LATE CENOZOIC DRAINAGE REORGANIZATION OF THE ARKANSAS RIVER, CENTRAL COLORADO, AND HISTORY OF THE ROYAL GORGE

The Arkansas River flows ~125km south-southeast through the Rocky Mountains of central Colorado and the upper Arkansas River Valley (UAV) before turning eastward near Salida and cutting a series of deep canyons (including the 400m deep Royal Gorge) before flowing onto the Colorado piedmont at Canon City. The river may have formerly flowed south from the UAV into the Rio Grande system. Though the timing is loosely constrained, the UAV appears to have integrated into the modern Arkansas River downstream of Salida during the Neogene. Two hypotheses for establishment of the modern river course have been advanced: (1) blockage of the paleo-Arkansas River valley due to uplift of the Poncha Pass block separating the UAV from the San Luis Valley segment of the Rio Grande Rift, resulting in spill-over into the lower reaches of the modern river and cutting of Royal Gorge, and (2) headward erosion of the canyon-segment of the river, driven by base-level lowering on the Great Plains or climate change, resulted in capture of the UAV.

Based on identification of >2000 clasts from >12 sites in mapped terraces and other alluvial deposits, we constrain provenance and therefore drainage organization as a function of age and distance from the UAV. The lack of UAV-sourced clasts of Mt Princeton quartz monzonite within a Tertiary (?) deposit ~400m above the modern river on the southern slope of Royal Gorge and 10% Mt Princeton-derived clasts within a mid-Pleistocene terrace ~100m above the modern Arkansas River further downstream demonstrates that communication between the UAV and Canon City was established in the interval between deposition of these two deposits. Two Quaternary terraces below Royal Gorge correlate with knickpoints at the head of Royal Gorge and ~15km further upstream in the middle of Bighorn Sheep Canyon. Above the higher of these two knickpoints, the Arkansas River is approximately at grade until reaching a ~200m high knickpoint in the UAV and tributary valleys. Quaternary alluvial gravels near the rim of Browns Canyon, ~200m above river level, suggest that canyon incision is a Quaternary phenomena possibly tied to the establishment of communication with the lower Arkansas River and incision of Royal Gorge.