PIONEER NANNOFOSSIL ASSEMBLAGES DURING THE INITIAL TRANSGRESSION OF THE NIOBRARA SEAWAY IN THE TURONIAN, SAN JUAN BASIN, NEW MEXICO

Nannofossils of the Cretaceous Western Interior Seaway have been extensively studied, but these investigations have primarily focused on the Niobrara and Greenhorn sea level highstands. We lack a firm understanding of the nature of early nannofossil communities in the initial stages of transgression of marine waters onto the continent. Here, we present our study on nannofossils from a core in the San Juan Basin, New Mexico. In this study, we compared nannofossil assemblages in the Turonian Carlile Formation to the more established communities of the Coniacian and Santonian Mancos/Niobrara highstand. The initial stages of transgression were marked by a rapid increase in abundance, with the earliest assemblages exhibiting low species richness dominated by holococcoliths. This assemblage also contained several undescribed species of hetero- and holococcoliths. This initial assemblage transitioned to a more cosmopolitan nannofossil assemblage coincident with an unconformity that likely represents subaqueous erosion or prolonged non-deposition. This unconformity is age-equivalent to the Fort Hays Member of the Niobrara, which may have been when the Tethyan water mass became more established on the continental shelf. Above this unconformity, nannofossils were abundant, diverse, and characterized by an open-marine assemblage that persisted through the Coniacian–Santonian. We suggest that the initial holococcolith assemblage was better adapted to a shallow marine, potentially hyposaline, turbid or unstable environment. X-Ray fluorescence data collected on the core shows high abundance of aluminum and other detrital elements in the section indicating substantial detrital clay input into the area at the time. This supports our hypothesis that this holococcolith-dominated assemblage represents a community that was adapted to high turbidity and possibly lower than marine salinities. The Turonian assemblage may be a unique endemic community or could represent a typical pioneer assemblage of Late Cretaceous marine incursions. Our integrated investigation of nannofossils and X-Ray fluorescence geochemistry sheds light on pioneer nannofloral communities in the initial phases of continental marine transgression.