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

BEACON STUDIES (K6A)

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BEACON STUDIES (K6A)

Triassic sedimentology in south Victoria Land - B. Walker.

Previous geological investigations of the Beacon Supergroup outcropping on the polar plateau edge of the Transantarctic Mountains have shown that the Triassic strata forming the youngest part of the Beacon were deposited by rivers that flowed over a broad, alluvial plain.

The object of this PhD study has been to visit the better exposed localities of Triassic strata throughout south Victoria Land so as to establish a set of palaeohydraulic models that best represent the fluvial depositional processes that operated during this time.

During the 1982/83 Antarctic field season the excellent exposures at Mount Bastion, Portal Mountain, Shapeless Mountain and Allan Hills were visited. These widely separated localities offered the coverage necessary for understanding the Triassic depositional basin.

The 400 metres of Triassic Lashly Formation is divided into four members. Member A, which is 100 metres thick, consists of laterally extensive trough-cross-bedded and micro-cross-laminated tabular sandstone bodies in the order of 2 metres thick that were formed by the lateral migration of river channels. Straight channels had widths of at least 200 metres but were seldom deeper than 2 metres. The channel sandstones are interbedded with root-bearing mudstones, siltstones and fine sandstones that occur in beds up to 6 metres thick. These beds commonly contain abundant white roots, Skolithus burrows and in some places pedogenic features (Gabites, this report). The fine grained beds are interpreted as overbank sediment that accumulated in a swamp environment.

Member B represents a marked change in river behaviour. Over 100 metres thick and consisting of over 90% sandstone it is dominated by massive, horizontally laminated and trough-cross-bedded sandstones. These deposits are interpreted as being of flood origin. The floods commonly attained upper flow regime conditions, were capable of scouring deeply in older flood deposits and transporting large mud-stone clasts, mats of peat and logs.

The 70 metres of Member C marks another change in depositional conditions. Carbonaceous shales, thinly bedded and laminated siltstones are abundant. Interbedded mudstone and micro-cross-laminated sandstones and coal beds up to 1.2 metres thick are also common. These deposits are interpreted as being formed in extensive shallow lakes and swamps. Vegetation was abundant (Gabites, this report). The common occurrence within Member C of tabular, trough-cross-bedded and micro-cross-laminated sand-stone beds around 1 to 2 metres thick indicate that the paludal environment was occasionally inundated by flood sands of sheet flow origin.

Member D is 200 metres thick and represents conditions similar to Member B.

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The sediments of the Lashly Formation were deposited by north-flowing rivers in a cratonic foreland basin now the present site of the Transantarctic Mountains. This basin may have extended south into the Beardmore Glacier area and possibly encroached upon some areas of North Victoria Land (Elliot, 1975; Walker, 1982). Basin width was at least 40 kilometres but most likely in the order of 100 to 200 kilometres.

Uplift of the Ellsworth and Pensacola Mountains supplied sediment to the Nilsen-Mackay Basin (Elliot, 1975). The rate of movement of sediment over the floodplain was largely controlled by differing intensities of tectonic uplift. Members A and C reflect relatively stable periods of crustal movement and near to equilibrium hydrologic conditions. Members B and D reflect deposition from periods of uplift, which provided an abundant source of sediment, increased the gradient of the palaeoslope and increased the amount of subsidence of the sedimentary basin. Secondary controls on sedimentation include a temperate climate with abundant rainfall and a well-vegetated floodplain.

The results of the geological data collected over three Antarctic field seasons will be presented ultimately as a PhD thesis in 1984, and then by a series of papers to internationally recognised sedimentological journals.

The results from this study will firstly provide an understanding of the environment of deposition for the Triassic part of the Beacon Supergroup and secondly will contribute to the understanding of similar alluvial plain deposits found elsewhere in the world.

References

ELLIOT, D.H. 1975. Gondwana basins in Antarctica. In: K.S.W. Campbell (ed.), Gondwana Geology, A.N.U. Press, Canberra, 493-536.

WALKER, B.C. In press. The Beacon Supergroup of Northern Victoria Land. Proc. Fourth International Symposium on Antarctic Earth Sciences, Adelaide, August 1982.

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.