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Geology of the Dundas-Mt Youngbuck area, Western Tasmania : the stratigraphy and structure of the regional geology, and the petrology, chemistry and petrogenesis of the ultramafic-mafic complexes in the Dundas-Mt Youngbuck area of Western Tasmania

Brown, AV 1986 , 'Geology of the Dundas-Mt Youngbuck area, Western Tasmania : the stratigraphy and structure of the regional geology, and the petrology, chemistry and petrogenesis of the ultramafic-mafic complexes in the Dundas-Mt Youngbuck area of Western Tasmania', PhD thesis, University of Tasmania.

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An onlap landscape unconformity has been proved between the Precambrian
Oonah Formation and Eocambrian Success Creek Group. This unconformity
represents a hiatus in sedimentation as well as a structural and low
grade metamorphic break.
The areal distribution of the Oonah Formation has been extended from
the type area to as far north as Mt Livingstone and west to Whaleback
Ridge, where rocks of the Oonah Formation are gradational with rock
sequences of the Arthur Lineament. The Oonah Formation consists of
two parts: a lower succession of fine-grained, siliceous, indurated
quartz sandstone and wacke interbedded with phyllitic mudstone and
siltstone, typical of the sequence at Oonah Hill; and an upper succession
of interbedded carbonate, mudstone and conglomerate with volcanic rock
horizons and minor sandstone.
Structurally the Oonah Formation is characterised by the presence of
outcrop-scale refolded isoclinal folds. Up to five cleavages can be
found in rocks of the Oonah Formation, the last two being the northerly
and northwesterly cleavages associated with Devonian deformation.
The Success Creek Group has been redefined, after Taylor (1954), to
consist of four mappable formations. All formations crop out in Taylor's
type section along the Pieman River, where they have a total measured
minimum thickness of 950 m. The basal formation is a mixtite. The
second formation, the Dalcoath Formation, is a sequence of interbedded,
clean, shallow-water quartz sandstone with minor siltstone, pebbly
sandstone and conglomerate. The Dalcoath Formation grades into the
third formation which is dominated by laminated mudstone and siltstone
with minor sandstone and conglomerate units. This formation is
characterised by pervasive intraformational soft sediment deformation
and slump structures indicative of an unstable basin of deposition.
The top formation is the Renison Bell Formation whose dominant units are
classic siliceous siltstone with mudstone partings. The upper member
of this formation is the 'red rock' of Conder (1918).
Overlying the Success Creek Group with transitional conformity is
the Crimson Creek Formation. This formation consists of a turbiditic
sequence of volcaniclastic lithic wacke with laminated siltstone and
mudstone interbedded with tholeiitic basalt. The proportion of
volcaniclastic lithic wacke and basalt in the succession increases
northwards from the type area along the Pieman River towards Cleveland. Following an implied hiatus in sedimentation, during which the
tectonic regime changed from tensional to compressional emplacing
dismembered ultramafic bodies into basins of deposition, the Dundas
Group (Elliston, 1954) was deposited. Remapping of Elliston's type area of the Dundas Group has shown that
it consists of two distinct successions which are now fault
juxtaposed, but may originally have been separated by a break in
sedimentation or period of shallow water deposition. The lower part
is sparsely fossiliferous and of Middle Cambrian age. Using the
terminology of Elliston, the lower part consists of all rock units
from the Judith Formation up to and including the lower part of the
Brewery Junction Formation. The upper succession is a fossiliferous
turbidite sequence, extending from the middle of Elliston's Brewery
Junction Formation, up to and including at least the Misery
Conglomerate and possibly including a white friable sandstone
sequence on the western slope of Misery Hill.
The Huskisson Group of Taylor (1954) is a biostratigraphic correlate
of part of the Dundas Group. The group'is composed of a succession
of clastic sedimentary rocks whose background sedimentation consists
of laminated and thinly bedded siltstone and mudstone with minor
sandstone and lithic wacke units into which numerous horizons of
mass-flow conglomerate and turbidite units were deposited. The
conglomeratic units in the lower 1000 m of the sequence were derived
from a mixed metasedimentary and active acid to intermediate
volcanic terrain, whereas those in the upper 200 m are from a
dominantly metasedimentary terrain. The dominance in most of the
sequence of acid to intermediate volcanic detritus is the main
difference between the Huskisson and Dundas Group sequences.
