REDOX GEOCHEMICAL STUDY OF THE DEVONIAN-CARBONIFEROUS SAPPINGTON FORMATION, SOUTHWEST MONTANA: PALEOENVIRONMENTAL CONDITIONS IN THE SAPPINGTON FORMATION AND COMPARISON TO THE BAKKEN FORMATION

The Sappington Formation outcrops in Southwest Montana and spans the Upper Devonian—Lower Mississippian transition. It consists of two carbonaceous shale intervals, one upper and one lower, and a middle dolomitic siltstone and silty sandstone interval. It was deposited in a relatively small basin, bound to the south by the Beartooth Shelf and the north and east by the Central Montana Uplift. This latter emergent area separated the Sappington Basin from the Williston Basin to the east, which contains the economically important Bakken Formation. Previous biostratigraphic and lithostratigraphic comparisons have related the Sappington and Bakken Formations as depositionally contemporaneous. Due to the primarily subsurface nature of the Bakken, the Sappington has thus emerged as a useful outcrop analogue system for its eastern cousin. This study presents new geochemical data from the Sappington Formation, which in combination with lithostratigraphic data provide insight into paleo-redox conditions during the Devonian-Carboniferous boundary interval in Laurentia and a comparison to the Bakken Formation. Specifically, previous analysis of iron speciation and trace metal data in the Bakken have been used to argue that the development of photic-zone euxinia was critical in the preservation of high total organic carbon (TOC) contents in that unit. Geochemical analysis of major- and minor-element composition, iron speciation, sulfur and organic carbon isotopes, and TOC are used here to test whether similar conditions occurred in the coeval Sappington Formation. Ultimately these data will help elucidate controls on the development of organic-rich intervals in these formations.