Basin evolution and structural setting of a late Cambrian–Early Ordovician siliciclastic rift succession, central northern Tasmania, Australia

The late Cambrian–Early Ordovician siliciclastic sequence in central northern Tasmania represents an excellent case study to document the transition from an active rifting succession to a post-rift system. Initial rifting created a complex system of half-graben, providing accommodation space for large volumes of basement-derived material. These half-graben were initially filled with coarse-grained alluvial fan sediments and a thick, syn-rift, low-sinuosity, multiple-channel braided fluvial succession. The uppermost sequence comprises a post-rift, regional, shallow marine, transgressive sandstone and mudstone package.

Six broad lithofacies have been recognised in the late Cambrian–Early Ordovician siliciclastic sequence: Four lithofacies are interpreted to represent terrestrial alluvial fan (displaying debris flow, sheet-flow and channel-flow geometries) and braided fluvial deposits (transitioning between proximal and distal sedimentological characteristics upsection), while two represent marginal-marine (tidal) to shallow marine environments (dominated by heterolithic bedding, and varying degrees and styles of bioturbation). Adjacent lithofacies often display marked lateral variations in both thickness and grain-size, suggesting accelerated changes in the volume of sediment flux and/or the rate of tectonic subsidence, particularly of the basal, coarse-grained, conglomerate-rich sequences where the varying composition and texture suggests considerable relief on hinterland palaeotopography.

A revised stratigraphic framework for the late Cambrian–Early Ordovician siliciclastics of central northern Tasmania is presented based on lithofacies and lithofacies associations. The Roland Formation forms the lower stratigraphic unit and comprises generally fining-up, conglomerate-dominated alluvial fan and proximal, low-sinuosity, multiple-channel braided river successions associated with syn-rift sedimentation. A hiatus in the extensional tectonics is recorded by a regionally extensive unconformity between the Roland Formation and the overlying Moina Formation, named the intra-Owen Group Unconformity. The Moina Formation marks a transition from high energy, terrestrial, conglomerate-rich sequences of the Roland Formation, to more moderate energy sandstone-dominated sequences that typically display an increasing marine signature towards the top of the unit. Three distinct sedimentary successions are recognised within the Moina Formation, and member status has been given to each of these since the distribution and relationships between these stratal packages is fundamental in understanding the mature phase of the late Cambrian–Early Ordovician extensional event, and the post-rift transgressive system. The terrestrial braided fluvial, channelised sandstone and minor conglomerate of the Badgers Range Member are superseded by interbedded, fine- to medium-grained sandstone deposited in a tidal regime (Deloraine Member) that is in turn overlain by thinly bedded, heavily bioturbated, shallow marine mudstone and fine-grained sandstone (Caroline Creek Member).

Sedimentological principles, including provenance studies and palaeocurrent analysis, are used to document the basin configuration and structural framework of the rift system. The spatial and temporal migration of depocentres can be documented such as in the Badgers Range where two fining-up successions are separated by the intra-Owen Group Unconformity with the lower quartzite-dominated Roland Formation being succeeded by the chert lithic-rich Moina Formation. It is apparent that depocentres were compartmentalised by topographic highs, and the sedimentary fill of these depocentres reflects the lithological composition of these highs. In addition, the observed general decrease in grain-size is not uniform, and there are numerous changes in sedimentation as a consequence of fluxes in uplift and subsidence in the source and basinal areas.

The distribution of thickly-bedded, coarse-grained conglomerate sequences, the juxtaposition of differing lithological successions, and the construction of geological cross-sections give insights to the location and position of several major bounding faults. These faults were subsequently reactivated during the Early–Middle Devonian Tabberabberan Orogeny, and a significant amount of reverse movement is recorded on the basis of structural restorations.

Regional palaeogeographic reconstructions of the Early Ordovician (Tremadocian; 480 Ma) (Cocks, 2001) demonstrate that Tasmania was located outboard of the eastern margin of Gondwana, situated north of the palaeoequator between latitudes 10° and 20°. The late Cambrian through Ordovician tropical climate was dominated by the influence of a strong, extended greenhouse effect. The lack of sediment stabilisation by plants and rootlets in this Early Palaeozoic, vegetation-free, terrestrial landscape resulted in alluvial and fluvial depositional processes being markedly different from their present day counterparts, since vegetation exerts significant influence on a variety of environmental factors. In particular, energy levels and run-off rates would have been greater, fluctuations in energy levels more extreme, and high energy events more frequent. These conditions would likely have been amplified by the relatively high precipitation rates associated with the tropical palaeogeographic position of Tasmania. Likewise, wind processes would have played a far greater role than present with regards to the removal of clay- and silt-sized particles from the depositional setting, and this may explain the paucity of claystone and siltstone in these terrestrial environments.