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Title:
Implications of old, glaciated surfaces at high elevations in the Sierra Nevada, California
Authors:
Brocklehurst, S. H.; Granger, D. E.; Whipple, K. X.
Affiliation:
AA(Department of Geological Sciences, CIRES, University of Colorado at Boulder, Boulder, CO 80301 United States ; ), AB(Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN 47907 United States ; ), AC(Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 United States ; )
Publication:
American Geophysical Union, Fall Meeting 2002, abstract #H22B-0884
Publication Date:
12/2002
Origin:
AGU
AGU Keywords:
1625 Geomorphology and weathering (1824, 1886), 1815 Erosion and sedimentation, 1824 Geomorphology (1625)
Bibliographic Code:
2002AGUFM.H22B0884B

Abstract

Among the numerous arêtes and sharp peaks at the crest of the Sierra Nevada, California, are a number of remarkably low relief, diamict-covered surfaces. We propose that these widespread, enigmatic, old surfaces represent former glacial valley floors. The head of Sardine Canyon is a beheaded cirque. Thick piles of diamict and gruss attest to the absence of any recent glaciation. At the point where the valley floor is truncated by the cliffs leading down into the Golden Trout Lake basin, we found boulders of a distinct biotite leucogranite not found elsewhere within the Sardine Canyon basin. Furthermore, glacial polish on the undersides of these boulders indicates that they were transported into place by a glacier. Cosmogenic exposure age dating on a large, flat boulder just downvalley of these erratics suggests that there has not been a cirque glacier in the basin in the last ~190 ka. This exposure age represents a minimum time since glaciation in the basin due to an unknown amount of erosion of the rock surface. The cirque on the western side of Baxter Pass is in the process of being beheaded. The deposit here is much coarser than that in Sardine Canyon, and exposure dates in the range ~20-50 ka confirm that this surface is younger. At the other extreme in terms of age, the isolated, small patch of diamict on the northeastern ridge of University Peak is surrounded on all sides by steep cliffs, and presumably represents a much more advanced stage in the evolution of these surfaces than Sardine Canyon. Given the variety of ages for these surfaces, and modelling studies that have shown that glacial headwall erosion is an important process, we infer that the glacial drainages of the eastern Sierra Nevada are subject to a cycle of drainage capture and relief inversion events, similar to the evolution of fluvial drainage networks. These processes produce diamict-covered surfaces at various stages of evolution from clearly U-shaped forms to stranded, flatter surfaces at the range crest. We suggest that this continued process of drainage network reorganisation might limit major relief production by glaciers.
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