NEOGENE FAULTING IN COLORADO’S HIGH PLAINS

Neogene uplift of the Colorado Rockies was accomplished by faulting. Historically, coeval tilting of the Great Plains was considered to involve only broad warping. However, several lines of evidence indicate that Neogene and Quaternary faulting are an important deformational component in the Great Plains of Colorado.

A digital elevation model reveals a graben 40 kilometers wide bounded by scarps 20 to 30 meters high. The graben is expressed in the surface morphology and subsurface structure. The western scarp trends N35W and is at least 135 kilometers long, extending from 28 km south of Akron to 25 km north of Cheyenne Wells. Trenching of the western scarp reveals that it is composed of truncated and tilted Quaternary sediments. Four different lines of evidence suggest that the western fault may have a strike-slip component: 1) linearity of the scarp, 2) presence of a left-stepping segment of the fault, 3) presence of two synthetic, Riedel shears splaying off the main scarp, and 4) presence of three left-lateral, stream offsets of 1.6 km each.

Geomorphologic and geologic analysis of the Colorado High Plains reveals a number of horst and graben structures throughout the area. Stream profiles show strong to moderate incision on the horsts and probable aggradation in the grabens. These faults have significance for groundwater resources, earthquake hazard, and hydrocarbon exploration and development.