An Alternative Interpretation for the Origin of Black Shale in the Bakken Formation of the Williston Basin

Abstract

Recently, most researchers have argued for a deep-water origin for black shale in the Bakken Formation of the Williston basin based on sedimentology, paleontology and geochemistry characteristics that place black, laminated, sparsely fossiliferous, radiolarian, pyritic shale as the seaward lithology in an assemblage that originally transitioned landward into shallow-water facies. The shallow-water interpretation advocated in this paper is based on stratigraphic characteristics that include the absence of strata that represent a landward equivalent to the shale, gradual landward shale pinchouts, the restriction of Bakken strata to basinal areas, the Bakken onlap depositional style, and placement of a major maximum flooding surface near the base of the overlying Lodgepole. Black mud deposition in the Bakken is interpreted to have occurred in a low-relief, semi-enclosed, epeiric-lagoon environment with typical water depths of 0–30 m. It is theorized that a high-rainfall climate caused salinity stratification that produced bottom-water anoxia, which preserved organic material. Perhumid climate conditions (year-round rainfall) fostered thick soils and dense vegetation that limited sediment release and induced mud-dominated, sand-poor deposition. During middle Bakken deposition, an arid to semi-arid climate eliminated the bottom-water anoxic conditions and caused carbonate-siliciclastic deposition. The abrupt vertical transitions from black shale (lower Bakken), to carbonate-siliciclastic lithologies (middle Bakken), to black shale (upper Bakken) were caused by paleoclimate change.