ALBERT

Description: Geosciences, Vol. 8, Pages 268: Diffuse Versus Conduit Flow in Coastal Karst Aquifers: The Consequences of Island Area and Perimeter Relationships Geosciences doi: 10.3390/geosciences8070268 Authors: Erik B. Larson John E. Mylroie 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.

Description: Remnant caves formed within the dolomitized Seroe Domi Formation on Curaçao were documented in order to determine their origin. Petrographic thin sections and geochemical analyses of samples from outcrops and cave wall rock agree with previous interpretations of a mixing zone origin for the replacement dolomite. Further, stable isotope analysis suggests migration of fluids from the underlying basalts along fracture networks in the Seroe Domi Formation. Most of the caves represent remnant phreatic caves based on morphology; however, a number of pseudokarst features were identified. Minimum cliff retreat rates were estimated (5 mm/1000 yrs to 52 mm/1000 yrs) using cave morphologies and elevations. These remnant caves, many of which were previously overlooked, have been used as indicators of diagenetic events within the Seroe Domi Formation and contain significant information when placed in the proper geologic framework. Key words: Flank Margin Cave, Dolomitization, Seroe Domi Formation, Curaçao. Jame v dolomitiziranem profilu formacije Seroe Domin na otoku Curaçao, Nizozemski Antili Dokumentirali smo ostanke jam v dolomitizirani formaciji Seroe Domi na otoku Curaçao in raziskovali njihov nastanek. Petrografske analize zbruskov ter geokemiče analize površinskih izdankov in vzorcev jamskih sten, potrjujejo predhodno domnevo o izvoru (replacement) dolomita v območju mešanja slane in sladke vode. Analiza stabilnih izotopov kaže na prenos tekočin iz bazaltne podlage vzdolž mreže razpok v formaciji Seroe Domi. Oblika večine jam kaže, da gre za dele freatičnih sistemov, odkrili pa smo tudi več psevdokraških pojavov. Morfološke značilnosti jam in njihova nadmorska višina sta nam omogočili oceno minimalne hitrosti umikanja otoških klifov, ki je med 5 m in 52 mm na tisoč let. Ostanki jam, ki so bili prej največkrat spregledani, so kazalci diagenetskih dogodkov v formaciji Seroe Domi in vsebujejo druge pomembne informacije, če jih le postavimo v ustrezen geološki okvir. Ključne besede: Jame tipa “flank margin”, dolomitizacija, formacija Seroe Domi, Curaçao.