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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.

There are two extreme views of the evolution of the Pliocene Antarctic Ice Sheet. Dynamicists argue for ice-sheet reduction and reexpansion on the basis of Pliocene marine diatoms in a glacial deposit, the Sirius Group, that is widespread in the Transantarctic Mountains. Stabilists argue from other evidence that the Antarctic cryosphere remained essentially constant in area and volume; they propose marine diatom transport by eolian processes and emplacement into terrestrial glacial strata. Hence, the inferred source area and transport mechanism of marine diatoms are of critical importance. We tested the reduction hypothesis on an important outcrop of the Sirius Group at Mount Fleming, South Victoria Land. We observed very few, unidentifiable, marine diatom fragments in Sirius Group strata. In contrast, marine diatoms enclosed in a surface diamicton covering the Sirius Group were more abundant and identifiable. Our study further indicates that the Sirius Group at Mount Fleming was not deposited by the East Antarctic Ice Sheet but rather by alpine ice originating on the Transantarctic Mountains. On the basis of both data sets, we infer that marine diatoms postdate Sirius Group deposition at Mount Fleming and that transport was by wind, and we advance alternative scenarios for their source and transport pathways.