Structure and genesis of the Boulder-Weld allochthon, Denver Basin, Colorado - Gravity slide or Laramide thrust sheet?

This study was undertaken to determine the structure and genesis of the Boulder-Weld allochthon (BWA), the 216 mi2 (559 km2) remnant of a once larger feature, that moved east from the flank of the Front Range into the western part of the Denver Basin. This review of surface and subsurface data revealed new aspects of the BWA, especially in its western part. There, the decollement of the BWA ramps 900 feet up-section to the east from a near bedding-parallel detachment low in the upper transition member of the Pierre Shale to a bedding-parallel detachment near the base of the Fox Hills Formation. Repeated sections found in wells east of the decollement ramp demonstrate up to two miles of translation in the system. Secondary faults in the hanging wall of the allochthon include antithetic thrusts bounding pop-up structures and occasional normal faults that almost exclusively overprint the decollement ramp. The hanging wall is also cut by a postulated tear fault separating areas exhibiting different amounts of translation. The western, trailing edge of the decollement shows attenuation in its hanging wall that increases to the west. This part of the decollement either represents a very low-angle breakaway normal fault or a thrust fault cutting slightly down-section in the direction of transport. Past studies perceived a southeast transport direction for the BWA in contrast to the northeast slip directions on nearby Laramide thrusts, a difference used to interpret the allochthon as a gravity slide. However, similar east-west oriented slickenlines on thrusts across the western part of the allochthon and into the neighboring Front Range leave open the possibility the BWA originated as a Laramide thrust sheet. Furthermore, both the BWA and Laramide thrusts in the neighboring Front Range utilized detachments near the top of the Pierre Shale, suggesting a possible common genesis. Given the available data, both the gravity slide and Laramide thrust models provide viable explanations for the BWA.

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

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.