Triassic and Jurassic strata at Coombs Hills, south Victoria Land: stratigraphy, petrology and cross-cutting breccia pipes

Abstract The Triassic Lashly Formation occurs to the east of Mount Brooke at Coombs Hills. Previously established informal members B, C, and D of the Lashly Formation are now identified at Coombs Hills. Lashly Formation member D passes up into a poorly exposed interval of silicic shard-bearing fine-grained sandstone and tuff, which is correlated with the Jurassic Shafer Peak Formation of north Victoria Land and Hanson Formation of the Beardmore Glacier region. Lashly Formation members C and D are intruded by three phreatic explosion pipes, resulting from emplacement of Ferrar Dolerite intrusions at depth and associated explosive steam generation. These pipes, ranging up to 180 m in horizontal dimension, comprise sedimentary clasts in a sand matrix, most of which was locally derived. Pipe margins are mainly ill defined and adjacent country rock is commonly disaggregated or shattered, although retaining stratigraphic order. Locally, thin basalt intrusions have interacted with coal beds.

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