Victoria University Antarctic Research Expedition Science and Logistics Reports 1980-81: VUWAE 25
Sediment Sampling in McMurdo Sound (B.L. Ward).
Sea floor sediments of McMurdo Sound were sampled to obtain material suitable for detailed analysis of foraminiferal populations and their relationships to sediment type and ecological factors. Areas sampled, using the sea-ice as platform, were New Harbour and near the McMurdo Station desalination plant discharge point. Later in the season during January and February, Drs. D. Bennett and F. Davey obtained five gravity cores for this work during a seismic profiling cruise on board the Benjamin Bowring. These were from open-water areas inaccessible from the sea ice earlier in the season. Figure 1 is a map of the McMurdo Sound area showing sample locations for all material collected this past season; depths ranged from 8 to 750m.
A total of 29 core and grab samples were obtained by dropping equipment through a twelve inch access hole drilled in the two metre thick sea ice (Plate II A.B.C.D.). Twenty-seven of these were large enough for analysis. The additional five gravity cores have been split horizontally into one or two centimetre segments to yield 24 samples. The total number of samples to be analysed from this season is 51.
Preliminary examination of material obtained during the 1979-80 season and this past season indicates variation of foraminiferal populations between open McMurdo Sound waters and the embayed New Harbour area. Agglutinated species predominate in the muddier, enclosed New Harbour sediments, while calcareous forms are more prevalent in the deeper open Sound waters. Varying amounts of sponge mat have been found at several sites but these seem to have little effect on the species present.
Next season (1981-82) we plan to continue the sampling programme, using our modified large-diameter sphincter corer, as well as a salinity-temperature bridge, current meter, tide gauge, and underwater camera. The areas to be covered are southern McMurdo Sound and Granite Harbour.page 8
|A.||The sphincter corer being lowered by winch through the 2m thick sea ice. The winch is mounted and operated on a Tamworth sledge. Twelve inch diameter ice augers and powerhead used to drill the sea ice access holes in the foreground.|
|B.||Assembled corer ready to lower. The sphincter sleeve in the coring head is being checked.|
|C.||The undisturbed top of a core retained in the sphincter corer. The core comprises: muddy sand, silica sponge spicules up to 100mm long and calcareous bryozoan.|
|D.||Sediment and sponge spicules retained in an orange-peel grab.|
A seismic refraction survey on sea ice near Butter Point, New Harbour, McMurdo Sound (D. Iles and R.R. Dibble).
A seismic refraction survey was conducted on sea ice near Butter Point from November 26th to December 3rd, 1980. The aim was to provide data on sediment thickness for possible further drilling and to investigate the cause of a gravity anomaly reported by Sisson (1980). He suggested that the gravity anomaly could be attributed to a basement fault, downthrown to the northeast.
A 12-channel SIE/RS44 refraction seismograph was used. The seismograph was operated by the second author in a Snow-Trac at temperatures of approximately −10°C. The only adaption required for the low temperatures was the replacement of several large electrolytic capacitors in the RS44 recorder.
The 12 vertical geophones were spaced at 29.95m intervals as determined by the geophone cable. They were frozen into holes chipped in the sea ice, which was covered in most placed by 100-200mm of snow. Noice levels were extremely low, even during "blowing snow" conditions.
The explosives used were 1.1kg cartridges of AN60 and 1.6kg cartidges of AN95. They were suspended on detonating cord down 150mm diameter holes drilled through the 2m thick ice by means of a "one man" gasoline powered auger. Plain No. 6 caps and safety fuse were used by the first author to fire the charge. Except for one day, when an assistant was available, the survey was carried out by two people (the authors) using a Snow-Trac, a Snow-Tric snowmobile, and a sledge for transport.
Details of charge size and depth are tabulated in Appendix IA. At the depths used we did get bubble pulses which sometimes obscured later arrivals. However, bringing the charge closer to the surface of the ice to reduce bubble pulses destroyed the shot holes, which we needed to reuse in order to save time.
A shot instant detector (described in Appendix IF) switched on a tone about six seconds before each shot, and terminated it at the shot instant. The tone was transmitted by radio to the recorder and marked the shot instant on the records.
Positioning of Survey:
Two reversed lines were shot, using four shot points (SP.I to SP.IV) shown in Fig. 1 and Fig. 2. The four shot points were surveyed by Messrs. C. Fink and G. Neale of the N.Z. Lands and Survey Department. Line 2 crosses the positions of the proposed fault at SP.III. Using SP.III both halves of Line 2 were reversed.
