A MULTI-PROXY GEOCHEMICAL APPROACH TO IDENTIFYING THE HANGENBERG CRISIS IN THE BAKKEN FORMATION, WILLISTON BASIN, USA

The End-Devonian global Hangenberg Crisis was an environmental perturbation characterized by a mass extinction, a carbon isotopic excursion, large sea level fluctuations, and widespread marine anoxic-euxinic conditions. Sedimentological proxies provide a useful mechanism for identifying this global event in local stratigraphic sections and basins, and may also shed light upon the paleodepositional environment. However, due to non-deposition and/or erosion associated with the Hangenberg sea-level fall, it is often difficult to locate preserved uninterrupted sequences spanning the boundary.

In this study, we use previously published biostratigraphic data and new elemental and isotopic chemostratigraphic data to identify the Hangenberg Crisis interval in two cores from the Williston Basin. The studied succession includes, from oldest to youngest, the Three Forks Formation, Bakken Formation, and the Lodgepole Limestone. The Bakken Formation comprises three informal members: organic-rich upper and lower black shales, and a middle member composed of dolomitic siltstone and sandstone. The unit unconformably overlies the Three Forks. Conodont and miospores data compiled from the literature suggest that the lower Bakken black shale was deposited during the latest Devonian LN miospore zone, and therefore is correlative with the globally-recognized Hangenberg Black Shale. The middle and upper Bakken contain Tournaisian conodonts (Siphonodella sulcata and S. duplicata Zones) and miospores (VI Zone). This succession indicates that the Devonian–Carboniferous boundary likely occurs at an unconformable lower-middle Bakken contact. The unconformity is also supported by the stable carbonate carbon isotopic data, which exhibits an offset, likely driven by the depositional hiatus following the fall in sea level. Additionally, trace element data collected using portable x-ray fluorescence (pXRF) show enrichment of redox-sensitive elements (Mo, U, V, Zn) in the black shales, indicating pervasive anoxic-to-euxinic conditions in the Williston Basin. Our multi-proxy approach yields new insights into the environmental conditions of the Williston Basin during the Hangenberg Crisis, and the degree to which the studied succession records this crisis locally.