ARROYO CYCLES IN THE UPPER ESCALANTE RIVER DRAINAGE, SOUTHERN UTAH: UTILIZING OSL DATING TO EXTEND REGIONAL FLUVIAL CHRONOLOGIES

Arroyos in the SW US have been studied extensively over the past century because of the well-documented period of arroyo cutting in the late 1800s and early 1900s. Given climate model predictions for the SW US of greater temperature extremes and more frequent floods and droughts (Diffenbaugh, 2005; Kerr, 2008), research involving the response of these dynamic fluvial systems to climate change is more relevant than ever.

Recent improvements in dating techniques (AMS ¹⁴C and optically stimulated luminescence (OSL) dating) provide an opportunity to test existing hypotheses regarding causes of arroyo cutting with greater precision than previously possible. ¹⁴C dates can be problematic in fluvial settings due to reworking. Further, charcoal may not be suitably located in a deposit to best constrain the timing of aggradation and degradation, limiting resolution of the record. OSL samples can be collected from any unit containing quartz sand, allowing better chronologic constraints on arroyo cycles. However, OSL ages may have problems with partial bleaching in fluvial settings. For this reason, a sampling strategy using both ¹⁴C and OSL dating is best.

The headwaters of the Escalante River in S Utah provide an excellent setting to examine the timing of arroyo cycles in relation to climate change. The modern arroyo provides ~30 km of nearly continuous outcrop exposures. This research involves mapping of alluvial deposits, detailed stratigraphic descriptions, and age control from ¹⁴C and OSL samples to extend the fluvial chronology in this watershed. So far, 10 OSL samples have been collected and are being analyzed at the USU Luminescence Laboratory. Also, 5 new ¹⁴C samples have been collected and will be examined along with ¹⁴C dates from previous studies which indicate several cut and fill cycles over the past 2 ka (Webb, 1985; Webb and Hasbargen, 1997). Chronologies in other watersheds in the region have been recently improved and extended, and this project will address causal linkages to arroyo cutting. Are the chronologies synchronous, as would be expected with a climatic forcing? If so, are these synchronous events correlated with documented paleoclimate events (e.g. increased ENSO or monsoon activity)? Improved chronologies using new tools such as OSL should offer greater insights into answers to these questions.