Abstract

CIROS-1, sited 12 km off Butter Point in western McMurdo Sound (Lat 77° 34′55″; Long 164° 29′55″), was successfully drilled to a depth of 702 m below the sea floor in marine glacial strata dating back to the early Oligocene (based on a preliminary study of foraminifera). The continuous nature of the core makes it an excellent record of glacial advances and retreats and will make it easier to date using microfossils and paleomagnetic stratigraphy.

The core also records changes in sand and pebble composition that reflect the progressive erosion of the Transantarctic Mountains. The abundance of coaly fragments and the dolerite boulders near the bottom of the hole suggest proximity to local bedrock of Permian coal measures intruded by Jurassic dolerite.

An asphaltic residue was found in sand 632 m down the hole. It indicates that petroleum has been generated in the Victoria Land Basin, and at this site was trapped but later released naturally.

Background

The aim of the CIROS project was to obtain a record of the Cenozoic history of the southwest corner of the Ross Sea by coring sedimentary strata offshore (Barrett 1982). Early Cenozoic strata are of particular interest because there are no strata of this age exposed on the Antarctic continent, and it is a period that is supposed to have seen the beginnings of Antarctic glaciation (Kennett 1977) and the rise of the Transantarctic Mountains (Fitzgerald et al. in press).

Previous drilling in the Ross Sea at DSDP-270 (Hayes, Frakes et al. 1975) and MSSTS-1 (Barrett 1986) had shown that Antarctic glaciation went back at least 25 and 30 million years respectively. The CIROS-1 drillhole was designed to core even older strata beneath the floor of McMurdo Sound (Fig. 1) to record the major glacial advances and retreats and to find out when they began. The core was also expected to show evidence of the uplift history of the Transantarctic Mountains.

Programme

The operation was staged from a camp at Butter Point camp 70 km west of Scott Base on the Victoria Land coast, and followed closely the schedule set out in the drilling operations plan (Antarctic Division 1986). A party of 10 flew to the ice in late August to open the camp and set up the rig, which is 12 km offshore, on the 2 m thick annual ice. Drilling began on October 14, after the arrival of the main group of drillers and scientists (Table 1).

The team of 30 worked 12 hour shifts for the next 2 months commuting each day between Butter Point and the drill site. Coring proceeded without interruption or mishap to a depth of 702 m below the sea floor, which was reached on November 16. The hole was terminated partly because it was approaching the manufacturer's stated limit of the rig's capability but also because mud loss had become high as a result of fractures encountered at 700 m.

Figure 1. Map of McMurdo Sound and Geological cross section showing CIROS-1 and MSSTS-1 drill hole.

Results

The CIROS-1 drillhole is the 4th to be drilled offshore in western McMurdo Sound (see Fig. 2), but has been by far the most successful. The hole was continuously cored from a depth of 27 m below the sea floor to the bottom at 702 m, and over 98% of that interval was recovered. The quality of the core is excellent with some of the core being taken as single 3 m lengths.

Figure 2. Antarctic bedrock holes.

Figure 3. Sequence at MSSTS-1 and CIROS-1 showing correlation and age.

The core is all of Tertiary age, and a preliminary study of microfossils from the lower 20 m of the hole by Professor P. N. Webb indicates an Early Oligocene age (33 to 38 million years). The strata are mainly shallow marine sandstone and mudstone with scattered stones deposited by floating or grounded ice. The stones, many of which are striated, and other features in the core are a positive indication of the continued presence of ice throughout the time period represented by the core. This is the oldest physical record of Cenozoic ice in Antarctica.

The core is particularly valuable because of its length and continuity. Variation in stone content and sedimentary features record the advance and retreat of the ice, and indicate two distinct phases to the glacial history - an early one represented by core from the base of the hole to 366 m sub-bottom for which ice was quite limited in extent, and a later phase represented by core shallower than 366 m during which grounded and floating ice periodically covered the site and was much more extensive than today. Planned studies of the microfossils and magnetic reversal stratigraphy will allow these events to be accurately dated.

