Two distinct and short-lived flooding events of shallow marine waters covered western Amazonia during the Miocene.

The Cocinetas Basin of Colombia provides a valuable window into the geological and paleontological history of northern South America during the Neogene. Two major findings provide new insights into the Neogene history of this Cocinetas Basin: (1) a formal re-description of the Jimol and Castilletes formations, including a revised contact; and (2) chronostratigraphy presented here provides the necessary background information to explore the complete evolutionary and biogeographic significance of the excellent fossil record of the Cocinetas Basin.

The Neogene sedimentary fill of the Cocinetas Basin in northern Colombia preserves a rich record of marine invertebrates and can be analyzed in the context of a high-resolution stratigraphy and excellent chronostratigraphy. Molluscan fossils are highly diverse and often well preserved, offering a window into the rapidly changing paleoenvironments and biogeography of northern South America during parts of the Early to Middle Miocene and latest Pliocene to Pleistocene. Before the evolutionary and biogeographic implications of these fossils can be understood, however, their associated depositional environments and geologic ages must be determined. Here, we present preliminary results from paleoenvironmental, biostratigraphic, and strontium isotope chronostratigraphic analyses of sediments and fossils from the Uitpa, Jimol, Castilletes, and Ware formations found in Cocinetas Basin. The basal unit in the Neogene succession, the Uitpa Formation, comprises mudstones redeposited sandstones and molluscs typical of bathyal to outer shelf environments at its base. It is a shallowing-up sequence and is conformable with the overlying Jimol Formation, which comprises coarsegrained lithic calcarenite, coquina, and mudstone that represent a regressive-transgressive-regressive sequence. This sequence includes foreshore and transition zone through lower inner shelf environments, but generally poorly preserved invertebrate assemblages. The conformably overlying Castilletes Formation contains a varied suite of depositional environments with better-developed shell beds and thicker successions of intervening siltstone. A significant unconformity exists between the Castilletes Formation and the overlying Ware Formation, which represents a deltaic to coastal shoreface deposition environment, rich in shallow marine molluscs from a variety of ecotopes. Biostratigraphic assessment and strontium isotopic results from the Jimol and Castilletes formations indicate that these units contain fossils of latest Early Miocene through Middle Miocene age, while those of the Ware are approximately Late Pliocene in age. These results help to place the shallow marine assemblages of Cocinetas Basin into a wider geologic context that aids our understanding of how these faunas relate to the broader evolutionary and biogeographic history of the southern Caribbean during the Neogene. Additionally, the isotope dating and paleoecology of this fauna help to place cooccurring terrestrial and aquatic vertebrate assemblages into a local paleoenvironmental and chronostratigraphic framework.