Date of Award

2002

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Quaternary and Climate Studies

Advisor

George H. Denton

Second Committee Member

Daniel F. Belknap

Third Committee Member

Terence J. Hughes

Abstract

A widespread erosion surface passes across bedrock and sedimentary deposits in the western Dry Valleys sector of the Transantarctic Mountains (TAM), southern Victoria Land, Antarctica. The surface includes stoss-and-lee slopes, channels, potholes, scoured basins, and corrugated bedrock. These features have been taken to represent subglacial meltwater erosion beneath a greatly expanded East Antarctic Ice Sheet (EAIS) in the mid-Miocene (Denton et al. 1984, Marchant et al. 1993a). Sedimentary deposits that are typically associated with ice-sheet wastage, such as outwash, are not present on the erosion surface. The lack of these deposits indicates that the expanded ice-sheet postulated to be responsible for the erosion surface probably sublimated from the mountains under a polar desert climate. By this hypothesis, the nearly perfect preservation of geomorphic features on the erosion surface suggests that the Dry Valleys region has been locked in polar desert conditions since the erosion surface was carved in mid-Miocene time. The adjacent ENS probably remained stable under such climate conditions. An alternate hypothesis suggests that the erosional features are the product of wind-deflation and chemical weathering of the land surface (Selby 1985, Isaac et al. 1996, Healy 1976). The wind-deflation hypothesis implies that the surface may be forming under present-day polar desert processes and has nothing to do with ice-sheet history. Chronology is one means of discriminating between these hypotheses. When was the erosion surface carved? Is it ancient (consistent with the subglacial erosion hypothesis), or is it still forming (consistent with the wind-deflation hypothesis)? From a study of the glacial geology and geomorphology of the western Olympus Range in the Dry Valleys region, I present here the stratigraphic context and chronology of the erosion surface and three sedimentary units, Circe till, Electra colluvium, and Alpine till. Circe till is a fine-grained diamicton with striated clasts that was deposited beneath wet-based glacial ice. Circe till everywhere rests on bedrock. Electra colluvium is a highly oxydized, clast-rich diamicton that overlies Circe till in stratigraphic section. The areal distribution of both of these units is patchy and non-uniform. Circe till and Electra Colluvium are cut by the erosion surface. Alpine moraines drape over the erosional features in the western Olympus Range. The moraines are undssected and arcuate in plan view. This undisturbed morphology indicates that the erosion surface has not been active since at least the time of moraine deposition. Thus, the age of the moraines provides a minimum age on the carving of the erosion surface. This study presents 3 ~ e cosmogenic surface exposure ages of twelve boulders from three moraines that rest on the erosion surface. The preliminary ages range from 0.90 Ma to 1.12 Ma, possibly 5.50 Ma. These are minimum values because surface weathering of the boulders was not considered in the exposure-age calibrations. The oldest moraine has remained unaltered for at least 1.12 Ma. Therefore, the erosion surface has not been active in at least 1.12 Ma. These surface exposure age dates constitute an argument against the wind-deflation hypothesis.

Comments

As of 2002, Degree of Master of Science (MS) Quaternary and Climate Studies published under the auspices of the Climate Change Institute.

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