Redox and paleoenvironmental conditions of the Devonian-Carboniferous Sappington Formation, southwestern Montana, and comparison to the Bakken Formation, Williston Basin

The Sappington Formation outcrops in southwestern 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. It was deposited in a relatively small basin, with the Central Montana Uplift separating the Sappington Basin from the Williston Basin to the east, which contains the economically important Bakken Formation. Previous biostratigraphic and lithostratigraphic comparisons have identified the Sappington and Bakken Formations as depositionally contemporaneous, and, due to the subsurface nature of the Bakken, the Sappington has emerged as a useful outcrop analogue. This study presents new geochemical data from the Sappington Formation, providing a comparison to the Bakken and insight into paleo-redox conditions during the Devonian-Carboniferous on the western Laurentian margin. Geochemical signals in the Sappington Formation outcrop samples are partially obscured by oxidative weathering, but similar to the Lower Bakken Shale, the Lower Sappington Shale appears to record dominantly euxinic conditions, albeit with intervals when the water column was suboxic to ferruginous and with an absence of organic or inorganic geochemical evidence for photic-zone euxinia. The Upper Sappington Shale appears to have been deposited in ferruginous to dysoxic conditions. Thus, the Sappington appears to be an excellent analogue to the Bakken on a lithostratigraphic level, but the geochemical environment for black shales/source rock deposition appears less reducing. Closer proximity to the open ocean and consequent lower levels of restriction may have been responsible for these redox differences; however, Sappington core samples and an increased focus on integrating geochemical data into the regional stratigraphic record will be necessary to further understand paleoceanographic and biogeochemical drivers. •Lower and upper shales of the Sappington Formation were both deposited under reducing conditions.•Lower shale was deposited under broadly euxinic conditions, with some ferruginous intervals.•Upper shale was deposited under dysoxic to ferruginous conditions.•Sappington Basin was less reducing than the co-eval Williston Basin in Devonian-Carboniferous.