Diagenesis of the Oligocene-Miocene Bluff Formation of the Cayman Islands: A petrographic and hydrogeochemical approach

Dolostones of the Oligocene-Miocene Bluff Formation on the Cayman Islands serve as host for major fresh water lenses (4-15 km;Based on evidence from aragonite dissolution, mineralogy, petrography and isotopic geochemistry, dolomitization of the Bluff Formation probably occurred at two separate times. The two phases of dolomitization replaced the metastable carbonates of the Oligocene Cayman Member and Miocene Pedro Castle Member shortly after the deposition of their respective sediments. Dolomitizing fluids of mixed marine-fresh water were delivered by gravitational flow, downward reflux, upward convection and tidal pumping.;The Bluff Formation has been affected by at least three phases of karstification. The extent of karsting is controlled by past sea levels, paleoclimatic regimes and antecedent tectonic fracturing.;Moldic cavities, caves and joints are commonly filled by numerous phases of dolomite and calcite cements, dolomitized caymanite, dolomitized skeletal grainstone, speleothemic calcite and terra rossa. The complex distribution of diagenetic fabrics is attributed to successive cycles of regressions and transgressions, variation in hydrochemistry, and activity of micro-organisms. A paragenetic sequence of cements suggesting increase in the degree of meteoric influence is clear or zoned dolomite, alternating calcite and dolomite, and clear or zoned sparry calcite.;Detailed analysis of the ground water indicates that saturation states with respect to calcite and dolomite generally increase with increasing salinity. Carbonate cement distribution relating to present day hydrochemistry suggests a progressive decrease in fluid salinity from dolomite to calcite precipitation. This relationship is in agreement with...