Tectonic evolution of western Colorado and eastern Utah
— D. L. Baars and G. M. Stevenson

The boundary between the Colorado Plateau and Southern Rocky Mountain provinces lies somewhere within the western Colorado-eastern Utah region, and is arbitrary in the geological sense, because the stratigraphy and tectonic relationships remain relatively constant throughout the area.

 

The area of this report lies astride the Uncompahgre Uplift segment of the Ancestral Rockies orogenic system (see fig. 2). To the west of the ancient Uncompahgre fault block lies the northwest- trending Paradox basin and to the east is the strikingly similar Eagle evaporite basin, both of late Paleozoic age. Bounding the region on the east are the marginal Gore and Mosquito faults of the Front Range segment of the Ancestral Rockies. Within this vast area the early Paleozoic stratigraphy is similar and essentially uniform within the paleotectonic basins, suggesting that the two down- warps were not so distinctly separated until after Mississippian time. However, in Pennsylvanian time the main uplift of fault blocks of the Ancestral Rockies abruptly separated and accentuated the Paradox and Eagle evaporite basins, although the stratigraphy of the two depressions remained remarkably similar. Both the Uncompahgre and Front Range uplifts became high topographic features and shed untold cubic kilometers of coarse, arkosic sediments into the eastern reaches of the structural basins. Whether the two salt basins were interconnected in a common sea is still a matter of personal interpretation.

 

The margins of the Paradox basin usually are defined by the geographic extent of salt deposited during Middle Pennsylvanian time in the Paradox Formation. Consequently, there is little or no reflection of the buried basin at the surface, except for the salt diapirs in the "Paradox fold and fault belt." The basin is bounded on the northeast and east bythe Uncompahgre Uplift and is surrounded elsewhere by paleotectonically controlled shallow-water shoals. The ovate basin has a northwesterly orientation, extending from Durango, Colorado and Farmington, New Mexico on the south-east to Green River, Utah on the northwest.

 

The Uncompahgre Uplift, which is best known for its exposures on the Uncompahgre Plateau and Black Canyon of the Gunnison River, extends for some 700 km northwestward from near Santa Fe, New Mexico almost to Provo, Utah. It is complexly fault- bounded on its southwestern margin and tilts gently eastward into the Eagle basin. Where exposed, the crest of the tilted fault block is Precambrian metamorphic and igneous rocks, overlain by various Mesozoic formations. Although it has a complex growth history, the Uncompahgre Uplift was a dominant feature throughout most of Permo Pennsylvanian time.

 

The Eagle evaporite basin lies generally between the tilted eastern margin of the Uncompahgre Uplift and the fault bounded Front Range Uplift on the east. The generally northwest-trending structural depression is a diminutive copy of the Paradox basin. The Eagle Valley Evaporites in the central part of the basin include gypsum and halite interbedded with drab-colored clastic rocks (Lovering and Mallory, 1962) that interfinger eastward with shelf carbonates and clastics of the Minturn Formation and arkose of the Maroon Formation. As in the Paradox basin, salt diapirs are present at a reduced scale near Glenwood Springs and Eagle, Colorado (see also Campbell, this guidebook).