Abstract: Deep Sinuous Channels and Giant Cross-Beds in Athabasca Oil Sands

Grant D. Mossop, Peter D. Flach

The Lower Cretaceous McMurray Formation consists of 50 to 70 m of uncemented quartz sand and associated shale, saturated with heavy oil in virtually all zones where primary porosity and permeability are developed. In the outcrop region of northeastern Alberta, the sands are of continental origin, except in the uppermost parts of the formation where sedimentation was influenced by the encroaching boreal sea.

Giant cross-beds dominate the sequence. These normally occur as solitary sets, up to 25 m thick, with cross-stratal dips of up to 12°. Depositional strike of adjacent sets varies widely. Subtle fining upward within the sets is manifest more by an upward increase in the proportion of shaly partings than by a decrease in mean sand size.

To date, it has been held that the giant cross-beds are representative of delta-foreset deposition, the sheets of sand accumulating through delta-front progradation into freshwater lakes or brackish lagoons. An alternative explanation, proposed here, is that the sloping beds were deposited on the inner banks of channel-meander bends, as epsilon cross-strata. In this context they represent the slip-off slope profiles of deep sinuous channels that underwent extensive lateral accretion. This concept accommodates the local cross-cutting relations between separate sets, and alleviates ambiguities in explaining the origins of vertically and laterally adjacent facies.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma