Updated May 2022
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 flooded previous higher-ground areas, as the Cretaceous Interior Seaway advanced from the north to the south into a large coastal complex of incised (north-northwest trending) valleys, bays and shorelines. Initial emplacement of the McMurray Formation was controlled by the pre-Cretaceous topography at the time of deposition. During pre-McMurray time, millions of years of deposition and erosion affected the landscape, with the net result that erosional, deep valleys were cut into the underlying Devonian carbonates, which later confined McMurray depositional systems.
McMurray depositional environments are extremely complex and consist mainly of fluvial channel deposits that are commonly preserved in the deepest parts of the Devonian valley systems. The coarse-grained pebbly/gravelly sand are generally porous sands 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 part 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 goes from fine to medium, and lithology such as mudstones breccias, low-angle inclined heterolytic stratification (IHS) deposits of mud and sand.
As transgression continued, and the Devonian valleys filled, there became more room for the fluvial systems to meander and estuarine systems to incise resulting in greater preservation of coarsening-upwards shoreface and bay-fill parasequences, which 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 act as a seal and caprock to the reservoirs of the underlying McMurray Formation.