Date of Award:

2015

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Tammy Rittenour

Abstract

Arroyos are entrenched channels characterized by near-vertical walls of alluvium and flat channel bottoms. Historic channel entrenchment in the southwest United States during the late AD 1800s and early 1900s has stimulated extensive research on these dynamic fluvial systems. The near-synchronous episodes of arroyo entrenchment and aggradation in Kanab Creek and other drainages in southern Utah during the last ~1 Ka has led many researches to argue that hydroclimatic forcings drive arroyo processes. These hypotheses remain largely untested, and there remains considerable uncertainty regarding the timing of these events and the specific mechanisms responsible for arroyo formations.

Previous work established an alluvial chronology for the kanab canyon reach of Kanab creek, but it remained unclear if arroyo events in this reach were continuous with those downstream or synchronous with events in the disconnected arroyo in the upper basin. Using deailed sedimentologic and stratigraphic descriptions coupled with AMS radiocarbon and optically stimulated luminescence (OSL) dating, a new chronostratigraphic record of arroyo entrenchment and aggradation for kanab creek is produced in this study. Results suggest at least five periods of fluvial aggradation and episodic arroyo entrenchment during the middle-to late-Holocene, with aggradation occuring from ~6.2 to 3.67 ka (Qfl), ~3.2 to 2.5 ka (Qf2), ~2.2 to `.45 ka (Qf3), 1.4 to 0.8 ka (Qf4), and 0.75 to 0.14 ka (Qf5). This record is compared to regional alluvial and paleoclimate records to explore potential allogenic and autogenic forcing mechanisms. Rapid transitions from exceptional drought to pluvial periods are quasi-synchronous with regional arroyo entrenchment over the last ~1.5 Ka, but the lack of clear correlations amongst the regional alluvial records and between paleoclimate records beyond 1.5 Ka suggests that internal geomorphic thresholds are important controls on the timing of entrenchment in individual catchments.

Previous research on arroyo dynamics has largely focused on the timing of entrenchment. The few studies that have investigated the processes related to aggradation have used historic observations, and not the stratigraphic record of arroyo deposits. In this study, the alluvial records from three reaches of Kanab Creek are combined to test models of the processes and geometric patterns of paleoarroyo aggradation. Results indicate that aggradation initially propagates upstream and then transitions to synchronous vertical aggradation along the entire channel profile as arroyos approach complete filling.

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