Date of Award:

12-2022

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Committee Chair(s)

Tammy M. Rittenour

Committee

Tammy M. Rittenour

Committee

Joel L. Pederson

Committee

Carol M. Dehler

Abstract

The famous landscapes of the Colorado Plateau have been created over millions of years, primarily by erosive forces of wind and water. Interruptions in the long-term erosion of the landscape occur when streams gain more sediment than they can transport, which causes deposition along channels and floodplains. The resulting sequences of terrace deposits are used by geologists to study when and how river systems have evolved.

Mammoth bones were recently discovered in stream deposits along Wahweap Creek, a tributary of the Colorado River in southern Utah. Previous work indicates that the deposits pre-date the last ice age, making it one of the oldest mammoth sites in the Colorado Plateau. The deposit is composed of uniquely fine-grained sediment and stands in stark contrast to gravels typically associated with stream terraces of similar ages. The geographic extent, precise age, and depositional setting of the fossil-bearing deposit was unknown, as was its relations to regional geology.

This research used mapping and stratigraphic assessment of stream deposits in the lower Wahweap Creek catchment to assess their locations and geologic characteristics. Deposits were dated using the luminescence properties of mineral grains and analyzed for their chemical properties. Results indicate that the mammoth site is part of a period of extensive deposition across the lower catchment occurring ~240-210 thousand years ago (ka), with sediment deposited in a combination of stream, wetland, and spring environments. Results also indicate that additional episodes of deposition occurred ~380 ka, ~320-290 ka, ~85-65 ka, ~20-15 ka, and ~3-1 ka. Using these ages and the elevations of their corresponding deposits, a reconstructed incision history of Wahweap Creek indicates the channel experienced an extended phase of little to no erosion ~240-100 ka. This period of landscape stability likely contributed to the high degree of fossil preservation and may reflect a delayed response to erosion recorded on the Colorado River. The results presented here have important implications for locating remains of extinct ice age fossils, as well as understanding of how tributaries to the Colorado River respond to climate and baselevel fall.

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