Return to the Table of Contents for this chapter.
In this study, tillites and glaciated surfaces in the Transantarctic Mountains were dated by cosmic-ray-produced noble gas nuclides, with a goal of improving knowledge on the Pliocene stability of the east antarctic ice sheet. These glacial surfaces and the tillites of the Sirius Group have apparently been formed during a maximum extension of the east antarctic ice sheet.
Cosmogenic nuclides are produced within the uppermost decimeters of an exposed rock surface as a result of cosmic-ray bombardment. The concentrations of nuclides such as helium-3 (3He) and neon-21 (21Ne) can be used to determine the age of the geological surface on timescales of thousands to millions of years. Therefore, exposure dating now offers a direct method of determining the time of exposures of geomorphological features such as glacial polish or clasts in a moraine. Such exposure ages are usually minimum ages for the formation or deposition of these features. The construction of an absolute timescale for terrestrial ice-age deposits has been hindered both by limitations in the dating methods and by a lack of datable material.
The samples dated in this study were quartz and clinopyroxene fractions separated from granites, dolerites, and sandstones collected at Table Mountain and Mount Fleming in the western dry valleys region of the Transantarctic Mountains. In addition, whole-rock samples have been analyzed from some dolerites from these sites. Until now, few or no age determinations have been based on cosmogenic noble gases in pyroxene and whole-rock samples of dolerite. All 53 samples analyzed in our study revealed mixtures of cosmogenic and atmospheric noble gases, without detectable amounts of other trapped components and also with at most minor contributions from nucleogenic noble gases. The pyroxenes show identical 3He and 21Ne ages, indicating complete retention of both these nuclides. Quartz ages are based on the cosmogenic 21Ne content only, because 3He is incompletely retained in quartz. Loss of 3He and even partial loss of 21Ne occurred from plagioclase in the dolerites. The cosmogenic 21Ne-based dolerite ages are thus minimum values.
Calculated exposure ages of our samples range from 0.2 to 7.0 million years. Some of these values are the oldest ages measured so far in any terrestrial samples. An age of 7.0 million years was determined for a dolerite clast on the Sirius tillite at Mount Fleming. This value affords a minimum age for the formation of this Sirius tillite deposit and perhaps others like it in the Transantarctic Mountains. An age of 6.1 million years at Table Mountain has been measured on a sample from the bedrock plateau surface just slightly higher in elevation than the nearby Sirius deposit. In comparison, an age of only 2.6 million years was determined for the surface of the Sirius tillite at Table Mountain. The relatively young age indicated for this tillite is probably due to erosion.
Constraints on the uplift rates of the Transantarctic Mountains have been estimated from the concentrations of cosmogenic nuclides in our samples from Mount Fleming and Table Mountain. If it is postulated that the samples were located at sea level at the beginning of exposure and were subsequently uplifted at a constant rate, then the maximum uplift for Mount Fleming is 160 meters per million years and for Table Mountain, 180 meters per million years. Different uplift rates would affect the altitude correction of the production rates and thus the exposure ages. For example, uplift of 100 meters per million years as reported by some authors would change the minimum ages for the deposition of the Mount Fleming tillite from more than 7 million years to more than 11 million years and the minimum age for the cutting of the Table Mountain plateau from 6 million years to prior to 8 million years. Uplift rates of 300 meters per million years or even 1,000 meters per million years suggested by other workers seem to be very unlikely, even if they occurred episodically.
The data presented here imply a stable behavior of the east antarctic ice sheet since exposure of the sampled Sirius tillite at Mount Fleming. The exposure ages of 6.1 and more than 7.0 million years for glacial surfaces and deposits, respectively, contradict the postulate of Pliocene ice sheet overriding of Transantarctic Mountains between 2.5 and 3.1 million years. The age difference between the Table Mountain and the Mount Fleming tillites is most likely due to partial erosion of the Table Mountain deposit. If the key samples were eroded since they were exposed to cosmic rays, the limiting ages for the maximum extension of antarctic ice sheet would be even older, and the maximum uplift rates would have been smaller than those reported here.
This research was supported by the National Science Foundation grant OPP 90-20975, ETH-Zürich research grant 0-20-624-92, and Swiss National Science Foundation grant 2118-28971.90.
References
Bruno, L.A. 1995. Dating of moraines and glacially overridden surface areas in Antarctica using in situ induced cosmogenic noble gases. [Datierung von Moränen und glazial überprägten Oberflächen in der Antarktis mit in situ-produzierten kosmogenen Edelgasen.] (Ph.D. Dissertation No. 11,256, ETH-Zürich, Abt. Xc; Aug. 11, 1995.) [In German]