A CHRONOSTRATIGRAPHIC RECORD OF ARROYO INCISION AND AGGRADATION IN KANAB CREEK, SOUTHERN UTAH

During the late 1800s, many alluvial valleys in the American Southwest became entrenched, forming continuous arroyos characterized by near-vertical walls (up to 30+ meters) and flat bottoms. Stratigraphic evidence in the newly exposed arroyo walls suggests that these fluvial systems experienced repeated periods of entrenchment and re-aggradation during the mid- to late-Holocene. Previous research suggests arroyo dynamics were regionally synchronous, implying that hydroclimatic fluctuations are the dominant drivers; however, many of these interpretations rely on records with limited age control and correlations between distant drainages subject to potentially different climate regimes. While hydroclimatic fluctuations must play a role, geomorphic slope/concavity thresholds related to high sediment yields and aggradational conditions throughout the Holocene may instead ultimately govern arroyo dynamics, implying that cut-fill events need not necessarily be regionally contemporaneous. The goal of this study is to determine if periods of arroyo entrenchment and aggradation between adjoining drainages in the Grand Staircase region of the Colorado Plateau were synchronous. Methods include development of a detailed chronostratigraphic record of cut-fill events in the two basin fills of Kanab Creek, southern Utah, using optically stimulated luminescence, radiocarbon, and detailed sedimentologic and stratigraphic descriptions of 15 to 20 arroyo-wall exposures, focusing on sites with cut-fill relationships bounding allostratigraphic units. This record will be compared to recently completed records from adjacent catchments in the region and an intermediary reach of Kanab Creek to test for both inter- and intra-basin correlations. Contemporaneous arroyo cut-fill dynamics would suggest a climate driver, while differing chronologies would suggest the importance of reach or catchment-specific geomorphic thresholds. This record will contribute to the development of the comprehensive history of entrenchment and alluviation in the region necessary to provide a base upon which future studies will explore the predominant mechanisms controlling these arroyo dynamics.