.gif)
Victoria University Antarctic Research Expedition Science and Logistics Reports 1982-83: VUWAE 27
SCIENTIFIC ACHIEVEMENTS
SCIENTIFIC ACHIEVEMENTS
EREBUS STUDIES AND IMESS (K4) - R.R. Dibble.
1982 was our best year so far in terms of data collected. The summit induction loop, which was broken by a bomb, was repaired by W.I. McIntosh, R. Mason and party plus Helo on 25 January 1982, and recording tapes up to 16 February were dispatched to Japan before the last flight out. All sensors, transmitters, receivers and recorders were fully operational until late May when the loop stopped working. Abbott transmitter died in early June. Early on 21 July, Summit, Hooper and Bomb began recording a swarm of earthquakes. On 22 July they were occurring every minute or so, and died away during the next 3 days. There were vague reports of a glow in the sky on or about 21 July at this time.
By the 3rd of August, the Summit, Bomb and Abbott transmissions had all become very weak and intermittent, but Hooper and Terror were still O.K. Terror dropped out by 11 August, and on 17 August, Ross Mason wrote that all the equipment had stopped working. Fortunately, he did not turn the receiver/recorder off, because by 23 August there was enough sky light reaching the solar cells to revive Hooper and Abbott. Bomb was intermittent until about 24 September, but Summit stayed dead because it did not have a solar cell.
When the Japanese party (Kaminuma, Ueki and Koyama) arrived at McMurdo in November they examined the recordings from 23 August onward. Between 13 and 25 September, earthquakes were recorded at a rate of 92/day (average). From 25 to 30 September it was 117/day and then jumped to 184/day until 2 October, after which it was again 117/day until 7 October.
Suddenly at 0248 UT on 8 October a swarm of earthquakes began. In the next 24 hours at least 640 earthquakes occurred, initially separate earthquakes and with sharp P and S phases at Abbott (the closest station); S-P 2.8s), but after a few minutes continuous tremor began, and clear P and S phases were not often discernible. Five of the best recorded earthquakes in the first 7 minutes were grouped near Fang Ridge under the Fang Glacier.
Within a day after the swarm, earthquakes were back to 106/day, and slowly declined to 74/day by 9 November, the date up to which the recordings have been replayed.
Observations were made of Erebus crater activity in the 13 day period over which equipment in the summit area was being repaired, and are presented in Table 1.
page 8
.jpg)
TABLE 1. Observed eruptions of Erebus from 22 December, 1982 to 4 January, 1983.
McMURDO SOUND SEDIMENT STUDIES (K5)
PART I - SEA ICE PROGRAMME
This season a relatively small bathymetry and seafloor sampling programme was designed to cover an area in the southwest corner of McMurdo Sound, which we had not previously sampled and also to start a detailed study in Granite Harbour. Samples were collected in front of the Blue Glacier and 10km east of Cape Chocolate near the Dailey Islands. In addition, bathymetry transects were done in this area and for 3km off Scott Base.
Granite Harbour can be considered a "closed basin" which collects sediment deposited from the Mackay Glacier Tongue. Study of this "sedimentation system" will enable a more precise interpretation of the marine glacial contribution to recent McMurdo Sound-Ross Sea sediments and the older Cenozoic sediments that will be cored in the next few years.
The other major part of the sea ice programme was in support of CIROS, and comprised seismic refraction studies for drill site location at Butter Pt., Dailey Islands, sea ice movement monitoring off Butter Pt and tide movements at Cape Roberts. The only project not completed was seismic refraction work east of Cape Roberts which was to be a co-operative project with Northern Illinois University. A spreading crack in the sea ice 3km east of Cape Roberts could not be crossed by bulldozer (D4) or the vehicles used by the seismic party.
Granite Harbour Sediment Studies (GHSS) - A. Macpherson.
The 1982-83 GHSS science programme can be considered successful not only due to the favourable results obtained this season but also as a result of the initiation of long-term monitoring, (glacial movement and sedimentation rates), where results will not be available for a year or more.
1. Pole network.
In an attempt to determine both rate of movement and amount of plastic deformation of the Mackay Glacier Tongue, a network of eight poles was established up and down glacier from the grounding line, and was subsequently surveyed by K26. Difficulty was experienced in identifying the poles using a theodolite alone, and it proved necessary to have people moving to each successive pole on the glacier at the time of surveying.
