Early Cretaceous continental-scale sediment dispersal: towards resolving the McMurray Conundrum

ABSTRACT Alternative interpretations of Early Cretaceous McMurray Formation environments of deposition in the Western Canada Sedimentary Basin of Alberta have been referred to as the McMurray Conundrum because they have been debated for more than a decade. Recent empirically grounded research has continued to support long-held views that favor both fluvial and estuarine depositional models, but have provided no further resolution of the controversy. Here, we use multiple quantitative macro-, meso- and micro-scale approaches to further advance discussion of the McMurray Conundrum. First, we present published results of a recent paleogeographical reconstruction of the McMurray river system paleodrainage basin area, which places McMurray deposition within the context of a continental-scale system. We then use a high-density McMurray subsurface dataset, including published 3-D seismic observations, to test for downstream morphological and sedimentary transformations that indicate a paleogeographic position in the normal flow reach, perhaps extending into the backwater reach, which indicate that McMurray environments of deposition were terrestrial, and formed far upstream from marine influence. Last, we deploy oxygen and carbon isotope analyses of unaltered mollusk shells in the McMurray, which indicate freshwater conditions during and shortly after deposition, and provide no evidence of brackish-water marine conditions. Our results from three types of data therefore converge on the following characteristics for the McMurray system: a) the McMurray, in the Alberta foreland, represents the axial river of a south-to-north-flowing continental-scale fluvial system that drained much of North America, serving as the Amazon River or the Mississippi River of its time, b) data collected in the Alberta foreland basin over a downstream distance of > 400 km, from the Cold Lake to Athabasca Oil Sands areas, shows no evidence of downstream morphological and sedimentary transformations that are characteristic of rivers in their backwater reach, indicating that the axial McMurray river system was characterized by normal flow, and was likely > 1000 km upstream (to the south) from a coeval shoreline, and c) the recently published carbon and oxygen isotope analyses from unrecrystalized mollusk shells (Hasiuk et al. 2024) show that McMurray deposition took place in a fully freshwater continental environment, significantly upstream from the influences of saltwater penetration or tidal processes that would indicate marine proximity and influence.