FORAMINIFERAL RESPONSE TO ECOLOGICAL PERTURBATIONS ALONG THE EASTERN MARGIN OF THE CANADIAN WESTERN INTERIOR SEAWAY, CENOMANIAN-TURONIAN INTERVAL

Ecological perturbations during the Cenomanian-Turonian boundary interval, along the eastern margin of the Canadian Western Interior Seaway (WIS), are investigated in a subsurface core from eastern Saskatchewan (distal) and outcrop along the Manitoba Escarpment (proximal). Paleoecological controls on biota include transgressive/regressive cyclicity, the Oceanic Anoxic Event 2 (OAE 2), frequent ashfalls, and a stratified water column. Changes in foraminiferal assemblage composition, diversity, and abundance can be correlated between the core and outcrop. Rare agglutinated species only appear in the Cenomanian and Upper Turonian in the more distal section, whereas the Lower Turonian at both sites is made up exclusively of planktic species, reflecting the persistent bottom water anoxia in the Canadian WIS. The low-diversity assemblage is nearly entirely composed of opportunistic, surface-dwelling, species of the genera Muricohedbergella and Heterohelix, responding to water turbidity, reduced salinity, and shallow water depth. Abundances of planktic foraminifera are significantly higher in the distal core section. During the early Turonian diversity slightly increases with Whiteinella aprica, a larger trochospiral species, reflecting periodic improvement of the water column conditions at both sites. The appearance of clavate species Clavihedbergella simplex and C. subcretacea reflects an expanded oxygen minimum zone, also confirmed by biomarkers, particularly in outcrop. Dwarfing of foraminiferal tests is interpreted as a response to frequent ashfalls that affected the water column. Lowering of sea-level during Early to Middle Turonian allowed for seafloor winnowing and caused a short-lived disappearance of planktic foraminifera. The temperature gradient in the WIS resulted in a lower planktic foraminiferal diversity in Canada compared to the US and the absence of benthic and deeper dwelling keeled planktic species can be attributed to an increasingly well-developed oxygen minimum zone along the northern and southern margin. The appearance datum of planktic foraminifera is distinctly diachronous along a north-south transect.