STORM DEPOSITION IN THE "ANOXIC" LOWER MISSISSIPPIAN UPPER BAKKEN SHALE OF NORTH DAKOTA - HOW DEEP WAS THE WILLISTON BASIN?

Conventional depositional models for organic-rich mudstones typically envision tranquil sedimentation in anoxic marine environments. Recently, this paradigm has begun to shift because studies have revealed bed-load transport structures and erosion surfaces in mudstones. This study focuses on the influence of storms on sedimentary facies within the Lower Mississippian upper Bakken Shale, a major source rock in the Williston Basin, U.S. and Canada. Facies analysis of this shale unit reveals three distinct facies belts all containing significant amorphous organic material. On a transect from proximal to distal environments, these facies belts are: (1) a heavily bioturbated mudstone largely lacking sedimentary structures, (2) a laminated silt-rich mudstone with vertical bioturbation that partly disrupts sedimentary features, and (3) a radiolarian-rich mudstone with varying content of silt and clay. Storm deposits are evident in all facies belts as sub-millimeter thick fine silt laminae interpreted as distal tempestites. The occurrence of silt grains indicates that oxic/dysoxic water from the shallow-marine realm was transported into the deepest parts of the basin. Thus, it is unlikely that any facies belt was deposited in a continuously anoxic environment. The presence of vertical bioturbation in laminated silt-rich mudstones also argues against continuously anoxic conditions even some millimeters below the sediment-water interface. Only some of the radiolarian-rich facies, devoid of any trace fossils or tempestites, may reflect temporary anoxia, whereas others are rippled indicating bottom-current reworking during deposition. The upper Bakken shale represents a highstand unit, sandwiched between the overlying Mississippian Lodgepole Formation and the middle Bakken member. Although the basin center was at its greatest depth during periods of elevated sea-level, storms episodically influenced Bakken sedimentation, which indicates that this basin was a relatively shallow trough with maximum depth only slightly below storm wave base, perhaps <100 m. Thus, major organic-rich source rocks such as the upper Bakken shale can be deposited in relatively shallow water, above storm wave base, with tempestites being a major depositional feature.