Chronostratigraphy of Holocene Valley-Fill Alluvium and Arroyo Cut-Fill Events in the Upper Escalante River, Southern Utah

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Contribution to Book

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Geology of Utah's Far South: Utah Geological Association Publication




Utah Geological Association

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During the late 1800s to early 1900s, many fluvial systems in the southwestern United States incised into their alluvium, forming steep-walled arroyos and causing economic and environmental impacts to settlers. Many studies of arroyo systems have been conducted over the last century, and several hypotheses have been proposed regarding the conditions necessary for recurring arroyo entrenchment. However, most of these studies have relied on radiocarbon (14C) dating, limiting the temporal resolution of chronostratigraphic records due to limited sampling opportunities and reworking of older charcoal. Recent advances in optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS) 14C dating allow for more highly resolved fluvial chronologies from these sensitive semi-arid river systems. Research was conducted along the upper Escalante River in south-central Utah to develop a chronostratigraphic record of Holocene cut-fill cycles. Field work focused on recognition and description of unconformity bounded fluvial sequences in well exposed arroyo walls. Alluvial packages were dated using 18 14C samples and 20 OSL samples. Results suggest that arroyo cut and fill dynamics became an important agent of landscape evolution approximately 4.5 ka. Since that time, at least five aggradation/entrenchment events occurred, with evidence for aggradational packages separated by incision at approximately 4.4 to 4.2 ka, 2.9 to 2.5 ka, 1.8 to 1.6 ka, 1.0 to 0.8 ka, and during the historic period of arroyo entrenchment that commenced in A.D. 1909.

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