The Weld River Group: a major upper Precambrian dolomite sequence in southem Tasmania

Abstract

The Weld River Group is a folded Precambrian 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.