New Interpretation of the Paleogeography of the Ancestral Rocky Mountains, Colorado

Kluth, Charles F.¹ (1) Kluth & Associates, Colorado School of Mines, Golden, CO

Existing interpretations of the classical Ancestral Rocky Mountains are in need of revision.

N-S stratigraphy of the Fountain Formation along the Front Range suggests that the Fountain lapped onto a broad NW-SE arch that began to develop in early Pennsylvanian time. The Front Range was separated from a separate, narrow uplifted block in the Colorado Springs area, the Ute Pass Block. The southwestern margin of the Front Range was faulted and had approximately 6 kilometers of structural relief. The uplift was a NE dip slope that ended at the Colorado-Wyoming state line.

The Central Colorado Trough, located between the Ancestral Front Range and the San Luis Highland and Uncompaghre Uplift, contained complex faulting and crustal slivers within the trough.

The San Luis Highland was a separate uplift from the Uncompaghre Uplift and is interpreted to have been a west dipping fault block with approximately 8 kilometers of structural relief on its eastern side. It was uplifted in early Pennsylvanian time and shed coarse sediments eastward and northward into the Central Colorado Trough, and generally finer-grained sediments westward into the Paradox Basin.

The Uncompaghre Uplift formed in late Pennsylvanian and early Permian time, after the deposition of middle Pennsylvanian evaporites. The Uncompaghre front in Colorado is a stack of SW directed thrust faulted basement blocks. Loading by synorogenic sediments displaced salt into long walls by downbuilding. The development of accommodation space between salt walls, ended when the salt was squeezed out and formed a weld. The salt walls young toward the SW. The new interpretation of the Uncompaghre Uplift suggests that the Paradox salts and the Eagle Valley Evaporites were deposited in a continuous basin.