PRECAMBRIAN TEMPLATE FOR LARAMIDE STRUCTURES, EASTERN BLACK HILLS UPLIFT

The doubly plunging, NNW-trending Black Hills uplift is a broad arch, bound by two large monoclines on the west and a broad half dome on the east. Granite of the Wyoming Archean province underlies the western portion and the Late Proterozoic Trans-Hudson Province the east flank. Dominantly west-facing, second-order folds across the larger structure form monoclines on the west side and anticlines on the east.

In the east-central portion of the uplift anticline-syncline pairs and rare east-vergent monoclines with structural relief to 300 m plunge SSE or E affecting the east-dipping panel of Phanerozoic strata. In the core of the uplift the SSE-trending “grain” of steeply dipping large faults, contacts, schistosity, and bedding of the Precambrian basement parallels strong aeromagnetic and gravity gradients. A similar geophysical pattern continues eastward in the area of Phanerozoic cover. Folds extending for many kilometers lie along some of the steepest geophysical gradients. Coincidence of Laramide fold trends with steep geophysical gradients and Precambrian structural grain imply a genetic relationship of Laramide structures over a Precambrian basement template. South of Rapid City, in areas interpreted from easterly trending geophysical anomalies to be underlain by granite plutons, east-trending, south-vergent monoclines form a linking pattern with SSE-plunging folds. An example exposed along Spring Creek shows a Precambrian fault zone reactivated by a Laramide fault. In other areas the Precambrian-Cambrian contact is folded.