The Sub-Cretaceous Unconformity and the Devonian Subcrop in the Athabasca Oil Sands Area, Townships 87-99, Ranges 1-13, West of the Fourth Meridian

Devonian strata underlying the Lower Cretaceous McMurray Formation in the Athabasca Oil Sands Area (northeastern Alberta) between townships 87 and 99 and ranges 1 through 13 west of the 4th meridian were investigated in order to understand the structure of the Devonian, with emphasis on the effects from the dissolution of the Prairie Evaporite Formation and the nature of the sub-Cretaceous unconformity. Devonian strata subcropping beneath the McMurray Formation have been affected by tilting, subaerial exposure and erosion, and deformation and collapse because of the dissolution of underlying Prairie Evaporite Formation halite and anhydrite. Because Devonian strata differ in their resistance to physical and chemical erosion, the resulting topography of the sub-Cretaceous unconformity surface and the pattern of subcropping Devonian rocks can be highly variable, even within a single township.

Mapping of the Devonian strata underlying the sub-Cretaceous unconformity resulted in a refined delineation of their subcrop extents and the topography at the sub-Cretaceous unconformity in the eastern extent of the Athabasca Oil Sands. Results are at a higher resolution than previously available because of the abundance and density of well data used. The sub-Cretaceous unconformity surface contains a north-northwest and south-southeast-trending trough comprised of the Bitumount Basin in the north and smaller basins in the centre and south within the study area. Comparison of the new subcrop map with previous unconformity models indicates a complex interaction between subaerial erosion before the deposition of the McMurray Formation and hypogenic karst processes, in which the dissolution of the Prairie Evaporite Formation at depth resulted in the subsidence to collapse of overlying Devonian strata.