Basin development, environment and deposition and deformation of a precambrian(?) conglomeratic flysch sequence at Bathust Harbour, SW Tasmania.
Creators: name: Williams, Peter Roderick
Publisher: name: University Of Tasmania
Year: 2023

description: Low grade metamorphic rocks of Late Proterozoic age overlie metasedimentary rocks which have been metamorphosed to the garnet zone of the greenschist facies. The boundaries between the two sequences are faulted. The low grade metamorphic rocks form the Clytie Cove droup, which Consists of a pile Of remobi1iSed clastic detritus up . to.2000 m thick.. It Was deposited in a fault-controlled basin which was deepening as the pile accumulated, resulting in a thinning- and fining-upwards transgressive sequence. Individual cycles of deposition were established largely by the location and movement of main channels which descended from the strand line, and these are represented by depositional units in which the thickness of coarse-grained beds reduces upwards in the unit. Sediment accumulation in regions where these channels shallowed are represented by units in which the bed thickness increases upwards. Symmetrical bed thickness variations form in areas of slowly laterally migrating shallow channels. Conglomerate beds are common in each of these environments and possess very similar lithological traits. Sequences represented by thinning, thickening and symmetrical trends can'be distinguished lithologically only by the relative abundance of lithological types present in thesequences. Bed thickness periodicity, determined by power spectral analysis, also differs in the three trends. As a consequence an analysis of palaeoenvitonment could not be made by studying lithofacies variations of.the conglomeratic flysch sequences, alone. The mode of emplacement of conglomerate types also constrains the environmental model built up from sequence trend variations. A simple submarine fan model is not adequate to account for the deposits of the Clytie Cove Group and a model is preferred of multi-channeled flow into an elongated basin which is proximal to a strandline slope controlled and maintained by faulting, with deposition by flows caused by a change of slope. The model is favoured largely because of the lithological constraints on the submarine fan model. The sequence has been deformed during at least three compressional phases, producing an east-west trending fold system which was over-printed by a fold system trenching north-west. Later events produced box-folds and crenulation cleavages trending eastwest. These folding events were preceded by deformation and metamorphism in the underlying metamorphosed sediments. In a basin of such antiquity with a record of polyphase deformation, the analysis of the structural geology of the region was used to establish the juxtaposition of elements Within the basin as sedimentary and structural. An understanding of deformation was required to determine the nature of early, pre-folding changes in the rocks which yielded valuable information regarding palaeoslope and basin stability during deposition. The structural geology of the surrounding, metamorphosed rocks was used to time relationships of faulting, folding and basin formation in relation to orogenic events documented in other parts of Tasmania, and hence the likely tectonic setting of the basin. The structural, elements also determine the elongation of the basin and hence controlled the palaeogeographic interpretation.