Results

CIROS 2 was successfully drilled near the middle of Ferrar Fjord in 210.7 m of water, through 165.5 m of sediment into basement gneiss (Table 2). Basement was found to be slightly deeper than the preferred interpretation of the available seismic data, and somewhat shallower than the estimate based on geomorphic inference (Fig. 8).

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The sedimentary sequence is subdivided into 10 units (Fig. 9), representing alternations of "interglacial" and "glacial" conditions. The oldest interglacial units (13, 11 and 9) are thin (1 to 5 m) diatomaceous mudstones, like the muds accumulating in Granite Harbour today. Units 7, 5, 3 and 1 consist mainly of black sorted fine to medium-grained sand (Fig. 9B) similar to that on the sea floor around CIROS 2 today, where 3/4 of the sand grains are basaltic (Barrett et al., 1984). The sorting and the indistinct horizontal stratification with the occasional mud laminae indicate sedimentation by settling. However, the sand was probably derived ultimately from the McMurdo Volcanics to the east, glacially transported and deposited on the walls of Ferrar Valley and then blown by wind offshore. Both mud and sand units probably represent times when glacial ice was no more extensive than today.

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The 6 even numbered units are extremely poorly sorted mixtures of mud, sand and gravel (diamict, Fig. 9A) with occasional striated stones, deposited directly or indirectly from glacial ice. They represent periods when ice cover around Ferrar Fjord was more extensive than at present. The oldest diamicts (Units 12 and 10) contain horizontal striated surfaces that are interpreted as subglacial shear planes, indicating that these units at least are lodgement tills. The other gravelly units, however, show in a number of places signs of redeposition by gravity flows or sedimentation through the water column, but probably accumulated close to the ice front.

The ice that transported the debris forming the diamict units came from one of two directions - west through the Transantarctic Mountains, or east past the volcanic piles of McMurdo Sound, and the debris should reflect this. Basaltic clasts are abundant (30 to 60%) in all diamicts but unit 8, indicating an easterly source. Several small basaltic cones of the order of 100 m across are known from upper Ferrar Valley, and are a potential source for some basaltic debris, but are tiny compared with the large area of exposed basement rocks. The proportion of volcanic debris in the sand fraction should help resolve this question.

Unusually well developed stratification occurs at several levels in the core between 7 and 80 m. It consists of sets of parallel mud laminae 1 to 3 mm thick in a well sorted fine sand (Fig. 9A). They superficially resemble glacial varves but the mud laminae are discrete, rather than part of sand-mud couplets. Also they lack outsized clasts. As yet we have no explanation for them.

The chronology of events recorded in the CIROS 2 core will depend on the results of current paleomagnetic, micropaleontologic and radiometric studies (Table 3). However, a preliminary examination of well preserved diatom assemblages from the lower 30 m of the hole indicates an Early Pliocene age (around 4 m.y.) (Table 4), The abundance of basaltic debris throughout most of the core and to the bottom of the hole is another indication of Plio-Pleistocene age, for basaltic debris appears in cores from DVDP 10 and 11 in the fill of adjacent Taylor Valley only above the late Miocene-Early Pliocene unconformity (Elston and Bressler, 1981; Porter and Beget, 1981). This unconformity and the sediments just above it are taken by Elston and Bressler to represent a significant glacial advance from the Ross Sea. The basalt-bearing lodgement till at the base of CIROS 2 may also represent this event.

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The sediment resting on basement rock at CIROS 2 was expected to be older, for it was the lowest point (377 m below sea level) from which valley fill has been recovered in the Dry Valleys region. The deepest prior to CIROS 2, DVDP 11 in Taylor Valley, ended in diamict 7 m.y. old (Elston and Bressler, 1981) at 268 m below sea level, though still 150 m above basement (Hicks and Bennett, 1981). The CIROS 2 sequence is also young compared with the MSSTS 1 core 30 km to the northeast (30 m.y. at 420 m below sea level; Harwood, 1984). Nevertheless, the CIROS 2 core will be of special value for the Pliocene history of the region because it is the only sequence of this period in which paleomagnetic zones and diatom assemblages can be radiometrically dated.

The basement gneiss cored at CIROS 2 377 m below sea level has been sampled for apatite fission-track dating by P.G. Fitzgerald at the University of Melbourne to extend the vertical sections he has sampled up nearby Mount Barnes and Trig Herb. The age obtained by this sample should provide the youngest rate of uplift obtained thus far for this part of the Transantarctic Mountains.

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A sea ice deformation survey was carried out during drilling at CIROS 2. Results of this survey and the other CIROS surveying programmes will be presented in the NZARP Surveyors Report 1904-85. Tidal movement was also recorded at CIROS 2 and is summarised in Table 5.