DRAINAGE REVERSAL AND SOUTHERN COLORADO PLATEAU UPLIFT IN THE SALT RIVER AREA

The Salt River runs southwest through southern Arizona, cutting a 1-km deep canyon at the edge of the southern Colorado Plateau. Abundant geological evidence indicates that rivers in this area once flowed toward the NE. The geologic driver of this drainage reversal was Colorado Plateau uplift relative to the down-dropping Basin and Range. The goals of this study are to constrain the timing of this drainage reversal, calculate incision rates of the modern Salt River, and constrain the magnitude of any post-10 Ma southern Colorado Plateau uplift. Multiple generations of NE-flowing paleorivers drained the NW-trending mountain range known as the Mogollon Highlands. These rivers deposited the far-traveled gravels of the Music Mtn. Fm. (and equivalents) between 65-55 Ma and the Mogollon Rim Formation between 37-33 Ma. A NE-trending river incised a km-deep paleovalley in this area that filled with the locally derived Whitetail Formation between 36-22 Ma. Internally draining conditions continued in this paleovalley until after 14.6 Ma. The first SW-flowing, proto-Salt River cut a 200 m deep gorge in the paleovalley fill and drained into Tonto Basin. It deposited the Dagger Canyon Conglomerate which has a maximum depositional age of 7.5 Ma near the base and 6.5 Ma near the top according to detrital zircon and sanidine data. The Salt spilled-over from the Tonto Basin and became a tributary of the Gila River around 3 Ma based on cosmogenic burial ages. The Gila River was graded to sea level by 5 Ma. Post-3 Ma Gila incision is 10 m/Ma at Gila Bend. The Salt River formed a series of strath terraces in the Phoenix basin, the highest gives a maximum incision rate of 37 m/Ma over 3 Ma. The second highest terrace has a 0.5 Ma cosmogenic burial age giving a maximum incision rate of 100 m/Ma. The Salt River headwaters have steadily incised the southern Colorado Plateau at a rate of around 100 m/Ma between 3-1.39 Ma based on heights of basalt flows that infilled paleochannels. Headwater incision rates have increased to around 115 m/Ma since 0.73 Ma. Increased upstream and downstream rates of incision by a river already graded to sea level is best explained by southern Colorado Plateau uplift of at least 300 m over the past 3 Ma. We speculate that volcanism on the edge of the southern Colorado Plateau from 9-0.3 Ma drove the uplift and downward integration of the Salt River.