Evidence for a Prograding Tide-Dominated Delta Model for the McMurray Formation Reservoir of the Athabasca Oil Sands

Murray K. Gingras¹ and Michael J. Ranger^2
1 University of Alberta, Edmonton, AB
² Geological Consultant, Chestermere, AB

The McMurray Formation is commonly interpreted as an estuary preserved within an incised valley. This is supported by the mappable valley-form of the sub-Cretaceous unconformity, brackish-water ichnofossil assemblages, the presence of rare rhythmic deposits, and the ubiquity of inclined heterolithic stratification. But the depositional geometry of two key facies associations (FA1 and FA2) is inconsistent with the interpretation that McMurray strata represents an estuary.

FA1 consists of cross-stratified, rhythmically-grain-striped sand; it shows local flow reversals, and contains rare Skolithos, Cylindrichnus, Siphonichnus, and Conichnus. FA1 is commonly identified as fluvial due to overall low degrees of bioturbation. A more parsimonious interpretation suggests FA1 sediments were deposited in basinward marginal-marine locales.

Rhythmically inclined sand/mud beds (IHS) comprise FA2. Sedimentary features include: sedimentary couplets, partial tidal bundles, local flow reversals, flaser through lenticular bedding, and locally high degrees of bioturbation. Common ichnofossils are Gyrolithes, Cylindrichnus, Skolithos, Arenicolites, and Planolites. FA2 is interpreted as sediment accumulation in brackish channel point bars indicating deposition in more landward portions of the system, intermediate between marine FA1 and fluvial channels.

In outcrop FA2 always overlies FA1 with an erosional contact. The typical association of FA2 over FA1 implies that they represent a progradational succession. The architectural arrangement between FA1 and FA2 is demonstrated by linking them as depositional elements of an estuary in which the internal fill prograded basinward, thus possessing some of the sedimentary and stratigraphic characteristics of tide-dominated deltas. In this framework, FA1 represents an outer estuarine/deltaic tidal sand complex, and FA2 estuarine distributary channels.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005