Determining the Time-Averaged Position of the Turbidity Maximum Zone in Major Paleo-Valleys of the McMurray Formation, NE Alberta

Abstract

The tidal-fluvial transition (TFT) is a complex depositional zone wherein fluvial processes interact with tides and brackish-water inundates rivers as they approach the receiving marine basin. Variations in the relative importance of these processes lead to a complex distribution of sediments that accumulate on channel bars in the upper subtidal and intertidal zones. These deposits dominantly comprise inclined heterolithic stratification (IHS). We hypothesize that the sedimentological and ichnological aspects of IHS can be used to infer its depositional position across the TFT owing to the distribution of physico-chemical stresses generated by progressive changes in the hydrodynamic conditions. In this study sedimentological, ichnological, and quantified bed-thickness data from IHS successions contained within the A Valley trend in the lower Cretaceous McMurray Formation are compared to the sedimentary trends established from the modern tidally influenced Fraser River. Mud-bed thickness distributions and the mudstone proportions in IHS show that the time-averaged location of a turbidity maximum zone (TMZ) occurred within the vicinity of a Township 76. Bioturbation intensity and trace fossil diversity data reveal that periodic brackish-water conditions occurred basinward (north) of the TMZ. Together, these point to a broad TFT in which sedimentary characteristics progressively shifted northward from fluvial-dominated channels in the south, to channels in close proximity to a TMZ, and finally into channels that experienced persistent brackish-water influence. These datasets suggest that channel deposits in the middle and upper McMurray of the southern Athabasca area were deposited across a broad tidal-fluvial transition (TFT) zone. Moreover, the time-average location of the TMZ does not appear to have shifted significantly during deposition of the channel stories analyzed from Valley A. This work constitutes a framework for interpreting depositional position within the TFT, which will ultimately improve reservoir quality prediction in bitumen-bearing IHS of the McMurray Formation.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016