Low-temperature emplacement of phreatomagmatic pyroclastic flow deposits at the monogenetic Mt Gambier Volcanic Complex, South Australia, and their relevance for understanding some deposits in diatremes

Jozua Van Otterloo, Raymond Alexander Fernand Cas

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The occurrence of pyroclastic flow deposits is not restricted to polygenetic and/or felsic volcanic systems, but can
also occur at mafic monogenetic volcanic centres. At the c. 5 ka Mt Gambier Volcanic Complex, Australia, two small-volume
pyroclastic flow deposits resulted from phreatomagmatic eruptive phases of the Blue Lake East and the Valley Lake maar
craters. The facies descriptions of the two massive, poorly sorted lapilli tuff and tuff breccia deposits are given. Low-grade
carbonized wood fragments in the deposits mark low emplacement temperatures (180–270°C) for these deposits with only
c. 12–23% of the total thermal energy of the original magma preserved at the time of deposition. The Blue Lake East vent erupted
by forming a short-lived vertical eruption column, which could not be sustained and collapsed. The Valley Lake pyroclastic
flow deposit has a highly asymmetric dispersal to the west, indicating that this deposit was formed during an eruptive phase with
a large westward-directed lateral component. Such massive, poorly sorted deposits outside the maar crater indicate that some
similar deposits in diatremes may have originated from surface eruptions and not just from subsurface debris jets.
Original languageEnglish
Pages (from-to)701-710
Number of pages10
JournalJournal of the Geological Society
Issue number4
Publication statusPublished - 2016

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