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

Jack Schmidt

Committee

Tammy Rittenour

Abstract

A well-exposed suite of Colorado River fill terraces preserved at Lees Ferry records the oscillating history of this major river superimposed on its overall downcutting of the Colorado Plateau. Detailed mapping, sedimentology, cross-sectional surveys, and the use of two geochronometers have been used in order to establish a detailed chronostratigraphy for the area. Eight distinct deposits have been identified along the Colorado River (Ml -M7, and S3), and four deposits have been identified along the Paria River (Pl -P4).

Geochronology of six of these deposits using optically stimulated luminescence and cosmogenic 10Be exposure techniques indicates a long-term average bedrock incision rate of 290 to 470 m/my. These incision rates are approximately two to three times higher than others reported in Grand Canyon and the upper Colorado River basin, but are similar to the recently reported high incision rates near Glen Canyon and along the Fremont River. These results suggest that there is a region of faster incision along the Colorado River in the central Colorado Plateau in the vicinity of Lees Ferry and Glen Canyon. This apparent increase in central plateau Pleistocene incision rates may be caused by either epeirogenic uplift due to tectonics and erosional isostatic rebound, or transient waves of incision in response to original drainage integration.

In addition to recording the incision history of the Colorado River, the well-preserved Pleistocene fluvial terraces provide evidence regarding the timing and processes of terrace formation at Lees Ferry. Chronostratigraphic analysis indicates that aggradation was occurring at ~20 ka (M2), ~70 to 40 ka (M3), ~115 to 90 ka (M4), and ~130 ka (MS). Aggradation and incision along the Paria River appears to be occurring at the same time as that on the Colorado River. Deposits at Lees Ferry are generally younger than correlative deposits in headwater catchments and in eastern Grand Canyon. ln addition, the most prominent deposit in the Lees Ferry area (M4) correlates to MIS stage 5b-c, a time in which no glaciations have been reported in headwater drainages. Data from this study indicate that fluvial responses at Lees Ferry are a complicated integration of signals from climate change in headwater catchments and sediment production from local hillslopes and tributaries.

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