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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 69 (1985)

Issue: 8. (August)

First Page: 1195

Last Page: 1207

Title: Depositional Environments of Lower Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta

Author(s): Peter D. Flach (2), Grant D. Mossop (3)

Abstract:

Most of the bitumen resources in the Athabasca oil sands deposit are contained in upward-fining channel sands of the Lower Cretaceous McMurray Formation. The channels were part of a northward-flowing fluvial drainage system that was being encroached upon by the rising Clearwater Sea.

The lower member of the formation, present only in the deepest valleys on the underlying Devonian limestone surface, is dominantly fluvial in character. Rising sea level during middle member deposition resulted in a decreased gradient and a change from the shallow (5-10 m or 15-30 ft), commonly coarse-grained channels of the lower member to the deep (20-30 m or 65-100 ft), narrow, sinuous, high suspended-load channels of the middle member. Although there is evidence for some marine influence in the middle member channels, the processes of channel formation appear to be dominantly fluvial.

In off-channel areas of the middle member, rising sea level resulted in the development of widespread subaqueous conditions and rapid vertical aggradation. Thick sequences of mud accumulated in lacustrine and brackish-bay environments that existed contemporaneously with the channels. Coal beds and rooted zones, common in the lower and upper members, are notably absent from the middle member.

The upper member of the McMurray Formation was deposited during a time of slower aggradation and increased marine influence. It is characterized by smaller scale sedimentation units, commonly small upward-coarsening sequences interpreted as splay deposits of nearby large channels. In northern and western parts of the study area, open-marine conditions existed in upper member time, and upward-coarsening, offshore marine bar sands developed at the top of the formation.

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