The Bakken formation (late Devonian-early Mississippian) : a black shale source rock in the Williston Basin

The Bakken Formation (Late Devonian-Early Mississippian) is an entirely sub-surface deposit in the Williston Basin of Saskatchewan, Manitoba, North Dakota and Montana. It is composed of two black mudstone members (Lower and Upper), separated by a gray mudstone/sandstone member (Middle). The hemipelagic mudstone of the Lower and Upper members was deposited under stagnant bottom conditions, characterized by anoxic bottom water containing increased concentrations of dissolved H2S and sluggish bottom water currents, in water > 200 m deep. The Middle Member consists of a lower burrow-mottled offshore mudstone (Sub-unit A), a middle ripple-bedded and trough cross-bedded shoreface sandstone (Sub-unit B) and a second, upper burrow-mottled offshore mudstone (Sub-unit C). Abundant trace fossils in the Middle Member are characteristic of Nereites, Cruziana and Skolithos ichnofacies which reflect offshore and shoreface settings. Changes in depositional conditions reflected by Bakken members were caused by relative sea-level change. The Lower black mudstone was deposited during an increase in relative sea-level. The mudstone and sandstone of sub-units A and B (Middle Member) were deposited during a forced regression as relative sea-level rapidly dropped. The mudstone of Sub-unit C and the Upper Member were deposited during renewed sea-level rise. The Lower member contains up to 20% total organic carbon (TOC) because of high algal productivity and increased siliciclastic sedimentation which improved burial efficiency. The Upper black mudstone contains up to 35% TOC that resulted from high rates of algal productivity. Anoxic bottom water enhanced the concentration of organic matter by inhibiting substrate colonization that leads to sediment bioturbation. During Bakken deposition, estuarine-like circulation in the Williston Basin was caused by the semi-enclosed basin geography and prevailing east to west winds. Upwelling associated with estuarine-like circulation brought nutrientenriched water from the equatorial counter current in the adjacent Pacific Ocean Basin to the surface photic zone resulting in increased algal productivity. Restriction of estuarine-like circulation to the upper water column in a semienclosed Williston Basin could have isolated basin bottom water which, along with high rates of organic sedimentation, caused anoxia in bottom water. Deposition of organic-rich Bakken mudstones demonstrates the combined effects of increased productivity and enhanced preservation.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat