EXTENDING THE RECORD OF ARROYO CYCLES FOR THE UPPER ESCALANTE RIVER, S. UTAH USING OSL AND RADIOCARBON DATING

During the late 1800s to early 1900s, many fluvial systems in the southwestern United States incised into their alluvium, forming steep-walled arroyos. Because of the devastating economic impact to settlers, many studies regarding arroyo cycles were conducted over the past century, and several hypotheses have been proposed regarding the conditions necessary for arroyo incision. However, most studies have relied on radiocarbon (¹⁴C) dating, limiting resolution of arroyo cycle records due to limited sampling opportunities. Recent advances in dating methods (particularly optically stimulated luminescence (OSL) dating) allow new insights into the mechanisms that drive degradation and aggradation in these dryland river systems by allowing extended fluvial chronologies to be linked to paleoclimate studies.

Research was conducted along the upper Escalante River in south-central Utah to develop a chronology of Holocene arroyo cycles. Field work focused on recognition of unconformity-bounded fluvial sequences in well-exposed arroyo walls. Stratigraphic descriptions and environmental interpretations were recorded at key outcrops showing cut-and-fill relationships or aggradational sequences. Alluvial packages were dated using both ¹⁴C and OSL dating in order to constrain the timing of arroyo cycles. A total of 18 ¹⁴C samples and 21 OSL samples were collected. Ages are in proper stratigraphic order once partial bleaching is accounted for by using a minimum-age model to determine OSL ages. Ages obtained indicate that arroyo cut and fill cycles became an important agent of landscape evolution following ~4.5 ka. Since that time, at least 5 cycles have occurred, with evidence for 5 distinct aggradational packages, and incision occurring at approximately 4 ka, 1.9 – 1.5 ka, 1.0 – 0.8 ka, 0.5 – 0.3 ka, and the historic period of incision which began in 1909. These new data are discussed in conjunction with recently updated fluvial chronologies from adjacent watersheds and with existing regional paleoclimate data to explore the response of this semi-arid fluvial system to past climate change.