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Victoria University Antarctic Research Expedition Science and Logistics Reports 1996-97: VUWAE 41

1 Popular Summary of Scientific Work

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1 Popular Summary of Scientific Work

The main goal of scientific research for the project is to understand depositional and post-depositional history of the Sirius Group tillite in the Dry Valleys area. The significance of this deposit centres on an intense international debate concerning the extent of the East Antarctic ice sheet three million years ago. In the debate, the dynamic view, which relies on the occurrence of rare marine diatoms in the Sirius, favours a nearly complete deglaciation of the East Antarctic ice sheet three million years ago. On the other hand, the stabilist view favours an ice sheet which formed nearly 14 million years ago and retained its shape through until the present day. Cores from within the Sirius will provide information not available from surface exposures that will help resolve this debate.

The goal of the field work was to test a diamond drilling technique and core as much as possible of the Sirius at Table Mt. During the 23 days in the field, a total of about 49 metres was drilled, and of this, about 42 metres of core was collected, giving an average recovery rate of 87%. On average, the core holes were 3.5 metres deep, but two of them reached depths of 9.5 and 8 metres. In addition, detailed glacial fabric analyses of the Sirius were made at 12 sites and enough geologic and geomorphic data were collected to provide a detailed map of about four square kilometres on the northwest flank of Table Mt.

Sirius Group deposits on Table Mountain appear to result from both advancing and retreating glaciers. Ice retreat has left an extensive ridge and hollow topography. Ridges generally consist of glacial diamictite, covered by a boulder lag, while the hollows consist of either conglomerate or sand. The deglacial environment is dominated by water-lain deposits of which the conglomerate and sand suggest that both high and low energy regimes were involved.

Petrographic analysis shows that authigenic zeolites, quartz, and calcite occur in various quantities throughout the pore network of Sirius sediments. Pores near the surface appear to result from freeze-thaw processes associated with periglacial activity, while those below three metres formed as the sediment was deposited. For authigenic minerals to precipitate, large volumes of water must have flowed through the pores. While this is inconsistent with present climatic conditions, the large amount of water is consistent with geomorphic observations and deposition of the sandstones and conglomerates. It is also consistent with the significant amounts of ice found in the pores and fractures of the core. Stable isotopic measurements of this ice may help determine the origin of this water.

In the future, more diamond coring of glacial deposits in the Dry Valleys area is needed to provide a detailed history of the Antarctic ice sheet. Modification of the core drilling equipment used in this project will make it lighter weight, more reliable, and allow a variety of sites to be cored in a single season.