Refinement of the stratigraphic framework for the Regional C depositional unit of the McMurray Formation and implications for the early transgression of the Alberta Foreland Basin, Canada

Abstract

ABSTRACT The Lower Cretaceous McMurray Formation in Alberta and Saskatchewan, Canada, comprises a series of depositional units (DUs) consisting of stacked parasequences bounded by flooding surfaces and incised by fluvio-estuarine channel belts. The fluvio-estuarine channel belts of the McMurray Fm have been the focus of numerous studies whereas the regional DUs have received substantially less attention. Of the regional DUs, Regional C (equivalent to the middle McMurray) is the most understudied, yet this interval records the history of the McMurray Formation between deposition of fluvial strata in the lower McMurray and marine facies in the upper McMurray and overlying Clearwater Formation. Determining the history of the Regional C DU is fundamental for accurately reconstructing the stratigraphic evolution of the McMurray Fm and, by extension, the early evolution of the Alberta Foreland Basin. The Regional C is divided into two DUs separated by a regionally mappable flooding surface. This surface occurs 11 to 15 m below the top of the Regional C and is traceable over a 2,550 km2 area. This flooding surface divides the thick interval of undifferentiated Regional C into a lower C2 DU and an upper C1 DU, each with a maximum thickness of < 15 m. The thickness of the C2 and C1 DUs indicates that deposition at this time also occurred in a setting of low to moderate accommodation creation, which is consistent with the rest of the McMurray Formation. The limited available accommodation space was easily surpassed by sediment supplied by the paleo-distributive channel system, leading to a basinward progradation of the shoreline. The C2 and C1 DUs are retrogradationally stacked, with the maximum regressive paleo-shoreline of C1 lying landward of that of C2. This stacking arrangement indicates that the shoreline backstepped during the early stages of transgression of the Boreal Sea. The backstepping of the paleo-shoreline from C2 to C1 time is consistent with previous studies that show continued and stepwise retrogradation and/or transgression of the paleo-shoreline from the onset of deposition in the lower McMurray Formation through to maximum transgression in the Clearwater Formation. Together, these studies demonstrate that the early drowning of the Alberta Foreland Basin was persistent and slow.