Structure and sedimentology of the Dundas Group, Western Tasmania
Selley, D (1997) Structure and sedimentology of the Dundas Group, Western Tasmania. PhD thesis, University of Tasmania.
Middle and Late Cambrian sediments and minor intrusives of the Dundas Group
form the western most exposures in the Mt Read volcanic belt of western Tasmania. The
Dundas Group comprise at least 2500 m of clastic sediments with rare pumice-bearing
quartz and feldspar-phyric volcaniclastics, minor quartz-feldspar porphyritic intrusives
and andesitic to dacitic lavas. Volcanogenic deposits become less abundant towards the
western limits of the unit, where lithologies are characterised by thick polymict
conglomerate packages which occur intercalated with mudstone and thinly bedded
sandstone. Structural and stratigraphic complexities of the Dundas area, with particular
emphasis on the Dundas Group, provide the focus of this study.
The provenance of the Dundas Group is linked to the presently exposed basement
lithologies and coeval volcanics. These include the tholeiite dominated Crimson Creek
Formation, boninites, gabbros and peridotites of the Serpentine Hill Complex, older
mature meta-sandstones, and the volcanics of the Mount Read volcanic 'arc'. All these
sources were detected in the conglomerates and sandstones of the Dundas Group. Suite 1
samples are dominated by Crimson Creek Formation with some contamination from
mature sedimentary rocks and the mafic/ultramafic complexes. Suite lB samples have
been identified as Crimson Creek Formation rather than Dundas Group on the basis of
their provenance. Suite 2 samples have been derived from the mafic/ultramafic complex
and a felsic source. Suite 3 has a broad range of compositions consistent with thorough
mixing of detritus derived from two or more sources. Suite 4 samples are mature
sandstones which have a metasedimentary source. The distinction of Dundas Group suite
4 samples from the basement sandstones is very difficult with presence of distinctive
traces of chromite very useful in recognising the younger Dundas Group sandstones.
Suites lB and 4 sandstones from the eastern zone have been tentatively identified as
basement lithologies faulted up within the footwall of the Rosebery Fault.
Deposition of suite 3 siliciclastics occurred during the mi~dle Middle Cambrian in
the western area, but continued into the earliest Late Cambrian in the central area. During
the latter period, suite lA sediments were being deposited along the western margin of the
basin, whereas coeval finer-grained suite 3 sediments occupied central or eastern portions
of the basin. This asymmetry in lithofacies distribution from west to east reflects
proximal derivation of suite lA conglomerates and sandstones from rapidly uplifted
basement sources to the west, with supply of thoroughly mixed sediment to the east.
The western-most exposures of the Dundas Group involve petrographically and
chemically similar conglomerate-greywacke-mudstone successions which crop out at
Dundas and at Que River to the north. These range in age from latest Middle Cambrian to
prob:tble Late Cambrian and represent a marked change in basin geometry characterised
by rapid basement uplift. This phase of tectonism and associated sedimentation is coeval
with the onset of thrusting and molasse-type Owen Conglomerate deposition further to the east. In the western parts of the Mt Read volcanic belt, however, basin subsidence and
quiescent marine sedimentation persisted until the middle late Cambrian. This asymmetry
in facies architecture and basin evolution across the Mt Read volcanic belt corresponds to
diachronous Late Cambrian E-W and downwarping of thin continental crust to the west of
an advancing fold and thrust belt.
The earliest deformation recognised in the Dundas Group (D1) is characterised by
pre-lithification deformation features which include coherent slides along bedding parallel
surfaces, chaotic zones of liquefaction, brittle fault zones and slump folds. These
structures relate to syn-sedimentary seismic shock and/or gravitational collapse following
Regional cleavage development and upright, open to tight folding correspond to
the earliest phase of Middle Devonian orogenesis (D2) . The S2 cleavage is the dominant
penetrative fabric developed throughout the Dundas region. Mesoscopic and macroscopic
folds related to S2 are upright to moderately inclined and shallowly to moderately
plunging, however the trend of hinge lines is quite variable, ranging from NNW-SSE to
NE-SW. The S2 cleavage is almost always non-axial planar. The cleavage transection is
interpreted to be the result of imposition of an ENE-WSW directed D2 shortening axis on
an earlier, pre-D2 generation of NNE- toNE-trending mesoscopic and macroscopic folds.
The pre-D2 folds generation is tentatively correlated with a regionally developed phase of
Late Cambrian E-W to NE-SW compression.
Structural relationships in the Dundas region are most complicated towards the
east and culminate in tightly folded and disrupted strata positioned within the footwall of
the Rosebery Fault. These rocks are characterised by the dissection of a N-S trending
upright folds by an anastomosing array of steeply dipping shear zones. Where shear
zones were developed in originally well stratified lithotypes with marked competency
contrast, melange-type textures have formed. Domains of melange-type deformation are
well exposed in the Ring River. The dominant texture common to all disrupted units is
partially to completely fragmented sedimentary layers enclosed within a fine-grained and
frequently fissile argillaceous matrix. Partitioning of strain and variation of deformation
styles throughout the disrupted domains occurs primarily as a function of the original
sedimentary lithotypes. Three broad mesoscopic styles are defined: i) "high strain"
phacoid zones , ii) domains of large-scale boudinage and pinch-and-swell structure and
iii) chaotic block-in-matrix structure. Bulk flattening-type strains in the Ring River
melange is indicated by chocolate tablet boudinage structure and development of
orthogonal extensional vein generations. The favoured interpretation for the development
of the Ring River melange is the tightening and rotation of upright, shallowly plunging,
NNE,. to NE-trending pre-D2 folds during oblique imposition of NNW to N striking D2
slaty and spaced cleavages.
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