--> Complexities in Lithostratigraphy and Chemostratigraphy of the Bakken Formation, Williston Basin, USA, Along a 200-Kilometer Core Transect

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Complexities in Lithostratigraphy and Chemostratigraphy of the Bakken Formation, Williston Basin, USA, Along a 200-Kilometer Core Transect

Abstract

The Bakken Fm was deposited in the Williston Basin (WB), North America, under a unique set of geographic, bathymetric, and oceanographic conditions during the Late Devonian and Early Mississippian. The integration of chemostratigraphic results from five drill cores that define a South-North, ~200-km-long transect across a large portion of the WB reveals the pervasive occurrence of the underlying Three Forks Gp, in general, a shaley-silty dolarenite. The lowermost informal member of the Bakken, the Pronghorn, is a ~12-meter-thick dolomitic-argillaceous mudstone in the southernmost core, but is much less dolomitic to the north, and is absent in the northernmost two cores of the transect. The thickness of the Lower Bakken, an organic-rich mudstone, trends from 0.2 meters in the south to a maximum of 9 meters at the north end of the transect. The Middle Bakken, a calcitic and dolomitic siltstone, is thin in the south (3 meters), but thickens to approximately 20 meters along the central- northern portion of the transect. The Upper Bakken (3–7 meters core thicknesses) is an organic-rich mudstone, similar to the Lower Bakken, but is much more laterally extensive across the basin. The overlying Mississippian-age Lodgepole Limestone occurs in all the cores, and an informal argillaceous unit termed the “False Bakken”, ranging from tens of centimeter to approximately two meters in thickness is present. Total organic carbon values in the Lower and Upper Bakken, both mudrock units, average 12–15%, by weight, suggesting that nutrient inputs were staggeringly high. The mudrock units are also elevated in %S, a proxy for pyrite, with values averaging ~2%. The dominance of biogenic silica in portions of the mudrock units indicates that the dissolved silica flux to surface waters was elevated. Redox-sensitive trace element (RSTE) concentrations are highly elevated in the Lower and Upper Bakken, with vanadium and zinc concentrations reaching >2000 ppm, nickel concentrations reaching 600 ppm, and molybdenum concentrations reaching 600–800 ppm. The elevated RSTE concentrations indicate that the Williston Basin water mass was connected with the open equatorial ocean such that marine conditions prevailed throughout the interval of Bakken mudrock deposition. However, deep-basin water mass restriction during sea-level highstands was severe, creating a stagnant, highly euxinic environment that enhanced the preservation of organic detritus and the accumulation of RSTEs.