Reconstructing depositional architecture and stratigraphy of coastal- to shallow-marine strata in a low-accommodation system: McMurray Formation, Alberta, Canada

A new and novel sedimentological-statistical approach is applied to Lower Cretaceous McMurray Formation strata in the southwest quadrant of the McMurray depocenter to further comprehend the impact of accommodation space creation on the preserved record of coastal- to shallow-marine deposits in a low-accommodation setting. Across the study area, the McMurray Formation consists of discrete depositional units bounded by flooding surfaces and/or transgressive surfaces of erosion. The facies characteristics, depositional architecture and sequence stratigraphy of these depositional units are investigated, focused in the three main areas: quantitatively defining the controls on localized accommodation creation, recognizing new depositional units within the McMurray Formation, and quantifying and characterizing the speed of the Boreal Sea transgression. First, depositional architecture work tied to statistical analysis in the Sparrow Paleovalley (≈1/3 of the study area) reveals the local overthickening of +21% to +45% of the depositional units. The local accommodation space creation is demonstrated to have resulted from syndepositional epikarst subsidence within the underlying Devonian carbonates. Second, the detailed facies analysis and depositional architecture analysis of the Regional C depositional unit reveals a regionally extensive allogenic flooding surface (Top C2) dividing the Regional C depositional unit. Recognition of this new stratigraphic surface further reinforces the persistent and slow drowning of the McMurray depocenter during accumulation of the McMurray Fm, and that deposition occurred in a low-accommodation setting. Finally, detailed facies analysis, depositional architecture analysis, sequence stratigraphic work and statistical methods applied to the entire McMurray Formation across the study area reveals that the thicknesses of depositional units decrease markedly upward, and that this thickness decrease correlates to a change in the facies character of the transgressive mudstones underlying each depositional unit. Together, these data record the acceleration in the rate of the transgression of the Boreal Sea across the McMurray depocenter.