Structurally, all folds and cleavage surfaces found within the
Success Creek Group, Crimson Creek Formation, Dundas and Huskisson
Groups and correlates are consistent with having been formed during'
Devonian deformation.
Litho- and biostratigraphic correlates of part of the Gordon
Sub-Group Limestone succession occur around the outer edges of the
Huskisson Syncline. On the southern nose of the Huskisson Syncline
the limestone sequence is in faulted contact with the Huskisson
Group, but a hiatus in sedimentation from the middle of the Late
Cambrian to the middle Middle Ordovician is indicated. Along the eastern
side of the Huskisson Syncline the transition between the limestone
sequence and basal Eldon Group correlate successions appears to be
Within the study area there exists an area of Tertiary basalt,
Devonian granite and numerous gabbroic phases. Volcanic activity
associated with the Eocambrian-Cambrian successions was dominantly
tholeiitic to andesitic in character. Three separate phases of
volcanism were differentiated. The first is the tholeiitic
volcanism associated with the Crimson Creek Formation, the second is
a succession of high-magnesian andesite lavas considered to be the
second stage melting of the same source as the Crimson Creek
Formation tholeiitic basalts. The third phase is a low-titanium
tholeiite suite which interdigitates with the basal conglomerate
units of the Dundas Group. The ultramafic rocks of western Tasmania fall into three groups.
A succession of Layered Pyroxenite-Dunite (LPD); a succession of
Layered Dunite-Harzburgite (LDH); and a succession of Layered
Pyroxenite-Peridotite and associated Gabbro (LPG). These three
ultramafic rock successions can be recognised both by field
characteristics and mineral chemistry. Comparison of mineralogy and
mineral chemistry with experimental work indicates that the
ultramafic rocks formed at high temperatures and low pressures, the
parental melt having low oxygen fugacity and a low water content.
All three of the ultramafic successions are consistent with being
cumulate bodies formed as crustal magma chamber products, each from
one of the three different magma events found within the
Eocambrian-Cambrian successions within the Dundas Trough.
The LPD succession is a monotonous, finely layered sequence
containing orthopyroxene of En89_89; olivine of F087-89; minor
chrome diopside (Ca:Mg:Fe = 47:49:4), and chrome spinel with an
average 100 x Cr/(Cr + Al) ratio of 65 and a variable
100 x Mg/(Mg + Fe 2+) ratio with an average of 43.
The LDH succession is well layered, contains a tectonic foliation
parallel to layering, the result of flattening of the mineral
grains, and contains olivine of F098-94; orthopyroxene of En93 - 94
with very low to undetectable Al 20 3and CaO contents. Chrome spinel
grains have a 100 x Cr/(Cr + Al) ratio of 87-93 and a variable
100 x Mg/(Mg + Fe 2+) ratio with an average of 49.
The LPG succession consists of multiple intrusions which formed an -
orthopyroxene-rich layered sequence, an olivine-rich sequence and a
gabbroic unit. Locally the ultramafic sequences are plagioclase
bearing, and the orthopyroxene-rich sequence contains numerous
primary structural features. The orthopyroxene-rich sequence is
intruded by the olivine-rich sequence which locally contains
chromite-rich zones. No relic olivine cores were encountered;
orthopyroxene is En89-37, chrome diopside (Ca:Mg:Fe = 48:48:4), and
there are two compositions of chrome spinel. Within the ultramafic
rocks chrome spinel has a 100 x Cr/(Cr + Al) ratio with an average
of 60, and an 100 x Mg/(Mg + Fe 2+) average ratio of 48. In the
chrome-rich zones within the olivine-rich sequence spinel grains
have an average 100 x Cr/(Cr + Al) of 69 and an average
100 x Mg/(Mg + Fe 2 +) ratio of 40. The gabbroic phase is a
two-pyroxene gabbro with a variable grainsize and texture. The shallow water nature of the original sediments which infilled
the proto-Dundas Trough; a gradual deepening of an elongate basin,
probably less than 70 km wide, into which tholeiitic lavas and
associated turbiditic sedimentary rocks formed; the low pressure
melting required to form the ultramafic rocks; high-magnesian
andesite lavas associated with terrigenous rocks; and the presence
of areas of Precambrian rocks in the Ramsay River and Dundas areas,
added to previous data, reinforces the implication that the Dundas
Trough represents part of a failed continental rift zone.

Item Type: Thesis - PhD
Authors/Creators:Brown, AV
Keywords: Geology, Igneous rocks, Ultrabasic rocks
Copyright Holders: The Author
Copyright Information:

Copyright 1986 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).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1986. Bibliography: leaves 163-180, v. 1

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