Deep channel sedimentation in the Lower Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta



ABSTRACT In the region of the Athabasca Oil Sands, Alberta, the Lower Cretaceous McMurray Formation comprises 50-80 m of uncemented quartz sand and associated shale, saturated throughout by bitumen. The sediments are dominantly of continental origin, except in the uppermost parts of the formation where sedimentation was influenced by the encroaching boreal sea.

In most outcrop and mine face exposures of the McMurray Formation, a sequence of three facies is recognized. In ascending order these are: (1) an erosionally based thick-bedded sand facies, 2-20 m thick, dominated by large-scale trough cross-beds; (2) an epsilon cross-stratified facies with solitary sets up to 25 m in thickness, consisting of decimetre to metre thick couplets of sand/mud, with depositional slopes of 8-12° and palaeocurrent indications parallel to the strike of the epsilon cross-set; and (3) a horizontally bedded argillaceous sand facies up to a few metres thick. The three-fold sequence is interpreted as a single upward-fining cycle of channel sedimentation, the trough cross-bedded sands resulting from channel bottom deposition, the epsilon cross-strata accumulating by lateral accretion of channel point bars, and the upper argillaceous sand representing floodplain sedimentation. Where the McMurray Formation is relatively thin (less than 50 m), virtually the entire formation is commonly composed of a single upward-fining channel deposit.

Details of the size and physiographic setting of the channels are somewhat uncertain, but the present evidence suggests that the epsilon-dominated McMurray Formation sequence in the Athabasca Deposit region represents the coastal plain culmination of a very large fluvial drainage system.