Acceleration in the rate of the Boreal Sea transgression recorded in the Lower Cretaceous McMurray Formation, Canada

Abstract

Depositional environments are typified as low-to high-accommodation space settings based on the sediment accumulation rate. In low-accommodation settings, persistent top truncation of regressive units renders it challenging to accurately determine the rate of accommodation space creation. Herein, we analyze a nearly continuous succession of the Lower Cretaceous McMurray Formation, which was deposited during the foreland development of the Western Canada Sedimentary Basin in northeast Alberta, Canada (the McMurray Depocenter). Across the study area, the McMurray Formation consists of a vertical succession of 6 discrete regressive depositional units (DU) bounded by flooding surfaces and/or transgressive surfaces of erosion. The sedimentology, architecture and stratigraphy of these DUs are investigated and statistical analysis is used to quantitatively assess variations in DU thickness. Additional statistical analysis is used to quantitatively assess the vertical distribution of transgressive mudstone subfacies in the McMurray Formation.The mean thickness of each DU is 10.4 m, and each DU represents approximately 1.7 million years of deposition. This indicates that the McMurray Formation preserves deposition in a low-accommodation space setting, and that creation of accommodation space was primarily controlled by transgression of the Boreal Sea. An abrupt decrease in DU thicknesses is interpreted to result from concomitant increased erosional truncation of DUs via wave action during transgression and changes in basin morphology. DU thickness variations correlate to a change in the marine expression of the transgressive mudstone underlying each DU. Together, these data suggest an acceleration in the rate of the Boreal Sea transgression, and we hypothesize that the duration of still-stands during which DUs accumulated diminished in the latter stages of McMurray Fm deposition. This study demonstrates that detailed sedimentologic and stratigraphic research tied to statistical analysis can be used to quantify changes in the rate of transgression, especially in low-accommodation space settings.