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Upper Cretaceous (Coniacian to Maestrichtian) pelagic limestones were recovered from the lower parts of several cores from Leg 15 of the Deep Sea Drilling Project. The limestones are characterized by a nanno-mosaic matrix and by the presence of microspar and spar infilling of foraminifera and radiolaria chambers. In the lower 50 meters of the limestone sections, there are indications of neomorphic processes, including the secondary grain-growth of micrite. Microstylolites and replacement veins of sparry calcite are also evident in the lower parts of the sections. Faunal composition and geologic setting indicate deposition in a deep-sea environment at depths that may have been close to the CaCθ3 compensation depth. This may preclude the presence of metastable carbonate phases (aragonite and high Mg-calcite) in the primary sediment. The limestones grade upward into indurated chalks and marls, and there is no evidence for uplift and exposure of the limestones to shallow-water or fresh-water conditions. Therefore, cementation in a deep-sea environment is indicated. The pelagic limestones from Leg 15 are underlain by partly intrusive basalt flows, which contain included blocks of re crystallized carbonates, and by intrusive dolerites. The occurrence of repeated basaltic ash beds above the flows and sills implies a nearly simultaneous series of basaltic eruptions over a wide area in the Caribbean during Coniacian to early Santonian time (Donnelly et al., this volume). Lithified carbonate sediments dredged from the sea floor are often associated with submarine igneous rocks and vulcanogenic sediments (Saito et al., 1966; Cifelli et al., 1966; Thompson et al., 1968; Milliman, 1971). Bonatti (1966) has suggested that the occurrence of apparently authigenic calcite grains dispersed in pelagic zeolite clays in several cores from the Pacific can be attributed to the effects of local submarine igneous activity. Submarine igneous activity can promote the cementation of carbonate sediments in several ways. First of all, an increase in temperature resulting from local igneous activity could lead to the precipitation of CaCC>3 from interstitial pore water since the solubility of CaCC>3 decreases with temperature. Submarine weathering of volcanics and of ash would lead to an increase of both Ca + concentration and pH value in pore waters, enhancing the possibility of carbonate precipitation. Another effect may be the dissolution of primary carbonate sediment by acidic volatiles degassing from magma, followed by the precipitation of

Stable Isotope Relationships in Pelagic Limestones from the Central Caribbean, Leg 15, Deep Sea Drilling Project