THE END OF THE ANCESTRAL ROCKY MOUNTAINS

The Late Paleozoic Ancestral Rocky Mountains constitute a collection of amagmatic, largely basement-cored uplifts and linked basins that formed in western equatorial Pangaea far from any recognized plate boundary. They are enigmatic in several ways: What caused these isolated but penecontemporaneous uplifts? What is the significance of their spatial distribution and orientation? Why are their geologic histories so complementary? How did these uplifts disappear during the latest Paleozoic geologic record? Here we highlight paleogeomorphic and sedimentologic data from two prominent and widely separated ARM uplifts to focus on this last question.

The Uncompahgre uplift of the western ARM (Colorado) exhibits at least 6 km of structural relief that formed during ARM orogenesis. The conglomeratic, Pennsylvanian-Permian Cutler Formation of the proximal Paradox basin wedges toward the uplift; newly collected field data indicate the youngest Cutler strata onlap the basement and bury approximately 1 km of relief carved in the uplift, largely preserved in the form of the modern Unaweep Canyon, which harbors a basal unit provisionally dated as late Paleozoic in age.

The Wichita uplift of the eastern ARM (Oklahoma) was apparently worn down to a near sea-level peneplain in the latest Pennsylvanian after about 7 km of uplift. A distinct and late-stage uplift of this block occurred in earliest Permian, yielding relief of ~1 km. This is documented by the existence and character of the Post Oak Conglomerate, by the existence of regional paleotopography, and by the character of the surrounding Permian strata. Burial and preservation of this topography requires active subsidence of at least 1 km in the early to middle Permian.

Preservation in both these widely separated sites of significant paleorelief beneath Permian strata, together with regional evidence for the broad disappearance of the coarse clastic mantles around ARM uplifts by mid-Permian time, suggests the ARM uplifts and their immediate surroundings succumbed to active (tectonic) subsidence soon after reaching their apogee. Absence of obvious collapse structures argue against orogenic collapse isolated to the uplifts. Rather, the regional character of this signature, and uniform magnitude, imply an as-yet unrecognized epeirogenic control.