Papers and Proceedings of the Royal Society of Tasmania. Volume 123, 1989 43 THE WELD RIVER GROUP: A MAJOR UPPER PRECAMBRIAN DOLOMITE SEQUENCE IN SOUTHERN TASMANIA by C. R. Calver (with two text-figures and two plates) CALVER, C.R., 1989 (31:x): The Weld River Group: a major upper Precambrian dolomite sequence in southern Tasmania. Pap. Proc. R. Soc. Tasm. 123: 43-53. https://doi.org/10.26749/rstpp.123.43 ISSN 0080--4703. Geological Survey of Tasmania, PO Box 56, Rosny Park, Australia 7018. The Weld River Group is a foldedPrecambrian sequence (several kilometres thick and predominantly of dolomite) that crops out in the northern Jubilee region, southern Tasmania. It overlies older predominantly siliciclastic sequences with paraconformity or angular unconformity that does not correspond to any major (cleavage-forming) tectonometamorphic event in the older successions. The group consists of a basal, relatively thin, siliciclastic unit (the Annakananda Formation), locally overlain by an impersistent pebbly mudstone (the Lake Timk Formation), followed by massive, fine-grained dolomite (the Gomorrah Dolomite), then bedded dolomite largely consisting of catagraphic, oolitic grainstone (the Devils Eye Dolomite), and then a second massive dolomite unit (the Styx Dolomite). In faulted contact with these units is a probably younger sequence of massive dolomite intercalated with dolomite-derived mixtite, mudstone and sandstone (the Cotcase Creek Formation). The older Precambrian siliciclastic sequences and the Weld River Group are unconformably overlain by a lithicwacke and conglomerate succession, probably of Middle or Upper Cambrian age. The stratigraphic setting of the group, and some lithologic features, suggest correlation with upper Precambrian dolomitic sequences in northwestern Tasmania (e.g. Black River Dolomite, Success Creek Group) implying that the Penguin Orogeny may be absent or only weakly manifested in the Jubilee region. Key Words: Precambrian, Tasmania, dolomite, tillite. INTRODUCTION correlation with sequences elsewhere in Tasmania; the nature and timing of major tectonic phases; and Inaccessibility, thick vegetation and the apparent the origin of the mixtites and related rocks that lack of economic mineral deposits have long occur in younger parts of the succession. Some discouraged any significant geological interest in aspects of this work, in particular dolomite petro­ the catchments of the upper Weld and upper Styx graphy and Pedder Quadrangle geology, will be Rivers. Lewis (1924) provided an exploratory treated in more detail in Calver et al. (in prep.). account and a precursory geological map. Spry (1962), in a synthesis of the Precambrian of Tasmania, included the area in a larger region of relatively unmetamorphosed Precambrian rocks, GEOLOGICAL SETTING later called the Jubilee Block (Corbett I 970) or the Jubilee region (Turner I 989a). Only recently were The Weld River Group is the youngest of at least large tracts of Precambrian dolomite recorded in three distinct Precambrian successions that the upper Weld valley (Hall 1967). Geological comprise the northern part of the Jubilee region mapping of the Pedder I :50 000 Quadrangle (fig. I). All the Precambrian rocks are thick, (Turner et al. 1985) first established the essential cratonic, shallow-water sedimentary sequences unity of the dolomite sequence, its structure, (orthoquartzites, pelites, dolomites) in which internal stratigraphy and relationships with other regional metamorphism is slight (sub-greenschist sequences. A brief outline of this work was given facies) to negligible. First-order folds, with by Calver (1989). wavelengths of a few kilometres, face northwest, This paper is a more detailed description of the plunge steeply northwest and are upright or over­ dolomite sequence, incorporating new information turned to the northeast. The folds are thoroughly from mapping on the Styx Quadrangle. New dismembered by major faults. stratigraphic names are proposed and matters of The rocks are multiply deformed but the intensity broader interest are briefly dealt with: e.g. of deformation and metamorphism decreases in a 44 C. R. Calver ~WurjwinaSUP8rgroup MIDDLE CAMBRIAN D lithitwacke,minolconglomerate LATE PRECAMBRIAN - CAMBRIAI (1} Dli,hicwecke,vokaniCI,ultramafics ' .. ECAMBRIAN ~. ~O;~nl:ix~:::kU~i~ shown ~~" o 0 Devils ...Eye Dolomite WHO RIVER GROUP. / ~:~:~::"::',':::,;oo lomilted where <10m thick) ITIIIIJ Pefite. dolomite, minor quartzarenite PA"'DANI GAOUP ~DOlomite,dOlomiticmudstone MT ANNE GROUP; D',';"·"",,,,,,,,,", SEQUENCE ON JUBilEE RANGE 1::::::1 Orthoquartzite 1 2 S Km I====I....._"==...."""'==', ....,.