LARAMIDE BENEATH THE PRAIRIES: GEOMETRIES AND RELATIONSHIPS OF THE CHADRON DOME, HARTVILLE UPLIFT AND BLACK HILLS UPLIFT TO THE POWDER RIVER AND DENVER-JULESBURG BASINS

The Laramide orogen, which comprises topographically and structurally distinctive basins and uplifts forming portions of the Rocky Mountains, USA, continues as much as 250 km to the east beneath the Prairies. Near the common boundary of Nebraska, South Dakota and Wyoming, three Laramide uplifts (Hartville on the west; Black Hills on the north; Chadron in the southeast), join. The north-northwest-trending Black Hills uplift and Chadron dome lie along a continuous structural arch that extends from central Montana on the northwest to western Kansas. The uniquely northeast-trending Hartville uplift, extending from the Laramie Range on the southwest to the southern terminus of the Black Hills uplift, represents a later phase of Laramide development which divided the formerly continuous Powder River and Denver-Julesburg basins.

Structure contours on the upper contact of the Niobrara Shale illustrate structural relief of 11,000 ft (Black Hills uplift), 9,000 ft (Hartville uplift) and about 6,000 ft (Chadron dome). Much of the character of the Hartville and Chadron structures is obscured beneath the cover of the Eocene-Oligocene White River Group.

Monoclines define the western margin of the Black Hills uplift and the north flank of the Hartville uplift. The western limb of the Chadron dome shares a long, continuous dip slope with the northern end of the Denver-Julesburg basin which, in turn, is separated from the southeast flank of the Hartville uplift by 4,000 feet of throw along the NE-trending, Wheatland-Whalen fault zone. This fault zone continues to the northeast, separating the Chadron dome from the southern limit of the Black Hills uplift. Based upon highly variable well spacing, elongated structural highs along this trend suggest either northeast-trending anticlines or positive flower structures. Combined with the en echelon, south-plunging fold train that forms the southern limit of the Black Hills uplift, this geometry is suggestive of a component of right slip along the Laramide-aged boundary.

The Wheatland-Whalen fault zone and the Fanny Peak monocline along the west flank of the Black Hills uplift appear to overlie terrain boundaries within the underlying Precambrian basement. The basement weakness underlying the Wheatland Whalen fault zone may have guided the anomalous NE trend of the Hartville uplift.