2. Bottom Sediment Sampling.
Collection of undisturbed bottom sediment enables detailed paleontological and sedimentological studies of various core to be related spatially
page 10
and possibly temporally. Two gravity cores, one from a crack about the grounding line, and another from in front of the snout were retrieved using a sphincter corer developed and successfully operated during previous seasons by the Antarctic Research Centre. Core 82/1, approximately 30cm in length was taken from a depth of 303m, half a kilometre east of the grounding line. Core 83/2 was retrieved from a depth of 796m immediately east of the glacier snout.
In both cases, corer penetration appeared to be stopped by a coarse sand layer observed at the base of each core. It is unlikely that the addition of more weight would have increased the penetration. Distinct black mottling, thought to relate to reducing conditions at the time of deposition (and shortly after), was observed in both cores, and in addition, a general fining up sequence was recognised in core 83/2.
3. Sediment Trap Deployment.
Sediment traps enable the capture of settling sediment within a known area and hence are a quantitative method of estimating rates of sedimentation.
Two traps were deployed at depths of 400m and 750m respectively, immediately east of the Mackay Glacier Tongue. It was planned to retrieve these traps during February 1983. However, due to calving of the Mackay Glacier these traps were lost.
A sediment trap was later successfully deployed using the "USCGS GLACIER" (see Part II).
4. Current Monitoring.
Currents have the ability to distribute and rework sediment, but the importance of currents within a semi-enclosed basin such as Granite Harbour is not widely known.
A "savonius rotor" current meter which measures currents greater than 0.02m/sec−1 was used immediately east of the Mackay Glacier snout at 1/2 hour intervals, at depths of 5, 10, 20, 40, 60, 80, 100, 150, 200, 400, 650 metres. The programme was terminated when no measurements were recorded at any depths after 4 consecutive hours of monitoring.
Sea ice crack off Granite Harbour - A. Pyne.
A "spreading crack" formed in sea ice has been encountered east of Cape Roberts during the last two seasons (December 1981, November 1982). The crack has proved impassable by heavy surface transport (D4 and IH500 bulldozers) thereby preventing access further seaward. This season the crack was visited twice in November (6th and 18th) and detailed measurements were made both times at the same site.
page 11
On November 6, 1982 the crack was located 2.91km due east of Cape Roberts. It was traced for 5km south and north, and remained undiminished over the total 10km. The orientation was northwest-southeast over this short distance but it is thought to curve towards Mt Bird further south based on previous seasons' sea ice breakout observations. North of Cape Roberts the crack cut close to a distinctive grounded iceberg, as it had done in December the previous season. While the location and orientation of the crack is not yet well known it has probably been the same in both the 1981-82 and 1982-83 seasons.
The spreading crack is filled with ice that has a ridged and terraced surface and thins towards the central crack (Fig. 2). On November 6 the distance between the edges of the first year ice where initial rifting began was 33m. Ice cores were taken from the western side of the crack to define the thickness profile of the infilling ice on November 6 and 18 (see Fig. 2). Two pegs straddling the central crack were fixed in the ice on November 6 and resurveyed on November 18 to determine a spreading rate for that period. The total spreading for the 12 days was 1.2m (10cm/day) and 24 "growth lines" that ranged in width from 2.5 - 10cm were counted in the new ice. The growth lines for this period appear to form daily probably at "night" when temperatures are cooler. The variation in width of the growth lines and terraced surface of the infilling ice indicate that the spreading rate is not constant and was probably greater earlier in the season.
Tide Monitoring - A. Pyne.
A tide gauge designed to automatically record tidal amplitude was established on the sea ice for 11 days at both Scott Base and Cape Roberts. The tide gauge is similar to that described in Pyne and Waghorn (1980) and records tidal amplitude by measuring the relative movement between the floating sea ice and a counterweighted line anchored to the seafloor.
At Scott Base recordings were made from 19-30 October in 80m of water approximately 200m off Scott Base. Unfortunately, good records were only obtained during the first five days (19-24 October), but showed a maximum tidal range of 0.75m for this period. Later recordings at Scott Base appeared to be dampened due to ice freezing on the steel anchoring wire immediately below the sea ice. The steel wire was replaced by monofilament nylon at Cape Roberts and no further dampening occurred.
Recording at Cape Roberts was made over the period 8-18 November with a recording break from 10-13 November, due to unwinding of the clockwork recording mechanism. Sufficient records were obtained, however, to show that the tidal cycle at Cape Roberts is similar to other parts of McMurdo Sound. The change from the "higher" amplitude single daily cycle to a "lower" amplitude double daily cycle was recorded. The maximum range measured was 0.76m and it is estimated that no tides would exceed 1m in the area.
page 12
.jpg)
FIG. 2. Spreading crack in 2.5m thick sea ice off Cape Roberts. Profiles are of the west side of the crack, ice thickness was measured by coring (lower left). Lower right photo shows new ice growth lines on November 18.
