Paleogeographic evolution and sequence stratigraphy of the Upper Cretaceous Bearpaw Formation (west-central Alberta, Canada)

This study is the first high-resolution stratigraphic analysis concerning a large and unstudied area (∼97000 km2) of the west-central Alberta foreland basin. We present the first extended correlation of late Cretaceous marine deposits of the Bearpaw Fm. (southern Alberta) to the continental succession of the Wapiti Fm. in the Grande Prairie area (north-central Alberta). We provide new tools used to understand the stratigraphic patterns in correlative marine and non-marine deposits of two time-constrained transgressive-regressive (T-R) cycles. Here, we integrate gamma ray analysis (977 well logs), cross-sections, recent datings and paleontological data to determine sediment distribution, accumulation rates and paleogeographic evolution during the second-order T-R cycle of the late Campanian Western Interior Seaway, when marine waters retired from west-central Alberta for the last time. Recent chronostratigraphic data constrain the two T-R cycles of the Bearpaw Fm. with an unprecedented resolution (∼200 kyr). Seven reference stratigraphic surfaces were mapped from the marine deposits into the alluvial domains and the tridimensionally modelled stratigraphic intervals resulted in isopach and cumulative thickness maps for consecutive systems tracts (ST). Data allowed a detailed interpretation of sediment distribution patterns, ST architecture and high-resolution rates of sediment accumulation mapping. This analysis led to interpret the paleogeographic evolution of west-central Alberta, focusing primarily on inferences of the paleo-coastlines and coastal environments during the two T-R cycles. Data support the interpretation that eustasy provided the main control on the evolution of the Western Interior Seaway during the deposition of the Bearpaw Fm. in late Campanian. The distribution of fine-grained, primarily marine sediments of the Bearpaw succession resulted in an effective seal for hydrocarbon accumulation within the underlying Belly River Group, preventing oil migration upsection, within the Edmonton Group.