Immediate report of Victoria University of Wellington Antarctic Expedition 1988-89: VUWAE 33
BEACON STUDIES (K047) - Ken J. Woolfe
BEACON STUDIES (K047) - Ken J. Woolfe
Exposures of Beacon strata in the Darwin Mountains, Cook Mountains and Britannia Range and in the Convoy Range-Allan Hills area were visited during the 1988-89 Antarctic field season The field program involved measurement of stratigraphic sections, paleocurrent directions, facies interpretation and a systematic study of trace fossil distributions.
|1)||Beacon strata in the Darwin Glacier area are interpreted as non-marine, thus requiring previous interpretations of the Hatherton Sandstone to be revised.|
|2)||Taylor Group strata in the Darwin Glacier area are similar to the sequence in the Knobhead area and new correlations are proposed.|
|3)||A previously unrecorded Devonian fish fauna was collected from a newly discovered exposure of Aztec Siltstone in the Cook Mountains and contains an unusual association of sharks and phylloloids.|
|4)||A 150 m thick sequence recording a lacustrine to glacio-lacustrine transition was discovered at the base of the Darwin Tillite, below the diamictite phase, in the Hatherton Glacier.|
|5)||The trace fossil Skolithos was observed in close association with overbank ephemeral pond deposits and paleosols.|
|6)||Diplichnites is restricted to large tabular and trough cross bed facies which are interpreted as alluvial deposits. The tracks were probably formed by terrestrial arthropods walking across exposed bed forms.|
|7)||Large scale slump folding at Elkhorn Ridge (Convoy Range) is shown to be associated with emplacement of Ferrar Dolerite.|
|8)||A ten degree angular unconformity previously recorded within the Weller Coal Measures at Allan Hills was found to result from juxtaposition of very large point bar and channel floor deposits.|
|9)||Bed forms resembling hummocky cross stratification were observed in the Weller Coal Measures at Allan Hills and are inferred to be the result of antidune formation.|
Devonian strata from southern Victoria Land have been variously interpreted as marine (Vialov 1962, Allen 1962, Gevers et al 1971, Bradshaw 1981, Gevers and Twomey 1982) and non-marine (eg Gunn and Warren 1962. Webb 1963, Barrett and Kohn 1975, Plume 1978, Barrett 1979, Sherwood et al 1989); the debate largely results from the presence of trace fossils that have previously been considered to indicate marine environments in sediments which are strongly indicative of alluvial plain deposition.
The large scale setting of the Beacon Supergroup is also under debate with some authors holding the view that the Beacon was deposited in a foreland basin while others believe that the strata were deposited in a intracratonic basin.page 21
This program is examining both the local and large scale depositional setting of the Beacon in southern Victoria Land. Field work is concentrating on solving problems such as paleoenvironment, paleoslope, basin shape and provenance of the sediment, With computer based modelling being used to examine the regional tectonic setting.
Scientific Endeavours and Achievements
A three man party was put in to the Darwin Glacier near Richardson Hill, by LC-130 aircraft, on November 16. The party travelled by sledge and motor toboggan visiting exposures in the Darwin Mountains, Cook Mountains and in the Britannia Range. The second phase of the expedition was seriously effected by a 17 day aircraft delay on our pull out from the Darwin Glacier. It was hoped that 1:50,000 geological mapping of the northern Convoy Range and Allan Hills would be undertaken and published in conjunction with the NZGS however air photos ordered by Antarctic Division for this work were not forwarded to the field party in time for the work to be undertaken.
Two Grizzly toboggans and three Tamworth sledges were used in the Darwin Mountain phase of the program one Tamworth being reserved to carry fuel while the others carried camp plus general cargo. The duration of camps ranged from three to ten days during which time the party worked locally, using motor toboggans and a sledge for local transport.
The Darwin Glacier area is in places heavily crevassed and areas of blue ice are common, travel was generally slow and accident free. The party was returned to Scott Base on Jan 5 and Henare returned to New Zealand the same day. A two man party was flown to Elkhorn Ridge (Convoy Range) on Jan 7 and worked from a fixed camp for ten days. The party was joined by Peter Barrett on Jan 17 and moved to Allan Hills for six days before returning to Scott Base on Jan 24.
