Victoria University Antarctic Research Expedition Science and Logistics Reports 1980-81: VUWAE 25
The field work follows on from VUWAE 23 of 1978-79 season when the Beacon sandstones at Mt. Bastion, Beacon Heights and Table Mt. were sampled. From that work, it was found that those rocks were very weakly magnetised and their magnetic direction had been reset at the time of the massive dolerite intrusions during the Jurassic period (160 My ago). The reason for this result is not clear and is the subject of further investigations now that our cryogenic magnetometer is operating.
For the above reason, the object was to sample only the red beds which occur at two periods within the Beacon Supergroup. The first period is near the base of the section - the Terra Cotta siltstone and probably Lower Devonian in age (Plume, 1975). The second is in the Upper Devonian, near the boundary with the Permian, called the Aztec siltstone (McPherson, 1976). The red colouring is due to the haematite and is usually more stable magnetically than the magnetite which is the principal magnetic constituent of the sandstones. The Terra Cotta siltstone outcrops at Mt. Kempe, Table Mt. and Knobhead. The Aztec siltstone outcrops at Portal Mt., Alligator Peak and Mt. Crean.
Our rock coring equipment was basically the same as for VUWAE 23, incorporating the modifications to equipment and technique suggested by that work. We used heavier coring barrels than previously and they lasted very well; we used pure anti-freeze which did not freeze in the pipes (although at times it became thick and difficult to pump). It was fortunate that our sampling methods worked so well, as with weather and transport holdups our time at some sites was severely curtailed. We now have a very portable rock sampling system which could have many uses besides sampling for paleomagnetic purposes.
The samples of the dykes (of Ordovician age) in the Wright and Miers Valleys as well as the volcanics of Ross Island, during my second visit, were also valuable additions to our collection.
The strongly magnetised basalts from a series of flows on Observation Hill have been measured and analysed. Measurements had previously been made by Kyle and Treves (1974) and Funaki (1978). We have sampled a more complete sequence of flows and although we cannot judge what time period is represented, the virtual Geomagnetic Pole positions from successive flows show a consistent trend not found by the previous workers. The rocks (1.2 My) are reversed and these measurements make a contribution to almost non-existent information on past patterns of secular variation of the earth's magnetic field in Antarctic regions - see Figure 3.
Pilot measurements have been made in the red bed samples from all localities. They are sufficiently weakly magnetised to require measuring on the cryogenic magnetometer which has operated for a limited period only. Most of the initial measurements on the red samples give the same directions as the Jurassic dolerites, indicating that they acquired a component of magnetisation, either through heating or chemical alteration.page 15
Figure 3: Virtual Geomagnetic Pole positions from a sequence of about 6 volcanic flows at Observation Hill, Ross Is. The time sequence is unknown, but it can be seen that the position of the North geomagnetic pole varied in a smooth manner, consistent with a period of hundreds or thousands of years. North Pole plots on lower hemisphere as all samples are reversely magnetised. Arrow paths are stylised in progression from oldest to youngest.
No significant changes are expected until the samples have been thoroughly demagnetised beyond 600°C. There are signs of changes beginning (see Pig. 4) but we have to await the completion of further paleomagnetic measurements before reaching our final conclusions.