Sedimentology of Weller Coal Measures at Mt Fleming, Antarctica

Abstract

The Weller Coal Measures are an alluvial plain sequence of early Permian age, and are particularly well exposed at Mt Fleming, where they are 186 m thick. In this study they are mapped, described and interpreted in terms of three facies associations. FA1 forms the basal and upper units and is a cyclic association of sandstone, shale and coal deposited by meandering channels. FA2, which overlies the basal occurrence of FA1, contains in the lower part interbedded sandstone and shale with channelling and sand lenses indicating deposition on a levee. This grades into shale with thin sandstone stringers and limestones deposited in a lacustrine setting on the floodplain. FA3, which overlies FA2, is dominantly sandstone with minor shale and coal lenses and was deposited in braided channels. The sandstone beds of FA1 and FA3 include intervals of small and large planar and trough cross-stratification. The large scale trough cross-stratification is considered to form from migrating in-channel linguoid dunes and hence provide paleocurrent directions that are close to the channel direction. A study of FA1 based on 187 measurements shows a symmetrical bimodal distribution indicating high variability in channel direction (sinuosity greater than 4) and preferential preservation of channel deposited sandstone from within meander loops. The symmetrical bimodal paleocurrent pattern can be simply explained by the downstream migration of a circular meander loop. The 88 measurements from FA3 are also bimodal but have a lower angular range, giving a sinuosity of 1.6, close to the low sinuosity channel pattern inferred from facies analysis. Regional paleoslope has been determined for the Weller Coal Measures by considering patterns of point bar migration and preservation as well as channel direction measured from large trough cross-stratification. Paleoslope is taken as the direction with least readings and one that bisects the two main modes. These data indicate an east-sloping flood plain throughout most of the deposition of the Weller Coal Measures, consistent with few flow directions for the Carboniferous-early Permian ice sheet, but covered the region immediately prior to Weller deposition. The study indicates that simple averaging is not a reliable way of estimating paleoslope for high sinuosity current systems. Deposition of the Weller Coal Measures followed a climatic warming and the final retreat of the early Permian ice sheet. However, vertical facies changes in the coal measures are thought to have resulted from regional tectonic events influencing slope and sediment supply rather than further climatic change.