Abstract

The eastern facies of the Cretaceous System in middle North America is distinguished by relatively thin, but significant units of carbonate-rich sediment, chiefly chalky shale, chalk, and thinly bedded limestone, deposited during periods of peak marine transgression. The facies extend from the area of the Manitoba Escarpment southward toward Kansas and are dated as Albian to Maastrichtian. For years, the American segment of the eastern facies belt has been the empirical base for modelling marine cycles of sedimentation. Recent studies on the Canadian segment of the eastern facies belt have provided a revised and refined stratigraphic scheme for the Canadian section that is in concert with the classical facies patterns and cyclothemic interpretations for the Western Interior Cretaceous basin as a whole.

In the Manitoba Escarpment, the eastern facies consists of some 600 m of varied sands, clays, chalk-speckled shales, chalky shales, marlstones, and minor amounts of limestone, including coquinoid and calcarenitic. The section is dominated by marine sediment, approximately 15 percent of which is calcareous, and affected by numerous unconformities, but nevertheless provides a variety of lithotypes and biotypes in sequences that reflect the transgressive-regressive histories of the Cretaceous epicontinental seas.

The eastern facies of the Cretaceous System in the vicinity of the Manitoba Escarpment were deposited during two primary cycles of marine sedimentation, the Greenhorn and the Niobrara, which reached their transgressive acmes respectively in the Early Turonian and in the Santonian or Early Campanian. The Greenhorn Cyclothem in Manitoba comprises the Albian to Middle Turonian sequence of the Swan River, Ashville, and Favel Formations and the Morden Shale. A secondary cycle (Skull Creek cycle) is recognized within the primary Greenhorn cycle in the lower part of the Ashville Formation. The Niobrara Cyclothem in Manitoba comprises the Late Turonian to Maastrichtian sequence of the Niobrara Formation, Pierre Shale, and Boissevain Formation. The primary Niobrara cycle contains one, or possibly two, secondary cycles. The first is centred on the Late Campanian calcareous sediments of the lower Millwood Member of the Pierre Shale (equivalent to the chalky Gregory Member of the Pierre Shale in North and South Dakota), and the tentative second is centred on the Late Campanian siliceous shale of the Odanah Member of the Pierre Shale.