Seismic Refraction Surveys at New Harbour and Dailey Islands - R.R. Dibble.
These surveys were made to aid the selection of drill sites for the Cenozoic Investigations in the Western Ross Sea, and extended the work begun by lies and Dibble in 1980. The team consisted of Bay Dibble and Ian Paintin (geophysicists), Alex Pyne and Tony Macpherson (geologists), and Malcolm McLeod (mechanic/field assistant). We used the SIE-RS44 and Nimbus 1210 seismographs belonging to the Institute of Geophysics. Each has 12 recording channels, and a 330m cable spread.
At New Harbour, both seismographs were operated in a heated Wannigan, and the Nimbus was used with an extension cable so that the two spreads along the seismic line were 330m apart. Two seismic lines, each 8.66km long with shot points at each end and at the centre, were laid out in the form of a cross with 6 Wannigan sites and 12 evenly spaced spreads on each line. (A 7th site was added in the centre of line B). Lines were laid out dead straight and flagged at 320, 330, 340m intervals by means of a precision odometer fitted to one of the 2 motor toboggans.
Two shooting parties, each with a shot instant tone encoder and Compak radio, fired charges of between 1kg at 5m depth and 8.8kg at 15m depth at all 3 shot points for each Wanigan site. Shot depths were chosen to avoid damaging the shot holes, rather than avoiding bubble pulses. At the Wannigan a Compak receiver and tone decoder started the Nimbus automatically at the shot instant, and the tone code was fully recorded by the RS44. At each Wanigan site a 30cm hole was drilled through the 2m sea ice for the echo sounder, and the water depth was measured.
At the Dailey Islands, Tim Stern and Peter Whiteford (DSIR geophysicists) and Brian Smith (field assistant) joined us with their Nimbus seismograph and Snotrac vehicle, enabling three shooting parties and two separate recording parties to be used. Here, sea bottom depth and dip along the seismic line were determined at each spread by stacking sledge hammer blows on the ice in the Nimbus digital seismograph.
Two 8.66km lines similar to those at New Harbour were laid out in the form of a "T". Four extra shot points were included on line A because a complex sea bottom was expected near the islands.
References
PYNE, A.R. and WAGHORN, D.B. 1980. Immediate Report of Victoria University of Wellington Antarctic Expedition No. 24.
page 14
.jpg)
FIG. 3. Location of seismic refraction lines off Butter Point and the Dailey Islands.
PART II - SHIPBOARD PROGRAMME - P.J. Barrett
| 1. | Sediment in the deep water (about 850m) around Ross Island is mud deposited from suspension with a small ice-rafted component, mainly basaltic. Its variable firmness a few centimetres below the seafloor may be due to differences in sedimentation rate or composition. |
| 2. | Granite Harbour is a deep basin (890m) cut or cleaned out by an enlarged Mackay Glacier and little modified by sedimentation since that event. |
| 3. | Tillite pebbles from seafloor sediment off Granite Harbour suggest that the strata forming the south-trending ridge about 20km offshore include the Cenozoic glacial sequence cored at MSSTS. |
| 4. | The Western Shelf and Slope in McMurdo Sound have not been crossed by similar extensions of Taylor or Ferrar Glaciers, at least since the Early Cenozoic. The Slope is cut by at least two narrow valleys, but their origin is not yet understood. |
| 5. | The McMurdo Volcanics extend as scattered cones or cone complexes along the edge and possibly beneath the McMurdo Ice Shelf and for some distance offshore from Ross Island. |
| 1. | To take several deep water samples not accessible from the sea ice. |
| 2. | To sample off (and if possible in) Granite Harbour. |
| 3. | To obtain continuous depth records from previously uncharted areas in western McMurdo Sound. |
The ship's track and stations for bottom sampling are shown in Figure 4. Station data is given in Table 2.
.jpg)
FIG. 4. Map of McMurdo Sound area, showing the track and bottom sample locations from USCGC GLACIER Cruise IV. shallow areas thought to be of volcanic origin are marked by a star. Depths in metres.
sand lenses. The sediment has been deposited largely from suspension with a small coarse component rafted by floating ice, probably some of it glacial and some shore ice. The sediment below 2cm is softest at Station 6 in the northern part of Bird Basin, becoming firmer to the south, though the floor of the basin remains at about the same depth (Fig. 5A). The variation in firmness may be due to differences in sedimentation rate or composition.Samples from slightly shallower depths (Stations 1, 2, 3A, 7, 8, 26; 547-755m) were also taken for foraminiferal studies by B.L. Ward to cover the range that includes the carbonate compensation depth. Sample 1, a few miles north of Scott Base, had pebbles encrusted with bryozoans, indicating their exposure above the seafloor and again a very low sedimentation rate.