The hole terminated in a boulder conglomerate with clasts more than 47 cm across. All are dolerite, presumably the Ferrar Dolerite (Jurassic), which forms extensive sills through the flat-lying Devonian to Jurassic strata (Beacon Supergroup) in the nearby Transantarctic Mountains. The boulders are by far the largest clasts in the core and probably rolled off a nearby bedrock high. It seems most likely that the base of the Cenozoic sedimentary section is only a few metres below the base of the hole.

The rock types represented by the stones in the core vary widely but show a progressive change reflecting the depth of erosion as the Transantarctic Mountains grew. In the upper part of the core granitic basement rocks are common, but near the base most are dolerite. A significant proportion near the base are coal or black carbonaceous shale. The association with dolerite suggests they come from the Permian coal measures in the Beacon Supergroup high in the nearby mountains to the west, and that bedrock beneath the cenozoic section at the CIROS-1 site is Permian coal measures intruded by dolerite. This allows us to determine for the first time the net vertical displacement across the Transantarctic Mountain Front (over 2 km since the Early Oligocene).

Traces of methane were found in the upper part of the hole, but none was encountered lower down. A layer of dark-stained sand was encountered from 632.09 to 634.34 m. The stain is caused by hydrocarbons that appear to represent the residue of a deposit that has been naturally flushed. Nevertheless the material provides the first evidence from the Antarctic continental shelf of mature petroleum generation. Further tests are being carried out to determine in detail the composition of the substance.

Once coring had ended the hole was logged with several probes recording various properties such as density, natural radioactivity and temperature (which at the bottom was 25°C). The hole was then filled with a cement plug and abandoned. The surveyors who had been monitoring the site throughout the drilling found the ice had moved 9 metres northeast during the drilling and the weight of the rig and equipment on the site had depressed the ice by 13 cm (Belgrave, 1987). The drillhole itself has been located with an accuracy of better than 1 m.

A seismic survey between CIROS-1 and MSSTS-1 4 km to the northwest was carried out by Event K045 to trace the major reflectors and to determine depth to bedrock. The results of the survey will be reported separately.

Publication

This report will be submitted to the N.Z. Antarctic Record. The core logs (170 pages at 4 m/page and about 40 pages of core photographs will be published as another volume in the VUW Antarctic Data Series, and should appear about April 1987. An abstract has been submitted to the Fifth International Symposium on Antarctic Earth Sciences (August 1987). A comprehensive volume reporting a wide range of work on the core and similar in style to the recently published MSSTS-1 volume is also planned, and should be published in 1988. Also most of the 15 or so scientists proposing to work on core material are likely to publish papers on more specific aspects of the core in various international journals. When the chronology has been determined a joint paper will be submitted to Nature or a similar journal.

Future Work

References

Antarctic Division. 1986. Cenozoic investigations in the western Ross Sea. Drilling operations plan 1986/87. Antarctic Division, DSIR, Christchurch: 81 p.

Barrett, P.J. 1982. Proposal for Cenozoic investigations in the western Ross Sea (CIROS). N.Z. Antarctic Record 4(2): 32-39.

Barrett, P.J. 1986. Antarctic Cenozoic history from the MSSTS-1 drillhole, western McMurdo Sound. DSIR Miscellaneous Bulletin 237: 174 p.

Belgrave, D.V. 1987. Cenozoic investigations in the western Ross Sea - Surveyors' Report 1986/7. Dept. of Lands & Survey Report, February 1987.

Cooper, A.K., Barrett, P.J., Webb, P.N. and Davey, F.J. 1986. Proposal for drilling in the western Ross Sea. Submitted to the Ocean Drilling Program. Texas A & M College, College Station, Texas, July, 1986.

Fitzgerald, P.G., Sandiford, T., Barrett, P.J. and Gleadow, A.J.W. In press. Assymetric extension associated with uplift and subsidence in the Transantarctic Mountains and Ross Embayment. Earth Science and Planetary Letters.

Hayes, D.E., Frakes, L.A. et al., 1975. Initial reports of the Deep Sea Drilling Project 28. U.S. Government Printing Office, Washington D.C.: 1017 p.

Kennett, J.P. 1977. Cenozoic evolution of Antarctic glaciation, the circum-Antarctic ocean, and their impact on global oceanography. Journal of geophysical research 82(27): 3843-3860.