INTRODUCTION The Colorado Plateau provenance in the western Unit- ed States is characterized by its high elevation and largely undeformed strata. Drainage integration off the Colorado Plateau and along its margins has driven landscape erosion forming the canyons and mesas characteristic of this re- gion. Generally speaking, sediment production is high due to low vegetation density, thin soils, and easily erodible bedrock. Well-known historic luvial entrenchment into thick alluvial deposits has created arroyos along many of the streams in this semi-arid region. In the late 19 th and early 20 th centuries (~1860-1910, Hereford, 2002, and references therein), many luvial sys- tems in the desert Southwest abruptly incised their allu- vial valleys creating vertically-walled channels (arroyos) that are entrenched 5-30 meters below former loodplain surfaces. The abrupt transition from gently graded, shal- low, perennial streams with expansive loodplains and marshes to incised, ephemeral arroyos left many histori- cal settlements perched above local water sources (Webb and others, 1991). These newly formed arroyos deepened and widened during successive looding events, followed by small-scale renewed sedimentation in the arroyo bottom after the mid-1900s (Graf, 1987). Located in far southern Utah, the 30-meter-deep Kanab Creek arroyo is a classic example of a landscape sculpted by recent entrenchment. While the details on the historic arroyo cutting are well documented, the prehistoric alluvial history along Kanab Creek is largely unknown. This paper focuses on the sedimentary record exposed in arroyo and terrace walls along an approximately 25-kil- MIDDLE TO LATE HOLOCENE CHRONOSTRATIGRAPHY OF ALLUVIAL FILL DEPOSITS ALONG KANAB CREEK IN SOUTHERN UTAH ABSTRACT Kanab Creek in southern Utah lows through three geomorphically deined reaches between the White Cliffs and the town of Kanab, Utah. The upper reaches, particularly the middle (canyon) reach, are characterized by three topographically distinct terraces. Alluvial deposits that underlie these terraces are chronometrically associated with inset aggradational packages downstream in basin-ill alluvium. The town of Kanab is located in the basin-ill reach at the base of the Vermillion Cliffs, where Kanab Creek enters a broad alluvial valley. Historical records document that the loodplain of Kanab Creek was at the level of the second highest terrace in the upstream reaches and near the height of the basin-ill surface in the lower reach prior to the most recent arroyo cutting. Today, Kanab Creek occupies a 20-40-meter-deep arroyo that formed following a series of large-magnitude loods in the early 1880s. Suricial mapping and detailed stratigraphy of terrace and arroyo-wall exposures reveals evidence for past arroyo cut-ill dynamics along Kanab Creek. Results from optically stimulated luminescence (OSL) dating of quartz sand and radiocarbon dating of char- coal collected within ine-grained alluvial sediments suggest at least four periods of luvial aggradation occurred along Kanab Creek since the mid-Holocene: 6.2-3.6 ka (Qa4), 3.2-2.6 cal ka BP 2010 (Qa3’- basin-ill only), 1.9-1.2 ka (Qa3), and 0.8-0.2 ka (Qa2), each separated by arroyo entrenchment (Summa, 2009; Summa-Nelson and Rit- tenour, 2012). The lowest terrace (Qat1) and alluvium (Qa1) formed in the arroyo bottom following early 1880s en- trenchment. The uppermost terrace (Qat4) is underlain by a single alluvial ill package (Qa4), while the middle ter- race (Qat2/3) is underlain by multiple ills (Qa4, Qa3, Qa2) and in places is a ill-cut (alluvial strath) terrace. These middle to late Holocene alluvial packages are primarily composed of broadly lenticular, ine-grained silty sand with interbedded clay beds and weakly developed soil horizons. Gravel facies are rare to non-existent in Qa4, Qa3, and Qa2 but can be found in Qa1 and are conined to small lenticular channel bodies in the basin-ill (Qabf) stratigraphy. While not the focus of this study, mapped units also include historic and late Pleistocene deposits and landforms. by Michelle S. Nelson 1* and Tammy R. Rittenour 1,2 1 USU Luminescence Laboratory, Logan, UT 2 Utah State University Department of Geology, Logan, UT *Corresponding author; michelle.summa@usu.edu Nelson, M.S., and Rittenour, T.R., 2014, Middle to late Holocene chronostratigraphy of alluvial ill deposits along Kanab Creek in southern Utah, in MacLean, J.S., Biek, R.F., and Huntoon, J.E., editors, Geology of Utah’s Far South: Utah Geo- logical Association Publication 43, p. 97–116.