The Beacon Supergroup in the McMurdo Sound region is a flat-lying continental sequence about 2000 m thick and ranging in age from Early Devonian to Late Triassic (400 to 180 m.y. B.P.). It was extensively intruded by sheets of Ferrar Dolerite in the Early Jurassic. The ultimate objective of our programme - to determine a polar wander curve for Antarctica for this 200 m.y. period - has yet to be achieved. However the secondary aim - devising and carrying out a field programme of collecting oriented rock samples - was successfully accomplished.

The field programme was to take oriented samples at time intervals of no longer than 0.5 m.y., roughly one every 3 m. Coring is the only feasible technique, but to date coring attempts in the Antarctic had been unsatisfactory. The main problems had been the large quantities of cooling fluid needed, the inability to use water, and the weight and unreliability of the equipment.

A 22 cc TAS back-pack motor (Plate XVI) used for tree trimming and grass cutting was adapted by attaching a specially designed coring head (Plate XVII) to the flexible drive. The coolant, a glycol and water mixture, was metered by a stock drenching handpiece. The entire unit, including 5 litres of antifreeze, weighs 14 kg.

Under 'normal' operating conditions, at least 2 litres of water was used per core. The aim was to reduce this consumption to less than 1/2 litre per core. For 600 cores, approximately 150 litres of antifreeze with a 1:1 mix would be required. In the event, 30 gallons (135 litres) was taken since the motor was low powered, thin walled coring stems were obtained as they require less driving power. These would be more likely to wear than the normal heavier corers so 10 core stems were taken.

Technique of taking cores

A suitable site, with exposed weathered bedrock is selected. This often requires much clearing with a geological hammer. It is then cored to a depth of approximately 100 mm. The orientation of the sample is measured and recorded (in our case the dip and strike of the top surface). This is done by inserting in the hole a tube (Plate XVIII) with an adjustable plane table. The table is levelled and the strike measured, preferably with a sun compass, but at least with a magnetic compass. The sample is then removed, marked and bagged. Even under good conditions in temperate climes, two operators take about 10 minutes per core.

Operation in Antarctic Conditions

We quickly found that the magnetic readings of both our geological compasses were unreliable. It was thus necessary to work with the sun on the rock face. At Mt Bastion, 75% of the section was on the southern face. This required working in the very early morning when temperatures were low (−25° to −30°C). Even so, the motor performed very well, usually starting first pull. However, the 1:1 antifreeze mixture froze. It was increased to two parts antifreeze to one of water and finally 100% antifreeze, when no further trouble was experienced. The handpiece metering system worked well and on average, four cores per litre were obtained.

Mount Bastion

A major difficulty here was the weather and the snow that accumulated during our stay. The weather was normally fine in the early morning, but by mid-day, had clouded over and snow was falling. This normally continued for the rest of the day. By the end of our time, most of the slopes were covered by about 300 mm of snow. We were fortunate in having the initial two fine days with clear rock. By carrying page 16 out the reconnaissance with P.J. Barrett who had described the section previously (Barrett and Webb, 1973), we were able to select the locations we wished to sample in the first few days and were subsequently able to carry on almost blind, which was a distinct advantage considering we worked much of the time in near white-out conditions and our tracks from one day to the next were almost obliterated. We are very grateful to Dr Barrett for making this time available.

West Beacon

Sampling at West Beacon was much more rapid. Our camp on the northeast ridge was almost half way up the section, and a minimum of time was spent in travelling to the sites. We benefited considerably by a third person in the party. Two people orienting and collecting the samples can slmost keep up with one driller.

The weather was better and although there was some snow on the rocks near the summit, it was easily cleared.

The slopes containing the chosen section faced north and northeast, enabling sampling to be carried out during the normal day. Sun compass orientations were obtained on a large proportion of samples.

The effect of these factors is reflected in the fact that it took 7 days to sample an equivalent length of section that took 14days at Mt Bastion.

We started sampling near the bottom of the section, at the New Mountain Sandstone - Altar Mountain Formation boundary in the Devonian - the exposures below this were very scattered beneath scree and it was difficult to tell whether they were in place.

Sampling was straightforward up and through the Arena Sandstone to the very prominent Beacon Heights Orthoquartzite (BHO) boundary. The rock was rather friable near the contact with dolerite sill, presumably due to baking. Some cores were taken through the 150 m thick dolerite sill. The section was then continued further around to the North and more directly up to the main peak. The BHO - Aztec Siltstone contact was a prominent erosion surface. The Aztec siltstone was characterised by hard layers of dark green siltstone which gave good cores.

The Weller Coal Measures (Permian) comprised the topmost part of the section to the summit dolerite cap rock. It was fairly coarse and extensively cross-bedded, but had no coal beds here - very similar to the lower Weller at Mt Bastion. The sampling overlap between here and Mt Bastion should provide a useful check on the reliability of the paleomagnetic measurements.

During the coring of the last few samples, the motor was behaving erractically and had considerably reduced power. With the day remaining after finishing the Beacon Heights section, the motor was decarbonised and the spark plug checked. In addition the samples were packed for transport.

Table Mountain

In the three remaining days of our programme, we had been scheduled to join Event K12 at New Harbour. Since they had already completed their programme we instead moved to Table Mountain in order to complete our sampling of the Beacon sandstones, which, in this region rest at the Kukri Erosion Surface, which is cut in plutonic basement rocks.

On Friday 15 December, we arrived at Table Mountain. Karen Williams replaced Peter Garden, who had twisted his knee on the final sampling day at Beacon Heights and returned to Scott Base as a precautionary measure.

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The remainder of the 15th and part of the 16th were spent surveying the section. Then the motor malfunctioned and the rest of the day was used in repairing it.

Consequently, Sunday 17th, the last day began at 0100. We started at the bottom of the section, about 1 1/2 hours' walk from the camp. Although clear, it was very cold. The petrol hose froze stiff and fractured, but after being retained and restarted, the motor ran for about 15 minutes and stopped, due to no spark. We returned to camp, stripped the motor, to find a short at the contact breaker and returned to the top of the section by 1200. It had started to snow, but this time we worked down. We completed our sampling near the base of the section at 1900 hours. By this time approximately 300 mm of snow had fallen and visibility was very restricted.

Conclusions

The sampling of cores in the Beacon section was very successful. The lightweight coring equipment worked well, enabling us to collect samples more efficiently than our coring equipment used in New Zealand. The system was ideally suited to a team of three. The hand piece, used for dispensing the coolant, finished up being held together by string and wire, but modifications should overcome this problem. Significant saving in cooling fluid was achieved so that bulk and weight of equipment and samples were reduced to a minimum.