MID-LATE HOLOCENE ARROYO CUT-FILL DYNAMICS: HYDRO-CLIMATE AND COMPLEX INTER-BASIN RESPONSE

Many streams in the semi-arid southwest U.S. oscillate between two end-member channel forms, an entrenched arroyo and a shallow aggraded channel. Historic arroyo entrenchment took place at the turn of the 20th century and exposed unconformity-bound sediment packages that record mid-to-late Holocene arroyo cut-fill dynamics. Previous studies have suggested that the timing of historic and prehistoric arroyo entrenchments in the southwest were regionally synchronous over the late Holocene, implying a hydroclimatic forcing. A new chronology of arroyo cutting and filling in Johnson Wash, located in the Grand Staircase region of the Colorado Plateau in south-central Utah, was reconstructed using field observations and age control from radiocarbon (n=57) and optically stimulated luminescence dating (OSL; n=27) collected from 15 stratigraphic sections. Four episodes of entrenchment separating longer-duration periods of aggradation have been bracketed: 1900-1700, 1600-1400, 1300-1200, and 1000-700 cal yr BP₂₀₁₀. The sediment record exposed in arroyo walls of Johnson Wash archives a complex intra-basin alluvial history that is likely due to 1) incomplete preservation of the sedimentary record, 2) limited number and locations of exposures, and 3) at times limited precision and accuracy of the available age control for the unconformity bound sediment packages. Based on these observations, challenges to reconstructing the alluvial history of Johnson Wash and correlating cut-fill sequences between exposures may be due to complex geomorphic response within the basin or differing geomorphic response times and thresholds to entrenchment. However, the record of alluviation as a whole, obtained from the 15 different stratigraphic sections, is approximately consistent with the timing of cut-fill dynamics identified in other regional basins, suggesting a hydroclimatic forcing. Specifically, the timing of entrenchment in Johnson Wash is approximately consistent with entrenchment in the Upper Escalante River (1800-1600 and 1000-800 yr BP₂₀₁₀), the Paria River and Kitchen Corral Wash (1450-1300 and 800-700 yr BP₂₀₁₀), Kanab Creek (1200-800 yr BP₂₀₁₀), and the Virgin River (800-600 yr BP₂₀₁₀).