THE UTILITY OF KUPHUS INCRASSATUS BIVALVES FOR DETERMINING ABSOLUTE AGES AND SHALLOW WATER MARINE ENVIRONMENTS IN TERTIARY CARBONATE AND SILICICLASTIC SYSTEMS IN THE CARIBBEAN

Kuphus incrassatus bivalves, composed of low-magnesium calcite, are common in many Tertiary sedimentary systems in the Caribbean. They are proving to be of considerable value for providing reliable ⁸⁷Sr/⁸⁶Sr isotopic values for absolute age dating of sequences and determining depositional environments. Little previous work has concentrated on depositional environment aspects indicated by Kuphus. The focus of our study is to document various occurrences of in-situ Kuphus to better understand and constrain environments and water depths, and to use Kuphus to provide absolute age dates. The initial results presented here come from study sites along the north and south coasts of Puerto Rico and northwestern Dominican Republic. For depositional environment analyses, sizes, orientations, morphology (presence or absence of siphons and their sizes) and grouping (isolated or clusters) of Kuphus were documented, as were facies, sedimentary structures, and associated faunal constituents. Kuphus mostly occur in bedded gravel and sandstone facies associated with shoreface to beach environments in the Dominican Republic. In Puerto Rico, Kuphus mostly occur in packstones-grainstones related with high-energy shallow water; some occur in carbonate breccias deposited in very shallow water. The data from both initial study areas, and reconnaissance study of other areas, demonstrate shallow to very shallow water environments for Kuphus. Ongoing studies are aimed at further constraining environments and water depths. Strontium isotopes from Kuphus incrassatus were used for age dating. Petrographic data, and oxygen and carbon stable isotopes were used to determine and avoid diagenetically altered samples prior to strontium analysis. Puerto Rican northern coast ⁸⁷Sr/⁸⁶Sr isotopic values from Kuphus indicate Oligocene-Miocene ages, and Puerto Rican southern coast indicate Middle to Late Miocene ages. ⁸⁷Sr/⁸⁶Sr isotopic values from northwestern Dominican Republic indicate a Late Miocene age. Kuphus incrassatus are proving valuable for constraining the chronology of Puerto Rico and Dominican Republic as well as improve environmental interpretations.