Bentonite deposits of the northern Black Hills district, Wyoming, Montana, and South Dakota
The northern Black Hills bentonite mining district includes parts of Crook County, Wyo., Carter County, Mont., and Butte County, S. Dak. Within this district, many beds of bentonite occur interspersed with sedimentary strata of Cretaceous age that have an average total thickness of about 3,000 feet and consist chiefly of marine shale, marl, and argillaceous sandstone. The bentonite beds occur in formations ranging upward from the Newcastle sandstone to the lower part of the Mitten black shale member of the Pierre shale. Tertiary (?) and Quaternary deposits of gravel, sand, and silt are present on extensive terraces, and deposits of such materials also extend along stream courses in all parts of the district. The overall geologic structure of the district is that of a broad northwestward- plunging anticline, in which the strata dip gently toward the northeast, north, and northwest. The overall structure is interrupted, however, by several subordinate folds which bring the bentonite beds to the surface repeatedly, so that large resources of bentonite are present under light overburden. The northern Black Hills district is an important source of commercial gel-forming sodium-type bentonite. During the period 1941-56 more than 5 million tons of raw bentonite was mined, most of which came from the Clay Spur bed near the top of the Mowry shale; a few thousand tons was mined from bed A in the Newcastle sandstone. Calcium-type bentonite occurs in bed B in the Mowry shale and in bed I at the base of the Mitten black shale member. Seven other beds are sufficiently thick and continuous to warrant consideration as prospective sources of bentonite for industrial use. Most of the bentonite produced is sold for use (a) as an ingredient of drilling mud; (b) for preparing metallurgical molding sand of superior dry strength; and (c) for the bonding material used in pelletizing taconite iron ore of the Lake Superior region. The results of drilling-mud and foundry-sand bonding-clay tests of several hundred samples, as well as analyses of selected samples, chiefly by X-ray, differential thermal, base exchange and spectrographic methods, are included in this report.