EXPLORING THE EVOLUTION OF THE SOUTHERN COLORADO PLATEAU THROUGH THE DRAINAGE REVERSAL AND INCISION RATES OF THE SALT RIVER

The Salt River of southern Arizona has cut a 1-km-deep canyon as it flows SW across the transition from the Colorado Plateau to the Basin and Range. Detrital grain ages (detrital zircon (DZ) and detrital sanidine (DS)) from paleoriver deposits show that several precursor rivers in this area flowed NE during the Paleogene from the Laramide Mogollon Highlands onto the Colorado Plateau depositing the Mogollon Rim and Whitetail Formations. These precursor rivers also carved the Salt River paleovalley, which would later become today’s Salt River Canyon. The timing of this regional drainage reversal is not well constrained. The maximum depositional ages (MDA) for 3 different Mogollon Rim Formation outcrops are: 64 Ma near Highway 60 (DZ); 37 Ma for Trout Creek (DZ), and 30 Ma at Blue Ridge (DZ). The depositional age of the Whitetail Formation is constrained between 26 Ma, the youngest DZ grains, and 22 Ma, the Ar-Ar age of an overlying dacite flow; this location is the only paleoriver deposit firmly linked to carving of the Salt River paleovalley. The paleovalley then experienced a period of internal drainage as indicated by the fanglomerate, marsh carbonate and mudstone facies of the Gila Group. Regional SW-flowing drainage was established by about 7 Ma based on both DZ and DS MDA for the Dagger Canyon conglomerate, a 300-m-thick aggradation succession that has MDA of 7.4 Ma at bottom and 6.4 Ma at top. This indicates that drainage reversal across the transition between the Colorado Plateau and Basin and Range occurred between 22 and 7 Ma. Based on Ar-Ar ages of basalts that filled paleovalleys, incision rates of the Salt River over the past 3.2 Ma have been semi-steady around 100 m/Ma on the southern edge of the Colorado Plateau. Incision rates farther downstream for the Gila River (parent river to the Salt River) in the Basin and Range are around 20 m/Ma over the past 3 Ma near Gila Bend. This indicates that the Colorado Plateau has experienced approximately 300 m of post-3 Ma uplift relative to the Basin and Range. This research provides additional support for headwater uplift associated with the Springerville volcanic field and ongoing mantle-driven uplift of the Colorado Plateau.