JURASSIC MAWSON FORMATION TUFF-BRECCIAS AND RELATIONSHIPS TO PERMO-TRIASSIC BEACON STRATA AT ALLAN HILLS, ANTARCTICA

Basaltic tuff-breccias of phreatomagmatic origin occur in thick deposits over 8,000 km2 in the Transantarctic Mountains. Those at Allan Hills consist of varying proportions of igneous particles and rounded to angular sedimentary clasts set in an ash to lapilli matrix. Sedimentary clasts consist of quartzose sandstone and carbonaceous rocks. Volcanic debris consists of pyroclasts and variably altered basalt clasts. Primary pyroclasts occur in two contrasting settings: plug and dike-shaped bodies up to 200 m across, and massively bedded tuff-breccia on a 100+m scale. Plugs and dikes contain spatter, bombs, and large amounts of sedimentary debris. A boulder bed occurs below a massively stratified tuff breccia unit that extends in outcrop for hundreds of meters. Stratigraphically below the boulder bed is a massive grey sandstone breccia with basaltic pyroclasts. Minor intrusive bodies of holocrystalline basalt and lapilli-tuff cut the grey sandstone breccia. The uppermost Feather and Lashley Member A (both Triassic) exhibit in situ brecciation and are overlain by landslide debris (reworked sedimentary material and sparse tuff-breccia clasts). Below the landslide and juxtaposed between in situ brecciated Triassic rocks and the grey sandstone breccia are megaclasts of intact and deformed carbonaceous strata similar to Permian Weller coal measures. The process of in situ brecciation and emplacement of megaclasts remains under investigation. Later plug and dike intrusion is clear. However, the origin of the landslide remains unclear and may, perhaps, be glacial. The association of large amounts of sedimentary debris with magma fragmentation and pyroclast production is indicative of phreatomagmatic processes. The occurrence of bombs and spatter suggest varying amounts of available water to interact with the magma. The large volumes of sedimentary debris and the range in ratio of sedimentary clasts to pyroclasts, together with apparent thicknesses of tuff breccia (up to 400 m) make this a unique example of phreatomagmatism.