Date of Award:

5-2007

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

Joel L. Pederson

Committee

Joel L. Pederson

Committee

Tammy Rittenour

Committee

Jack Schmidt

Abstract

Climate variability has had a dramatic impact on eastern Grand Canyon tributaries over the past ~100 ky. This is readily observed in the Lava Chuar and Comanche catchments, which host well preserved colluvial remnants and river terraces that resulted from several climate-induced cycles of aggradation and incision. This study investigates these climate responses using surveying, sedimentology, and luminescence geochronology methods to investigate the mechanisms and timing of their deposition.

The survey data demonstrate that the concavity of terrace treads is lower than modern drainages. The sedimentology suggests the prevalence of stream-flow reworking of debris flow deposits and portrays an expected down-stream fining of deposits. Results of geochronology indicate that Lava Chuar Creek aggraded from ~100-90 ka (S4), 61-55 ka (S3o), 50-35 ka (S3y), and 14-7 ka (S2). Comanche Creek similarly aggraded twice during the time period from 77-35 ka (S3) and also from 26-22 ka (S2).

The response of eastern Grand Canyon tributaries to glacial-interglacial climate cycling appears to be more complicated than formerly thought. Previous conceptual models assumed that stream aggradation in arid settings occurred as a response to one specific set of climatic and biologic conditions, but new field data and luminescence dates on terrace materials reveal that there are at least two sets of conditions that were conducive to aggradation along local streams. First, streams aggraded during cooler, wetter climates, which is inferred to be due to increased sediment supply (e.g. S4 and S3y in Lava Chuar). With the transition to warmer interglacial climates came higher intensity storms and decreased vegetation density in catchments, based on previous paleoclimate studies. These conditions caused remobilization and redeposition of older sediment and resulted in a second set of stream aggradation phases (e.g. S3o and S2 in Lava Chuar). Meanwhile, the Colorado River aggraded only once per climate cycle during glacial advances and subsequent climate transitions. Thus, tributaries have responded more frequently and more sensitively to changing local conditions compared to the relatively insensitive Colorado River. This is supported by tighter correlation of tributary records to trends in regional paleoclimate records than to global ice records.

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