Petrography and stable isotope geochemistry of Upper Cretaceous carbonate concretions and veins in the Cariblanco Formation of south-central Puerto Rico

This work describes the petrography and stable isotope geochemistry of concretions and calcite veins present in various lithologies of the Upper Cretaceous Cariblanco Formation in south-central Puerto Rico and postulates the geochemical conditions that existed during fracture development and subsequent calcite mineralization. In general, the two diagenetic trends shown by concretion components and vein calcite show progressive δ18O depletion and δ13C enrichment. δ18O depletion in both cases resulted from progressive warming of circulating seawater as magma emplacement proceeded and/or from the fluid interaction with the volcaniclastic materials evolving during burial diagenesis. The 13C-depleted CO₂ involved in calcite cementation in both concretion matrix and vein calcite is most likely sourced from the oxidation of methane. The source of the methane related to matrix formation is probably of biogenic origin, whereas the source of methane associated with the vein calcite cements is most likely of thermogenic origin. There is a marked difference between the oxygen isotopic composition seen in concretion components during early diagenesis and the oxygen isotopic compositions recorded by late concretion diagenetic cement phases as well as vein calcite precipitation. This indicates that the fluids and/or the temperature regime of the fluids precipitating the late concretion cements and vein calcite are very different. The early diagenetic fluids were most likely marine in origin and presumably circulated through the sediments as a result of thermally driven advection. The fluids precipitating vein calcite and late concretion cements were sourced deeper in the basin and were certainly warmer than earlier fluids, and it is likely that they had significant changes in their isotopic composition as they altered the volcaniclastic and volcanic materials in the basin. The presence of hydrocarbon fluid inclusions, the common fluorescence of vein calcites, and the exclusive association of the live hydrocarbon with calcite veins and late cements in concretions clearly indicate that this later fluid mobilized the hydrocarbons. A better knowledge of the source rock potential of the Cariblanco Formation will provide a better foundation for assessing the paradigm that no appreciable quantities of hydrocarbons exist in the Cretaceous rocks in and around Puerto Rico.