Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3–5, 2004)

Paper No. 6
Presentation Time: 9:40 AM


ELLIOT, David H., Geological Sciences and Byrd Polar Research Center, The Ohio State Univ, 125 South Oval Mall, Columbus, OH 43210 and FORTNER, Everett H., Byrd Polar Research Center, Ohio State Univ, Columbus, OH 43210,

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.