Abstract

The Lower Cretaceous McMurray Formation of northeastern Alberta is the major reservoir of the Athabasca Oil Sands deposit. Its position at the foreland basin margin, high up on the craton, implies that it was deposited under conditions of relatively low subsidence, resulting in considerable lateral and vertical stratigraphic complexity. Present debate regarding the depositional setting of the McMurray Formation has focused particularly on the extent of a marine influence on the depositional environment of the middle member, which contains much of the bitumen reserves. North of the present study area, in the outcrops and shallow subsurface around Fort McMurray, recent studies of the sedimentology and ichnology of the middle member demonstrate a definite marine influence — probably a brackish water estuarine environment.

Core studies from the Primrose area, which is 200 km to the south and paleogeographically landward from Ft. McMurray, reveals that evidence of a significant marine influence is still present in both the upper and middle members. The lithofacies comprise a complex of units dominated by thinly interbedded to laminated, very fine sand and silty shale. The occurrence and relative abundance of distinct ichnofossil suites are crucial in the differentiation of distinct facies. In particular, the specific recurrence of an abundant but low diversity association of ichnofossil forms indicates a stressed environment. The ichnofossils are interspersed with a widespread shale facies containing thin coals and rootlets. Preliminary interpretation of the vertical succession in this area suggests a restricted marine estuary, dominated by brackish water conditions, with intermittent development of brackish to freshwater marsh environments.

This interpretation is consistent with what is known about the paleogeographic setting. Dissolution of Devonian evaporites appears to have been active during most of Early Cretaceous time. Structural collapse along the dissolution front controlled the axis of a major north-northwest draining, low-gradient, trunk valley system on the sub-Cretaceous unconformity. Early Cretaceous transgressive events resulted in aggradation and accumulation of the McMurray Formation, but also must have resulted in a southward marine incursion deep inland up the trunk valley system. This is an ideal geomorphological setting for the development of brackish estuarine environments.