Blackwell Publishing Journal Backfiles 1879-2005
The Late Permian Bainmedart Coal Measures form part of the Permo-Triassic Amery Group, which crops out in the Beaver Lake area of the Northern Prince Charles Mountains, MacRobertson Land, Antarctica. The exposed strata are believed to have formed in graben or half-graben sub-basins on the western edge of the Lambert Graben, a major failed rift system. 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 the basin, and were associated with waterlogged floodbasin and peat-forming wetlands (Facies B1-B4). A third Facies Association (comprising Facies C1-C3), interpreted as the deposits of lake floor and delta environments, is exclusively developed within a distinctive, fine-grained interval here named the Dragon's Teeth Member. The proportion of Association B facies within the succession increases markedly above the level of the Dragon's Teeth Member (at about 300 m above the base of the formation).Flat, low-angle and undulatory bedding structures preserved within channel deposits are suggestive of sediment deposition in flow conditions which were often critical or supercritical. Presence of massive and chaotic intervals of sandstone further implies some deposition from high-concentration aqueous flows. Alluvial channel bodies show evidence of incision into underlying substrates, both during initiation and at later stages in channel belt construction. The lack of interfingering between channel deposits and coals suggests that thick peats formed only in areas and at times of minimal clastic sediment supply.Analysis of well-developed cyclicity within the coal measures suggests that the dominant control on sequence architecture was climatic, related to precessional Milankovitch fluctuations of c. 19-kyr periodicity. Cycles began abruptly with the deposition of coarse-grained material in high-energy alluvial channels, which contracted with time in response to changes in water supply (rainfall). Upper parts of cycles are dominated by finer-grained sediments and then coal, indicative of progressively reduced coarse sediment input. Tectonic processes overprinted this pattern at least once during the period of sediment accumulation, to form the Dragon's Teeth Member.
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