In Albian–Turonian time, the interior of North America was flooded by a seaway extending from the present Gulf of Mexico to the Arctic Ocean. Detailed studies of this interval north and south of the Canada – United States international border have not usually been integrated. The present foraminiferal biostratigraphic study includes a 38 000 km2 area straddling the Alberta–Montana border from the Lewis thrust in the west to the Sweetgrass Hills in the east, including the Sweetgrass Arch.Stratigraphic cross sections and isopach maps of six Albian – early Turonian stratigraphic units prepared from 57 surface and subsurface sections demonstrate that sedimentation was controlled primarily by (i) sporadic volcanism to the west and (ii) tectonic activity coincident with the present location of the Sweetgrass Arch.The occurrence of the late Cenomanian planktonic foraminifer, Rotalipora cushmani, in association with three other keeled species, suggests an east–west marine connection between the eastern Pacific and Western Interior. This interpretation is consistent with the facies model described by Kauffman, the paleogeographic model developed in the present study, reported gastropod paleozoogeographic data, and reevaluation of pelecypod and ammonite paleozoogeographic interpretations.

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