IMMEDIATE SCIENCE REPORT
K047: Petrology of Sirius Tillite Antarctica New Zealand 1996/97
Antarctica New Zealand November 1996 - December 1997
1 Popular Summary of Scientific Work
The main goal of scientific research for the project is to understand depositional and post-depositional history of the Sirius Group tillite in the Dry Valleys area. The significance of this deposit centres on an intense international debate concerning the extent of the East Antarctic ice sheet three million years ago. In the debate, the dynamic view, which relies on the occurrence of rare marine diatoms in the Sirius, favours a nearly complete deglaciation of the East Antarctic ice sheet three million years ago. On the other hand, the stabilist view favours an ice sheet which formed nearly 14 million years ago and retained its shape through until the present day. Cores from within the Sirius will provide information not available from surface exposures that will help resolve this debate.
The goal of the field work was to test a diamond drilling technique and core as much as possible of the Sirius at Table Mt. During the 23 days in the field, a total of about 49 metres was drilled, and of this, about 42 metres of core was collected, giving an average recovery rate of 87%. On average, the core holes were 3.5 metres deep, but two of them reached depths of 9.5 and 8 metres. In addition, detailed glacial fabric analyses of the Sirius were made at 12 sites and enough geologic and geomorphic data were collected to provide a detailed map of about four square kilometres on the northwest flank of Table Mt.
Sirius Group deposits on Table Mountain appear to result from both advancing and retreating glaciers. Ice retreat has left an extensive ridge and hollow topography. Ridges generally consist of glacial diamictite, covered by a boulder lag, while the hollows consist of either conglomerate or sand. The deglacial environment is dominated by water-lain deposits of which the conglomerate and sand suggest that both high and low energy regimes were involved.
Petrographic analysis shows that authigenic zeolites, quartz, and calcite occur in various quantities throughout the pore network of Sirius sediments. Pores near the surface appear to result from freeze-thaw processes associated with periglacial activity, while those below three metres formed as the sediment was deposited. For authigenic minerals to precipitate, large volumes of water must have flowed through the pores. While this is inconsistent with present climatic conditions, the large amount of water is consistent with geomorphic observations and deposition of the sandstones and conglomerates. It is also consistent with the significant amounts of ice found in the pores and fractures of the core. Stable isotopic measurements of this ice may help determine the origin of this water.
In the future, more diamond coring of glacial deposits in the Dry Valleys area is needed to provide a detailed history of the Antarctic ice sheet. Modification of the core drilling equipment used in this project will make it lighter weight, more reliable, and allow a variety of sites to be cored in a single season.
2 Proposed Programme
- To collect data for a geologic and geomorphic map of the Sirius Group at Table Mt and Mt Feather.
- To obtain cores at least one metre long of the Sirius Group outcrops from at least three sites each at Table Mt and Mt Feather. Coring locations were to be selected on the basis of geomorphic mapping and previous sites where study samples were taken.page break
- To complete a comparative petrological study of the vertical core sequences taken through the Sirius. This includes determining the cause and relative timing of mineral dissolution and precipitation (mineral diagenesis). In addition, the stable isotope ratios of ice in pores and fractures will be measured. These will provide important clues about the post depositional history of the Sirius.page break
- To document the occurrence of diatoms through the vertical core sequence of Sirius. This will help determine the origin of the rare marine diatoms which may be crucial in determining the history of the East Antarctic ice sheet.
3 Scientific Endeavours and Achievements
On 10 April 1997, a meeting was held at the School of Earth Sciences, Victoria University of Wellington, to present the preliminary findings of Antarctic field work carried out in the summer of 1996/97. The abstracts of three talks, presented at that meeting, provide a summary of scientific endeavours and achievements of the field work completed at Table Mt.
3.1 A Preliminary Report on Sirius Group Deposits, Table Mountain James Goff and Ian Jennings
Sirius Group deposits on Table Mountain appear to result from both advancing and retreating glaciers. The topography, however, is the result of glacial retreat and includes patterned ground, water-lain deposits, and mass movement features. Cores taken through the Sirius Group should help explain the role played by water in ice advance and retreat at the site, and in dating the event.
