Eastern Boulder-Weld fault zone, Colorado: A gravity slide with pop-up structures

Abstract

The Boulder-Weld fault zone, located southeast of Boulder, Colorado, is about 10 km (6 mi) wide, 34 km (21 mi) long, and involves at least 335 m (1100 ft) of upper Cretaceous sedimentary rocks. It affects the Cretaceous upper Pierre Shale, Fox Hills Sandstone, and the coal–bearing lower Laramie Formation. This study is a detailed examination of the eastern portion of the fault zone which consists of undisturbed areas separated by three long, narrow, fault-bounded uplifts that have received a variety of interpretations over the years. The fault zone geometry is determined from 21 closely spaced cross sections that use more subsurface data than previous studies, incorporate the elevations of the major economic coal seam derived from a published composite structure-contour map, and are area balanced using area-depth-strain (ADS) analysis. The most common structural style is a pop-up structure in which the uplifts are bounded on both sides by reverse faults. At larger-displacement the pop-ups are at the tip of the ramp and a second fault has formed close to the base of the ramp. A few sections show simple ramp anticlines developed above listric thrusts. The lower detachment for all structures is the distinctive Kp2 marker in the upper Pierre Shale. ADS analysis of the best-controlled uplifts shows that the uplifts are area balanced and confirms the lower detachment to be near Kp2. The structures are interpreted to have formed as a gravity slide because they formed in a break-back sequence, a characteristic of gravity gliding, and because the transport direction is approximately down the current southeast dip of the Kp2 detachment.