Abstract

The regional deposits of the Middle and Upper Mannville subgroups of east-central Alberta are characterized by a series of stacked offshore-paralic successions which represent transgressive–regressive cycles superimposed upon a longer term regressional trend. This trend eventually resulted in the complete withdrawal of the lower Albian Clearwater sea from eastern Alberta and western Saskatchewan. The marine deposits of the Clearwater sea (the Clearwater shales) carry the lower Albian Verneuilinoides cummingensisMarginulinopsis collinsi foraminiferal assemblage. The next diagnostic foraminiferal assemblage, the upper Albian Haplophragmoides gigas fauna, is found in the Joli Fou shales of the Colorado Group which overlie the Mannville Group. Middle Albian strata are absent between the Mannville and the Colorado in eastern Alberta, but the exact placement of the maximum corresponding unconformity has been controversial. This study indicates that the uppermost unit of the Mannville (the Colony unit) should be regarded not as a basal Colorado transgressive sand but as a Mannville unit deposited prior to the maximum break.Sampling of microfauna from successively younger units of the Middle and Upper Mannville indicates a shift away from normal marine environments and towards increasingly brackish depositional conditions. This shift may have resulted from the development of embayment-like conditions during the withdrawal of the Clearwater sea, as well as from increases in fluvial drainage during the late stages of Mannville deposition.

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