1) Darwin Glacier Area
Sections were measured at numerous localities in the Darwin Glacier area, the Devonian sections show the lower half of the Taylor Group (below Hatherton Sandstone) to be of similar 0 thickness in both the Darwin Glacier and Knobhead areas. The upper part of the Taylor Group (Hatherton Sandstone and above) is significantly thinner in the Darwin Glacier area. A number of sections were also measured through the Victoria Group and these will be compared with strata in the central part of the basin following the 1989-90 field season.
Taylor Group New Correlations
The reappearance of abundant pebbles near the top of Junction Sandstone (?Lower Hatherton Sandstone, Haskell et al 1965) is correlated to the appearance of scattered pebbles near the top of Altar Mountain Formation in the Knobhead area. A platform developed at the boundary between Junction and Hatherton Sandstone is littered with well rounded quartz pebbles which are very similar to the quartz pebbles that litter the platform identified at the Attar Mountain Formation Arena Sandstone boundary in the Knobhead area. Field characteristics (including trace fossil assemblage) of the Hatherton Sandstone are very similar to those of the Arena Sandstone and the units are inferred to be equivalent. The discovery of Aztec Siltstone by a party led by Margaret Bradshaw page 22 from Canterbury Museum (K221) supports this correlation and shows the correlation suggested by Haskell et al (1965) is incorrect.
Paleoenvironments of the Taylor Group
The presence of paleosols, desiccation polygons, red beds, small scale channels, intraformational conglomerate, plant fossils and carbonaceous shale in all formations except the Hatherton Sandstone indicates the Taylor Group to be non-marine. The absence of many of these features in the Hatherton Sandstone makes this part of the Taylor Group harder to interpret, but the presence of dominantly unidirectional trough cross bed axes and similarity to the Arena Sandstone and Beacon Heights Orthoquartzite suggests that the Hatherton Sandstone is also non-marine. This interpretation is supported by trace fossil evidence, the trace fossil assemblage in the lower part of Hatherton Sandstone is very similar to assemblage found in the Junction Sandstone which is considered to be fluvial.
New Fossil Fish Locality
Abundant fish plates, scales and spines were found in the Aztec Siltstone near the summit of peak 1960 m, 5 km southeast of Mt Hughes in the Cook Mountains. Preliminary investigation by J.A. Long has indicated an unusual association of sharks and phylloioids. Further work on this association will be conducted in order to determine both paleoenvironmental and biostratigraphic significance.
A systematic study of the relationships between trace fossil distributions and sedimentological controls was conducted in the Darwin Glacier area. Initial results confirm observations made in 1988-89 which indicated that ichnogenera which have formerly been used as marine indicators were in fact occurring in non-marine strata. Skolithos in the Taylor Group is generally closely associated with ephemeral pond deposits and massive green sandstone beds interpreted as overbank deposits.
Diplichnites is restricted to large trough and tabular cross-bed surfaces in the Junction and Hatherton Sandstones. We believe that these tracks were made by terrestrial arthropods walking on exposed bed forms during periods of low flow and consider it unlikely that the tracks could be preserved if they were made in submerged or saturated sand.
Two previously undescribed facies sequences were recorded in the Darwin Tillite in the Hatherton Glacier area. At one locality, a red shale sequence overlies an erosion surface cut in Hatherton Sandstone, abundant sand blebs indicate periodic influxes of sand and up section sand beds become increasingly common and the unit passes upwards into a bluff forming sandstone. The sand beds are up to 40 cm thick and grade from medium to fine sand, load casts occur at the base of beds and ripples are preserved on the top surfaces. The sandstone units are inferred to have been deposited by turbidity flows in a glacio-lacustrine setting. The slumped sandstone unit is truncated by an erosion surface containing glacial striations and is overlain by a green diamictite phase of the Darwin Tillite. The sequence is 150 m thick and records a glacial advance prior to diamictite emplacement.
