The Origin of Dolomites in Tertiary Sediments from the Margin of Great Bahama Bank

Based on an integrated geochemical characterized by extremely high Sr and petrographic investigation of dolomites concentrations, which reflect high from two cores drilled on Great Bahama concentrations of Sr in the pore fluids. Bank, we have determined three different The high concentrations of Sr in the pore mechanisms of formation for the dolomites fluids arise through the continued which are common throughout the Pliocene recrystallization of meta-stable aragonite and and Miocene aged portions of these cores. high-Mg calcite to dolomite and LMC driven The first mechanism of dolomitization occurs by the oxidation of organic material by in association with development of nonsulfate. Sulfate reduction not only provides depositional surfaces. Dolomite typically the thermodynamic drive for recrystallization, forms below each of these surfaces, the but as the absolute concentration of strontium concentration and extent of which is governed in the pore fluids is governed by the solubility by the length of the period of non-deposition. of celestite, allows the Sr /Ca ratio of the These dolomites are recognized by their interstitial fluid to become much higher than association with the non-depositional normally encountered. The final type of surfaces, characteristic heavy oxygen isotopes dolomite is a massive dolomite which occurs indicative of formation from cold bottom in coarse grained reefal sediments. The waters, and δ O and Sr profiles with depth 18 which suggest formation in the presence of diffusive temperature and Sr gradients. The second mechanism of dolomitization, occurs in pore fluids where the cation and anion profiles are governed by diffusive processes and forms what we term background dolomite. This is a microsucrosic dolomite and forms both by the recrystallization of the existing sediment and precipitation directly into void space. Dolomitization by this mechanism uses a local source of Mg and 2+ consequently the dolomite never comprises more than between 5 and 10% of the sediment. This type of dolomite is c 2+