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Victoria University Antarctic Research Expedition Science and Logistics Reports 1982-83: VUWAE 27

Triassic paleoecoloqy in south Victoria Land - I. Gabites

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.

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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.