LATE CRETACEOUS PLANKTIC FORAMINIFERA FROM THE SANTONIAN-CAMPANIAN BOUNDARY OF THE MANITOBA ESCARPMENT

A major turnover in the taxonomic composition of Cretaceous planktic foraminifera occurred ~83.5 Ma across the Santonian-Campanian boundary (SCB). Many deep-sea records of the SCB are truncated by an unconformity signaling a global reorganization of ocean circulation likely due to the opening of the Equatorial Atlantic Gateway. Within the Western Interior Seaway of North America, the SCB is marked by a change in lithology from calcareous sediments (Niobrara/Carlile Formation) to siliciclastic deposits (Pierre Shale), and a suite of bentonites collectively referred to as the “Ardmore” succession in the lower Campanian (~80.5 Ma). Microfossil assemblages preserved in sediments exposed along the Manitoba Escarpment (Canada) record a series of striking faunal changes across the SCB. Planktic foraminiferal faunas within the organic-rich Boyne member of the Carlile Fmn (Coniacian-Santonian) are dominated by the genus Hedbergella; however, this assemblage is succeeded at ~15 meters below the Ardmore succession (Santonian-Campanian) by a fauna dominated by the genus Globigerinelloides. Planktic foraminifera are absent through the overlying Gammon “Ferruginous” and Pembina members of the basal Pierre Shale, which rest unconformably atop the Carlile Fmn. Above the Ardmore succession, within the lower few meters of the Millwood member of the Pierre Shale (lower Campanian), planktic foraminifera briefly reappear in association with a rich, diverse (>70 species) benthic foraminiferal fauna. Microfossil assemblages within the remaining Millwood member (Campanian) contain no planktic foraminifera and a preponderance of radiolarians. This dramatic change in microfossil assemblages reflects an incursion of colder, boreal waters sourced from Arctic regions during a period of renewed transgression. Overall, the succession from organic-rich shale (Carlile Fmn) into the oxidized Gammon member (basal Pierre Shale) is broadly similar to pelagic records of black shale deposition during ocean anoxic events (OAEs) followed by “Cretaceous oceanic red beds” (CORBs). Thus, the SCB turnover may have been related to the end of “super plume” magmatism as indicated by its coincidence with the termination of the Cretaceous Normal-Polarity Super-Chron and cessation of black shale deposition (OAE3) in pelagic settings.