1981_Bacon_pyroclastics.pdf (398.65 kB)
Pyroclastics in the Upper Parmeener Supergroup, near Bicheno, eastern Tasmania
journal contributionposted on 2023-11-02, 05:47 authored by JL Everard, CA Bacon
Extensively altered, poorly welded ash-fall tuffs of rhyolitic to rhyodacitic composition occur both in situ within the Upper Parmeener Super-Group and as loose talus, at five localities up to twelve kilometres apart in the vicinity of Bicheno in eastern Tasmania. Three closely spaced tuff layers up to 0.7 m thick have been intersected in a drill hole. These and the four outcrops may represent the same stratigraphic interval. The rocks contain phenocrysts of embayed quartz, kaolinized feldspar and altered vermiculite in a devitrified matrix of quartz and kaolinite, in which may be discerned relict glass shard and bubble structures. In some localities, the vermiculite has been thermally exfoliated, suggesting heating to more than 3000 C during the intrusion of nearby Jurassic dolerite. Rhyolite, probably derived from unlocated Triassic flows, occurs as clasts within Triassic conglomerate float and as pebbles within Quaternary river gravels.