Geology and tectonic setting of the Tawallah Group, southern McArthur Basin, Northern Territory

The southern McArthur Basin compnses a Late Palaeoproterozoic to Mesoproterozoic sequence of clastic sediments, evaporitic carbonates, basaltic and rhyolitic volcanics and mafic intrusions that crop out between the Urapunga Tectonic Ridge and the Early Proterozoic Murphy Inlier. Within the NNW-trending Batten Fault Zone, the four recognised stratigraphic groups (Tawallah, McArthur, Nathan and Roper Groups) attain a maximum collective thickness of 12 km. The basal Tawallah Group unconformably overlies the Early Proterozoic Scrutton Volcanics, and comprises the lower 7 km of southern McArthur Basin stratigraphy. In the study region, a series of N- to NNWtrending fault blocks contain well-exposed sections of the Tawallah Group succession. Three main cycles of sedimentary depositi6n and igneous activity are recognised in the . Tawallah Group, with each cycle marking major changes in the basin architecture and palaeo-environmental conditions. The basal Tawallah Group Package represents a transgressive transition from alluvial braidplain (Yiyintyi Sandstone and Sly Creek Sandstone) through braid-delta (Rosie Creek Sandstone) to shallow subaqueous (Aquarium Formation) depositional environments. A period of widespread and largely subaerial mafic flood volcanism (Seigal Volcanics) separated Yiyintyi and Sly Creek Sandstone deposition. A second extension/subsidence depositional cycle and transgressive phase (middle Tawallah Group package) incorporates the Wununmantyala Sandstone (braidplain) and Wollogorang Formation (shallow subaqueous). Mafic magmas intruded the lower and upper Wollogorang Formation lithologies during this cycle to form the Settlement Creek and Gold Creek Volcanics respectively. The depositional architecture of the middle Tawallah Group package was locally controlled by intrabasinal tectonic uplift of lower Tawallah Group and Scrutton Volcanics stratigraphy along the Tawallah Fault. The upper Tawallah Group package marks a regressive transition from nearshore (Warramana Sandstone) to braided river depositional environments (Nyanantu Formation), with local subaerial felsic volcanism (Tanumbirini Rhyolite). Elevation of the ambient geothermal gradient and the addition of mafic material to the crust by underplating and intrusion during extension are tentatively proposed as uplift mechanisms for this regressive transition. The Seigal Volcanics, Settlement Creek Volcanics and Gold Creek Volcanics have sub-alkaline tholeiitic geochemical compositions. Ti-Y-Nb-Zr relationships are consistent with genesis in a within-plate tectonic setting. A marked Nb-Ta trough on multielement variation patterns supports a continental flood basalt affinity for the Tawallah Group mafic suite. Zr/Nb, Y INb and REE relationships indicate that the Settlement Creek and Gold Creek Volcanics were derived from a more enriched mantle source than the Seigal Volcanics. Progressively lower degree partial melting of an asthenospheric mantle source in response to slower rates of tectonic extension is proposed to explain this trend towards more enriched compositions. Deformation in the study region is characterised by complex brittle faulting along primary N-striking faults (Tawallah, Lorella and Bauhinia Faults). Fault zones in the Tawallah Group formations are typified by narrow deformation zones containing cataclasite, hydraulic breccia and wall rock alteration (silicification). Palaeostress analyses of fault-slip data led to the recognition of three main compressional deformation events. D1 (E-W compression) occurred during the final stages of Aquarium Formation deposition and locally controlled later sedimentation patterns. A change in the rate of tectonic extension following D1 caused lower degree partial melting of asthenospheric source regions, producing the 'enriched' geochemical compositions of the Settlement Creek and Gold Creek Volcanics. D2 (NW-SE compression) involved sinistral strike-slip displacements along the Tawallah and Lorella Faults, and is correlated with synmineralisation deformation at the McArthur River Pb-Zn deposit. D3 (NE-SW compression) post-dated Roper Group deposition, and is characterised by dextral strikeslip deformation adjacent to the Tawallah and Lorella Faults, and NE-directed reverse displacement on the Bauhinia Fault. Quartz-hosted fluid inclusions in D3 hydraulic fault breccias contain moderate temperature (160 to 210°C), saline (8-13 eq wt.% NaCl) and oxidised fluids. An intracontinental tectonic setting is proposed for the southern McArthur Basin. The basin architecture was characterised by WNW -trending rift compartments that were bound by NNW -striking transfer faults during the early stages of basin evolution (ie. deposition of the Tawallah Group). Termination of the initial rifting stage was followed by deposition of carbonate- and evaporite- dominated McArthur and Nathan Groups in a nonvolcanic, sag-phase tectonic setting. Post -Roper D3 compression marked the conclusion of Proterozoic sedimentation in the southern McArthur Basin.