PALEOECOLOGY OF THE MAASTRICHTIAN EL RAYO FORMATION, SOUTHWESTERN PUERTO RICO

Puerto Rico, the easternmost island of the Caribbean Greater Antilles, is a complex island arc of accreted terranes with a geologic record of ophiolite, volcanic, volcaniclastic, and sedimentary rocks extending from the Early Jurassic. Three volcanic/tectonic events characterize the Cretaceous in southwestern Puerto Rico with each event followed by subsidence and/or rise in sea level and deposition of carbonate platforms. A Lower Maastrichtian volcanic event ended carbonate deposition of the Guaniquilla Limestone, and an ensuing transgression resulted in the deposition of volcaniclastic sandstones and subsequent massive to bedded limestone lenses of the El Rayo Formation. Sites in the San German and Sabana Grande quadrangles in southwestern Puerto Rico were examined for fossils, lithology, carbon/sulfur geochemistry and stratigraphy with the objective of characterizing the paleoecology of the carbonate deposits within the El Ray Formation. Fossils identified were predominantly rudists and corals, which dominated back reef and reef sites. Silicified sponges dominated an eastern site in a slope setting. Carbon/sulfur geochemistry analyses of samples from one site yielded low levels of organic carbon for limestones samples, but high levels of organic carbon for shales. The sulfur levels were extremely low suggesting there was no input of sulfur from volcanic activity. Age of the El Rayo Formation, based on identification of the ammonite Eubaculites labyrinthicus by W. Cobban (personal communication, 2001), is suggested to be high in the Maastrichtian, with Parastroma guitarti and Titanosarcolites giganteus as additional age diagnostic fossils. The depositional environment of the El Rayo Formation is characterized by the development of a shallow-water carbonate platform deposited over a volcanic, volcaniclastic, and siliciclastic base. The first phase consisted of volcanic, volcaniclastic, and siliciclastic deposition exposed at three sites with an increase in complexity of lithologic units from west to east. The second phase consisted of relatively pure carbonates. Paleoecologic indicators show carbonates with a general trend of increasing water depth from west to east, though the slope of the platform is unknown across the study area.