CRETACEOUS CARBONATE SEQUENCES IN THE CENTRAL IGNEOUS PROVINCE OF PUERTO RICO: FURTHERING AN UNDERSTANDING OF EVOLUTIONARY PALEOECOLOGY IN THE TROPICS

Geologic data from low latitude, shallow-water carbonate sequences strengthen our knowledge of the evolutionary history of tropical life, and inform on future processes of evolution in the tropics. Yet paleotropical data assimilated from carbonate rocks can remain largely unexplored because of the faulting, recrystallization, and etching that occur frequently in the densely vegetated Caribbean. Our analysis is focused on the integrated stratigraphy, sedimentology, paleoecology and geochemistry derived from low-latitude, middle Cretaceous carbonate rocks and fossils from the island of Puerto Rico in the Caribbean Sea. We investigate the Río Matón Limestone Member of the Robles Formation, the Barrancas Limestone Member of the Magueyes Formation, and the Aguas Buenas Limestone of the Torrecilla Breccia. Each discontinuous carbonate member extends more than 40m in stratigraphic thickness, and is surrounded by igneous, volcaniclastic and siliciclastic rocks of the formation. Basal carbonates were rarely free of volcanic or detrital clasts. Lithologies with greater than 50% siliciclastic input yielded TOC/TS ratios that allowed for interpretation of rare brackish and hypersaline intervals within the marine depositional sequences. Brackish water deposits occurred more frequently in the lower to middle parts of stratigraphic sections, whereas sustained carbonate deposition with decreasing volcaniclastic and siliciclastic input, and thick-to massive-bedded packstones with moderately abundant gastropods and rudists, indicated deposition in deepening marine paleoenvironments upsection. Relative stratigraphic position, macro- and microfossil biostratigraphy, and published Sr isotope interpretations from selected rudist bivalve fossils converge on an Albian age. Together, the age, geochemical signatures, and overall deepening sequence enhance potential for correlation of tropical settings, such as the ones presented here from Puerto Rico, to published Gulf Coast deposits citing evidence for a regional Albian sea level rise. This work advances an understanding of the evolutionary paleoecology of important reef builders of the middle Cretaceous greenhouse world.