SCAPHITID AMMONITE BIOSTRATIGRAPHY FOR THE LOWER CALCAREOUS SHALE MEMBER OF THE NIOBRARA FORMATION AT PEMBINA GORGE, NORTH DAKOTA

The “calcareous shale” member of the lower Niobrara Formation is exposed in outcrops along the bluffs of Pembina Gorge in northeastern North Dakota. This stratigraphic unit represents the early stages of the Niobrara transgressive sequence and was deposited on the passive eastern margin of the Cretaceous Western Interior Seaway (WIS) in a shallow, neritic setting. Drillers informally refer to it as the “first speckled” shale owing to the white specks of carbonate disseminated throughout its dark, organic-rich matrix. Yet, despite its carbonate content, age diagnostic fossils and microfossils are scarce. Earlier work (Reiskind, 1986) tentatively assigned the lower calcareous shale member to the early Santonian based on the presence of rare specimens of the ammonite, Clioscaphites saxitonianus. This age assignment seemed to be supported by nannofossil biostratigraphy, which suggested a Santonian age for the overlying “chalky” member of the upper Niobrara Formation. However, uranium-lead radioisotope dating of zircons derived bentonite beds suggest a younger age (81.5 ± 0.5 Ma) for the overlying chalky member of the upper Niobrara Formation, placing it firmly within the early Campanian (Bamburak et al., 2016). To further complicate matters, specimens of the index fossil, Clioscaphites saxitonianus, are preserved as flattened compressions with no visible sutures, making taxonomic identification difficult. Work is currently underway to collect more ammonite specimens from the calcareous shale member of the lower Niobrara Formation at various localities throughout the Pembina Gorge area of North Dakota. Cursory sampling of these outcrops has already found scaphitid ammonites of two different sizes, which may constitute an example of sexual dimorphism. Another field excursion to the Pembina Gorge area of North Dakota is planned for early August of 2022 in an effort to carry out a more detailed study. A more definitive identification of the scaphitid species present within the calcareous shale member of the lower Niobrara Formation will provide sorely needed age constraints on the timing of the Niobrara Transgression in this area, as well as to solidify the temporal range represented by this stratigraphic unit.