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

Abstract

ABSTRACT Based on an integrated geochemical and petrographic investigation of dolomites from two cores drilled on Great Bahama Bank, we have determined three different environments of formation for the dolomites that are common throughout the Pliocene and Miocene parts of these cores. The first environment of dolomitization occurs in association with development of nondepositional surfaces. Dolomite typically forms below each of these surfaces, the concentration and extent of which is governed by the length of the period of nondeposition. These dolomites are recognized by their association with the nondepositional surfaces, characteristic positive 18O values indicative of formation from cold bottom waters, and 18O and Sr profiles with depth that suggest formation in the presence of diffusive temperature and Sr gradients. The second environment of dolomitization occurs in pore fluids where the cation and anion profiles are governed by diffusive processes. The dolomite forming here is termed background dolomite. This is a microsucrosic dolomite and forms both by recrystallization of the existing sediment and precipitation directly into void space. Dolomitization by this mechanism uses a local source of Mg2+, and consequently the dolomite never constitutes more than between 5 and 10% of the sediment. This type of dolomite is characterized by extremely high Sr concentrations, which reflect concentrations of Sr2+ in the pore fluids from which it formed. The high concentrations of Sr2+ in the pore fluids arise through the continued recrystallization of metastable aragonite and high-Mg calcite to dolomite and low-magnesium calcite driven by oxidation of organic material by sulfate. Not only does sulfate reduction provide an additional thermodynamic drive for recrystallization, but because the absolute concentration of Sr2+ in the pore fluids is governed by the solubility of celestite (SrSO4), removal of sulfate allows the Sr2+/Ca2+ ratio of the interstitial fluid to become much higher than normally encountered. The final type of environment of dolomitization is associated with coarse-grained reefal sediments. The pervasive nature of the dolomitization and the relatively normal Sr concentrations suggest the circulation of normal marine water in a relatively open system.