ESTIMATED AGES OF TERRACES AND PLEISTOCENE MIGRATION OF THE COLORADO RIVER NEAR GRAND JUNCTION, COLORADO

Recent geologic mapping in the Grand Valley of west-central Colorado has identified a series of five distinct terraces underlain by gravel of the Colorado River. The terraces range from 10 to about 170 m above the present river. The average long-term (past 10 my) rate of regional down-cutting, 0.15 +/- 0.01 m per 1000 years (m/ky), can be used to estimate the ages of the terraces. Thus terrace T10 is about 70 ky old, T30 is about 200 ky old, T60 is about 400 ky, T100 is about 670 ky and T170 is about 1.1 million years old. A possible terrace between T60 and T100 would be intermediate in age. This long-term rate probably over estimates the ages of the younger two terraces thought to correlate with the Pinedale (12-35 ky) and Bull Lake glaciations (130 ky). A small patch of gravel 610 m above the river near its exit from DeBeque Canyon is the highest Colorado River gravel mapped in the area and is estimated to be about 4 million years old. The presence of well-defined terrace levels suggests that the down-cutting was episodic, perhaps related to periods of glaciation in the mountains to the east and south. Mapping also identified a series of Colorado River terrace remnants above and west of the present course of the Gunnison River. Gravels in these remnants correlate with the T170 gravels several km to the east in the Clifton Quadrangle and indicates that about 1.1 million years ago the Colorado River flowed several km to the south into what is now the Gunnison River valley. At the eastern margin of the Uncompahgre Plateau, the river turned northwestward around the plateau, following the approximate contact between the Mancos Shale and the Dakota Formation. The river has subsequently migrated northward as it cut downward and down dip. The present course of the river is about 6 km north of the southernmost segment of the T170 terrace remnant. Buried gravels beneath the city of Grand Junction indicate that the river once flowed an additional 1.5 km to the north. There has been a long-standing controversy as to whether the Colorado River once flowed through Unaweep Canyon, a spectacular canyon incised into the Uncompahgre Plateau. Terrace gravels of the Colorado River are aligned toward the canyon and extend to within 4 km of it. Although the gravels do not prove that the river formerly flowed through Unaweep Canyon, they do provide strong supporting evidence.