THE TECTONIC DEVELOPMENT OF THE UNCOMPAHGRE UPLIFT AND THE PARADOX BASIN, UTAH AND COLORADO

The Uncompaghre Uplift and Paradox Basin are large, intraplate structures that formed in late Pennsylvanian time as part of the Ancestral Rocky Mountains. The subsurface data indicates that published interpretations of the development of these structures should be revised. The data show that the geometry of the Uncompaghre front to the SE from the Utah/Colorado state line contrasts to the single large fault in Utah, and is a stack of SW directed thrust faulted basement blocks that also carry Mississippian and older, pre-salt rocks. Distribution of synorogenic sediments was largely by axial river systems and post dates the deposition of the Paradox Salt and the more uniform thickness, post-salt, but pre-growth strata. The growth strata geometry record the development of accommodation space, and moving of the salt due to differential loading. the loading by the sediments caused the underlying, mobile salt to move into salt walls that nucleated on earlier, fault-caused, basement topography. The salt walls grew by a process called 'downbuilding' due to the differential loading and some salt was at the surface during the entire growth history of the salt wall growth. The development of accommodation space for each mini-basin between salt walls, ended when the pre-salt and post-salt sections welded together as the last of the salt moved from beneath parts of the basin and closed salt movement pathways. The locus of deposition then shifted to the SW, farther away from the Uncompaghre front, and a younger salt wall and mini-basin formed. This process was repeated several times and resulted in a younging of the salt walls toward the SW. The coarse alluvial fan material was preserved and prograded away from the mountain front only after the locus of deposition moved to the SW and axial rivers no longer redistributed the erosional debris. The new interpretation of the geometry and timing of the Uncompaghre Uplift suggests that the Paradox salts and the Eagle Valley Evaporites were deposited in a continuous basin that existed at times across the site of the later Uncompaghre Uplift. Finally, it appears that there may be too much sand in the synorogenic deposits in the Paradox Basin to have been derived solely from the Uncompahgre Uplift.