CHARACTERIZATION AND MAPPING OF PARASEQUENCES AND PARASEQUENCE SETS OF THE LOWER CRETACEOUS MCMURRAY FORMATION, ALBERTA, CANADA

The regional stratigraphic architecture of the Lower Cretaceous McMurray Formation in the McMurray Sub-Basin of Alberta, Canada comprises a series of regressive parasequences cut into by nested and deeply incised fluvio-estuarine channel belts. Parasequences (PS), generated by autogenic processes, stack vertically and shingle to form allogenic parasequence sets (APS). Distinguishing between autogenic PS and APS requires detailed characterization of sedimentological and ichnological structures and mapping of bounding discontinuities. This was undertaken in the NE-SW trending Sparrow Paleovalley of the McMurray Sub-Basin.

Based on facies analysis of 80 cores along the axis of the Sparrow Paleovalley, the characteristics of autogenic PS and APS are apparent. Autogenic PS exhibit a coarsening-upward pattern marked by a scoured basal contact overlain by a basal mudstone facies that passes upward into a sandy heterolithic facies, and capped by a coarser-grained sandstone facies with combined-flow structures. The coarsening-upward successions of autogenic PS normally terminate at a scoured upper contact. The character of the basal mudstone facies is critical in distinguishing between autogenic PS and the more regionally extensive APS. Basal mudstones marking the APS bounding surfaces are associated with major flooding events and exhibit a darker color, a higher degree of bioturbation, and a higher gamma-ray response (>100 API) and lower density porosity (< 18%) on well logs than mudstones separating autogenic PS.

The most reliable way to distinguish between autogenic PS and APS is through mapping. Autogenic PS reflect delta avulsion or lobe shift in the depocenter and have limited lateral extents (<100 km²). By contrast, APS represent major periods of progradational regression bounded by regional transgression, and correlate throughout the entire Sparrow Paleovalley (3352 km²) except where removed by channel incision. Overall, younger APS are the best preserved, with preservation of all APS increasing towards the southwest (landward) end of the Sparrow Paleovalley. Only the uppermost APS is preserved throughout the paleovalley, marking the end of channel incision in the southwestern part of the McMurray Sub-Basin.