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.

Shapeless Mountain can be divided into several geologically distinct areas, based on the relationships between the Mawson Formation, Ferrar Dolerite and the Beacon Supergroup:

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.

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.