HOW HIGH WERE THE ANCESTRAL ROCKY MOUNTAINS?

The Late Paleozoic Ancestral Rocky Mountains (ARM) formed in western equatorial Pangea (western U.S.) during the late Paleozoic icehouse, and consist of a series of basement-cored block uplifts and intervening basins. The ARM system remains tectonically enigmatic; its paleoelevation is unknown, but key for improving models of ARM geodynamics. Here, we document alpine glaciation within the equatorial ARM, and use this finding to construct preliminary estimates of ARM topography.

Alpine glaciation in the ARM is recorded by proglacial strata and glacially carved topography of late Paleozoic age. The inner, Precambrian-cored gorge of Unaweep Canyon, a geomorphologic enigma of the Uncompahgre Plateau, was glacially carved in the late Paleozoic, buried by the earliest Mesozoic, and fluvially exhumed in the late Cenozoic. Evidence for this history includes the observations that Mesozoic strata bury paleotopography exhibiting characteristic glacial features, and palynologically dated Permian diamictite occurs within the inner gorge. Further, we infer that the Pennsylvanian-Permian Cutler Formation exposed at the western mouth of Unaweep Canyon (near Gateway, CO) records proglacial deposition, in contrast to the long-accepted alluvial-fan interpretation for this system. Evidence includes the presence of (1) facies inconsistent with an arid alluvial-fan environment, such as cross-bedded gravelly sandstones that record deep sustained flow, and (2) deposits consistent with ice-contact and proglacial sedimentation, such as apparent glaciolacustrine strata containing m-scale dropstones.

Documentation of alpine glaciation in the late Paleozoic ARM implies more extensive icehouse conditions and significantly higher elevations than previously envisaged for this time period and region. Analogy to tropical glaciers of the Last Glacial Maximum suggests minimum ARM elevations in excess of 3600 m, and remarkably high relief, because the ARM basins were epeiric seas. Geodynamic models for ARM tectonism must therefore encompass uplifts that were high elevation, high relief, and short wavelength.