HR: 16:15h
AN: V52G-02    [PDF]
TI: Mawson Formation at Allan Hills, Antarctica: Evidence for a Large-scale Phreatomagmatic Caldera
AU: * Elliot, D H
EM: elliot.1@osu.edu
AF: Ohio State University, Department of Geological Sciences and Byrd Polar Research Center, 125 South Oval, Columbus, OH 43210 United States
AU: Fortner, E H
AF: Ohio State University, Department of Geological Sciences and Byrd Polar Research Center, 125 South Oval, Columbus, OH 43210 United States
AU: Elliot, R J
AF: Ohio State University, Department of Geological Sciences and Byrd Polar Research Center, 125 South Oval, Columbus, OH 43210 United States
AB: At Allan Hills, Transantarctic Mountains, Jurassic Mawson Formation pyroclastic rocks are more than 300 m thick. Previously described as unconformable on older Permian and Triassic Beacon strata, the Mawson is now known to be, at least in part, intrusive. Triassic Feather Formation country rocks at the contact display a zone of in situ brecciation. This is followed inward by a zone of megaclasts (10s of m long) derived from younger Triassic Lashly Formation strata, and then by a structureless, grey, sand-rich breccia which has increasing proportions of pyroclasts laterally and vertically. The grey breccia is overlain by units, up to 10s of m thick, of stratified tuff breccia and lapilli tuff, both of which consist of high proportions of Beacon and dolerite clasts set in a matrix of pyroclasts and sand-sized debris also derived from Beacon rocks. All but the brecciated country rocks are cut by basaltic diatremes, and by tuff-breccia and lapilli-tuff intrusive bodies. Megaclasts are mainly Lashly C strata which were a minimum of 120 m stratigraphically and topographically above any extant country rock, and demonstrate that the Mawson rocks are filling a collapse structure. The sequence of events is interpreted to be: 1) initial phreatic activity, on emplacement of a Ferrar Dolerite sill at depth, causing in situ brecciation; 2) withdrawal of magma and collapse of overlying strata to form a caldera containing megaclasts of Lashly strata; 3) renewed magma emplacement which initially caused phreatic activity but increasingly became phreatomagmatic, and formed the grey breccia by disaggregation of the collapsed Beacon strata; 4) full scale phreatomagmatism that erupted the stratified tuff breccia and lapilli tuff; 5) intrusion of basalt diatremes, bodies of tuff breccia and lapilli tuff, and dolerite plugs and dikes. Away from the mapped area, on the southern arm of Allan Hills, tuff breccia and lapilli tuff are crudely stratified and could be either outflow facies or part of the caldera fill; if the latter, the areal extent of this caldera exceeds 50 km2. Locally, base surge deposits cap the coarse pyroclastic rocks. Jurassic pyroclastic rocks at two other localities in the Transantarctic Mountains appear to fill comparable large-scale calderas.
DE: 8400 VOLCANOLOGY
DE: 9310 Antarctica
DE: 9609 Mesozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting