AN OVERVIEW OF THE VARIETY OF STRUCTURAL INTERPRETATIONS PROPOSED FOR THE EASTERN FLANK OF THE COLORADO FRONT RANGE

The geologic marker at Red Rocks Park shows a section across the margin of the Colorado Front Range that is familiar to most of us. It depicts the range, comprised of Precambrian basement and capped by east-dipping strata of the Fountain Formation through Benton Shale, thrust eastward over Pierre Shale upturned at the margin of the Denver Basin. The range carrying thrust, the Golden Fault, appears as a planar reverse fault dipping 50 degrees to the west and cutting to the surface. While this section has become the iconic structural picture for the Front Range, a review of the geological literature over the last 140 years reveals that a consensus has remained surprisingly elusive, and geologists have found reason to interpret the range margin in all manner of ways. A partial list includes: 1) east-dipping, west-directed, younger-over-older thrust faults, 2) unconformities folded by multistage deformation, 3) underthrusting of the range front from the east beneath a west-dipping reverse fault, 4) west-dipping, east-directed reverse faults that steepen upward and daylight as east-dipping normal faults, and 5) east-dipping, low-angle normal faults. Such diversity of opinion may come as no surprise to geologists, or it could speak to the underlying complexity of the problem. In an effort to reconcile these disparate views and address apparently conflicting geologic data, the speaker will offer a model showing stacked triangle zones at three principal detachments, breaks to the free or erosional surface, and a mix of older-over-younger and younger-over-older thrust relationships set up by backthrusts in the intercutaneous wedge.