Background

Seismic surveys over the last decade (Northey et al., 1975; Davey and Bennett, 1981; Wong and Christoffel, 1981),have revealed along the Victoria Land coast a sedimentary basin (Fig. 1) of probable post-Jurassic age, the margins of which are accessible to shallow offshore drilling. The structure of the western margin of the basin in South Victoria Land is shown in figure 2. The MSSTS 1 core has shown that the uppermost strata (to 226 m sub-bottom) are of marine glacial character (Barrett and McKelvey, 1981), and extend back to 30 m.y. (Harwood, 1984).

The first phase of CIROS was to drill two holes in the same area as MSSTS 1. CIROS 1 was to core to the limit of the rig (around 500 m) near the MSSTS 1 site, where earlier seismic refraction data taken from the annual ice showed a marked velocity increase from 2.7 to 3.5 km/sec about 300 m below the sea floor (lies and Dibble, 1981) interpreted as the change from glacial to preglacial strata (Barrett, 1982). The lower part of this core would provide a record of climatic changes preceding Cenozoic glaciation in the region. The value of the site was increased with the running of a
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Figure 1. The Ross Sea region showing the location of the Victoria Land basin and the area of CIROS drilling (Fig. 2)

multichannel seismic line in February 1983 by the S.P. LEE from near MSSTS 1 north along the basin margin (Eittreim, Cooper et al., 1984). The new data confirmed the marked velocity increase, though at a slightly greater depth (360 m), and offered the possibility of extending the drill hole stratigraphy further out into the Sound and to the dipping sequence off Cape Roberts, 80 km north and the target for the second phase of CIROS.

The second hole in the first phase (CIROS 2) was to be drilled as far landward as the ice in Ferrar Valley would permit. This hole was to core the sediment deposited on the valley floor to basement to work out the glacial history of the valley and also to obtain a minimum age on the cutting of the valley from the sediment just above the basement. In addition, the core was expected to contain reference planes to correlate with CIROS 1 and hence allow us to gauge the timing and extent of vertical movement within the fault zone between the two sites. The basement core was also of interest for it would be the lowest sample obtained to date for apatite fission-track dating, and hence provide the youngest possible point on the uplift curve for this section of the Transantarctic Mountains.