UNUSUAL LARGE-SCALE PHREATOMAGMATIC DEPOSITS OF JURASSIC AGE AT ALLAN HILLS, ANTARCTICA

At Allan Hills, south Victoria Land, Jurassic phreatomagmatic deposits (Mawson Formation), which are at least 300 m thick, are in part intrusive and in part extrusive. Intrusive relations are shown at all observed contacts with country rocks, which show brecciation and/or deformation. North of Watters Peak, Triassic Feather Formation country rocks display a zone of in situ brecciation followed inward by a zone of megaclasts, predominantly of younger Triassic Lashly Formation strata, which are up to 10s of meters long. This zone is succeeded by a grey sand-rich breccia which has increasing proportions of primary pyroclasts both laterally and vertically. The grey breccia is overlain by stratified tuff-breccia and lapilli-tuff units up to 10s of m thick. Both consist of high proportions of sandstone and dolerite/basalt clasts set in a matrix of pyroclasts and sand-size debris derived from Triassic strata; some basaltic clasts show peperite textures. All but the brecciated country rocks are cut by diatremes and by tuff-breccia and lapilli-tuff intrusive bodies, and a Jurassic dike cuts the in situ brecciated rocks and megaclast unit. Megaclasts are mainly Lashly C strata which were originally 125 m higher stratigraphically and topographically than Feather Formation country rock, and demonstrate that here the Mawson pyroclastic rocks fill a collapse structure. The sequence of events proposed is: 1) phreatic activity, on emplacement of dolerite sills at depth, which caused widespread brecciation; 2) withdrawal of magma and formation of a collapse structure containing megaclasts of the overlying strata; 3) renewed magma emplacement causing phreatic activity that became increasingly phreatomagmatic, and which formed the grey breccia; 4) phreatomagmatism with eruption of thick stratified tuff-breccia and lapilli-tuff units; and 5) emplacement of diatremes and other intrusive bodies. Locally base-surge deposits cap the coarse pyroclastic rocks. To the south of Watters Peak, the pyroclastic rocks are crudely stratified and could be an outflow facies or the fill of a collapse structure; if the latter, the areal extent of the collapse structure is 50+ km². Comparable thick pyroclastic deposits of similar age and setting are known at the adjacent Coombs Hills and 750 km distant at Otway Massif.