THE GEOMETRY AND TECTONICS OF THE UNCOMPAGHRE MOUNTAIN FRONT, WESTERN COLORADO

The Uncompaghre Uplift-Paradox Basin structural front trends NW-SE in east-central Utah and western Colorado. The boundary is usually depicted, based on published data in Utah, as a single reverse fault that cuts synorogenic strata in the basin, with structural relief up to approximately 8 kilometers. Coarse clastic arkoses shed from the uplift are interpreted to have filled the basin and interfingered with Paradox Formation evaporites. Seismic and well data, however, indicate that, in the Colorado part of the Uncompaghre front, Cutler Formation arkoses form a large growth wedge that overlies and displaces Paradox salt. In addition, thickness patterns in the Cutler Formation indicate depocenters developed progressively southwestward from the mountain front. Sediment loading caused the underlying salt to move into the diapiric walls, which grew by down building of the adjacent depocenters. The top of the walls was always near the earth's surface and provided topography that influenced sedimentary accommodation space. Once the salt was completely evacuated beneath a depocenter, no more accommodation space was possible and younger sediments were deposited farther from the mountain front where salt was still present. The loading of the new depocenter resulted in moving salt from beneath that location into adjacent salt walls. The sediments and salt walls are progressively younger to the southwest, away from the mountain front. Syn-tectonic growth wedges of Cutler Formation thin onto the mountain front, above the bounding fault zone, which is made up of several thrust imbricates that have basement on their hanging walls. Restoration of mountain front structure suggests that the Uncompaghre Uplift had little or no topographic effect on the deposition of the salt. The data are permissive then, of the interpretation that the Paradox Basin the Eagle Valley evaporites were continuous before uplift of the Uncompaghre.