The Friis Hills are located in the McMurdo Dry Valleys region of southern Victoria Land, Antarctica (77˚45’ S, 161˚30’E). The Friis Hills is a 12 km-wide isolated plateau that rises 600 m above the surrounding topography. Scattered patches of ancient till sequences are preserved on the upper surface. The oldest preserves a phase of glaciation in which glaciers flowed out of the Transantarctic Mountains to the SW, toward the interior of the continent. These tills were deposited 19.76 Ma ago. The second set of tills preserves a glacial phase in which the direction of ice flow switched from SW to SE. The youngest tills preserve a phase of glaciation in which ice flowed to the east and overtopped the Friis Hills, cutting deep troughs and stripping away most of the previously deposited material. These tills were deposited at least 10.76 Ma ago. The tills contain varying proportions of granite cobbles. More than 15 granitic plutons make up the basement rock of the area, and as glaciers carved their way through the Transantarctic Mountains they picked up granitic cobbles and transported them through the Friis Hills. It is difficult to know in the field whether cobbles are derived from local bedrock sources or are exotic, i.e., transported from outside the Friis Hills. We used textural observations and whole rock major and trace element geochemistry to characterize granite cobbles and compare them to likely bedrock sources. Of eight samples, all but one are consistent with the local Pearse Pluton. One sample, ALG-08-20, has different chemical and physical characteristics that resemble those of an older granitic suite, possibly from the Cavendish or Bonney plutons, that are exposed as much as 50 km away. The Friis Hills lie entirely on the Pearse Pluton, a heterogeneous granitic pluton, so any exotic cobble found here was transported in a glacier. Further geochemical testing of cobbles can be an important indicator of ice source regions. It is extremely difficult in the field to determine if a cobble is exotic, but geochemical testing can prove that ice was derived from outside the Friis Hills. Our results provide important constraints on the ice source region of at least one till sheet.

Using geochemical analysis to identify local vs. exotic sources of glacial clasts, Friis Hills, Antarctica

249th National Meeting (2015)

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