Abstract

Ecological perturbations during the Cenomanian-Turonian boundary interval along the eastern margin of the Canadian Western Interior Seaway (WIS) were investigated in a subsurface core from eastern Saskatchewan (distal) and outcrop at the Manitoba Escarpment (proximal). Paleoecological controls on biota included transgressive/regressive cyclicity, the Oceanic Anoxic Event 2, frequent ashfalls, and a stratified water column. Changes in foraminiferal assemblage composition, species richness, and abundance can be correlated between the core and outcrop. The lower Turonian biofacies 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 species are significantly higher in the distal core section. During the early Turonian, diversity slightly increased, with the presence of Whiteinella aprica reflecting periodic improvement of the water column conditions at both sites. The appearance of the clavate species, Clavihedbergella simplex, 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 the middle Turonian allowed for seafloor winnowing and caused a brief disappearance of planktic foraminifera. The north-south temperature gradient in the WIS resulted in a lower planktic foraminiferal diversity in what is now Canada compared to the waters that covered what is now the United States and the absence of benthic and deeper-dwelling, keeled planktic species can be attributed to an increasingly well-developed oxygen minimum zone or increasingly stratified water column. The appearance datum of planktic foraminifera is distinctly diachronous along a south to north transect.

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