Ross, Pierre-Simon; White, James D. L.
(2005).
Mafic, large-volume, pyroclastic density current deposits from phreatomagmatic
eruptions in the Ferrar large igneous province
Journal of Geology
, vol. 113
, nº 6.
p. 627-649.
DOI: 10.1086/449324.
Résumé
Mafic volcaniclastic deposits are increasingly recognized as important components of continental flood basalt
provinces, but the mode of origin of these deposits and the possible environmental influence of the eruptions that
formed them remain poorly constrained. The Mawson Formation (Ferrar large igneous province, Jurassic) contains the
best known examples of large-scale, mafic, primary pyroclastic deposits from any flood basalt province worldwide. At
southern Allan Hills, in contrast with other sites, the Mawson Formation consists predominantly of sub-horizontal,
laterally-continuous, meter-thick layers which fall into four facies: (1) poorly sorted, coarse lapilli-tuff or tuff-breccia
beds; (2) tuff or fine lapilli-tuff beds; (3) block-rich layers; (4) accretionary lapilli-bearing beds. We interpret the first
facies as deposits of high-concentration pyroclastic density currents (PDCs) resulting from the collapse of
phreatomagmatic eruption plumes, some from a source probably several kilometers away in Coombs Hills. The finer-grained layers were probably deposited by dilute PDCs, whereas block-rich layers originated from local sources
predominantly by lateral ejection and ballistic fall. Dilute, moist turbulent currents were also likely responsible for the
generation and deposition of large (up to 4.5 cm) rim-type accretionary lapilli. Emplacement of meters-thick basaltic
PDC deposits over areas of at least 100-200 km² is suggested, with individual deposit volumes on the order of a cubic
kilometer. Pre-lava pyroclastic eruptions of the Ferrar mafic magmas were explosive, produced significant eruption
clouds, and distributed their products over substantial areas.
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