SEISMIC IMAGING OF AN OVER-DEEPENED ALPINE VALLEY: IMPLICATIONS FOR LATE PALEOZOIC GLACIATION OF THE UNCOMPAHGRE UPLIFT (WESTERN COLORADO)

Unaweep Canyon is a unique landform located within the Uncompahgre Plateau in western Colorado. Two small creeks flow from the middle of the canyon, but are clearly underfit, such that the formation of the canyon remains debated. Most suggest that the canyon was formed by the ancestral Gunnison River in the late Cenozoic and later abandoned, but many attributes are inconsistent with a purely fluvial origin. An alternative hypothesis posits that the canyon was carved by glaciation in the late Paleozoic, and later exhumed by the ancestral Gunnison River. A glacial origin remains controversial since the Uncompahgre uplift was equatorial during the late Paleozoic. Previous geophysical surveys suggested that the valley might be over-deepened, but were inconclusive regarding the exact depths and also the valley geometry.

To better understand the subsurface structure and genesis of Unaweep Canyon, new high-resolution seismic reflection data were acquired in 2017. Receivers (500) were deployed at a 5 m interval along a 2.5 km-long profile, and a nitrogen-pressured impact source was fired at 228 shot locations. Data are of excellent quality and enable imaging of a U-shaped valley with maximum depths to basement reaching 400 to 550 m. In addition to reflection processing, refraction travel-time tomography and shear-wave velocity inversion were applied to the data. Results support the interpretation of significant over-deepening, and steep valley walls. The seismic velocity structure of the sediment fill in the over-deepened section suggests a discontinuity at ~ 200 m depth, possibly recording a transition into Paleozoic strata.

Given the lack of a structural explanation in the area, the over-deepening and U-shape are unambiguous indicators of a glacial origin for the paleovalley. The Uncompahgre Plateau was not glaciated in the Quaternary, thus Paleozoic glaciation appears to be the most likely explanation. This implies that our understanding of the climate of the late Paleozoic needs to be refined. Our study also showcases how seismic acquisition can be optimized to image geologic targets in shallow- to intermediate depths.