Controls on organic matter accumulation in the Bakken Formation, Williston Basin, USA

Abstract The environmental processes and conditions controlling productivity and organic matter accumulation/preservation in the lower black shale (LBS) and upper black shale (UBS) members of the Devonian-Mississippian Bakken Formation were evaluated utilizing molybdenum (Mo), total organic carbon (TOC), total sulfur (ST), and iron (Fe) data. High-resolution (1- to 3-cm scale) chemostratigraphic records were generated for twelve drill cores, four of which closely flank the north-south-trending axis of the Nesson Anticline, proximal to the center of the Williston Basin in northwest North Dakota, USA. Degree of pyritization (DOPT) values (0.25 to 1.0) indicate that bottom waters were frequently dysoxic with intermittent aerobic and anoxic/euxinic conditions. However, the presence of organic sulfur in black shales can complicate the use of DOPT as a redox proxy requiring a multi-proxy approach to adequately elucidate depositional conditions. In such samples, the use of ST to approximate pyritic sulfur can lead to an overestimation of DOPT values. Total organic carbon contents of 2–20%, by weight, and consistently elevated Mo concentrations (>100 ppm) in both the LBS and UBS imply that the Williston Basin was often hydrographically unrestricted, allowing for ample nutrient resupply that resulted in enhanced primary productivity. Enhanced organic production (as indicated by the elevated TOC contents) was most likely supported by an abundant supply of nutrients in the basin. Using published Mo–TOC relationships from modern anoxic-euxinic basins, it is estimated that renewal time of the sub-chemoclinal water mass during accumulation of the LBS and UBS approximated 10 and 30 yr, respectively. Key differences between LBS and UBS redox proxies highlight different bottom-water redox conditions during deposition, suggesting differences in paleo-water mass geochemical characteristics and general circulation.