BENTHIC FORAMINIFERAL COMMUNITY TURNOVER ALONG THE EASTERN MARGIN OF THE WESTERN INTERIOR SEA DURING THE LATE CRETACEOUS COOLDOWN: MANITOBA ESCARPMENT

Although the Late Cretaceous is typified by a greenhouse climate state, global climate cooled near the end of the Santonian stage (ca. 83.6 Ma) and over the course over the ensuing Campanian stage. Here we build upon the seminal work of McNeil and Caldwell (1981) by reexamining benthic foraminiferal community turnover during this Late Cretaceous “cooldown” as recorded in geologic exposures of the Niobrara Formation and Pierre Shale along the Manitoba Escarpment in southern Canada. The study sections were deposited as parts of the Niobrara and Claggett marine transgressions along the passive eastern margin of the Western Interior Seaway and are punctuated by numerous hiatuses. The Niobrara transgressive sequence progresses up-section from (1) depauperate, arenaceous benthic microfaunas of the organic-rich Morden Shale possibly of mid-Turonian age to (2) mixed microfaunas composed of arenaceous and calcareous benthic taxa and rare planktic foraminifers within the lower Niobrara Formation (Coniacian to mid Santonian) to (3) hemipelagic microfaunas composed predominantly of planktic and calcareous benthic foraminifers within Late Santonian to Early Campanian chalky marlstones of the upper Niobrara Formation. An unconformity truncates the uppermost Niobrara Formation, signaling a sea-level fall and end of the Niobrara transgressive sequence. Renewed sea-level rise during the Claggett transgression is indicated by deposition of the overlying Pierre Shale, but the succession of microfaunas differs markedly from that seen during the antecedent Niobrara transgressive sequence. Specifically, depauperate, arenaceous benthic foraminifer assemblages within the organic-rich shales of the Pembina member (mid-Campanian) abruptly transition to the remarkably diverse benthic assemblage of the Glomospira corona Zone, which in turn is overlain by microfaunas composed chiefly of agglutinated benthic foraminifers and a preponderance of radiolarians within the calcareous, silty clays of the upper Millwood member (middle Late Campanian). Thus, the Tethyan affinities of the warm-water, calcareous microfaunas within the upper Niobrara Formation contrast starkly with the boreal affinities of siliceous, cool-water microfaunas found within the Millwood member of the Pierre Shale.