Pioneer nannofossil assemblages from the initial transgression of the Niobrara seaway in the Turonian, San Juan Basin, New Mexico, USA

Abstract

Abstract Nannofossils of the North American Late Cretaceous Western Interior Seaway have been extensively studied, but these investigations have primarily focused on the diverse communities of the Niobrara and Greenhorn sea-level highstands. We lack a firm understanding of the nature of early nannofossil communities from the initial stages of the transgression, and of how the diverse assemblages of the Coniacian–Santonian became established. We investigated nannofossils in a core from the San Juan Basin, New Mexico, with concentrated sampling across this transgression. We then compared nannofossil assemblages from the initial transgression of the seaway to the more established, cosmopolitan assemblages of the Niobrara highstand. The initial stages of transgression were characterized by unusual, low-diversity assemblages dominated by holococcoliths and seldom reported and previously undescribed species of hetero- and holococcoliths. Herein, we describe five new species and one new genus from these assemblages: Ahmuellerella frankiae sp. nov., Gartnerago waszczakii sp. nov., Orastrum schollei sp. nov., Pharus clarescopoli sp. nov., and Wisea sanjuanensis gen. et sp. nov. These initial assemblages transitioned to more cosmopolitan, higher-diversity assemblages, coincident with an unconformity that likely represents subaqueous erosion or prolonged non-deposition. Above this unconformity, the nannoplankton were abundant, diverse, and characterized by open-marine, cosmopolitan taxa that persisted through the Coniacian and Santonian. X-ray fluorescence data collected from the core show a high abundance of the detrital elements Al and Ti in the section, indicating substantial detrital clay input into the area, coincident with deposition of the holococcolith-dominated assemblages. The detrital influence lessened as cosmopolitan nannoplankton became more abundant in the area. We suggest that the initial holococcolith-rich assemblages are representative of a shallow-marine, potentially hyposaline, turbid or unstable environment. Such assemblages have been observed from the Late Turonian elsewhere in northern New Mexico and may represent typical pioneer nannofloras of the Late Cretaceous Western Interior marine transgressions. Our integrated investigation of nannofossils and X-ray fluorescence geochemistry adds to the meager knowledge base of pioneer nannofloral communities in the initial phases of continental marine transgression.