Glacier-fed(?) sandstone sheets in the Weller Coal Measures (Permian), Allan Hills, Antarctica

Abstract

The upper member of the Weller Coal Measures (Permian) at Allan Hills, Victoria Land, Antarctica, consists of repetitive coal layers separated by sheetlike bodies of predominantly sandstone and subordinate siltstone and conglomerate, all of fluvial origin. Investigation of the lowermost inter-coal sheet (WC1) reveals abrupt planar basal contacts, persistent upward fining, and a strong westward palaeocurrent trend. Three lithofacies are recognized in the WC1 sheet: (A) small-scale cross-bedded pebbly sandstone, which occupies most of the basal portions of the sheet, (B) large-scale cross-bedded medium-coarse sandstone, confined to a linear NE–SW belt overlying or locally replacing facies A, and (C) siltstone and rippled fine-grained sandstone, which drapes and overlaps facies A and B and contains Glossopteris leaves and Vertebraria root traces. Facies distribution patterns indicate the WC1 body initiated as an abrupt sheetlike incursion of coarse sediment over a low-lying mire (facies A), followed by entrenchment and confinement of flow as the sheet extended distally (facies B). Facies C represents low-energy floodplain sedimentation co-existing with and later succeeding facies B as deposition evolved toward abandonment and eventual renewal of peat deposition. Relatively rapid deposition of the sandstone sheet is implied by the lack of any significant hiatus markers. Outsized clasts up to 45 cm in diameter, one observed to display prominent striations, occur sporadically throughout the WC1, including within siltstone beds of facies C. These are inferred to be ice-rafted deposits which, together with other evidence, suggest meltwater influence, possibly glacial, on sediment transport and deposition. Preliminary examination of other sandstone sheets in the upper Weller indicates attributes similar to the WC1, including abrupt bases, upward-fining character, and predominantly westward palaeocurrent patterns. The Allan Hills exposures are believed to represent a succession of low-lying mire complexes interrupted by abrupt but infrequent incursions of coarse sediment, possibly by extreme meltwater floods.