Structure and sedimentology of the Dundas Group, Western 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 basin-floor tilting. 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. "