ABANDONMENT OF UNAWEEP CANYON, WESTERN COLORADO: STREAM PIRACY AIDED BY MAJOR LANDSLIDE

The deep windgap of Unaweep Canyon, which transects the Uncompahgre Plateau southwest of Grand Junction, is critical to understanding Colorado River drainage history. A recent test well in the west part of Unaweep Canyon drilled by University of Oklahoma researchers encountered a thick section of fine-grained lacustrine sediments. These apparently were deposited in a lake formed behind a landslide which is preserved as coarse breccia exposed in a roadcut ~ 6 km west of the test well. The test well verifies older seismic and electrical resistivity interpretations of thick valley fill within central Unaweep Canyon, including > 150-300 m of fill 2 km northeast of the roadcut breccia. A landslide can explain the abrupt southwestward steepening of West Creek's gradient from 80 m/km to 200 m/km and thinning of valley fill to 20-55m near the southwest canyon mouth.

New mapping indicates the landslide is much larger than suspected, extending ~ 4 km northeast of the roadcut breccia. Individual landslide blocks of basement rock capped by Mesozoic sandstone float form rounded hills 200-1000 m across and 135-330 m high. Tops of blocks range from 2035-2465 m in elevation and rise above broad alluvial fans at 1900-2135 m. Four blocks comprise an undulating N-S ridge that descends from the headscarp on the north canyon wall at elevation 2630 m, near a mapped fault and the Uncompahgre structural axis. A fifth block abuts the south canyon wall.

Headward erosion by tributary streams set the stage for stream piracy, but diversion was caused by landslide damming and subsequent spill-point overflow at > 1926 m, the highest elevation of lake sediments reported from Cactus Park. In the test well, an upward change of pollen from Paleozoic and Cenozoic forms to Cenozoic-only forms might reflect a two-stage abandonment sequence: Colorado River first and Gunnison River second. The substantial rise in local base level resulted in Unaweep streams being superimposed or "pinned" on basement rock at three locations: the northeastern and southwestern canyon mouths, and adjacent to the roadcut breccia exposure itself. These factors effectively halted incision of Unaweep Canyon and may have greatly reduced erosion of the canyon walls, providing a rare opportunity for comparative studies of erosion.