The water wave velocity was determined using shots fired from one shot point to another, since those distances were known to within a few metres. The mean velocity was 1443m.s−1 over 6 results, which ranged from 1441.2 - 1446.1m.s−1.
Results and Analysis:
Appendix IB lists the spread offset distances and the apparent velocities and intercepts of the refraction arrivals. The velocities have been corrected for angle between spread and shot directions.
The time distance graph for line 1 (Appendix IC) shows a single refraction arrival of high apparent velocity typical of basement rock. The interpretation by the first author using a novel ray tracing method (Appendix IG) and a velocity of 5655m.s is shown in Fig. 2A.
Refractions above the basement were not recorded from line 1, but depth sounding data nearby (Ward - this report) require a low velocity layer between the sea floor and basement. The interpretation in Fig. 2A assumes for the layer a velocity of 2600 m.s, the same velocity determined for the sediment layer at SP.III on line 2.
The time distance graph for line 2 (Appendix ID) indicates one sedimentary layer over the basement at the West end and three sedimentary layers at the East end. Guided by the least squares line segments for each spread, straight time distance lines satisfying the theoretical requirements for a plane-layer structure were drawn on (Appendix ID). The arrivals to the East from SP.III are very poor and the 2207m. s−1 line has been drawn in using known bathymetry and the time interrupt of the 2322m.s−1 line.
The adopted lines for the sedimentary layers were interpreted in terms of dipping plane layers (Fig. 2B), using the formulation of Mota (1954). Then the basement arrivals were interpreted by the first author using the ray tracing method (Appendix IG). An average sedimentary velocity of 2600m.s−1 has been assumed on the West half of the line.
The 3547m.s−1 layer would appear to be cut off as you move towards the west since the 3681m.s−1 refraction was very weak and the arrivals on spreads 6 and 7 from SP.II (3398m.s−1 and 3146m.s−1 arrivals) are believed to be basement refractions and their low apparent velocity is not consistent with an intermediate velocity layer with as high a velocity as 3547m.s−1, on such a shallow dip. This cut-off is consistent with but not indicated by our arrivals from that layer. The alternative is that the interface between the 2730 and 3547m.s layers has undulationsin it so as to increase its dip to the east below and west of spreads 6 and 7. There is no arrival from SP.IV on spread 6 supporting this, but if it were the case, the basement positions shown between 3750 and 4650m east of SP.II would move westward and downward, and the cross at 4650m east of SP.II would move eastward and downward, i.e. the basement page 13 would bend downward more rapidly when approaching SP.III from the west.
A mean sedimentary velocity of 2600m.s was used for analysis of the basement arrivals on spreads 3-7 from SP.II but for the remaining two spreads (8 and 9) the three plane layers were used. The arrivals from shot points III and IV were analysed with 2600m.s−1 sediment to the west and the plane layer case to the east of SP.III.
The basement velocity of 5655m.s−1 was obtaining by averaging the slopes of the segments where a basement refraction was recorded from both directions on the same spread. A simplified interpretation by the second author, using plane layer interpretation theory, is given in Appendix IE.
This survey has defined the shape and depth of the basement-sediment interface for a line east from Butter Point. Nearshore the surface is gently domed, averaging 400m below sea level, but dip increases eastward reaching up to 65° at 6km from Butter Point. Beyond this distance the data is insufficient but indicates a depth to basement greater than 1.8km.
The present interpretation shows that SP.II (nearshore)is underlain by 300m of sediments (2600m.s−1) overlying basement (5655m.s−1). SP.IV 8km east is underlain by a thin (60m) layer of low velocity sediment (2210m.s−1) another sedimentary layer of 250m (2730m.s−1) and a further and thicker sedimentary layer (3547m.s−1).
Barrett, P.J. and Froggatt, P.C. 1978. Densities, porosities and seismic velocities of some rocks from Victoria Land, Antarctica. N.Z. Jour. of Geol. and Geophys. Vol. 21, No. 2 (1978): 175-187.
Barrett, P.J. 1979. Proceedings of the Seminar III on Dry Valley Drilling Project, 1978. Proposed Drilling in McMurdo Sound - 1979. Reprinted from Memoirs of National Institute of Polar Research. Special Issue No. 13.
Dobria, M.B. 1976. Introduction to Geophysical Prospecting. McGraw-Hill Book Co., 3rd ed., 630p.
McGinnis, L.D. 1979. Initial Report on a Refraction Seismic Study in Western McMurdo Sound. (Unpublished).