With Granite Harbour virtually ice-free we first carried out a bathymetric survey, 250km of track in an area of about 400km2. The work in Granite Harbour also involved the setting of a sediment trap in 800m of water at the tip of the Mackay Glacier Tongue (to be retrieved in November 1983) and the recovery of 13 bottom samples. The latter were mainly grab samples, but included 2 cores 60cm long and 20cm across for detailed sedimentological study. A grab sample near the middle of the Harbour (Station 16, 849m) contained black mud, but a piston core attempt nearby (Station 16A, 880m) yielded only 25cm of core because the corer encountered stony till, showing that the pattern of sedimentation in Granite Harbour is not simple. The bathymetry is also quite complex for although the Harbour can be described in gross terms as a broad basin almost 900m deep, the floor is hummocky with a relief of 200-300m (Fig. 5B).
A bathymetry line was run east from Cape Roberts and two bottom samples were taken to help with CIROS site selection. The seafloor east of Cape Roberts is sandy or gravelly, even in 489m of water at Station 19, where a boulder of granite 30cm long was taken by the grab. Worm trails of cemented sand over part of the surface of the boulder show that it was partly buried in sand. Grabs from near Cape Roberts and the 108m deep shoal 18km offshore represented a gravel pavement, judging from the encrusting bryozoans and other organisms. Bathymetric data obtained off Granite Harbour can now be used to chart the North-South-trending ridge and West-facing scarp that is cut in the East-dipping sequence to be sampled by CIROS 3 and 4. Some idea of the sequence was obtained from pebbles in the till recovered at Station 8 which is on a broad, flat "delta" surface at 550m. Although most pebbles were granite and presumably from Granite Harbour, a number were sandy mudstone with scattered coarse sand grains, like the glacial mudstones in the Early-Mid Cenozoic MSSTS core. The west-facing scarp was also sampled by grab (Station 11) but recovery was poor and no new lithologies were seen. Nevertheless, we hope that processing the sand fraction may yield micro-fossils that indicate the age range of strata forming the scarp (Pig. 5C).
.jpg)
FIG. 5. Topographic features in McMurdo Sound and Granite Harbour. Depths in metres. Horizontal scale bars 5km long.
| A. | Southern part of Bird Basin, contrasting the smooth flat floor with the rough topography beyond the margin. |
| B. | Cross-section near the head of Granite Harbour, with the glacially moulded topography still evident in the deepest parts. |
| C. | Bathymetry across the "scarp" off Granite Harbour. |
| D. | Submarine valleys glacially carved by the Debenham Glacier 10km south of Station 19. |
| E. | Narrow valley of unknown origin off New Harbour. |
| F. | Submarine peaks along the edge of the McMurdo Ice Shelf, and inferred to be volcanic cones. |
reports (including Barrett et al., 1974) show submarine valleys extending from the Dry Valleys into the "moat" around Ross Island. However, the Western Shelf is in fact a rather flat feature (average depth 180m) with two very narrow (about 1km) valleys running from its edge to near the foot of the slope. These features seem far too small and well-defined to be glacial in origin, especially when compared with the broad submarine extension of the valley now containing the Debenham Glacier (5km wide and 500m deep) or the 800m deep basin carved by the Mackay Glacier off Granite Harbour (Figs. 5D, E). The lack of broad glacial valleys in the Western Shelf east of the Dry Valleys, in contrast to those cut by the outlet glaciers north of 77 10′S, suggest that none of the Dry Valleys have served as outlet glaciers since the deposition of the near surface strata of the Western Shelf, which at MSSTS-1 are as old as Early Cenozoic (Webb, Leckie and Ward, in press).The bathymetric data also revealed another feature no doubt seen before, but not to our knowledge taken into account in discussions of McMurdo Sound geology. Tracks along the edge of the McMurdo Ice Shelf showed a hilly topography superimposed on the gentle slope of the Western Shelf (Fig. 5F). Because of the nearby Dailey Islands, scattered basaltic cones that rise to 132m above sea level, these hilly features are also thought to be of volcanic origin. Defense Mapping Agency Hydrographic Center Chart 29321 shows a number of unusually shallow soundings in deep water around Ross Island, and one of these is crossed by Track XXX of Northey et al. (1976), showing it to have a conical section and an apparent height of 165m above the floor of Bird Basin. These are also judged to be of volcanic origin, and may present a significant extension of the Erebus volcanic Province (Figure 1).
