High-Resolution Calcareous Nannofossil Biostratigraphy of the Santonian Stage Boundary, Western Interior Basin

Blair, Stacie A.¹, David K. Watkins¹ (1) University of Nebraska-Lincoln, Lincoln, NE

The Ten Mile Creek area (Dallas, TX) is a proposed Global Stratotype Section and Point (GSSP) candidate for the Coniacian/Santonian stage boundary. The Santonian Working Group has nominated the speciation of Inoceramus (Cladoceramus) undulatoplicatus as the diagnostic macrofossil bioevent for the base of the Santonian stage. Calcareous nannofossils were examined from sediments of the Bruceville Marl at the Ten Mile Creek proposed GSSP site and from well-preserved sediments of the coeval Smoky Hill Member type area (northwestern Kansas). Nannofossil bioevents were correlated with the lowest stratigraphic occurrence of I. undulatoplicatus to create a high resolution nannofossil biostratigraphic framework and stratigraphic proxy for the Coniacian/Santonian stage transition.

Six nannofossil bioevents are useful for recognition of the upper Coniacian/lower Santonian transition within the Bruceville Marl and Smoky Hill Member. The first appearance datums (FADs) of Prediscosphaera grandis and Amphizygus n. sp. A, as well as the FAD of two rare taxa, Orastrum campanensis and Tortolithus n. sp. A, are concurrent with the lowest stratigraphic occurrence of I. undulatoplicatus. In addition, two nannofloral acmes occur near the boundary: Watznaueria quadriradiata and Zeugrhabdotus scutula.

The succession of nannofossil bioevents noted from these two outcrop sections within the Western Interior Basin provide a link for correlation with more coarsely-resolved nannofossil schemes developed from cored, deep-water sections.