Fabric data from deposits at the southern end of Table Mountain indicate that this area was a confluence zone for ice emanating from the directions of the contemporary Tedrow and Ferrar Glaciers. However, the imprint of "Ferrar" ice dominates Sirius Group sediments at Table Mt. Other minor contributions of sediments were made from small mountain glaciers emanating from the saddle area between Table Mt and Navajo Butte. We believe that ice from these sources may have been sufficient to occupy the anomalous hollow (Figure 1) at Table Mountain which is devoid of glacial deposits.
Deposits that contain a small percentage of granitic clasts are found several hundred metres upslope from the prominent dolerite sill at Table Mt (Figure 1). The abundance of granitic clasts appears to increase down slope suggesting it was sheared up by glacial flow immediately downglacier of a confluence zone. Fabric measurements, taken from south to north along the length of Table Mt, indicate a reorientation of ice flow from west to southwest. Reorientation was caused by Table Mt obstructing ice flow which has resulted in the deposition of thrust-faulted lodgment tills and associated deposits. Lodgment and thrust-faulting may represent a period of glacial advance.
Ice retreat and down wasting has left an extensive ridge and hollow topography. Ridges generally consist of glacial diamictite, covered by a boulder lag, while the hollows consist of either conglomerate or sand. The deglacial environment appears to be dominated by water-lain deposits of which the conglomerate and sand suggest that both high and low energy regimes were involved.
Three distinct mass movement features cut across the ridge and hollow topography. Patterned ground, which is pervasive throughout the Table Mt area, is most prominent on these mass movement features but less prominent on the ridge and hollow topography. It is not clear if the degree of prominence expressed by the patterned ground, represents different degrees of activity, different ground materials, or different periods of generation.
3.2 On-Land Coring: New Developments Warren Dickinson, Pat Cooper, Bain Webster
Core drilling of permafrosted sediments is common and well understood in most Arctic environments, but in the cold, dry page break Antarctic environment it is at its infancy. Coring the Sirius deposits at Table Mountain was largely an on-site experiment because such permafrosted sediments do not exist in NZ or on Ross Island.
All drilling equipment had to be hand portable, and available in the field camp. Off the shelf HQ and NQ diameter core barrels and drill rods were used with a flushing medium of compressed air. The purpose-built, portable compressor could produce 50 cubic feet of air per minute at 30 psi which was pumped down the hole via an air swivel. The drill rods were rotated by a Sthil 056 motor mounted on frame with a torque bar which was pinned to the ground. Bit weight was controlled by the weight of the operator and any additional weight from the helpers. Pulling and lifting of the drill assembly was controlled by a hand winch via a single running block attached to a tripod.
Initial drilling showed that experimentation and modification of bit types was necessary to properly core the variety of lithologies contained in the Sirius. The flushing and cooling of the drill bit with compressed air was found to be critical. At all times the drill bit must be kept at sub zero temperatures to prevent melting of core and nearly instantaneous freeze in of the bit should rotation stop unexpectedly. Cooling of the bit depends on the temperature and volume of air entering the hole as well as the kerf and diameter of the bit. For a given air supply, a thin kerf and small diameter bit runs cooler than a thick kerf and large diameter bit. Diamond bits must be used to core hard and firmly cemented dolerite clasts, but tungsten bits must be used to core ice lenses and soft friable sands. Core recovery of conglomerates in ice-free horizons, which usually occur from the surface to 50 cm deep, was not possible. Loose clasts which are jarred from this horizon and fall into the hole must be either pulverized by further drilling or scooped out of the hole if core is to be recovered.
