IMMEDIATE SCIENCE REPORT
K045 : Polar Wander of Gondwanaland:
New Zealand Antractic Programme 1994/95
Antarctica New Zealand November-December 1994
IMMEDIATE SCIENTIFIC REPORT EVENT K045: INVESTIGATION OF
|i)||THE POLAR WANDER PATH OF GONDWANALAND;|
|ii)||THE STABILITY OF THE ANTARCTIC ICE SHEET THREE MILLION YEARS AGO|
the determination of a Polar Wander path for the East antarctican portion of Gondwanaland.
Investigators: David A Christoffel, Research school of Earth Sciences, Victoria University of Wellington, Adam Wooler, RSES, VUW and University of Plymouth, United Kingdom.
determining the climatic environment during the formation of the Sirius glacial deposits three million years ago.
Investigator; Nerida Bleakley, RDRC Scholarship holder, RSES, VUW.
Popular Summary of Scientific Work Achieved
The Beacon sediments of East Antarctica were deposited over the Devonian to Triassic geological periods, from 400 to 200 million years ago. They show evidence of significant changes in climate over that time. Palaeomagnetic measurements on rocks having similar time spans, from Africa, Australia, India and South America, which were then also part of Gondwanaland, give conflicting results regarding the continental drift of that landmass. Many researchers have made palaeomagnetic measurements on Antarctic rocks, including those collected on our previous VUW Antarctic Expeditions in the 1978-79 and 1980-81 seasons. To date, all these have yielded a single pole, for a time 160 million years ago in the Jurassic period, corresponding to a massive intrusion of dolerite sills into the Beacon sediments. Heat and fluid from these intrusions largely destroyed the primary magnetisation acquired by rocks older than them. We have recently remeasured some rock samples from the VUW expeditions, using newly developed techniques. Some rock samples from Mt Crean, Portal Mountain and Alligator peak appear to have retained some primary magnetisation. However, insufficient unprocessed samples remained for us to obtain statistically acceptable results. It was thus necessary to obtain further samples from those promising sites in order to corroborate the measurements.
We obtained over 200 oriented cores, 25 mm in diameter and 50 to 150 mm long using a light portable drill specially adapted at VUW for Antarctic conditions. The sampling was carried out between 30 Nov and 6 Dec 1994 at Mt Crean, on the edge of the Polar plateau, 200 km west of Scott Base. The sediments sampled are red siltstones from the Aztec Group, of Upper Devonian age (approximately 350 million page 3 years ago). They were probably deposited in fresh water lakes and contain finely divided haematite which has the highest stability of any magnetic mineral.
Some preliminary measurements have been made on these samples. They give results similar to those of our remeasured samples. If confirmed by further measurements, it would mean that these rocks were deposited within 12° latitude of the equator. These results differ from those found from African and Australian rocks of similar age, A reassessment of the structure of Gondwanaland and its climate at the time will be necessary in the light of these new results.
Sampling and mapping the Sirius glacial moraines; the age of the Sirius Group glacial deposits in the Transantarctic Mountains has been the topic of considerable debate. The controversy centres on the source of the three million year old marine microfossil diatoms found in tillite beds in the glacial deposits. There are two conflicting models for the source of these microfossils. One is that the diatoms were from the antarctic interior and were subsequently eroded and deposited in the tillite. This implies extensive deglaciation and the existence of marine basins inland of the Transantarctic Mountains three million years ago. The second model is that the three million year old diatoms were blown by wind into the tillite, which could have been of much greater age (fourteen million years or more) and incorporated by periglacial processes. The aim of this project is to distinguish between these two models.
Two sites having Sirius deposits were surveyed and n series of rock sections was sampled at various depth intervals. Neighbouring snow samples were collected, melted, filtered and the residue collected to check for currently existing diatoms. The field work was carried out between the 18th and 30th November, 1994. The first site was at Table Mountain, 150 km west of Scott Base at an altitude of 1800 m on the south side of the Ferrar Glacier. The formation was draped for 2 km along the upper edge of the valley. The second site was at Mt Feather, on a bench at altitude 2500 m on the western edge of the Ferrar Glacier.
