Palaeobotanical Studies of the Permian and Triassic Victoria Group (Beacon Supergroup) of South Victoria Land, Antarctica

Abstract

Plant macrofossils and microfossils from the Victoria Group (upper Beacon Supergroup) of south Victoria Land are described, and tentative biostratigraphic units are proposed. The Victoria Group consists of 1000 metres of flat-lying continental strata, extending from Early Permian tillite at the base, through Permian coal measures, non-carbonaceous sandstone and conglomerate and into coal measures of Triassic age. Apart from burrows, plant remains are the only fossils found so far in the strata examined, and therefore are critically important for correlation. Early to Middle Jurassic igneous activity has thermally metamorphosed the strata and included plant material, initially by the increased geothermal gradient and depth of overburden with the extrusion of the Kirkpatrick Basalt, and subsequently by the extensive intrusion of the Ferrar Dolerite, This has resulted in the unusually high rank of the coal throughout the sequence. Carbonisation of the plant fossils is in many cases extreme, leaf cuticles are not preserved and preservation of plant microfossils is usually poor. The distribution of identifiable pollen and spores appears unrelated to the distribution of dolerite sills. On the basis of the palynological succession three informal stratigraphic zones are proposed: 1. The Parasaccites zone is the oldest, and is associated with the Darwin Tillite and with similar glacial sediments in other parts of the Transantarctic Mountains. It is of Early Permian age by comparison with Australian and Indian fossil microfloras. 2. The Protohaploxypinus zone includes the middle and upper parts of the Weller Coal Measures (no microfloras were obtained from the lower Weller), and by correlation with Australian microfloras is Middle Permian (late Early Permian) in age. 3. The Alisporites zone is divided into four informal subzones designated A (oldest) to D, and includes the top of the Feather Conglomerate and most of the Lashly Formation. By correlation with Australian microfloras subzone A (uppermost Feather Conglomerate and basal Lashly Formation) is Early Triassic in age, subzone B is Middle Triassic, and subzones C and D are Late Triassic. Plant macrofossils in the Weller Coal Measures include leaves of Glossopteris Gangamopteris and Noeggerathiopsis, and also fructifications, seeds, roots, stems and silicified tree trunks with thick, well-developed growth rings. They indicate an age from Early Permian for the lower part of the formation to Middle Permian. Similar but rare microfloras occur in the Misthound Coal Measures of the Darwin Mountains. Abundant leaf fossils and stems in the Lashly Formation represent the Dicroidium flora and are of Triassic age, in accordance with the associated plant microfossils. Palaeomagnetic data indicates that during the Permian and Triassic Periods Antarctica lay, as at present, in polar latitudes. Lith ological and palaeobotanical evidence suggests that the climate was humid cold temperate, and somewhat cooler during the Permian than during the Triassic Period. The floras were similar, though of lower diversity, to other Gondwana floras, especially those of eastern Australia, and exhibited similar trends in development.