CONCRETIONARY PALEOECOLOGY: PALEOCEANOGRAPHY AND FORMATION OF A BACULITES ELIASI CONCRETION FROM THE LATE CRETACEOUS WESTERN INTERIOR SEAWAY

The Western Interior Seaway (WIS) represents an important and rapidly changing ecological domain during the Late Cretaceous. Faunal assemblages contained within concretions are a pervasive mode of fossilization within the WIS, and typically contain well-preserved and diverse marine fauna. The goal of this study was to assess local environmental conditions (including paleocurrent direction) using an in situ embedded sub-tabular concretion from the Baculites eliasi zone of the Late Cretaceous (Campanian) WIS in Montana. We used a comparative approach, utilizing both computed tomography (CT) and systematic manual dismantling of the concretion to investigate the orientations of the fossils as a potential indicator of paleocurrent direction. From both the CT scan and systematic manual dismantling, a rose diagram was produced indicating paleocurrent direction or post-depositional migration of fossilized individuals during concretion formation. Additionally, several petrographic thin sections were made of each observed fossil-rich zone in the concretion to investigate microfauna and concretion formation. Most fossils found were Baculites oriented in a NE-SW direction longitudinally. No statistical difference was found between the CT scan and systematic manual dismantling. Systematic manual dismantling produced a larger number of Baculites individuals than were observed in the CT scan. We discuss further implications of the studied concretion on paleoenvironmental methodology and paleoenvironment of the WIS.