A contamination free laboratory has been prepared for processing the sediments to extract the diatoms. Experiments to find the most effective extraction method have been completed and the main extraction programme has commenced. Many diatoms have been found and meaningful interpretations will soon be possible.
|i)||Palaeomagnetic Programme. The objective was to collect oriented core samples from the Devonian Aztec red beds at Mt Crean and Portal Mountain to complement the samples collected at these sites in the reconnaissance expedition in 1980.|
|ii)||Sirius Programme. The objectives were twofold.|
- To accurately map the distribution of the Sirius Group tillite deposits on Table Mountain and Mt Feather and make measurements on the tillite to determine the origin and direction of the glaciers that deposited the sediments.
- To take samples to precisely document the distribution of Diatoms in and around the tillite. This was to be achieved by sampling at different depths in the tillite, from weathered regolith from adjacent older material and from melted snow by filtering water through a Ave micron sieve.
Scientific Endeavours and Achievements
The equipment and techniques have evolved from experience gained from the previous sampling expeditions in 1978 and 1980. Oriented core samples, 25 mm in diameter and 50 to 200 mm long are collected. The corers utilise a modified chain saw and weed eater which have been adapted for attaching diamond tipped coring stems. The cutting fluid is a 60/40 ethylene glycol/water mixture to prevent it freezing at the temperatures encountered. The equipment is similar to that developed and used by us in NZ except that connecting tubes and seals were chosen and tested to withstand the low temperatures. The orienting device is a barrel that fits over the in situ core to which is attached a levelling table holding both a magnetic and sun compass. When the core orientation has been recorded, it is removed and marked before being stored for transport in a magnetically shielded container. For statistical purposes, six closely spaced cores per site are required. The site locations have to be levelled with respect to a marker - preferably a trig station or surveyed summit.
Sirius tillite sampling
After mapping the tillite deposits with a staff and Abney level, the matrix is sampled at varying depth intervals, depending on the deposit thickness. Care must be taken to avoid cross contamination. The deposit, which can be very hard and may be permafrost cored were to be broken up with bolster and chisels. A hand operated percussion tool was specially made for breaking up the harder deposits. The residue from filtering 10 1 of water melted from snow through a five micron filter was to be collected to test for airborne diatoms.
Camp 1: Table Mountain, 77°58′S, 160°E, 18-23 November
This was the first site for sampling the Sirius tillites. We established camp within metres of the upper end of the deposit. This was not a planned palaeomagnetic sampling site so all three of us worked on the Sirius programme. We surveyed the page 5 deposits and found them to be draped for about 2 km along the edge of the slope facing the Ferrar Gl.; more extensive than previously reported. Exposures were good at the upper end but were sparse in the middle and became more continuous towards the lower end. It was misty for most of the time so we flagged a route along the ridge. There were frequent snow showers so we had to clear the rocks before commencing to take samples. Considerable effort was needed, at times even to loosen a few cm depth of deposit. Fifteen samples were taken in the top section and seven in the bottom section. The in situ fabric (ie the orientation and size distribution of the pebbles) was measured to find the mean flow direction of the glacier which deposited the moraine. For all but one day, when the survey was carried out, conditions were misty and it frequently snowed. Temperatures were between −22° and −27° so despite the effort of hammering, working conditions were a little cool and slowed progress. Nevertheless our objectives were achieved.
Camp 2: Mt Feather, 77°56′S, 160°28′E, 23-30 Nov.
