Middle to Late Holocene Chronostratigraphy of Alluvial Fill Deposits Along Kanab Creek in 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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Kanab Creek in southern Utah flows through three geomorphically defined 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-fill alluvium. The town of Kanab is located in the basin-fill reach at the base of the Vermillion Cliffs, where Kanab Creek enters a broad alluvial valley. Historical records document that the floodplain of Kanab Creek was at the level of the second highest terrace in the upstream reaches and near the height of the basin-fill 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 floods in the early 1880s. Surficial mapping and detailed stratigraphy of terrace and arroyo-wall exposures reveals evidence for past arroyo cut-fill dynamics along Kanab Creek. Results from optically stimulated luminescence (OSL) dating of quartz sand and radiocarbon dating of charcoal collected within fine-grained alluvial sediments suggest at least four periods of fluvial aggradation occurred along Kanab Creek since the mid-Holocene: 6.2-3.6 ka (Qa4), 3.2-2.6 cal ka BP2010 (Qa3’- basin-fill only), 1.9-1.2 ka (Qa3), and 0.8-0.2 ka (Qa2), each separated by arroyo entrenchment (Summa, 2009; Summa-Nelson and Rittenour, 2012). The lowest terrace (Qat1) and alluvium (Qa1) formed in the arroyo bottom following early 1880s entrenchment. The uppermost terrace (Qat4) is underlain by a single alluvial fill package (Qa4), while the middle terrace (Qat2/3) is underlain by multiple fills (Qa4, Qa3, Qa2) and in places is a fill-cut (alluvial strath) terrace. These middle to late Holocene alluvial packages are primarily composed of broadly lenticular, fine-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 confined to small lenticular channel bodies in the basin-fill (Qabf) stratigraphy. While not the focus of this study, mapped units also include historic and late Pleistocene deposits and landforms.

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