Sedimentology, ichnology and sequence stratigraphy of the Upper Devonian–Lower Mississippian Bakken Formation in eastern Saskatchewan

Abstract

The Upper Devonian–Lower Mississippian Bakken Formation in the subsurface of the Williston Basin in northeastern Montana, North Dakota, southwestern Manitoba and southern Saskatchewan typically includes three members: the lower and upper organic-rich black shale, and the middle calcareous/dolomitic sandstone and siltstone, which makes a “perfect” petroleum system including source rock, reservoir, and seal all within the same formation. In eastern Saskatchewan, the Bakken Formation is divided into eight facies, and one of which (Facies 2) is subdivided into two subfacies: Facies 1 (planar cross-stratified fine-grained sandstone); Facies 2A (wavy- to flaser-bedded very fine-grained sandstone); Facies 2B (thinly parallel-laminated very fine-grained sandstone and siltstone); Facies 3 (parallel-laminated very fine-grained sandstone and muddy siltstone); Facies 4 (sandy siltstone); Facies 5 (highly bioturbated interbedded very fine-grained sandstone and siltstone); Facies 6 (interbedded highly bioturbated sandy siltstone and micro-hummocky cross-stratified very fine-grained sandstone); Facies 7 (highly bioturbated siltstone); and Facies 8 (black shale). Our integrated sedimentologic and ichnologic study suggests that deposition of the Bakken occurred in two different paleoenvironmental settings: open marine (Facies 4 to 8) and brackish-water marginal marine (Facies 1 to 3). The open-marine facies association is characterized by the distal Cruziana Ichnofacies, whereas the brackish-water marginal-marine facies association is characterized by the depauperate Cruziana Ichnofacies. Isochore maps show that both open-marine and marginal-marine deposits are widely distributed in the study area and suggest the existence of a N-S trending paleo-shoreline. The Bakken strata in the study area represent two different transgressive systems tracts separated by a coplanar surface or amalgamated sequence boundary and transgressive surface. This surface has been identified in previous studies west-southwest of the study area, therefore assisting in high-resolution correlation of Bakken strata. The unusual stratigraphic architecture of the Bakken Formation in this area resulted from its proximal emplacement which favored intense erosion and cannibalization of previously accumulated deposits.