Results of the cruise will appear in several different places. Selected samples will be used by Ms B. Ward in her study of the distribution of modern foraminifera in McMurdo Sound, and the Granite Harbour samples and data will be used as part of a thesis on the Mackay Glacier-Granite Harbour sedimentary system by Mr. Macpherson. The bathymetry will be added to an earlier compilation to complete a bathymetric map of McMurdo Sound with the assistance of Geophysics Division and the Institute of Marine and Freshwater Science, DSIR. The samples and bathymetry will also provide the basis for papers on recent sedimentation in McMurdo Sound.
A cruise plan is being prepared for work in February 1984 mainly to carry out further site investigation for CIROS drilling. A shipboard survey has become necessary because sea ice conditions off Cape Roberts prevented the planned "land" based seismic survey off Cape Roberts. The survey will link with previous shipboard surveys in the area. The cruise plan also includes some time for gravity cores from Bird Basin (not collected in 1983 because of equipment problems and [unclear: lack] of time), and seafloor photography.
page 20
.jpg)
TABLE 2. Grab/Core Stations from USCGC GLACIER in McMurdo Sound, 12-19 February, 1983.
References
BARRETT, P.J.; CHRISTOFFEL, D.A.; NORTHEY, D.J.; SISSONS, B.A. 1974. Seismic profiles across the extension of Wright Valley into McMurdo Sound. Antarctic Journal of the U.S., 9(4), 138-140.
BARRETT, P.J.; PYNE, A.R.; WARD, B.L. In press. Modern sedimentation in McMurdo Sound, Antarctica. Proc. Fourth International Symposium on Antarctic Earth Sciences, Adelaide, August 1982.
McGINNIS, L.D. 1973. McMurdo Sound - a key to the Cenozoic of Antarctica. Antarctic Journal of the U.S., 8(4), 166-169.
NORTHEY, D.J.; BROWN, C.; CHRISTOFFEL, D.A.; WONG, H.K.; BARRETT, P.J. 1975. Dry Valley Drilling Project Bull. No. 5, Northern Illinois University, De Kalb, 167-179.
WEBB, P.N.; LECKIE, M.; WARD, B.L. In press. Paleogene-Neogene Foraminifera from the MSSTS-1 drill hole, McMurdo Sound, Antarctica. In: P.J. Barrett (ed.). Report on MSSTS-1 and associated studies, western McMurdo Sound, Antarctica, Department of Scientific and Industrial Research, Wellington.
BEACON STUDIES (K6A)
Triassic sedimentology in south Victoria Land - B. Walker.
Previous geological investigations of the Beacon Supergroup outcropping on the polar plateau edge of the Transantarctic Mountains have shown that the Triassic strata forming the youngest part of the Beacon were deposited by rivers that flowed over a broad, alluvial plain.
The object of this PhD study has been to visit the better exposed localities of Triassic strata throughout south Victoria Land so as to establish a set of palaeohydraulic models that best represent the fluvial depositional processes that operated during this time.
During the 1982/83 Antarctic field season the excellent exposures at Mount Bastion, Portal Mountain, Shapeless Mountain and Allan Hills were visited. These widely separated localities offered the coverage necessary for understanding the Triassic depositional basin.
The 400 metres of Triassic Lashly Formation is divided into four members. Member A, which is 100 metres thick, consists of laterally extensive trough-cross-bedded and micro-cross-laminated tabular sandstone bodies in the order of 2 metres thick that were formed by the lateral migration of river channels. Straight channels had widths of at least 200 metres but were seldom deeper than 2 metres. The channel sandstones are interbedded with root-bearing mudstones, siltstones and fine sandstones that occur in beds up to 6 metres thick. These beds commonly contain abundant white roots, Skolithus burrows and in some places pedogenic features (Gabites, this report). The fine grained beds are interpreted as overbank sediment that accumulated in a swamp environment.
Member B represents a marked change in river behaviour. Over 100 metres thick and consisting of over 90% sandstone it is dominated by massive, horizontally laminated and trough-cross-bedded sandstones. These deposits are interpreted as being of flood origin. The floods commonly attained upper flow regime conditions, were capable of scouring deeply in older flood deposits and transporting large mud-stone clasts, mats of peat and logs.
