Production and preservation of organic matter during deposition of the Bakken Formation (Late Devonian and Early Mississippian), Williston Basin
Abstract The organic-rich (up to 35% TOC) lower and upper black mudstone members of the Bakken Formation (Late Devonian and Early Mississippian) accumulated in response to both increased productivity and enhanced preservation that resulted from unique palaeogeographic, palaeoceanographic and palaeoclimatic conditions. During Bakken deposition the semi-isolated Williston Basin was located close to the Equator (5° to 10°N) in a zone of prevailing east to west winds. A pattern of estuarine-like marine circulation between the Williston Basin and open-ocean conditions at the western craton edge of North America is proposed. Surface water discharged from the basin was replaced by nutrient-rich deeper water sourced from the equatorial undercurrent in the Pacific Basin. Undercurrent flow into the Williston Basin upwelled providing a potentially sustainable source of nutrients to enhance productivity in the surface photic zone during black mudstone deposition. In response to increased productivity, that led to a high rate of organic sedimentation, and quiescent, deep-water conditions, due primarily to the geographic isolation of the basin and the restriction of estuarine-like circulation to the upper water column, anoxic bottom water developed on the basin floor. As sea-level rose during lower and upper black mudstone deposition, anoxic bottom water expanded across the basin enhancing organic matter preservation. Conditions in the Williston Basin demonstrate that both increased productivity and improved preservation can be essential and complementary factors affecting organic-rich mud deposition. These results have potentially important implications for the accumulation of similarly organic-rich deposits across the North American interior during Late Devonian and Early Mississippian time.