OLIGO-MIOCENE RIVER EVOLUTION OF THE UPPER COLORADO RIVER SYSTEM

New geologic mapping and detrital sanidine ⁴⁰Ar/³⁹Ar and detrital zircon U-Pb geochronology show that integration of the upper Colorado River system began in the early Miocene following cessation of major San Juan volcanic field (SJVF) activity. The two most important ancient river deposits supporting this interpretation are located on Grand Mesa and the Uncompahgre Plateau in western Colorado.

Grand Mesa (elev. 3000 m) is capped by ca. 11-10 Ma basalt flows that overlie 5 m of fluvial gravel, sand and mud. Detrital sanidine and zircon data show that these sediments are late Miocene (14-11 Ma) in age, which is consistent with the ages of the overlying basalt flows. Gravel composition and detrital zircon indicate show that the sources for these sediments were the Colorado Rocky Mountains, which suggests that this ancestral Colorado River flowed west from the Rocky Mountains towards the Colorado Plateau by the late Miocene. The presence of andesitic gravel clasts and ca. 29-27 Ma detrital sanidine and zircon age populations further suggests that during the Miocene, the watershed of the ancestral Colorado River contained significant thicknesses of volcanic rock derived from the SJVF. Sanidine data also indicate that volcanic airfall from Miocene Yellowstone hotspot track eruptions is preserved in the fluvial sediments beneath Grand Mesa.

Columbine Pass (elev. 2800 m) on the Uncompahgre Plateau is represented by 4 m of fluvial gravel, sand and mud that locally mantle the crest of the plateau. The abundance of volcanic gravel clasts show that this ancient river system drained the SJVF, and flowed northwest towards present-day Unaweep Canyon. Detrital zircon data provide a maximum depositional age of ca. 23 Ma for the ancient Columbine Pass River, which also coincides with the end of SJVF ignimbrite activity. Stratigraphic reconstructions suggest that the ancient Coloumbine Pass River flowed through a canyon bordered by Mesozoic sedimentary rocks and Tertiary volcanic deposits, which probably totaled 2 km in thickness. This reconstruction further suggests that ancient rivers of the upper Colorado River system had begun to significantly incise through the widespread blanket of SJVF debris by the early Miocene.