Date of Award
Master of Arts (MA)
Southwestern McLean County is an area characterized by glacial topography and glacially-modified bedrock topography, with limited exposures of bedrock. Outcropping bedrock formations include the Cannonball, Tongue River, and Sentinel Butte Formations, all in the Fort Union Group of Paleocene age.
The Cannonball Formation crops out only in the southeastern part of the area studied (at Washburn), where 18 feet of laminated, gray-brown sandy mudstone is exposed. A yellow-brown, cross-bedded sand unit directly overlies the mudstone. I have tentatively selected the base of this sand unit, which is present in the subsurface throughout southwestern McLean County and reaches thicknesses of more than 125 feet, as the contact between the Cannonball and Tongue River Formations.
The contact between the Tongue River and Sentinel Butte Formations is exposed in the Fort Mandan Badlands. Lithologically, the upper part of the Tongue River Formation and the lower part of the Sentinel Butte Formation are similar. Sediment of the Tongue River Formation consists predominantly of light yellow-buff sandstone, with some buff to gray clay and silt, and lignite. Sediment of the over lying Sentinel Butte Formation consists mostly of somber gray-brown sand, silt and clay, and lignite.
Criteria used to define the contact between the Tongue River and Sentinel Butte Formations are (1) a change in color from yellowish beds in the Tongue River to somber gray-brown beds in the Sentinel Butte, (2) the presence of a lignite bed at or a few feet below the contact, and (3) the presence of a silt or sand bed, overlying either the lignite bed or a clay bed directly above the lignite.
The contact between the Tongue River and Sentinel Butte Formations is located in the subsurface by correlating marker beds and stratigraphic sequences. Marker beds that I have informally named, which are used for correlation between the outcrop area and the sub surface area, are the TR Bed, and the A and B Beds. The TR Bed is a lignite bed associated with the contact. The A and B Beds are both lignite beds in the Sentinel Butte Formation that are scheduled for future mining in the Underwood Lignite Field.
The environment of deposition for the upper part of the Tongue River Formation and the lower part of the Sentinel Butte Formation is interpreted as fluvial. Sediment that I have provisionally assigned to the lower part of the Tongue River Formation was probably deposited mostly as a transitional facies between the marine Cannonball Formation and the continental upper part of the Tongue River Formation. The upper part of the Tongue River Formation and the lower part of the Sentinel Butte Formation were probably deposited on an aggrading delta or a coastal plain as channel, levee, crevasse-splay, flood basin, and swamp deposits.
Hemish, LeRoy A., "Stratigraphy of the upper part of the Fort Union Group in southwestern McLean County, North Dakota" (1975). Theses and Dissertations. 133.