Abstract

The Sirius Group comprises a suite of consolidated glacial deposits that are widespread in the Transantarctic Mountains; some of the deposits contain Pliocene marine diatoms. Because of the interpretation that the Sirius Group tills and the marine diatoms were deposited by the East Antarctic Ice Sheet, it has been inferred that the East Antarctic Ice Sheet was much reduced in area and volume during the Pliocene. These interpretations were evaluated by studying Sirius Group lodgment tills on Mount Fleming in the Dry Valleys sector of the Transantarctic Mountains.

We infer that the Sirius Group lodgment tills at Mount Fleming do not support the hypothesis that the East Antarctic Ice Sheet was much reduced during the Pliocene. We demonstrate that Sirius Group lodgment tills on Mount Fleming were deposited by alpine ice. This conclusion is founded on ice-flow directions inferred from rattails, gravel fabric, and the orientation of bullet boulders, as well as on sediment and gravel characteristics of the tills. Only a very few unidentifiable diatom fragments were recovered. Better preserved, identifiable, Pliocene marine diatoms occur in moderate abundances in a 5–10-cm-thick unconsolidated and discontinuous drift that caps the lodgment tills. On the basis of the decline in diatom preservation with depth into the till, we dismiss transport of the better preserved diatoms by the ice that deposited the underlying Sirius Group lodgment till. Instead, we attribute diatom presence to eolian deposition and recycling into lodgment tills at a later stage.

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