The Cretaceous McMurray Formation was deposited 120 to 90 million years ago. Deposition occurred during an overall rise in sea level as shorelines shifted and areas of higher ground were flooded as the Cretaceous Interior Seaway advanced from north to south into a large coastal complex of incised (north-northwest trending) valleys, bays, and shorelines. The pre-Cretaceous topography controlled the initial emplacement of the McMurray Formation at the time of deposition. During pre-McMurray time, millions of years of deposition and erosion affected the landscape, resulting in deep erosional valleys cut into the underlying Devonian carbonates, which later confined the McMurray depositional systems.
McMurray depositional environments are extremely complex and consist mainly of fluvial channel deposits, commonly preserved in the deepest parts of the Devonian valley systems. The coarse-grained pebbly/gravelly sands are generally porous and were deposited on the lower McMurray fluvial channels within the erosional valley complexes. Deposition changed upwards into more transgressive estuarine systems in the middle of the McMurray Formation. The tidally influenced estuaries were dominated by fluvial-tidal estuarine point bar deposits, with changes of the sand grain sizes that go from fine to medium, and a lithology such as mudstones breccias, low-angle inclined heterolytic stratification deposits of mud and sand.
As transgression continued and the Devonian valleys filled, more room became available for the fluvial systems to meander and estuarine systems to incise, resulting in greater preservation of a coarsening-upwards shoreface and bay-fill parasequences that tend to have more laterally continuous horizontal mud beds. Finally, at a point of maximum transgression, thick laterally continuous shales and mudstones of the Wabiskaw Member and Lower Clearwater Formation were deposited and acted as a seal and caprock to the reservoirs of the underlying McMurray Formation.
