MOGOLLON RIM GRAVELS, DETRITAL ZIRCON EVIDENCE FOR NORTH-NORTHWEST-FLOWING EARLY MIOCENE STREAMS DURING EXHUMATION OF THE CHUSKA ERG

The Mogollon Rim gravels are distributed for more than 100 km along the length of the Mogollon Rim drainage divide in eastern Arizona, the southern margin of the Colorado Plateau. These “rim gravels” are residual basal conglomerate of the late Laramide Mogollon Rim Formation derived from ancestral highlands to the southwest. Most of the arkosic clastic rocks of the 375 m thick Mogollon Rim Formation were eroded from the Mogollon Rim, yet a detrital zircon signature in overlying sands provides evidence that waning fluvial sedimentation from the south continued across the Mogollon Rim after emplacement of the 18.63 Ma Apache Leap Tuff in the Salt River paleocanyon. This is supported by clast provenance and northwest paleocurrent measurements near Juniper Ridge on the rim. The continuance of northward drainage following deposition of the Oligocene Chuska Erg indicates that Early Miocene streams of southerly source contributed in some measure to approximately 500 m of plateau denudation prior to deposition of the Bidahochi Formation. Late Eocene streams of the northeast-distributed Mogollon Rim Formation were deflected from the Baca Basin depocenter toward the northwest by lava flows and volcaniclastic aprons from the emerging Datil volcanic field, terminating stream flow from the Mogollon Highlands to the Atlantic Ocean and initiating northwest-flowing streams of the proto Little Colorado River basin.