CRETACEOUS (ALBIAN-TURONIAN) FORAMINIFERAL BIOSTRATIGRAPHY AND PALEONTOLOGY OF NORTHERN MONTANA AND SOUTHERN ALBERTA

Abstract 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 (transgressive phase, Greenhorn cyclothem). Detailed studies of this interval, north and south of the international border, have usually not 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 6 Albian-Early Turonian stratigraphic units prepared from 57 surface and subsurface sections demonstrate that sedimentation was controlled primarily by (1) spasmodic volcanism to the west; (2) 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 the Western Interior at the maximum Greenhorn transgression. This is consistent with the facies model described by Kauffman(1967, 1969, 1977), the paleogeographic model developed in the present study, reported gastropod paleo-biogeographic data, and reevaluation of pelecypod and ammonite paleo-zoogeographic interpretations.

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