DISTRIBUTED BASEMENT DEFORMATION IN LARAMIDE DOMAL GENESIS, EASTERN BLACK HILLS UPLIFT, SOUTH DAKOTA, USA

The eastern flank of the Black Hills uplift extends 300 km from southwestern Montana to southwest South Dakota as a broad half-dome. Outcrop patterns of Phanerozoic strata from Montana to the latitude of Rapid City generally trend N.40W.: South of Rapid City across a distance of 50 km, the strike changes to N.30E. forming a broad reverse-C pattern. Greatest structural relief across the uplift (~2,100 m) occurs in the area of the curving flank where exposures of the Proterozoic Trans-Hudson Province are exposed in the topographic Black Hills. The basement fabric of faults and multiply deformed metasedimentary and metaigneous rocks parallel gravity survey high and low contours and SSE-plunging anticline-syncline pairs in the Phanerozoic strata, suggesting a basement control for later folding. With few exceptions, the multiple Laramide folds of the uplift are west-vergent forming asymmetric anticlines on the east side and monoclines on the west, including those separating the uplift from the adjoining Powder River basin.

Recent 1:24,000-scale geologic mapping in the Cretaceous shale section exposed in the prairies southward from Rapid City shows that the strata do not curve in a continuous arch. Rather, a mosaic of SE-plunging anticline-syncline pairs (to 40 km long) and east-plunging, south-facing monoclines bound triangular, gently east-dipping, planar segments (to 10 km on a side). North-trending structure contour lines in each segment show a consistent right-step across such folds and structural relief to 100 m. The half dome does not form a continuous arch, therefore, but results from integration of adjustments on multiple basement-cored blocks.