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
\nMine show the effects of extreme isoclinal folding on which has been superimposed
\nshear folding crossing the isoclinal axial planes at small angles. An attempt
\nhas been made to distinguish the two types of folding in all scales of folds from those
\nof microscopic size to those large enough to appear on the regional map.
\n
\nEnough Cambrian cover remains in this area to permit contouring the top of the
\npre-Cambrian rocks. These contours, which essentially measure post-Cretaceous deformation, show a close correlation between doming of the district and the intrusion
\nof a stock of Eocene age. Several other domes in the northern Black Hills,
\npreviously called laccoliths, are believed to be due to stock intrusion.
\n
\nThe deformation of pre-Cambrian rocks by the Tertiary intrusive bodies is more
\ndifficult to interpret. There are, however, many small-scale deformations attributable
\nto igneous intrusion, and probably there are large ones as well. Whether
\nthe shear folds crossing the isoclinal axial planes are due to Tertiary igneous intrusion
\nor, like the isoclinal folds, are of pre-Cambrian age cannot be answered at this
\ntime.