Considering the budget restraints and limited helicopter support the drilling project was extremely successful. With moderate adjustment and modification the existing equipment could be refined into a highly reliable and portable Antarctic drilling unit.
|Final score at Table Mountain:|
|Total core recovered:||42m (86% recovery rate)|
|Total time in field:||24 days|
|Equipment loss:||1 diamond core bit and tungsten reamer|
|Cost of equipment and labour:||$28,000 (this excludes rental and depreciation of equipment) or about $650 per metre of core|
3.3 Ice and Temperature Signals from Sirius Group, Table Mountain Warren Dickinson
Core hole drilling of the Sirius Group at Table Mountain provided the opportunity to measure closely spaced (10-30cm) ground temperatures from the surface to four metres deep. These temperatures will provide background data for two areas of study. 1) Determination of the potential for periglacial or active ground movement that could produce patterned ground on Sirius Group sediments. 2) Calibration of oxygen isotope temperatures obtained from ice in fractures and in pores of the sediment. Although the Sirius was thought to have ice-filled pores below 50 cm, the amount and number of ice filled fractures found in the core was a surprise. Although this ice cannot be directly dated, stable isotopic measurements are critical to understanding its origin.
The equipment and methods used for measuring the temperatures were designed to be simple and economic. They also had to page break accommodate a range of unknown conditions. At the outset of the project, the depth and number of core holes was uncertain as well as the ability to actually measure the core hole temperatures.
The measuring system consisted of a digital thermometer calibrated for K-type thermocouples and 15 thermocouple wires from 0.5m to 4.5m in length. The core holes varied in diameter between 70 and 90mm and for this particular range a 50mm OD plastic pipe worked best for holding the wires down the hole. The relatively loose fit ensured that the plastic pipe would not get stuck in the hole. Thermocouple wires were brought down the centre of the pipe and out through holes at 0.25m intervals. The wires were taped to the outside of the pipe and bent to form whiskers protruding about 10mm outwards from the pipe. In this way, the wire whisker with the thermocouple junction on the end contacted the side of the core hole. Temperature measurements were taken over a period of five days at one hole but only for one day periods at four other holes.
Temperatures decreased with depth by 3.5_C per metre up to four metres depth (Figure 2). At four metres depth the average temperature, which varied by 2_C between the five holes, was −21.5_C. Temperatures measured 3-5cm below the surface in loose soil showed large variations. This was probably due to the degree of sunlight exposure at the surface. These variations appeared to affect temperatures down the hole to 0.5 m and possibly 1.0 metre deep. To confirm these temperature variations, measurements must be made over a period of weeks and probably through the winter.
As event leader, my most sincere appreciation and thanks are given to the five other members of the event who were largely responsible for the success of the field season. All of the personnel at Scott Base were extremely helpful and provided the support that made the field season possible. In particular, Paul Woodgate in Christchurch and Bridget Troughton at Scott Base were extremely efficient in shipping extra drilling equipment at the last minute. Numerous discussions with Alex Pyne greatly helped in organizing the field season. Graeme Claridge provided much needed background information as well as the spark that initiated the diamond core drilling. Nine metres of drill rod provided by Jim Cowie and the Cape Roberts Project made the drilling eight and nine meter holes possible. Jeff Ashby gave much needed assistance with the drilling reports, and Peter Barrett saved the PGSF bid from going under in the early stages. Funding for the event was provided by the Foundation for Research, Science and Technology under contract number DIC601.page break
Small cairns of one to two cobbles high were left to mark and cover open core holes at TM-1, TM-2, TM-6, TM-7, and TM-8 (see locations on drill hole map). These holes were left open for future monitoring of ground temperatures. In addition, it may be feasible in the future to deepen the hole at TM-7 to penetrate through the Sirius to the underlying bedrock contact.
The main impact at the Table Mt sites consisted of tramping over the fragile desert pavement. While this is unavoidable if field work is to be conducted in the area, such an impact is visible for perhaps 5 to 10 years after the event. This is because of slow the process by which the pavement forms. When working around a drill site, event personnel attempted to follow the same track. When the drill site was vacated, pavement stones were swept and raked over the tracks in an effort to reclaim the surface. In most cases this mitigation measure, significantly reduced the visual impact and should allow the pavement to reform completely in 3-5 years.