Mid-Late Holocene Arroyo Stratigraphy in Southern Utah; Balance between Climate Forcing and Geomorphic Thresholds

Abstract:

Historic arroyo entrenchment at the turn of the 20th century signified a rapid and widespread change in stream dynamics throughout much of the southwest U.S.A.. Arroyo walls along modern channels expose multiple unconformity-bound sediment packages that record mid-to-late Holocene arroyo cut-fill dynamics. Many of these different-aged periods of aggradation appear to have reached a similar tread height through time, suggesting that a 'geomorphic threshold' may partially control end-member stream grade and the timing of channel entrenchment. However, observations of near-synchronous regional cut-fill events support an alternative hypothesis that climate is a primary control of arroyo dynamics. In order to test the role of allogenic forcing versus autogenic processes on arroyo cut-fill dynamics, three datasets were constructed and analyzed from Johnson Wash (JW), a drainage containing a ~40 km long arroyo in the Grand Staircase region of the Colorado Plateau in south-central Utah. The chronostratigraphy of arroyo cut-fill events was reconstructed using a combination of field observations and age control from radiocarbon (n=57) and optically stimulated luminescence dating (OSL; n=27) collected from 15 stratigraphic sections that bracket episodes of incision and characterize alluvial-fill packages. These data are compared to regional cut-fill chronologies from other arroyo systems. Temporal and spatial variability in catchment averaged erosion rates was quantified using terrestrial in-situ Beryllium-10 measured in quartz from alluvial and colluvial sediment samples (n=24) collected from the modern channel and paleo-arroyo walls located in JW and the adjacent upper Kanab Creek watershed. The third dataset consists of longitudinal profile concavities of the currently entrenched channel and the relict aggraded valley-fill surfaces and is used to identify systematic trends in aggraded versus entrenched channel forms.