BENTHIC FORAMINIFER COMMUNITY TURNOVER ALONG THE PASSIVE EASTERN PLATFORM OF THE WESTERN INTERIOR SEAWAY DURING THE LATE CRETACEOUS COOLDOWN

The Campanian stage (83.6 – 72.1 Ma) is marked by the cooling of Late Cretaceous climate and a global reorganization of ocean circulation patterns. A record of this Late Cretaceous cooldown is preserved within marine strata exposed along the Manitoba Escarpment in southern Canada. The stratigraphy is divisible into the Morden (Turonian-Coniacian), Niobrara (Santonian-Campanian), and Pierre Shale (Campanian) formations. These formations were deposited as part of the Niobrara, Claggett, and Bearpaw transgressions on the passive eastern margin of the Western Interior Seaway (WIS). Benthic foraminifer communities from this neritic (inner to outer shelf) setting were examined to better understand how the Late Cretaceous cooldown affected the WIS circulation and its biota. A succession of at least five benthic foraminifer faunas is recorded. The early stages of the Niobrara Transgression are marked by depauperate faunas consisting of rare arenaceous taxa in the organic-rich shales of the Morden Formation and lower parts of the Niobrara Formation. The highstand systems tract is signaled by early Campanian faunas composed of rotaliids and an abundance of planktic foraminifers within the “chalky” marlstones of the upper Niobrara Formation. The Niobrara Formation is separated from the overlying Pierre Shale by an unconformity signaling the end of the Niobrara Transgression and the beginning of the Claggett Transgression. The basal Pembina Member of the Pierre Shale is a black carbonaceous shale almost completely barren except for a few thin seams containing planktic and benthic foraminifers. The ensuing Bearpaw Transgression is signaled by a remarkably diverse fauna of calcareous and arenaceous benthic foraminifers assigned to the Glomospira corona Zone at the base of the Millwood Member. The remainder of the overlying Millwood fauna is composed of an agglutinated fauna not seen during the Niobrara Transgression and is associated with a preponderance of radiolarians. We posit that the unique G. corona Zone fauna demarcates an oceanic front separating warm Tethyan waters from cooler boreal waters, and that the radiolarian-rich faunas of the Millwood Member reflect a southerly incursion of cool waters from the circum-Arctic region as global climate gradually cooled over the course of the Campanian stage.