On the south side of the Hatherton Glacier a similar red shale with thin sandstone beds and blebs occurs above the green diamictite. The interbedded sandstone and shale grades upwards into varved black shale containing scattered (probably ice rafted) pebbles. The unit is directly overlain by the basal Misthound Coal Measures and records glacial retreat following diamictite emplacement.
Misthound Coal Measures and Ellis Formation
Section measuring and facies description was conducted with in the Victoria Group at a number of locations in the Darwin and Hatherton Glacier area. Comparison with the Victoria Group in the central part of McMurdo basin will be carried out following the 1989-90 field season.
2) Convoy Range-Allan Hills Area
Beacon Heights Orthoquartzite
Exposures of Beacon Heights Orthoquartzite in the Fry Glacier area were examined and found to be similar to exposures at the type section and at knobhead.
The Metschel Tillite does not occur in any of the sections visited in the Towle and Northwind Valleys and the Weller Coal Measures directly overlie the Maya Erosion Surface in this area.
Large slump folds have been described in the Beacon Heights Orthoquartzite from several locations in the Convoy Range (Burgess et al 1981), only one such occurrence was observed on Elkhorn Ridge. About 50 m of Beacon Heights Orthoquartzite is incorporated in a slump fold on the southside of Elkhorn Ridge, however, the presence of a thin sliver of Weller Coal Measures within the slump shows it is not the result of soft sediment deformation. Geometry of adjacent dolerite bodies suggests that the folding is associated with the intrusion of Ferrar Dolerite.
Allan Hills Section Measuring
Sections were measured though all the Victoria Group formations exposed at Allan Hills. The section data along with similar data collected in the Darwin Glacier area will be compared with the sequence in the Skelton Neve following field work in 1989-90. Facies descriptions and paleocurrent measurements were collected during section measuring. The ten degree unconformity reported by Ballance (1977) was found to result from the juxtaposition of very large dipping point bars and horizontal channel floor deposits.
Structures resembling hummocky cross-stratification were observed in the upper part of the Weller Coal Measures at Allan Hills. Further work will be required before the significance of the discovery can be fully evaluated. HCS is normally considered a marine indicator its occurrence in an acknowledged alluvial setting (Weller Coal Measures) is likely to be significant.
The results of this field work will be written up as a PhD thesis at VUW, a paper on the correlations with in the Taylor Group has been submitted to NZJGG, a detailed study of the fish fauna from the Cook Mountains is being prepared (with J.A. Long, Hobart) and a paper addressing sedimentological controls on trace fossil distributions is planned.
Victoria Group sediments in the Skelton Neve region will be examined in 1989-90 and this work should permit correlation of all units to be made throughout the McMurdo Basin. The nature of the page 24 present day boundaries to the basin, near the Byrd Glacier in the south and the Mawson Glacier in the north, is uncertain and field work north and south of these areas may be required. Allan Hills provides unequalled exposures of coal measures (Welter Coal Measures) and braid plain (Feather Sandstone) sequences. There is still much that could be gained from further sedimentological work at this site.
Management of Science in the Ross Dependency
|1)||Planning for an alternative landing site was undertaken prior to our pull out date, it was known from the put in that the site was unsuitable due to the presence of crevasses yet alternate sites were not looked for.|
|2)||A list of previous landing sites and there characteristics were readily available. Ant. Div. should maintain a log of all NZARP related landings and this should be available at Scott Base.|
The planned 1:50,000 scale geological mapping of the Convoy Range and Allan Hills could not be undertaken due to the non-arrival of the required air photos. This is of concern because the photos were ordered from USGS some months before hand and arrived in Christchurch on or before Dec 1st. However they did not arrive in Antarctica until after our last camp move which was on Jan 17. It is of serious concern that our co-operative project with NZGS was unsuccessful due to the slowness of mail between Christchurch and Antarctica, every effort should be made to upgrade the system before next season.
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Ballance, P.F. 1977. The Beacon Supergroup in the Allan Hills, central Victoria Land Antarctica New Zealand Journal of Geology and Geophysics 20: 1003-1016.
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