Stratigraphy of the Upper Cretaceous Wapiti Formation, west-central Alberta, Canada

Abstract

The lithostratigraphic interval between the marine Puskwaskau Formation (Smoky Group, Santonian–Campanian) and the fluvial Scollard Formation (early Maastrichtian) in west-central Alberta and easternmost British Columbia (Canada) is represented by the nonmarine deposits of the Wapiti Formation. Its subdivision into regionally mappable stratigraphic units and the correlation of such units with the better known successions of central and southern Alberta are the main goals of this study. We present a detailed stratigraphic revision of the Wapiti Formation in the Grande Prairie region, where the entire succession crops out extensively and intensive oil and gas exploration activity provides excellent subsurface control. This study indicates that the Wapiti Formation consists in five stratigraphic units: their description has been based in particular on facies analysis and well-log signatures. In ascending order, units 1 to 5 record major differences in depositional architecture related to variation in accommodation and climatic conditions. Upper and lower contacts of these units are represented by regionally mappable subaerial unconformities or conformable facies contacts. Three major coal zones are identified within the Wapiti Formation, the Basal, Red Willow, and Cutbank: coals referred to these intervals have been documented in both outcrop and subsurface in the entire study area, thus representing a reliable tool for regional correlations. Furthermore, results presented here indicate that the maximum flooding surfaces of the Bearpaw seaway and the Drumheller Marine Tongue, both marine reference units in central and southern Alberta, lie, respectively, within coals of unit 3 and the Red Willow coal zone.