STRONTIUM ISOTOPE STRATIGRAPHY FROM KUPHUS INCRASSATUS, CENOZOIC LIMESTONES, PUERTO RICO

Low magnesium calcite shells of Kuphus incrassatus were collected from basal units of the Lares Limestone and Ponce Formation, northern and southern Puerto Rico, respectively. Petrographic examination in plane polarized light, cathodoluminescence, trace element and stable isotope geochemistry identified diagenetically altered shell areas. Eleven calcium carbonate samples were collected from bivalve shell regions with lowest probability of alteration. ⁸⁷Sr/⁸⁶Sr isotopic compositions were converted to numerical ages to obtain the age of shell formation and an estimate of the age of the limestone where the Kuphus shells were in original growth position. Northern coast ⁸⁷Sr/⁸⁶Sr isotopic values from Kuphus collected above the San Sebastián - Lares Limestone contact range from 0.708087 to 0.708105 and span the interval from 26.58 to 27.17 Ma. Based on foraminiferal assemblages and a sequence boundary, the age of this contact was previously estimated as Late Oligocene. The strontium isotope data are consistent with the previous age assignment and increase the chronologic resolution. Southern coast ⁸⁷Sr/⁸⁶Sr isotopic values from shells collected above the contact between the Ponce Formation and the Angola Limestone equivalent range from 0.708889 to 0.708880 and suggest deposition from 9.92 to 11.56 Ma. ⁸⁷Sr/⁸⁶Sr values are again consistent with previous work and refine the chronologic resolution of the contact from the Middle Miocene based on foraminiferal assemblages to the earliest Late Miocene. This preliminary study suggests that strontium isotope values extracted from Kuphus incrassatus shells can be used successfully to better constrain the chronostratigraphy of Cenozoic limestones in Puerto Rico.