The 70 metres of Member C marks another change in depositional conditions. Carbonaceous shales, thinly bedded and laminated siltstones are abundant. Interbedded mudstone and micro-cross-laminated sandstones and coal beds up to 1.2 metres thick are also common. These deposits are interpreted as being formed in extensive shallow lakes and swamps. Vegetation was abundant (Gabites, this report). The common occurrence within Member C of tabular, trough-cross-bedded and micro-cross-laminated sand-stone beds around 1 to 2 metres thick indicate that the paludal environment was occasionally inundated by flood sands of sheet flow origin.
Member D is 200 metres thick and represents conditions similar to Member B.
page 23
The sediments of the Lashly Formation were deposited by north-flowing rivers in a cratonic foreland basin now the present site of the Transantarctic Mountains. This basin may have extended south into the Beardmore Glacier area and possibly encroached upon some areas of North Victoria Land (Elliot, 1975; Walker, 1982). Basin width was at least 40 kilometres but most likely in the order of 100 to 200 kilometres.
Uplift of the Ellsworth and Pensacola Mountains supplied sediment to the Nilsen-Mackay Basin (Elliot, 1975). The rate of movement of sediment over the floodplain was largely controlled by differing intensities of tectonic uplift. Members A and C reflect relatively stable periods of crustal movement and near to equilibrium hydrologic conditions. Members B and D reflect deposition from periods of uplift, which provided an abundant source of sediment, increased the gradient of the palaeoslope and increased the amount of subsidence of the sedimentary basin. Secondary controls on sedimentation include a temperate climate with abundant rainfall and a well-vegetated floodplain.
The results of the geological data collected over three Antarctic field seasons will be presented ultimately as a PhD thesis in 1984, and then by a series of papers to internationally recognised sedimentological journals.
The results from this study will firstly provide an understanding of the environment of deposition for the Triassic part of the Beacon Supergroup and secondly will contribute to the understanding of similar alluvial plain deposits found elsewhere in the world.
References
ELLIOT, D.H. 1975. Gondwana basins in Antarctica. In: K.S.W. Campbell (ed.), Gondwana Geology, A.N.U. Press, Canberra, 493-536.
WALKER, B.C. In press. The Beacon Supergroup of Northern Victoria Land. Proc. Fourth International Symposium on Antarctic Earth Sciences, Adelaide, August 1982.
Triassic paleoecoloqy in south Victoria Land - I. Gabites.
Four mountains were visited during the nine-week field season to record and sample Triassic plant fossils and palaeosols (fossil soils) with the aim of establishing a model of vegetation on the Triassic alluvial plain. The results of this field work will go towards a MSc thesis. Work will concentrate on describing the Triassic vegetation in terms of communities, soil/plant relationships, climate and palaeohydrology.
Palaeosols with associated vegetation were recorded at Portal Mt. and Allan Hills, and undeveloped palaeosols with root traces at Mt. Bastion.
page 24
No macro-fossils were found in the Fleming Member. Well-preserved macro-fossils were recorded in Members A, B, C and D of the Lashly Formation. Equisetalean stems ("horsetails") are abundant in A; well-preserved gymnospermous logs in channel deposits of B and D; and a wide range of flora in lake, swamp and quiet backwater deposits of C, including a rich corytosperm ("seed-fern") assemblage. The corytospermous Dicroidium flora is restricted to the Triassic, but was recorded at Shapeless Mountain in a bed previously designated as Permian Weller Coal Measures.
Member A: At Portal Mt. the oldest recorded Triassic macroflora (Dicroidium) is found at the contact between Fleming Member and Member A. Neocalamitid stems and? Lycopod stem impressions are the most abundant macroflora. Grey-green undeveloped mudstone palaeosols contain white rootlet traces and often have a densely burrowed upper surface where not eroded. Developed palaeosols seen at Portal Mt. display gammate structure and eluviated horizons resulting from periodic waterlogging and drying in a "subhumid" environment. They support a small woody vegetation and large Neocalamitid stems with trunk bases around 6-8cm diameter.
Member B: Where stabilised muds and overbank deposits are preserved, a corytosperm association of 3 or 4 species and Neocalamitid stems can be found. The broad, deep, medium-grained flood channel deposits of Member B contain much coaly and silicified wood material. This may reflect an overall stability of channel course in the region disturbed by infrequent large floods which destroyed forested areas. (Some stumps at Mt. Bastion may be 200 years old). At Allan Hills a flood deposit bearing logs is exposed over almost 0.3km2. Trunks up to 10m in length with roots and branches are oriented with the channel flow; compression ranges from 10-60%. When the flood subsided, large rafts of peat 30cm thick were deposited. The only recognisable fragments in the peat are Dicroidium.
