Kaolinitic clay deposits occur in the lower member of the Golden Valley Formation of western North Dakota. The Eocene deposits are from 10 to 40 feet thick, flat lying, and stratiform. They consist of three consistent stratigraphic units; sand in the lower part, clay in the middle, and silt in the upper part. Limonite concretions lie in a definite horizon and stain the clay bright orange where weathered. A thin lignite or carbonaceous silt overlies the clay. Poorly crystallized kaolinite is the dominant clay mineral in the deposit, and 2M-type illite, halloysite, montmorillonite, mixed-layered illite-montmorillonite, and montmorillonite-chlorite are present in varying amounts. Chlorite is present in the enclosing sediments but absent in the clay deposits. The clay mineral content is very uniform within the area studied, but a definite stratigraphic variation in clay mineral proportions is indicated. The kaolinite content increases upward within the deposits as does montmorillonite; the illite content decreases.
Various theories of genesis are considered, but sedimentary structures and mineralogic and textural characteristics support the theory that the deposits originated detritally. Deposition in a large, freshwater, lake basin in a sub-tropical climate is postulated. The source of the kaolinitic clay may have been pre-Eocene rocks that lay to the west, but a widespread, deep-weathered zone in these rocks has not been established.