Date of Award
Doctor of Philosophy (PhD)
Outcrop and test hole data for 225 sites in a 33,700-km2 area of southwestern North Dakota were examined. Seven sedimentation intervals were identified for the Paleocene Bullion Creek and Sentinel Butte Formations. The intervals extend from the top of the Harmon coal (lower Bullion Creek) to the top of the Twin Buttes coal (upper Sentinel Butte). Each interval consists of medium and fine elastics underlying a persistent lignite coal, or some other lithology at the stratigraphic position of the coal. Clastics are finer-grained upwards within intervals and within both formations to the upper Sentinel Butte.
Sand-rich zones align northwest-southeast, parallel to the axis of the Williston Basin. Southwest of the axis, clay and silt dominate, and coal beds are few but thick; to the northeast, fine sand and silt dominate, and coal beds are many but thin.
The Williston Basin received sediments whenever the Powder River Basin fluvial system episodically diverted between the Cedar Creek and Nesson Anticlines. Flow was initially southeasterly along the axis of the basin, beginning each new sedimentation interval. Northwest-southeast-trending sandy belts resulted, and thick, single-bedded coals formed southwest of the axis. As sediments accumulated, the base level migrated north, forcing the system to flow easterly to northeasterly, down regional paleoslope. Diffuse, distributary-like sandy belts resulted, and thin coals, in multiple beds, formed northeast of the axis. Finally, sediment accumulation choked the diversion, and only a small amount of sediment entered the area. Fine and limy elastics and capping coals were deposited at the top of the interval.
During Bullion Creek time, the basin axis plunged southeast and the basin depocenter was south of Billings County, near what is now the southern edge of the basin. Anticlines in Billings County were not actively rising during Bullion Creek time. During Sentinel Butte time, the anticlines rose, but at less than the rate of basin subsidence. The depocenter migrated northeast to east of Billings County. After Sentinel Butte time, the southern end of the basin rose, producing the present subcircular southern margin and reversing the plunge of the basin axis to its present direction. The depocenter moved northwest to its present position north of Billings County.
Winczewski, Laramie M., "Paleocene coal-bearing sediments of the Williston Basin, North Dakota : an interaction between fluvial systems and an intracratonic basin" (1982). Theses and Dissertations. 329.