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Abstract

In the Black Bluff region of Northwestern Tasmania, Cambrian sedimentary
and volcanic rocks are distributed in an arcuate belt around the northwest
margin of the Tyennan Geanticline, which is composed of Precambrian greenschistgrade
metaquartzites and metapelites. The volcanics, which are dominated by
heavily altered intermediate to acid pyroclastics, are concentrated in the
southern part of the belt which borders the margin of the geanticline. The
Cambrian rocks are unconformably overlain by Lower Ordovician quartzose
clastics and Middle Ordovician limestones. Movements responsible for the
angular discordance are inferred to have been dominantly vertical and responsible
for steep tilting of the Cambrian sequences and considerable uplift of the
Tyennan Geanticline, which became the source area for the sediments immediately
overlying the unconformity. The quartzose clastics were mainly deposited in
terrestrial alluvial fan environments, although marginal marine sedimentation
became progressively more important upwards in the sequence. Intertidal
sedimentary environments persisted during deposition of the limestone sequence,
but relief of the Precambrian source areas had by early Hiddle Or.dovician time
been reduced to such an extent that terrigenous influx was insignificant.
Hiddle Devonian deformation of the Palaeozoic rocks, at high crustal
levels, produced open, upright, horizontal to gently plunging folds of
variable trend. The Tyennan Geanticline behaved as a relatively rigid block
during this deformation, which has been correlated with the Tabberabberan
Orogeny, of regional importance in southeastern Australia. The structural
development now recognized involves a progressive sequence of regional folding
on axes which trend east (Dl), northeast (D2), north (D3) and northwest to
north-northwest (D4). The last three phases of folding were associated with
development of axial plane cleavage in the more ductile lithologies.
Localized folding on east-northeast axes and associated cleavage postdates
02, and may have occurred between D2 and D3. The relative timing of fold
and cleavage development on west-northwest trends in the limestone at
Loongana and Gunns Plains is uncertain. Previous conclusions regarding
dating of the Tabberabberan deformation may need reconsideration in view of
the newly proposed structural sequence.
Development of tectonite fabrics in the altered volcanics \

Item Type:	Thesis - PhD
Authors/Creators:	Seymour, DB
Additional Information:	Copyright the Author-The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s)
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