ABSTRACT: Decollement Faulting in the Western Portion of the Denver Basin,

Ken Kittleson

Detailed correlations of 825 well logs, within an 80 km² portion of Weld County, Colorado, has revealed the Boulder-Weld fault zone was formed by decollement faulting. The Upper Cretaceous sedimentary rocks here, from the surface to a depth of about 366 m, have been displaced downdip by gravity sliding along bedding plane faults, as a result of the uplift of the Front Range to the west. The faults in this structural zone were previously mapped as either a series of horstgraben faults or as growth faults. Additional unmapped decollement faults probably exist in the 230 km² area between the westernmost mapped decollement fault and the Front Range Uplift. A recognition of a decollement origin for this fault zone has permitted a reevaluation of the minea le coal resources in the Boulder-Weld coal field, as well as more efficient production of ground water from the faulted Laramie-Fox Hills aquifer. Reverse faults with up to 183 m of throw, which were produced by decollement faulting, may have created oil and gas traps just east of the Front Range Uplift. The Boulder-Weld fault zone probably extends beneath the Rocky Flats Nuclear Plant, a recognized source of ground water contamination. If the Laramie-Fox Hills aquifer underlying this site is contaminated, the cleanup of this unit will depend on its accurate structural interpretation.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990