Mota, L. 1954. Determination of Dips and Depths of Geological Layers by Seismic Refraction Method. Geophysics 1954, Vol. 19, pp 242-51.
Robinson, E.S. 1963. Geophysical investigations in McMurdo Sound, Antarctica. Jour. Geophys. Res. V. 68 pp. 257-262.
Sissons, B.A. 1980. In: Pyne and Waghorn Immediate Report of VUWAE 24.
Paleomagnetic Studies (D.A. Christoffel).
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.
Funaki, M., 1978. The paleomagnetic investigation of Antarctica. 1. Paleo-magnetism of Hut Point Peninsula volcanic sequence. Memoirs of Nat. Inst. of Polar Res., Japan, Special Issue #14, pp. 186-193.
Kyle, P.R. and S.B. Treves, 1974. Geology of Hut Pt. Peninsula, Ross I., Antarc. J. U.S., 9 pp. 232-234.
McPherson, J., 1975. The Aztec Siltstone: An Upper Devonian Alluvial Plain Red Bed Sequence, Southern Victoria Land, Antarctica. Ph.D. Thesis held VUW Library.
Plume, R., 1976. Stratigraphy, Sedimentology and Paleocurrent Analysis of the Basal part of the Beacon Supergroup (Devonian and Older (?) to Triassic), Southern Victoria Land, Antarctica. M.Sc. Thesis held VUW Library.page 17
Figure 4: Strike and dip of red bed samples from Knobhead (KH), Table Mt. (TMt), Mt. Kempe (MK), Alligator Peak (AP) and Mt. Crean (MC). Numbers beside the samples show the thermal demagnetising temperatures. The cross shows the mean strike and dip of the Jurassic dolerite.
It can be seen that at the higher temperatures, the samples are beginning to move from dolerite position.page 18
Sampling a red bed sequence of the Terra Cotta Siltstone at Mt. Handsley. The red beds have been extensively burrowed.
Granites and Metamorphic Rocks of the Areas between the Miers and Salmon Valleys (Frank Reid).
The area between the Miers and Salmon Valleys has been remapped in greater detail than before and several corrections have been made to the small scale maps of Blank et al. (1963). A simplification of the stratigraphy of the metasedimentary rocks will be proposed and areas mapped before as undifferentiated granitic intrusions have been mapped as different intrusive events. The initial distinction between different granitic bodies has only been made on field relations and hand specimen inspection at this stage, but this is presently being extended to petrographic and geochemical classification. The area has been extensively sampled and it is hoped that this may provide a basis for a coherent nomenclature of some of the granitic rocks in the dry valley region which is at present somewhat confused. Samples were collected from type areas of Granites in the Wright Valley and these will be used for comparison.
At least 5 distinct granite bodies intrude the area and these can be arranged in a time sequence by field relationships between the different granites and the extent of deformation suffered by the different bodies. Four of these granites are shown to be mutually intrusive in the north wall of the Miers Valley (Plate IVA).
Preliminary thin section petrography has established that most of the 'granites' in the area are granodiorites and diorites rather than true granites with different types often having distinctive mineral assemblages, with the presence or absence of hornblende being one of the most important distinguishing features.
There is little evidence of the mineralogy of the granites being affected by the high grade metamorphic events known to have occurred in the area.
Blank et al. (1963) reported that the metasedimentary rocks of the area had undergone regional metamorphism to amphibolite facies. Preliminary thin section petrography has confirmed this but in the section chosen no minerals suitable for geobarometry or geothermometry were found. The meta-sediments intruded by the later granites show little sign of assimilation and retrogressive metamorphism is uncommon.
Blank, H.R., Cooper, R.A., Wheeler, R.H. & Willis, I.A.G. 1963. Geology of the Koetlitz-Blue Glacier Region, South Victoria Land, Antarctica. Transactions of the Royal Society of New Zealand, Geology 2: 79-100.
Skarn studies in the area between the Miers Valley and the Salmon Valley (Steven Simmons).
Six weeks were spent in the field sampling and mapping granite-marble contacts. Suitable outcrops of skarn were mainly restricted to the ridge crests where the exposure was relatively free of any scree cover. The coarsely sacchroidal Salmon Marble was often difficult to sample as specimens would readily crumble when struck with a hammer.
Contact aureoles were usually several centimetres in thickness. However, some localised pods, rich in either garnet or sulphides, were some tens of centimetres wide (Plate IVB).
