Date of Award:

12-13-2011

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

Tammy Rittenour, Ph.D.

Abstract

Arroyos are steep-walled, entrenched, typically ephemeral streams commonly found in dryland river systems that form when streams incise into previously deposited alluvial fill. Arroyos in the southwestern U.S. have been studied extensively following the historic period of arroyo cutting in the late 1800s and early 1900s A.D. The upper Escalante River in south-central Utah similarly began incising in 1909, and records evidence for past cut and fill cycles in well-exposed walls along the now continuous arroyo.

Establishing robust geochronologies of past arroyo cycles in these fluvial settings has been difficult. Recent improvements in accelerator mass spectrometry (AMS) radiocarbon (14C) and optically stimulated luminescence (OSL) dating provide an opportunity to link more highly resolved fluvial records to existing paleoclimate records. This allows hypotheses regarding the causes of arroyo cycles to be tested, and for the role of climate versus intrabasinal characteristics to be examined.

One major objective of this research was to examine the applicability of OSL and AMS 14C dating in the upper Escalante, as both methods have proved problematic in similar settings. In total, 37 ages were obtained, 21 OSL and 16 14C ages. The Holocene fluvial history of the upper Escalante River was reconstructed using these age results and stratigraphic relationships.

The chronostratigraphic record developed in this study suggests that at least six arroyo cycles have occurred in the upper Escalante since the middle Holocene, with incision occurring ~4.4 – 4.2 ka, ~2.6 – 2.4 ka, ~1.8 – 1.5 ka, ~1.0 – 0.9 ka, ~0.5 - 0.4 ka, and during the historic period of arroyo entrenchment. While semi-synchronous arroyo cutting (indicative of a climate signal) appears to have occurred in the Paria and Escalante drainages over the last 1 ka, correlations between proximal drainages are less clear prior to 1 ka, although this may be due in part to preservation effects. Overall, linkages to specific climate regimes and correlations to regional drainages are difficult to identify, suggesting that internal geomorphic thresholds are important in determining when arroyo entrenchment occurs in individual catchments.

Comments

Publication made available electronically December 21, 2011.

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