in which tectonics is limited, the rocks are entirely eogenetic and the diffuse to conduit flow transition is demonstrated.

Paleokarstic macroporosity development at platform margins: Lessons from the Paleocene of north Spain

Platform margins have been targets for carbonate exploration because they are commonly seismically resolvable and reservoir prone for several critical reasons including karstic porosity. Platform margin karst models, mainly based on the Quaternary of the Caribbean (tropical humid, including “flank margin caves”) are well documented, but analogs remain rarely identified in the stratigraphic record. Analysis of a remarkably well-exposed macroporosity paleokarst system from the Paleocene of north Spain, formed under an arid to semiarid climate, provided a model that differs from the Caribbean template. Implications for exploration and appraisal strategies, include provisos regarding (1) how early fracture intensity can be estimated at platform margins, (2) the absence of flank margin caves at the margin, (3) the absence of karstic features at or immediately below the main lowstand surface, (4) the presence of cave-free corridors such that the porosity zones could be missed completely, and (5) the stratigraphic inheritance of caves into successive levels adjacent to the cave-free zones. Quantitative data were evaluated on the main macroporous intervals. We also compared this model with other paleokarst models in the literature to emphasize the diversity of approaches that can be used to evaluate paleokarst targets: “one size does not fit all.”