YOUTHFUL, TRANSIENT RIVER INCISION OF MYSTERIOUS ORIGIN IS CAPTURED IN CHRONOSTRATIGRAPHY AROUND MOAB, UTAH

Previous work in the central Colorado Plateau has identified patterns of Colorado River incision and topography that suggest a wave of transient incision. Superimposed on this is local salt deformation that has been unquantified. Parsing out the patterns and rates of regional incision and active salt deformation are a rich challenge. We are using mapped stream terraces and basin-fill deposits dated by luminescence and cosmogenic isochron-burial dating to quantify both Quaternary incision and salt deformation along the Colorado River and its tributaries around Spanish Valley and Moab, Utah.

Downstream from Moab at the Jackson Hole Rincon (abandoned bedrock meander), new work on Colorado River terraces dated by OSL document that the bedrock meander-neck cutoff occurred at ~125 ka. Subsequent incision has continued at 300 m/my. Yet, a high (100m) terrace gravel predating the cutoff has a surprisingly young IRSL age, indicating a preceding episode of very rapid incision of up to 2000 m/my at this location. Recent work upstream of Moab on correlative late Pleistocene Colorado River terraces found upstream-increasing rates of Colorado River incision from 600 to 900 m/my. This is all consistent with a wave of incision passing through the system over middle-late Pleistocene time.

Similar evidence is found along the Mill Creek tributary of the Colorado River, which runs through Moab and drains the La Sal Mountains. The isochron-burial age and distribution of upland gravels along local drainage divides indicate that salt-dissolution subsidence and upstream bedrock incision did not begin in earnest until sometime after ~1.5 Ma. Tributary stream terraces dated by OSL record an acceleration of incision to ~600 m/my in the middle Pleistocene, as is also evident in relations between knickpoints and terraces and in patterns of long-profile metrics such as Chi. These results add to a growing body of evidence that youthful, transient incision is a hallmark of the Colorado River system, but a source of such recent baselevel fall in the central Colorado Plateau is unknown.