DRAINAGE REVERSAL AND INCISIONAL HISTORY OF THE SALT RIVER

The Salt River of southern Arizona has cut a towering canyon as it flows from the Colorado Plateau to the Basin and Range. Today the river flows SW; however, 35 million years ago, its precursor rivers flowed NE from the Laramide uplift known as the Mogollon Highlands onto the Colorado Plateau depositing the Mogollon Rim and Whitetail Formations. These paleorivers carved the Salt River paleovalley. Between the late Eocene and late Miocene, extensional reactivation of Laramide reverse faults down-dropped the highlands. The Colorado Plateau emerged toward the E and NE while Basin and Range style deformation dominated toward the W and SW. The Salt River paleovalley provided a key path for an emerging regional SW-flowing drainage that further down-cut the paleovalley into the modern canyon and deposited the Dagger Canyon Conglomerate. Estimates for the timing of this drainage reversal range between 18 and 3 Ma. Incision rates of the Salt River also remain inadequately understood. To determine the timing of the reversal we are dating detrital zircon and sanidine from river gravels deposited before and after the reversal as well as any overlying volcanics to determine the minimum and maximum depositional ages. We applied this dating method at Canyon Creek Butte in the Salt River Canyon where the depositional age of the Whitetail Formation is now constrained between 25.7 ± 1.3 Ma, the youngest detrital zircon grains (n=3), and 21.8 ± 0.03 Ma, the ⁴⁰Ar/³⁹Ar age of an overlying dacite flow. These constraints indicate that the upper Whitetail Formation at this location is apparently younger than the Mogollon Rim Formation. Detrital zircon data from Dagger Canyon Conglomerate indicate a maximum depositional age of 6.45 ± 0.99 Ma (n=6). These data lead us to hypothesize that (1) continued dating of pre-reversal river deposits will reveal separate paleoriver ages and river pathways and 2) drainage reversal was complete by 5-6 Ma, when the Colorado River fully integrated across the Colorado Plateau. We also hypothesize that differential incision by the Salt River occurred at rates between 100-150 meters/Ma across the plateau boundary reflecting 500-900 m of Neogene Colorado Plateau uplift. This research will provide insight into the ongoing debates surrounding the drainage evolution and uplift history of the Colorado Plateau.