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

Rogers, J 1996 , 'Geology and tectonic setting of the Tawallah Group, southern McArthur Basin, Northern Territory', PhD thesis, University of Tasmania,.

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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.

Item Type: Thesis - PhD
Authors/Creators:Rogers, J
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