Processes of paleoarroyo aggradation in Kanab Creek, southern Utah

Abstract:

Many alluvial valleys in the southwest United States have experienced repeated periods of arroyo entrenchment and re-aggradation during the Holocene. Previous research suggests arroyo dynamics were regionally synchronous, implying that climate fluctuations are the dominant drivers. However, intrinsic reach- or catchment-specific geomorphic thresholds to entrenchment are also hypothesized to partially control the timing of arroyo processes. This study focuses on the Holocene alluvial history of three entrenched reaches of Kanab Creek, southern Utah, to explore these competing hypotheses. Episodes of prehistoric arroyo cutting and filling are reconstructed by recognition of buttress unconformable contacts in the arroyo-wall stratigraphy and age control derived from optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) radiocarbon dating. A combined dataset of 47 OSL and 47 radiocarbon ages is produced, and results indicate at least five periods of aggradation occurred since ~6.0 ka, each interrupted by an episode of arroyo entrenchment. Comparison of this record to recently completed chronologies from arroyo systems in the region indicates near-synchronous arroyo processes over the last ~1.5 ka; however, beyond 1.5 ka correlations are less clear. Broadly contemporaneous alluviation suggests a climatic driver, and comparison to paleoclimate records suggests that arroyo entrenchment events may be driven by transitions from periods of multi-year drought to wetter periods. However, the detailed alluvial chronology indicates that the initiation of aggradation is transient, with each period of paleoarroyo aggradation beginning downstream and propagating upstream, which suggests that potentially regionally synchronous, climate-driven events may not appear as such in the stratigraphic record.