REINTERPRETATION OF THE GEOMETRY AND ORIENTATION OF THE LATE PALEOZOIC FRONTRANGE UPLIFT

Late Paleozoic Frontrange Uplift is typically shown trending N-S. Data suggest that the uplift was oriented NW-SE, and may have been a block faulted uplift with faults on the SW margin and a relatively unfaulted dipslope on its NE margin. The SW end of the structure is between Denver and Perry Park, approximately midway between Denver and Colorado Springs, Colorado. Marine rocks and preservation of pre-Pennsylvanian rocks between Perry Park and Colorado Springs indicate that arkoses of the Fountain Formation were deposited in a trough that separated the Ute Pass block at Colorado Springs from the Frontrange north of Perry Park. The data permit the interpretation that this trough completely separated the Frontrange from the Ute Pass block and, at times, connected westward to the Central Colorado Trough. Sediment in the northeastern Central Colorado Trough was shed southward from the Frontrange and the boundary of the uplift is indicated by faults and rapid thickness changes in syn-tectonic deposits. The NW end of the uplift is located in the vicinity of Meeker, Colorado, based on the preservation of pre-Pennsylvanian rocks, even on the hanging walls of Pennsylvanian structures. Seismic and well data indicate that the NW end of the uplift was located near the present-day Colorado-Wyoming state line. The NE side of the Frontrange is problematical due to lack of exposure or preservation of critical relationships. It is interpreted to have been a NE trending dipslope along a line approximately from Baggs, Wyoming to Lyons, Colorado. Marine rocks interbedded with arkosic sediments suggest that the area to the northeast of the Frontrange was a broad shelf that was occasionally inundated. The Fountain Formation along the present-day Front Range appears to have onlapped the SE plunge end of the uplift. There appears to be little reactivation of Late Paleozoic structures by the Laramide Front Range Uplift.