Member C: Rich in fossil material preserved in the shales, coals and carbonaceous sandstone beds characteristic of this member, in particular Dicroidium, Xylopteris and Johnstonia species. At Allan Hills a succession is preserved from Neocalamitid and Gingkophyte swamp vegetation to a drier Johnstonia flora. Approximately 20cm of Johnstonia - rich silt forms the substrate for evenly spaced trees up to 55cm diameter. Field growth ring counts give an estimated age of 100 years for this stand.
Member D: A similar situation on the floodplain as for Member B is envisaged.
Fossil types recorded were: Dicroidium, Xylopteris, Johnstonia, Sphenobaiera, Phoenicopsis, Neocalamites, Lepidopteris. ?Glossopteris, ?Taeniopteris, gymnospermous and ?lycophytic wood, seeds, microsporophylls, roots, infilled root traces, peat, horizontal and vertical burrows.
I can foresee value in extending the region of study at least as far as the Beardmore Glacier area, to compare floral diversity and environments of vegetation growth and deposition, with those 500km further down the alluvial floodplain.
BEDROCK STUDIES (K6B) - R.J. Korsch.
Basement Reconnaissance
Approximately six days were spent in the Miers Valley on a reconnaissance geological survey of the basement rocks so that Korsch could obtain a working knowledge of these rocks. The Miers Valley was selected because of its accessibility, facilities (Wannigan) and because a 1:25000 geological map which included the northern wall of the Miers Valley had been produced in the 1980-81 field season by G. Mortimer, F. Reid and S. Simmons (see report by Mortimer, 1981).
During our stay in the Miers Valley we were able to examine most of the rock units shown on Mortimer's map. Metasedimentary rocks of the Koettlitz Group include a wide variety of parent lithologies including pure and impure limestone, shale, sandstone, quartzose sandstone and intercalated basaltic volcanics. These rocks have been regionally metamorphosed to amphibolite facies and then suffered localised contact metamorphism during emplacement of various granitic plutons.
Deformation of the metasedimentary rocks produced isoclinal folds, an excellent example of which is well displayed in the Salmon Marble on the north wall of the Miers Valley. A second deformation refolded the first generation structures.
Granitic rocks observed in the field included the Buddha Diorite, Dais Granite, Miers Granite, Rivard Diorite and Grey Granite along with unnamed aplite, pegmatite, lamprophyre and basalt dykes. Wherever contacts were exposed good evidence for the intrusive nature of the plutons was observed. Detailed field descriptions of all the units can be found in Mortimer (1981).
During our reconnaissance study of the ridge between the Miers and Marshall valleys we found that the map of Mortimer (1981) stood up well under scrutiny, although minor modifications should be made. The biggest problem we faced was in the recognition of various plutonic units which in places seemed to grade into each other, but this will be overcome by experience with these rocks. One must question Mortimer's use of the term "Penance Pass Formation" as a suitable term - "Hobbs Formation" already exists in the literature and only needs redefining rather than the introduction of a completely new term.
This study represented an introduction to the basement rocks for Korsch, who is developing a programme to study various aspects of the basement geology over the next few field seasons.
References
MORTIMER, G. 1981. Event 15 1980/1981. Provisional Report on the basement geology between the Miers and Salmon Valleys, McMurdo sound, Antarctica. Unpublished Report to Antarctic Division, DSIR.
Structure of Shapeless Mountain
Shapeless Mountain appears so because it is quite irregular structurally in contrast to the consistent near-horizontal stratification in the Beacon Supergroup elsewhere. Shapeless Mountain also has outcrops of the Mawson Formation, which at Allan Hills has been interpreted as volcanic mudflow deposits. The aim of the field season at Shapeless Mountain was to determine the nature of the Mawson Formation, and to study the structural geology in detail to establish any relationship between the structure of Shapeless Mountain and the volcanics of the Mawson Formation.
