The 2001-2002 field season in the Coombs and Allan Hills, south Victoria Land.

Four weeks were spent in the Allan Hills during November and December, 2001, investigating the Mawson Formation. The objective of the study was to understand the mode of formation of the basaltic pyroclastic rocks that comprise the Mawson Formation and investigate the relationship to the underlying strata (Fig. 1).

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Fig. 1. View south from a platform of Triassic Feather Formation towards the Mawson outcrops. The grey rubbly rocks, above the long snow patch in the center of the image, are formed of sandstone megaclasts in a sandstone matrix and constitute a surficial debris deposit (landslide). Tuff breccia forms the main skyline summit and the distant skyline summit on the right hand side of the image. Tuff breccia extends downslope to the right (west) beneath dark basaltic surficial debris; on that west-facing slope it is also cut by resistant dolerite intusions which are 1-10 m across.

These basaltic pyroclastic rocks belong to the late Early Jurassic Ferrar Large Igneous Province and are the first manifestation of extrusive igneous activity in Antarctica associated with the break-up of Gondwanaland.

Although initially interpreted as a glacial deposit (Gunn and Warren, 1962), reinvestigation demonstrated that the rocks have a pyroclastic origin (Borns et al., 1969; Ballance and Watters, 1971). The rocks were interpreted as overlying an unconformity cut down more than 500 m through Triassic strata into the Permian Weller Formation. The Mawson Formation itself, interpreted as the product of pyroclastic activity, consists predominantly of thick (>400m) tuff breccia which contains varying amounts of sedimentary debris embedded in a basaltic matrix.

Investigations in 2001 showed that the contact may be interpreted as intrusive, and aspects of the Mawson Formation itself support that view. A zone of disturbed Beacon rocks separates the in situ stratified Permo-Triassic Beacon strata from the Mawson pyroclastic rocks. This zone includes brecciated Triassic Beacon strata, sandstone breccias, and megaclasts measuring 10s of meters in length and consisting of carbonaceous sedimentary rocks that are similar to the Weller coal measures (Fig. 1). An altered basalt dike, which is very similar in field aspects to Ferrar tholeiitic dikes, cuts the brecciated Beacon rocks, thus setting a minimum age for their brecciation. The sandstone breccias demonstrably overlie a surface that cuts down into the Lower Triassic Feather Formation and its overlying brecciated rocks (Fig. 2).

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Fig. 2. On the near outcrops, a contact can be seen between sandstone breccia (landslide) forming the gray layer which is about 2 m high, and underlying brecciated Triassic Feather Formation sandstones which here have streaked reddish-brown weathering. The distant summit is the top of the Triassic section, which is about 300 m stratigraphically above the foreground outcrops.

The megaclasts, if eventually demonstrated to be Permian, were lifted more than 100 m upwards by volcanic processes before being both overlain by sandstone breccias and incorporated in them. The breccias contain sedimentary megaclasts and clasts of Mawson Formation rocks which themselves may contain sandstone clasts and sand-size mineral grains. The contact of the sandstone breccias with Mawson pyroclastic rocks is obscured by talus. It seems likely that the sandstone breccias are the remnants of syn-Mawson landslide deposits (Fig. 3).

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Fig. 3. Sandstone breccia with coal clasts which form all the small dark areas to the left of the standing person. This outcrop is part of the unit that is interpreted as a landslide deposit.

Rocks assigned to the Mawson Formation are cut by irregular and poorly defined intrusive bodies of tuff breccia, and by two concentric plugs of pyroclast-rich lapilli tuff and tuff breccia (Fig. 4).

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Fig. 4. Tuff breccia forms a resistant plug, adjacent to the figure in the center of the photo, cutting a lapilli tuff body that also is an intrusive body. The latter cross cuts mainly unstratified tuff breccia deposits. The bluffs at the top of the hill are formed of weakly bedded tuff breccia .

The host rocks to these intrusive bodies locally appear to have a weak, gently dipping stratification, but much of the outcrop is massive. The host rocks range between two end members: grey, sandstone-rich breccia and tan, basalt-rich breccia (Fig. 5).

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Fig. 5. Tuff breccia outcrops adjacent to a dolerite body (just visible below and half way along the right hand ridge in Fig. 1). In the foreground the tuff breccia contains large basaltic clasts in a pyroclast-rich matrix; tuff breccias with differing clast and matrix compositions and proportions show up as outcrops of different color.

Contacts between Mawson rock units vary between gradational and abrupt, suggesting that many of these rock bodies were poorly lithified at the time of emplacement of cross-cutting tuff-breccia bodies. Although there is unequivocal evidence for intrusive tuff-breccia, such as has also been described by White and McClintock (2001) for Mawson rocks at Coombs Hills, some aspects suggest that individual units were deposited in a stratified sequence (Elliot and Hanson, 2001) and this is particularly the case for outcrops south of the study area.

Samples were collected for laboratory study of the volcanic textures to determine the mode of formation. Samples were also collected for pollen analysis to establish the age of the clasts in the Mawson Formation and in the landslide deposits, and hence their provenance within the Beacon succession.

Research was supported by NSF Grant No OPP 0087919 to the Ohio State University. Helicopter support was provided by PHI and is gratefully acknowledged.

References

Ballance, P.F., Watters, W.A. 1971. The Mawson Diamictite and the Carapace Sandstone Formations of the Ferrar Group at Allan Hills and Carapace Nunatak, Victoria Land, Antarctica. N.Z. J. Geol. Geophys., 14, 512-217.

Borns, H.W.Jr., Hall, B.A. 1969. Mawson "Tillite" in Antarctica: Preliminary Report of a Volcanic Deposit of Jurassic Age. Science, 166, 870-872.

Elliot, D.H., Hanson, R.E. 2001. Origin of widespread, exceptionally thick basaltic phreatomagmatic tuff breccia in the Middle Jurassic Prebble and Mawson Formations, Antarctica. J. Volc. Geotherm. Res., 111, 183-201

Gunn, B.M., Warren, G. 1962. Geology of Victoria Land between the Mawson and Mulock Glaciers, Antarctica. Bull. Geol. Surv. N.Z., no. 71, 157 pp.

White, J.D.L., McClintock, M.K. 2001. Immense vent complex marks flood basalt eruption in a wet failed rift: Coombs Hills, Antarctica. Geol., 29, 935-938.