ABSTRACT

The transition zone sediments between the Upper Cretaceous Bearpaw and Horseshoe Canyon Formations in the Drumheller area form part of a major regressive sequence laid down in late Campanian time over central and southern Alberta. During this time, the shoreline of the Bearpaw Sea was forced further to the east with a buildup of continental sediments along the margins. Within the present study area, located about 10 mi. southeast of Drumheller along the Red Deer River, six separate units can be recognized in the transition zone deposits, representing: prodelta and delta front; distributory channel; beach and interdistributary bay; mudflat; open bay; and lower floodplain environments of an ancient deltaic complex. Sedimentation occurred relatively rapidly in an unstable environment, with constantly shifting streams the primary agent of transportation and deposition of sediment.

The interpretations of environment and depositional processes were based on a detailed sedimentological analysis of lithology, bed morphology, sedimentary structures and vertical sequences present in the deposits. Grain size distributions, determined for the sands, in combination with sedimentary structures, showed the sediments to have been dominantly deposited under low energy conditions. A factor analysis of the grain size data produced four factors which were related to specific levels within the flow regime. Petrographic examination of the sands provided information on the source material and character of the sediment.