Evolution of the Western Interior Seaway in west-central Alberta (late Campanian, Canada): Implications for hydrocarbon exploration

This study presents the first integrated, high-resolution stratigraphic analysis of a large area of the Cretaceous Western Interior Basin in Alberta (western Canada), providing new tools to discriminate sedimentary processes and stratigraphic patterns of transgressive-regressive (T-R) cycles in correlative marine and non-marine domains. We integrate gamma ray well-log analysis, measured sections, and paleontological data to determine sediment accumulation and distribution during the second-order T-R cycle of the late Campanian Western Interior Seaway over a previously unstudied area encompassing approximately 97,000 km2 of the Alberta foreland basin. The Bearpaw Formation, historically regarded as the product of a single transgression, is shown to include two T-R cycles, whose timing is constrained by new chronostratigraphic data that provides an unprecedented resolution (~200 kyr) for the Cretaceous of western North America. Seven reference stratigraphic markers were mapped across the study area from the marine deposits into the fluvial domains. 3D-modelled stratigraphic surfaces and stratigraphic intervals resulted in isopach maps for consecutive systems tracts, allowing detailed interpretations of their architecture and patterns of sediment accumulation. Our analysis provides paleogeographic maps for the Western Interior Seaway, focusing primarily on the evidence of the paleo-coastlines during the documented cycles. The distribution of fine-grained, primarily marine, sediments resulted in an effective seal for hydrocarbon accumulation in the Belly River Group. Further oil migration upsection, within the Edmonton group, was prevented by the occurrence of these sealing units. Data support the interpretation that eustasy provided the main control on the evolution of the Western Interior Seaway during the late Campanian.