A detailed manuscript map of the geology of Shapeless Mountain was compiled by P.J. Barrett from earlier VUWAE work and we found this map invaluable during our field work.
| 1. | To the north and east of the summit a series of Beacon sediment blocks are caught up in, and completely surrounded by Ferrar Dolerite. These blocks vary in size from a few metres to a few hundred metres and in most cases the rocks have been tilted from subhorizontal with near-vertical dips being recorded in some places. Intrusive contacts and baking of the sediments adjacent to the dolerite implies that the blocks were tilted during intrusion of the dolerite. |
| 2. | In the extreme northern part of Shapeless Mountain the Aztec Siltstone, Metschel Tillite and Weller Coal Measures have been faulted and intrusive breccia, similar to Mawson Formation elsewhere on Shapeless Mountain, has been injected along the faults. Displacement on the faults varies from virtually zero up to several tens of metres. The intrusion breccia consists of blocks of dolerite, basalt and sediment surrounded by a basaltic matrix and in places the matrix was gradational into normal Ferrar Dolerite. |
| 3. | Within a kilometre of the summit, several localities contained Mawson Formation which consists of intrusive breccia. Steep, very often vertical contacts that were frequently irregular and baking of the adjacent sediments suggest that the breccia was intruded in a relatively hot state. Paleomagnetic work by Hunt and Mumme (1977) on samples collected from a site approximately 800m W of the summit indicated that the breccia had been intruded at temperatures in the range 300°C - 600°C. |
| 4. | At one locality approximately 1.5km SW of the summit (Section S5 in Barrett and Webb, 1973) volcanic breccia and basalt lava flows were interbedded and both occurred stratigraphically above the Lashly Formation. The contact between the two appeared conformable. Barrett and Webb (1973, Section S11) also reported a conformable sequence of Mawson Formation above Lashly Formation on Mistake Peak.page 27 |
| 5. | At the western extremity of Shapeless Mountain between sections S3 and S4 of the Barrett and Webb (1973) both sedimentary and volcanic lithofacies of the Mawson Formation were observed. Sedimentary rocks, mainly fine sandstones, contained rare horizons of basaltic cobbles indicating that the Mawson Formation had been reworked by surficial fluvial processes. These rocks were intruded by Mawson intrusion breccia and the contact sediments were baked, suggesting that the reworking was essentially contemporaneous with the intrusion of the breccias. |
| 1. | A gradation from normal Ferrar Dolerite into Mawson basaltic breccia over a distance of a few metres. |
| 2. | Breccia which consists of dolerite, basalt and sedimentary clasts from 2mm to 1m in length surrounded by a basaltic matrix. |
| 3. | Breccia which consists of dolerite, basalt and sedimentary clasts up to 1m long surrounded by a sedimentary matrix, usually medium to coarse sand, Which is similar to the adjacent unaffected sediments. |
| 4. | Intraformational sedimentary breccia which consists of singular, randomly-oriented blocks of sediment with a matrix of the same composition. This lithology was always found in close association with intrusive dolerite or breccia with a basaltic matrix and probably results from fracturing of the host rock during intrusion. |
| 5. | Sedimentary units, including fluvially deposited fine sandstone and silt-stone as well as some horizons which contain basalt clasts up to 10cm diameter. |
Based on our field work at Shapeless Mountain we conclude that the Mawson Formation is a product of volcanic eruptions and subsurface intrusion of magma which occurred at essentially the same time as emplacement of the Ferrar Dolerite sills. The present structure of Shapeless Mountain results from the effects of the intrusions on the host sedimentary rocks.
It is intended to publish at least two papers, one dealing with the relationships between the Mawson Formation and the structure of Shapeless Mountain and the other examining aspects of the lithofacies and geochemistry of the Mawson Formation.
Following this field season, a detailed examination of the Mawson Formation in the Allan Hills would be profitable. Based on observations made by members of K6A this field season the situation is more complex in the Allan Hills than has been described in the literature, and a field season longer than the week proposed by Korsch for the 1982-83 season would be required. This work
page 28
should also include an examination of the Mawson Formation and related Kirkpatrick Basalt at Carapace Nunatak and in the Coombs Hills. Near Mt. Brooke in the Coombs Hills a granite clast in the Mawson Formation has been reported by G. Claridge (pers. comm. 1983). No granite material was seen at Shapeless Mountain this season and its presence at Mt. Brooke is very significant geologically and thus an examination of it in the field is important.
References
BARRETT, P.J.; WEBB, P.N. (Editors), 1973. Stratigraphic sections of the Beacon Supergroup (Devonian and Older (?) to Jurassic) in south Victoria Land. Victoria University of Wellington, Department of Geology Publication No. 2, 165 pp.
HUNT, T.M.; MUMME, T.C. 1977. Laboratory investigation of remanent magnetisation in some rocks from New Zealand and Antarctica. Department of Scientific and Industrial Research Geophysics Division, Report No. 125, 68 pp.