Additional field interpretation of the Jurassic sequence at Carapace Nunatak and Coombs Hills, south Victoria Land Antarctica

Abstract The Jurassic volcanic sequence at Carapace Nunatak and Coombs Hills south Victoria Land, records an initial phase of explosive volcanicity, in part acidic, that was followed by quieter, flood basalt extrusion, a pattern repeated elsewhere in the Transantarctic Mountains. The explosive phase produced poorly sorted volcanogenic material (Mawson Formation) at Coombs Hills, and this was probably contemporaneous with the deposition of coarse lake sediments (Carapace Sandstone Formation) at Carapace Nunatak. The succeeding flood basalts are known as the Kirkpatrick Basalt Group. Features of the Mawson Formation diamictites do not support the laharic origin proposed by earlier workers. The development of breccia mounds ad)acent to phreatic vents, with avalanching of mixed Beacon Sandstone and volcanic ejecta seems more likely, with lahars developing distally Some finer, laminated diamictites may be airfall or gravity‐ flow deposits. The explosive eruptions carried megablocks of basement granite and Beacon Sandstone upwards for as much as 950 m. At carapace Nunatak a hyaloclastic breccia mass developed after eruption of lava directly into lakewater. Basalt “pillows” intrude from a sheet lava base into both breccia and disturbed beds of finegrained fossiliferous lake sediment. A comparable sequence does not occur at Coombs Hills where thick basalt flows overlie Mawson Formation, although rafts of fine sediment in the base of the fourth flow suggest lake formation during pauses In eruption. Thin pillow lavas and sedimentary interbeds in the predominantly subaerial Kirkpatrick Basalt sequence. indicate that subsidence kept pace with lava accumulation to maintain a persistent regional depression. This is also indicated by other Jurassic sequences in the Transantarctic Mountains.