DETRITAL ZIRCON ANALYIS OF HIGH TERRACES OF THE EARLY COLORADO RIVER SYSTEM (CROOKED RIDGE RIVER, GRAND MESA, AND UPPER GREEN RIVER); IMPLICATIONS FOR COLORADO PLATEAU DRAINAGE EVOLUTION

The history of integration of the Colorado River system is controversial, in part due to scarcity of Miocene and Pliocene paleo-river deposits. This study addresses three areas with gravels in this age range, from north to south: 1) Pliocene paleo-Green River terraces sampled near Flaming Gorge, Wyoming are 400 meters above the modern river. Detrital zircons from 2 samples show prominent peaks at 33 Ma, 1000 Ma, and 1800 Ma, with only four Archean grains in the age spectrum; suggesting a southerly source (Uinta Mountain Group) to explain the dominant 1000-1100 Ma peak, which is not observed in modern Green River samples from near its confluence with the Colorado River. 2) Gravels beneath the 11 Ma Grand Mesa basalt contain 1700 Ma and 1400 Ma Proterozoic peaks, a strong peak (20 grains) at approximately 22 Ma and youngest grains of about 16.5 Ma that we interpret to be sourced from the San Juan volcanic field; these gravels are interpreted to represent a paleo-Gunnison River rather than a paleo-Colorado River. Modern Colorado River sands from upriver of the Gunnison/Colorado River confluence (north of Grand Mesa) contain no grains of this age. 3) The Crooked Ridge River has been hypothesized to be a west-flowing Miocene paleoriver in northern Arizona; new detrital zircon data can constrain the age of the Crooked River deposits to younger than 24 Ma. The diverse zircon population in Crooked Ridge samples is similar to that of the San Juan and Little Colorado Rivers, which is compatible with a derivation from western Colorado Plateau Mesozoic stratigraphy, with limited input from volcanic sources on the Colorado Plateau.