Other formats

    Adobe Portable Document Format file (facsimile images)   TEI XML file   ePub eBook file  

Connect

    mail icontwitter iconBlogspot iconrss icon

Victoria University Antarctic Research Expedition Science and Logistics Reports 1986-87: VUWAE 31

Results

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.

page 4
Figure 2. Antarctic bedrock holes.

Figure 2. Antarctic bedrock holes.

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

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.

page 5

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.