CONSTRAINING BEDROCK SOURCE REGIONS OF SURFICIAL DEPOSITS IN THE MCMURDO DRY VALLEYS, ANTARCTICA

The McMurdo Dry Valleys of Antarctica contain a record of glacial fluctuations, lake level changes, and meltwater activity during the Quaternary. Samples were taken from surficial deposits in low-elevation study sites in Pearse Valley, the Rhone Glacier region in Taylor Valley, and the North Fork and South Fork regions of Wright Valley. The possible bedrock sources for unconsolidated deposits include the Skelton Group, Granite Harbor Intrusive Complex, Beacon Supergroup, Ferrar Dolerites and McMurdo Volcanics. Samples were analyzed using x-ray diffraction, x-ray fluorescence spectrometry, grain mount thin sections, and grain size analyses to constrain sample depositional environment, source regions, in situ weathering, and the possible variations in these parameters with depositional age and sampling sites. Results show that Pearse Valley, North Fork and South Fork contain material from all bedrock sources except McMurdo Volcanics. South Fork had the highest average SiO2 concentrations suggesting contributions from the Beacon Supergroup and Granite Harbor Intrusive Complex. Rhone Glacier region contained material from Skelton Group, Beacon Supergroup and Ferrar Dolerites. Rhone Glacier samples exhibited the lowest average SiO2 and highest average MgO weight percentages of all sampling regions, likely indicative of contribution from the Ferrar Dolerites. Regardless of whether samples sourced from local glaciers or the East Antarctic Ice Sheet, significant differences in mineralogy or geochemistry of the glacial tills and glaciofluvial deposits were not observed. The two silt-rich samples of lacustrine or glaciolacustrine origin from Pearse Valley and North Fork had low SiO2 (50.4% and 52.0%) and high Al2O3 (11.2% and 15.1%) compared to other samples. All samples exhibited little evidence of chemical weathering. The average chemical index of alteration value was 50.4% and the average chemical index of weathering value was 54.8%. For the previously optically stimulated luminescence (OSL) dated samples the older deposits did not consistently exhibit higher weathering values. The samples with higher values were often located near a moisture source. The data support slow chemical weathering rates in hyper-arid cold desert environments.