Geology of the northern Black Hills bentonite mining district

Extensive deposits of bentonite are present in the Cretaceous sedimentary rock strata cropping out on the northern flank of the Black Hills. These deposits have been the subject of a study by the U.S. Geological Survey as part of the program of the Department of the Interior for appraisal of natural resources of the Missouri River Basin. This report contains information concerning the geology of the bentonite deposits of the Northern Black Hills district. An attempt is made to explain some of the relations that exist between the physical properties and the mineralogical characteristics of the bentonite. Consideration is given to the occurrence of bentonite deposits and different types of sedimentary rocks and to depositional and post-depositional conditions which these rocks reveal. Evidence for sources of ancestral materials and conditions of alteration are discussed.