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Victoria University Antarctic Research Expedition Science and Logistics Reports 1981-82: VUWAE 26

Lower Feather Conglomerate at Mount Bastion (K7A) - P. Fitzgerald

page 12

Lower Feather Conglomerate at Mount Bastion (K7A) - P. Fitzgerald.

The lower part of the Feather Conglomerate at Mt. Bastion was deposited in late Permian times on a moderately steep west-to-northwest dipping paleoslope (Fitzgerald, 1982). Deposition was characterised by sheetflow during flood events and low sinuosity braided rivers during normal flow. The dominant facies present, trough cross-bedded sandstone, is a result of dune migration in the upper lower flow regime. The planar cross-beds are a result of linguoid bar formation, whereas other minor facies are a consequence of low water modification processes and overbank sedimentation. Sediments are from a dominantly granitic provenance and consist of submature quartzarenites and subarkoses. Roundness of quartz pebbles (0.55) and mineralogical immaturity indicate deposition within a few hundred kilometres of source.

The paleosols have been recognised from several features including vein networks, which result from desiccation produced by clay contraction. Mottling and gleying in the underlying sandstone along with gammate structure in the paleosols themselves indicate periodic, possibly seasonal, waterlogging of the soil profile. Concretions associated with the paleosol have formed both syngenetically and epigenetically. These along with absence of any carbonaceous material suggest an oxidising environment and a low level of contemporaneous vegetation. Climate was temperate to moderately cool and humid.

Trace fossils included Skolithos and hypichnial ridges. Skolithos occurs throughout the section, in some instances destroying almost all sedimentary structures. It was not found in fine grained sediments or in paleosols. The presence of Skolithos as well as clean, moderately well sorted sands indicate a high energy environment and abrupt sediment aggradation or degradation. Its presence here in alluvial plain sediments indicates that it can occur in non-marine as well as marine sediments.

A metamorphic overprint assemblage has resulted from the intrusion of the Ferrar Dolerite in Jurassic times. Both 7 Å and 14 Å chlorite are present in the sandstones but only 7 Å in the paleosols. 14 Å oxy-chlorite is a result of alteration of detrital biotite and 7 Å a result of recyrstallisation of clays due to thermal metamorphism. The presence of pyrophyllite also indicates metamorphism and its co-existence with kaolinite and quartz suggests a temperature of 310-315°C.

References

Fitzgerald, P.G., 1982: Environment of deposition of the Feather Conglomerate at Mount Bastion, south Victoria Land, Antarctica. Unpubl. BSc Hons Thesis, Victoria Univ. of Wellington Library, 89pp.