87Sr/86Sr-derived mean ages from low-Mg calcite Kuphus incrassatus and Ostrea haitensis bivalves provide an updated and refined chronostratigraphy for selected Oligocene-Miocene carbonate and siliciclastic units in Puerto Rico and the Dominican Republic. Results indicate ages of middle to late Oligocene for the San Sebastian Formation (ca. 29.78–26.51 Ma), the Lares Limestone (ca. 26.51–24.73 Ma), and the Montebello Member (ca. 27.30–24.10 Ma); middle Miocene to the early part of the late Miocene for the Cibao Formation (ca. 12.17 Ma), the Aguada (Los Puertos) Limestone (ca. 14.67–11.14 Ma), and the Aymamón Limestone (ca. 10.98 Ma) in northern Puerto Rico as well as for the Ponce Limestone (ca. 14.97–9.84 Ma) in southern Puerto Rico and the Yanigua–Los Haitises Formations (ca. 15.75–12.58 Ma) in northeastern Dominican Republic; and late Miocene for the Cercado Formation (ca. 6.31–5.88 Ma) in northwestern Dominican Republic. These results show some significant modifications to previous chronostratigraphic studies. The San Sebastian Formation marks the last major input of siliciclastics in Puerto Rico. The Lares Limestone is characterized by a diverse warm- and cool-water coral assemblage, whereas the Montebello Member is characterized by large benthic foraminifera and mollusks, which likely indicates differences in depth of deposition or environments that were differentially influenced by upwelling. The ultimate disappearance of warm-water corals in the late Oligocene coincides with the end of global warming and coral extinction events. The Cibao Formation, the Aguada (Los Puertos) Limestone, the Aymamón Limestone, the Ponce Limestone, and the Yanigua–Los Haitises Formations are predominantly composed of shallow marine deposits consisting of red algae, mollusks, small benthic foraminifera, and cool-water corals in the upper parts. These characteristics are consistent with an upwelling control, which has been documented as a regionally important process in the Caribbean during that time. The reappearance of corals indicates an environmental change that coincides with closure of the Central American Seaway. The closure resulted in circulation changes, warm temperatures, and low nutrients in the Caribbean, which created suitable conditions for diverse coral reef development, as exemplified by the Cercado Formation.

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