PRELIMINARY RESULTS FROM SEISMIC IMAGING OF THE BASEMENT PROFILE IN WESTERN UNAWEEP CANYON,COLORADO

The origin of Unaweep Canyon (western Colorado) remains debated. Some authors relate the genesis of the canyon to fluvial processes, presuming that either the Colorado and/or Gunninson rivers carved the canyon in the Plio-Pleistocene. Others suggest that it was created due to glacial processes in either Pleistocene or late Paleozoic time. To help distinguish among these hypotheses, it is important to determine the shape of the basement profile, and depth and character of the valley fill. To achieve this, a preliminary geophysical survey conducted within Unaweep Canyon in summer 2003 provided a first velocity model, and an estimated depth for the top of the Precambrian basement of 60 m. In summer 2004, seismic and gravity surveys were acquired within western Unaweep Canyon, approximately 2 km from the 2003 survey. Seismic reflection data were collected in CMP mode along two lines: a NW-SE transverse profile (540 m long) across the canyon, and a longitudinal profile (150 m long) that crosses the transverse line.

The transverse refraction profile was supplemented by five shots at one end of the profile, producing reversed refraction profiles. Travel times from the refraction profiles indicate a velocity jump to approximately 1600 m/s at a depth of approximately 10 m. These values corroborate earlier refraction results conducted in 2003. The persistence of the refraction branch from this boundary at large distances from the shot indicates that there are no velocity increases to a depth ≥100 m, and may indicate a thick section of heterogeneous conglomerate. The shallow refraction model will be used to correct the reflection profiles for variation in near-surface velocities. We are also integrating lithologic, petrophysical and depth data from a well drilled on the transverse profile to provide parameters for data processing and interpretation.

Ongoing analysis of these data will allow a more accurate assessment of the bedrock shape of the canyon. If the true transverse profile of Unaweep Canyon is U-shaped, a glacial origin is likely, whereas a V-shape would be more consistent with a fluvial origin for the canyon. The seismic results will also enable analysis of the thickness and character of the canyon fill in regions beyond the well bore, which will contribute to our understanding of the possible age and genesis of Unaweep Canyon.