Structure of a part of the northern Black Hills and the Homestake Mine, Lead, South Dakota

Abstract

The pre-Cambrian rocks of the northern Black Hills in the vicinity of the Homestake Mine show the effects of extreme isoclinal folding on which has been superimposed shear folding crossing the isoclinal axial planes at small angles. An attempt has been made to distinguish the two types of folding in all scales of folds from those of microscopic size to those large enough to appear on the regional map. Enough Cambrian cover remains in this area to permit contouring the top of the pre-Cambrian rocks. These contours, which essentially measure post-Cretaceous deformation, show a close correlation between doming of the district and the intrusion of a stock of Eocene age. Several other domes in the northern Black Hills, previously called laccoliths, are believed to be due to stock intrusion. The deformation of pre-Cambrian rocks by the Tertiary intrusive bodies is more difficult to interpret. There are, however, many small-scale deformations attributable to igneous intrusion, and probably there are large ones as well. Whether the shear folds crossing the isoclinal axial planes are due to Tertiary igneous intrusion or, like the isoclinal folds, are of pre-Cambrian age cannot be answered at this time.