FACIES AND CYCLICITY OF THE LATE PERMIAN BAINMEDART COAL MEASURES IN THE NORTHERN PRINCE-CHARLES-MOUNTAINS, MACROBERTSON-LAND, ANTARCTICA

The Late Permian Bainmedart Coal Measures form part of the Permo-Trias sic Amery Group, which crops out in the Beaver Lake area of the Northe rn Prince Charles Mountains, MacRobertson Land, Antarctica. The expose d strata are believed to have formed in graben or half-graben sub-basi ns on the western edge of the Lambert Graben, a major failed rift syst em. Sedimentological analysis has revealed that these rocks formed in alluvial environments in which swiftly flowing rivers of low sinuosity (represented by Facies A1 and A2) flowed northward down the axis of t he basin, and were associated with waterlogged floodbasin and peat-for ming wetlands (Facies B1-B4). A third Facies Association (comprising F acies C1-C3), interpreted as the deposits of lake floor and delta envi ronments, is exclusively developed within a distinctive, fine-grained interval here named the Dragon's Teeth Member. The proportion of Assoc iation B facies within the succession increases markedly above the lev el of the Dragon's Teeth Member (at about 300 m above the base of the formation). Flat, low-angle and undulatory bedding structures preserve d within channel deposits are suggestive of sediment deposition in Row conditions which were often critical or supercritical. Presence of ma ssive and chaotic intervals of sandstone further implies some depositi on from high-concentration aqueous flows. Alluvial channel bodies show evidence of incision into underlying substrates, both during initiati on and at later stages in channel bell construction. The lack of inter fingering between channel deposits and coals suggests that thick peals formed only in areas and at times of minimal elastic sediment supply. Analysis of well-developed cyclicity within the coal measures suggest s that the dominant control on sequence architecture was climatic, rel ated to precessional Milankovitch fluctuations of c. 19-kyr periodicit y. Cycles began abruptly with the deposition of coarse-grained materia l in high-energy alluvial channels, which contracted with time in resp onse to changes in water supply (rainfall). Upper parts of cycles are dominated by finer-grained sediments and then coal, indicative of prog ressively reduced coarse sediment input. Tectonic processes overprinte d this pattern at least once during the period of sediment accumulatio n, to form the Dragon's Teeth Member.