The field work will be followed up at Victoria University with a mineralogical study which aims to evaluate the thermal regime of skarn formation. Preliminary rock sectioning has revealed some striking contact mineralogy; in hand specimen garnet, epidote and some sulphides are identified. Detailed petrological and microprobe studies will be made on the granite-marble samples to determine the parameters (e.g. temperature, CO2 pressure) involved in the formation of skarn.page 21
Four mutually intrusive granitoid rocks from Miers Valley area. Relative ages are determined by cross cutting relationships, (a = oldest, to d = youngest).
International Mount Erebus Seismic Survey 1980/81 (R.R. Dibble).
IMESS is the brain-child of Dr. Philip Kyle, and was set up in collaboration with Dr. Jurgen Kienle (university of Alaska), Dr. Katsutada Kaminuma (Institute of Polar Research, Tokyo) and the writer. The principal objectives and responsibilities are listed in Appendix 2B.
Earthquakes in the range 1 to 10 kJ in seismic energy (M = −2.8 to −0.5) were occurring in 1980/1 at roughly half the rate in 1974/5 and 1978/9. Above 10 kJ, where the events on Erebus usually accompany eruptions and exceed the number predicted by the linear regression line for smaller earthquakes, there were also fewer events in 1980/1 than previously. The clean white snow around the vents in the crater showed there were far fewer bombs ejected. Furthermore, the eruptions were usually emissions of gas accompanied by a loud roar, and without an explosive onset. After 27 December, some of the eruptions were explosive, and Kyle was able to collect a few fresh lava bombs from the crater rim. A list of eruptions is given in Table 1.
TABLE 1: Eruptions of Erebus between 20 December 1980 and 9 January 1981. page 23page 24 page 25
A distinction between gas emission and explosion seems to be apparent in the microphone recordings. Roaring gas emissions of short (c. 3s) duration, such as at 1443 NZST on 31 December, were recorded as a congressional air pulse of duration about one second (Fig. 5a) followed by a single oscillation of period 1/7 Hz which is probably the organ pipe mode of the crater. The accompanying earthquake was small (160 J) and preceded the air wave by about one second. Fig. 5b shows another roaring gas emission of about 18s duration which occurred at 0949 on 27 December. The air-pulse onset is similar, but the organ pipe oscillation is not obvious, presumably due to the long duration of the discharge. Explosions which eject bombs, such as at 1644 NZST on 31 December, have a strong oscillatory onset of frequency 2 Hz superimposed on the compressional air pulse, followed by the 1/7 Hz coda (Fig. 5d) but the coda is of shorter period (c. 4s). This event was not observed well enough to know if bombs were ejected at the time.
Earthquakes not accompanied by eruptions, such as the 2 kJ earthquake seen recording at 2102 NZST on 31 December 1980, were not recorded on the microphone (Fig. 5e), whereas in 1978/9 the larger earthquakes were recorded (VUWAE 23, 1978/79 Immediate Report). Possible reasons are obscuration by the high wind noise level in 1980/1, and differences in the microphone. In 1978/9 it was a dynamic microphone in a Helmholst resonator, giving a sharply peaked response, whereas now its response to waves of constant pressure amplitude rises proportional to frequency (6 dB/octave) up to the resonance frequency of about 80 Hz - then flattens off and falls at 6 dB per octave rise up to the frequency for which Q = 1. Above this the response falls at 12 dB per octave rise in frequency. Both microphones also act as ground accelerometers to some degree.
The results expected from the figure of eight induction loop were, (i) the detection of magnetic signals from eruptions of conducting magma in the static magnetic field of the earth, and (ii) the detection of spectral effects in the magnetic micropulsations (dominantly of solar origin) recorded separately from the two halves of the loop. These expectations are not realised in the recordings presently available, due in part to the high static discharge noise prior to 1 January, and a break in the Crater loop between 1 January (when someone ploughed through it) and 4 January when the break was located and repaired.
No signals were detected on the Camp loop which correlated with any of the eight eruptions which occurred when correlation was possible. This result was expected. Of the three eruptions which occurred when correlation with signals on the Crater loop was possible, there was only a doubtful correlation with the eruption at 1749 NZST on 1 January. There was also one possible correlation with an earthquake at 19.0 hours on 5 January.
Figure 6: Frequency of occurrence of Erebus earthquakes of all types versus seismic energy. The frequency in 1980/81 was about half that in 1974/75 and 1978/79.
Further results must await the processing of the telemetry tape recordings in Japan, following purchase of the playback equipment, and the receipt of tape recordings made over the winter. At the time of writing (12 May 1981) the telemetry signals are still being recorded satisfactorily at Scott Base.