http://journals.cambridge.org Downloaded: 13 Dec 2010 IP address: 130.237.165.22 Geol. Mag. 134 (3), 1997, pp. 335-353. Copyright O 1997 Cambridge University Press 335 Revised stratigraphy of the Permian Bainmedart Coal Measures, northern Prince Charles Mountains, East Antarctica STEPHEN McLOUGHLIN & ANDREW N. DRINNAN School of Botany, The University of Melbourne, Parkville, Victoria, 3052, Australia (Received 4 April 1996; accepted 14 January 1997) Abstract - The Bainmedart Coal Measures constitute the middle portion of the Permian-Triassic Amery Group, which represents the only substantial Palaeozoic-Mesozoic sedimentary succession exposed in east- em Antarctica outside the Transantarctic Mountains. The coal measures disconformably or unconformably overlie alluvial fan deposits of the Radok Conglomerate and are conformably overlain by the dominantly fluviatile Flagstone Bench Formation. The coal measures were deposited within alluvial settings dominated by north to northeasterly flowing, low-sinuosity rivers chiefly confined to broad, fault-bounded, valleys of the Lambert Graben, a major late Palaeozoic to early Mesozoic failed rift system. Both climatic and local tectonic factors are considered to have been the major influences on the pattern of coal measure sedimenta- tion. Recent mapping has identified a much greater thickness of sediments within the coal measures than had been inferred previously. The Bainmedart Coal Measures are formally subdivided into six members. In ascending stratigraphic order these are: Dart Fields Conglomerate Member (< 3 m thick), Toploje Member (c. 300 m thick), Dragons Teeth Member (15-25 m thick), Glossopteris Gully Member (c. 670 m thick), Grainger Member (c. 350 m thick) and McKinnon Member (c. 530 m thick). 1. Introduction Permian-Triassic sediments of the Amery Group crop out in a small area around Beaver and Radok lakes in the northern Prince Charles Mountains (Fig. 1). These rocks represent the only significant exposures of Palaeozoic or Mesozoic strata in eastern Antarctica outside the Transantarctic Mountains. Previous detailed studies of these rocks were restricted to exposures in the Radok Lake, Flagstone Bench and Jetty Peninsula areas and resulted in the establishment of three stratigraphic subdi- visions of the Amery Group (Mond, 1972). In these areas, the Radok Conglomerate (at the base of the sequence) is overlain disconformably or unconformably by the Bainmedart Coal Measures, but an obscured contact between the coal measures and the succeeding Flagstone Bench Formation precluded identification of their precise stratigraphic relationship. Fieldwork over the 1994-95 summer documented for the first time the stratigraphic relationships of the relatively extensive Amery Group exposures along the western margin of Beaver Lake (Fig. 1). Two new and distinctive lithostratigraphic units were recognized within this succession, the upper unit being conformably overlain by Flagstone Bench Formation equivalents. This paper revises previous strati- graphic schemes, describes the physical and palaeonto- logical characteristics of each of the six members now recognized within the Bainmedart Coal Measures, and presents detailed measured sections and facies succes- sions for the previously unlogged upper two-thirds of the formation. 2. Regional setting Geophysical studies of MacRobertson Land and Prydz Bay indicate that the Lambert Glacier (the largest valley glacier in the world) occupies a narrow, fault-bounded, depression generally referred to as the Lambert Graben (Fedorov et al. 1982; Stagg, 1985). The Amery Group exposures in the Beaver-Radok Lake region occur within one or more small sub-basins (sometimes named the Beaver Lake Graben: Fedorov, Ravich & Hofmann, 1982) on the western side of the Lambert Graben system. The Lambert Graben has been interpreted as a failed arm of a continental rift system and was possibly continuous with the Mahanadi Graben of India before the breakup of Gondwana (Fedorov, Ravich & Hofmann, 1982). Underlying and surrounding Precambrian basement rocks comprise mostly felsic and mafic gneisses and charnockitic granites (Tingey, 1982). Basement rocks decrease in metamorphic grade from granulite and amphibolite facies in the northern Prince Charles Mountains to greenschist facies in the southern Prince Charles Mountains. Amery Group strata are gently folded with dips typically less than 10" Amery Group exposures are transected by a number of fractures and faults with vertical displacements ranging from neg- ligible to greater than 3- km. Localized, quartz-filled, gash-vein orientations and flexures associated with some faults suggest the occurrence of some dextral strike-slip displacement. Amery Group exposures extend about 50 km north- south and 30 km east-west. They are constrained by a