THE ANCESTRAL ROCKY MOUNTAINS AND THE CRATONIC SHELF OF WESTERN PANGAEA

The correspondence of Late Paleozoic tectonic and stratigraphic events in the Ancestral Rocky Mountains (ARM) and the Cordilleran margin on western Pangaea is not yet clear. New data in both areas suggest a previously underappreciated complexity in both regions, but correlations of these features through time and space may now be possible.

Two large uplifts in the ARM, the northeast-tilted Ancestral Front Range, and the west-tilted San Luis Uplift developed in Middle Pennsylvanian and were separated by a complexly block faulted Central Colorado Trough. Parts of the western Central Colorado Trough were part of a large salt basin that extended into eastern Utah. The bounding faults on the uplifts were covered by Desmoinesian time. Faulting in the Central Colorado Trough extended westward beneath the present day Uinta Basin, and possibly influenced the development of the Oquirrh Basin in northern Utah. Uplift of the northeast-tilted Uncompahgre Uplift segmented the earlier salt basin in western Colorado and eastern Utah. As the Uncompahgre Uplift and the adjoining Paradox Basin developed in the earliest Permian, the Oquirrh Basin became starved for sand input. The thick Cutler Formation in the Paradox Basin region attests to the large input of sand and shale in that area during that time. Other basins and uplifts in the region are small compared to the ARM. A shallow marine to nonmarine cratonic shelf was located southwestward, westward and northwestward of the ARM, and extended to the Cordilleran Hingeline. This shelf received a mixture of fine sands and carbonates during the Pennsylvanian and early Permian. This shelf separated the ARM from coeval tectonic events in the Cordillera. Features that record equivalent tectonic events and which demonstrate stratigraphic ties from both ARM and the Cordillera have not yet been recognized in the shelf region, but are the subject of renewed study.