High-resolution calcareous nannofossil biostratigraphy for the Coniacian/Santonian Stage boundary, Western Interior Basin

Abstract

The Ten Mile Creek area (Dallas, Texas) is a proposed Global Stratotype Section and Point (GSSP) candidate for the Coniacian/Santonian Stage boundary. The Santonian Working Group has designated the first appearance 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 proposed GSSP site and from well-preserved sediments of the coeval Smoky Hill Member-type area (northwestern Kansas) of the Niobrara Formation. Nannofossil bioevents were correlated with the lowest stratigraphic occurrence of I. undulatoplicatus to create a high resolution biostratigraphic framework and stratigraphic proxy for the Coniacian/Santonian Stage transition. Six bioevents are useful for recognition of the Coniacian/Santonian transition within the Bruceville Marl and Smoky Hill Member. The first appearance datums (FADs) of Prediscosphaera desiderograndis, n. sp. and Amphizygus megalops, n. sp. as well as the FADs of two rare taxa, Orastrum campanensis and Tortolithus dodekachelyon, n. sp., are in close stratigraphic proximity to the lowest occurrence of I. undulatoplicatus. In addition, two nannofloral acmes occur near the boundary: Watznaueria quadriradiata and Zeugrhabdotus scutula. This study describes eight new species from the Smoky Hill Chalk type area; Amphizygus megalops, Bifidalithus phenax, Pharus evanescens, Gartnerago margaritatus, Helicolithus tectufissus, Tortolithus dodekachelyon, Prediscosphaera desiderograndis and Helicolithus varolii. Light microscope images are provided for rare and well-preserved specimens of Reinhardtites clavicaviformis Varol, 1991, Orastrum campanensis (Cepek) Wind & Wise, 1977, Rhombolithus rhombicum (Bukry) Black, 1973, and Gartnerago clarusora Varol, 1991. This study extends the ranges of several species from those documented in previous literature.