- Pr"4 Representative dip and strike of bedding: right way up; 30 50 eo overturned: facing unknown [= J Approximate area enlarged in Fig 2 ~Unconformity/paraconformity FIG. J - Geological map of upper Weld Valley and environs. That part of map west of 59 km E gridline is adapted from Turner et al. (1985). general northeasterly direction. This is manifested consistent with known upper Precambrian rocks of chiefly by a gradual change in pelite character, the Tyennan and Rocky Cape regions (Turner from fine-grained phyllite south of Mt Anne, 1988). They are unconformably overlain to the through slate around Sandfly Creek (DN500500), north by a Middle or Upper Cambrian lithicwacke­ to weakly cleaved mudstone around Mt Bowes and conglomerate sequence, and faulted against a east of the Weld River. similar lithicwacke-mafic volcanics sequence of All these sequences, including the Weld River unknown age. All these rocks are unconformably Group, are presumed late Precambrian, because of overlain by upper Cambrian to Devonian rocks of the total absence of fossils other than simple algal the Wurawina Supergroup and by the flat-lYing remains (stromatolites, catagraphs) and because Permo-Carboniferous Lower Parmeener Super­ their rock types and tectonomorphic grade are group. Precamhrian dolomitic Weld River Group 45 STRATIGRAPHY catchment of the upper Weld River and extending east into the Styx River valley (fig. I). Litho­ Older Precambrian Sequences logically, it is marked by a great predominance of fine-grained, pale-grey dolomite that appears The base of the Weld River Group is regionally relatively pure except for local secondary transgressive. East of the Weld River it para­ silicification. Total apparent thickness of the Group conformably overlies an orthoquartzite-quartz is of the order of 5 km, but known faulting, and siltstone-mudstone sequence (here informally more especially the undoubted presence of called the "Jubilee Range sequence"'), while in a undetected faulting in this generaily poody separate area of outcrop north of Mt Anne, it exposed area, make estimates of stratigraphic overlies, with angular unconformity, a dolomite and thickness fraught with uncertainty. The sequence is mudstone sequence (the Pandani Group - Calver thought to be wholly Late Precambrian in age (for et al., in prep.). The relationship between the reasons outlined above). Jubilee Range sequence and the Pandani Group is The Weld River Group is regionally folded but uncertain. The former is lithologically very similar the thick dolomite units have behaved competently to the Mt Anne Group (Calver et al., in prep.), and appear unstrained at the scale of outcrop or which crops out extensively south of the Pandani thin-section. The volumetrically minor clastic units Group and may be older, as it occupies a southerly are, by contrast, usually cleaved and in a few places position in a predominantly north-facing structural display outcrop-scale folding. regime. Difficulty of access, inadequate exposure and the A thick (c. I km) unit of greenish-grey mudstone great thicknesses of the constituent formations have monotonously interlaminated with quartz siltstone precluded measurement of type sections: definition is generally the topmost unit of the Jubilee Range is by type area, and conformity and continuity of sequence. Orthoquartzite occurs lower in the the sequence are inferred from constancy of sequence and forms the high strike ridges of the bedding attitude and facing in type areas. Jubilee Range. At the South Styx River, however, Karst topography is developed over much of the the mudstone-quartz siltstone unit grades up into an area occupied by the Weld River Group. Scattered unusual unit of stromatolitic and oolitic dolomite surficial boulders of crystalline and chalcedonic and dolomitic mudstone, about 100 m thick, which quartz, often with much vuggy porosity, are very is then overlain by the Weld River Group. Although common, particularly in the Styx River catchment. showing greater compositional affinity with this These may be remnants of an ancient (Tertiary?) group than with the Jubilee Range sequence, the silcrete. unit is clearly part of the latter as it is gradational with, and to a small extent interbedded with the Annakananda Formation greenish-grey mudstone-siltstone unit and is overlain by a conglomerate bed correlated with the The Annakananda Formation is defined as the basal Annakananda Formation of the Weld River formation of conglomerate and lithic sandstone, Group. The localised distribution of this unit approximately 30 m thick, which overlies the suggests gentle overstep by the Weld River Group Pandani Group with angular unconformity on the over the Jubilee Range sequence, even though at northeastern ridge of Mt Anne (DN543463) and is known localities the two sequences show no conformably overlain by the Gomorrah Dolomite. discernible angular