Abstract

Neogene glacigenic strata, collectively referred to as the Sirius Group, are widely distributed throughout the Transantarctic Mountains. The group is particularly well exposed near the head of Shackleton Glacier (85°10′ to 85°40′S) on Roberts Massif and at Bennett Platform. These deposits are critical for examining the nature of former ice flow from the East Antarctic Ice Sheet into the Ross Embayment. The Sirius Group rests on a glacially grooved and striated pavement named herein the “Shackleton erosion surface.” The sub–Sirius Group surface on Roberts Massif was of low relief, and glaciogenic sediment was deposited on it as a sheet of uniform thickness. At Bennett Platform, stratigraphic sections attain a thickness of 110 m thick and are subdivided into the Shackleton Glacier Formation (maximum thickness 98 m) and the overlying Bennett Platform Formation (44 m), separated by an unconformity. A third, older, lithified diamictite containing wood fragments occurs as clasts within these formations and as boulders in the modern lateral moraines of Shackleton Glacier. The dominant facies, massive diamict, is interpreted primarily as lodgement till. Other facies indicate glaciofluvial and glaciolacustrine deposition. Facies associations suggest deposition either by sliding temperate or by polythermal glaciers, under much warmer conditions than those of today. Widespread large- and small-scale faulting has affected the Sirius Group and underlying rocks to the extent that inland exposures are over 500 m higher than those to the north, over a distance of 30 km. Thus, at the time of deposition of the Sirius Group, the mountains were probably lower, and the ice sheet was much thinner.

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