Mapping of the Devonian strata underlying the sub-Cretaceous unconformity resulted in a re ned delineation of their subcrop extents and the topography at the sub-Cretaceous unconformity in the eastern extent of the Athabasca Oil Sands Area. 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-trending trough comprising the Bitumount Basin in the north and smaller basins in the centre and south within the study area. Comparison of the new subcrop extents with unconformity models presented below 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.