WATER MASS DISTRIBUTION IN THE WESTERN INTERIOR SEAWAY DURING OCEANIC ANOXIC EVENT 2 BASED ON FORAMINIFERAL ASSEMBLAGES

The Western Interior Seaway (WIS) stretched from Utah to Kansas and from the Northwest Territories to Texas through the Late Cretaceous. Decades of foraminiferal and geochemical studies of marine deposits from this region, and the Cenomanian/Turonian boundary (CTB; 93.9 Ma) in particular, have identified patterns of foraminiferal assemblage change and a positive carbon isotope excursion that defines Oceanic Anoxic Event 2 (OAE2). These strata feature well constrained lithostratigraphy and macrofossil biostratigraphy and dateable bentonites (ash layers) that can be traced across the basin. In this study, we take advantage of these features to correlate equivalent stratigraphic units and to construct locality specific age models for OAE2 time slices. These timescales are used to evaluate how paleoenvironments changed before, during, and after OAE2 in different parts of the seaway and to estimate the timing of foraminiferal bio-events including the ‘Benthonic Zone’, the extinction of Rotalipora cushmani, the ‘Heterohelix shift’, the ‘Gavelinella acme’, and ‘Neobulimina dominance’. We compare records from five WIS sites: Big Water, Utah; Billings, Montana; Rock Canyon, Colorado; Carthage, New Mexico; and northeastern Kansas. Our comparison reveals north-south diachroneity, as well as some east-west isochroneity of bio-events. For example, the Benthonic Zone, Heterohelix shift, and Gavelinella acme are (nearly) isochronous across the southern WIS, while Neobulimina dominance is diachronous to the west, and all bio-events are diachronous to the far north. The northern WIS (Billings) was dominated by a Boreal water mass based on the dominance of agglutinated benthic foraminifera, while the central and eastern WIS (Rock Canyon, NE Kansas) feature few benthic foraminifera, and the southwestern side (Carthage and Big Water) support diverse calcareous benthic assemblages. Through OAE2 foraminiferal assemblages respond to transgression-driven changes in water mass distribution, mixing, and downwelling.