Our camp was at 2500 m altitude on a bench covered with Sirius tillite. We were flown in on the second attempt, one day later than scheduled. Rachel Brown joined our Event, to gain first hand experience of field work and to assist Nerida with sampling since we also hoped to collect palaeomagnetic samples. On the first day we had gale force winds and were confined to camp. The second was clear and we reconnoitred to determine the extent of the Sirius formation and to look for a section suitable for palacomagnetic sampling. The .Sirius was extensive but we wore unable to reach the Aztec red beds, which were overlain by a thick dolerite sill. In any case this would have made them unsuitable for sampling. However, we found an accessible exposure of Permian Weller coal measures and decided to sample them. The following three days were spent by Nerida and Rachel surveying and collecting 15 samples from three sites of the Sirius deposits. Adam and Chris surveyed the Weller coal section, finding a series of thin beds suitable for sampling. Again, by flagging a route along the valley edge both parties were able to work in poor visibility without danger of getting lost. Fifty cores were obtained - some from positions under overhangs, making drilling difficult. Weather conditions were again indifferent. By 1000 a mist would come up from the Beacon Valley to the north and stay all day. Temperatures were around −25°. Working conditions were similar to those at Table Mt except it was generally a little colder. We exceeded our objectives.
Camp 3: Mt Crean, 77°55′S, 159°32 E, 30 Nov - 6 Dec.
We camped at 2200 m on an ice covered section of the ridge leading to the south facing Aztec red beds. We arrived a day behind schedule, again due to poor weather conditions. There was no Sirius programme although a sample of glacial regolith and a sample from melted snow were collected to check for airborne diatoms. Rachel left our party and Nerida helped with the coring. Our section was located 30 mins from the campsite. During the next five days we experienced whiteout conditions, strong winds, snow and temperatures down to −27° with a storm on Dec 2 which confined us to our tents. However, we obtained 150 cores from sites we had previously identified as the most promising. This more than fulfilled our objective.
Camp 4: Lashly Mts, 6-8 Dec.
This site was on The Portal at the south end of the Lashly massif. Our objective had been to land at the foot of the SE ridge of Portal Mt but due to lack of fuel and some confusion about its location we were dropped here. No suitable section was accessible from this site and after some delay from helicopter mechanical problems, we returned to Scott Base on Thurs. 8 Dec.
Despite generally poor weather conditions - it was consistently cold, frequently misty, with regular snowfalls which partially covered the rocks we wished to sample, the objectives of both programmes were fulfilled at the sites where we worked. The disappointment was that we missed out on sampling at Portal Mountain.
Both programmes involved our working at climatically exposed sites and handling and marking of specimens, which necessitated the use of thin gloves. We had to take care to avoid frostbite and exposure. Adam Wooler had previous experience on an Arctic ice island, but this was Nerida Bleakley's first Antarctic experience. All members performed well and we cooperated well on both programmes.
For the palaeomagnetic programme, a preliminary set of measurements has been made on selected samples. In many of these from Mt Crean, a very weak primary magnetisation has been isolated from a strong overprint from the Jurassic dolorite intrusions. It gives a direction in agreement with the previous pilot results. Considerably more analysis is necessary before final results can be obtained. Results to date are in agreement with our previous deduction that the sites were 12 degrees in latitude from the equator when the sediments were deposited.
For the Sirius program, a contamination free laboratory has been set up for the extracting diatoms from the tillite samples. Some samples have been processed and diatoms have been found. Each extraction is a lengthy process. More samples need analysing before meaningful interpretations are possible.
Abstracts for conferences have been submitted and accepted as follows: A Wooler and D Christoffel, An Upper Devonian palaeomagnetic pole position for East Antarctica; International Symposium on Antarctic Research VII, Oct 1995. P Barrett, N Bleakley, W Dickinson and M Hannah, Occurrence of diatoms in Sirius diamectite on Mt Feather, Antarctica; Pliocene antarctic Glaciation workshop, Woods Hole, April, 1995. N Bleakley, abstract submitted to International Symposium on Antarctic Research, VII, Italy, Sept, 1995.
We are grateful for the support provided by NZAP, in particular by the staff at Scott Base. We thank Rachel Brown for her field assistance at Mt Feather.
NB acknowledges the RDRC scholarship which made her programme possible.
AW was in receipt of a Rotary International Scholarship and he also received a grant from the Transantarctic Association of UK. Additional funding was provided through the VUW Internal Research Grant. Our thanks also go to Eric Broughton for his thorough preparation and testing of equipment which functioned well in the field.page break page break