DOI: 10.26186/5c63c43259b6c
title: Geology of the southern McArthur Basin, Northern Territory

description: The McArthur Basin contains mainly mid-Proterozoic sedimentary rocks that form a platform cover sequence near the eastern edge of the North Australian Craton. The rocks are gently folded and faulted, unmetamorphosed, and appear to have been deposited in mostly shallow environments in an intracratonic basin, which at times was dominated by a prominent north-trending half-graben-the Batten Trough. The sequence in the southern half of the basin, the subject of this Bulletin, is divided 
by regional unconformities into four stratigraphic groups. The oldest, the Tawallah Group (about 4500 m thick), unconformably overlies crystalline basement that is about 1800 Ma old. Most of the group consists of thick formations of resistant quartz sandstone alternating with much thinner formations of deeply weathered basic volcanics and fine-grained elastics. The oldest rocks are arkosic and conglomeratic, and mainly of continental origin. They are succeeded by monotonous, probably marine and aeolian orthoquartzites. The basic volcanics are poorly known, but include subaerial flows, intrusions, and volcaniclastics accompanied by iron formations and other redbeds. Rare units of stromatolitic dolostone similar to those higher in the basin succession are also present. The uppermost units are potassium-rich felsic igneous rocks which host the copper-bearing breccia pipes at Redbank, in the southeast. The unconformably overlying McArthur and Nathan Groups (combined thickness about 5500 m), which are separated by an unconformity, are dominated by formations consisting of evaporitic and stromatolitic cherty dolostones interbedded with dolomitic fine-grained elastics. The formations, which are of shallow water origin and contain evidence of exposure and desiccation, were deposited mainly in peritidal, lagoonal, lacustrine, and possibly fluvial environments. A wide variety of evaporite pseudomorphs, solution-collapse breccias, and desiccation features, suggest dominantly arid climates. Tuffs in about the middle of the McArthur Group have yielded a U-Pb age of 1690125 Ma. The Roper Group (up to 2000 m thick), unconformably overlying the Nathan Group, consists of alternating resistant quartz sandstone and recessive micaceous siltstone and shale at least 1400 Ma old. Previous studies have identified three coarsening-up megacycles, which may represent major prograding episodes. Thin Cambrian and Cretaceous rocks (parts of other basin sequences) form discontinuous outliers in the west and south. The structure of the southern McArthur Basin is dominated by the Batten Fault Zone (the site of the earlier Batten Trough), an eastward-deepening half-graben now expressed as a horst containing up to perhaps 12 km of sedimentary rock; the shelves either side of it contain only about 4 km of rock. During part of its history the basin sediments were deposited in pull-apart style sub-basins related to major northwest trending right-lateral wrench-faulting. Although no mines are presently operating, the McArthur Basin shows promising base-metal potential. The large shale-hosted McArthur River Pb-Zn deposits are well known. Other prospects include 1) both discordant-vein and disseminated deposits of Pb-Zn, Pb-Ba, and Cu associated with karstically weathered rocks beneath unconformities, 2.) Cu-bearing breccia pipes, 3) stratabound disseminated Cu, 4) pisolitic Fe, and 5) U. More recently a promising hydrocarbon potential has been identified: organic-rich shales occur at several levels; gaseous and solid hydrocarbons have been encountered during drilling; appropriate maturation levels have been indicated; and suitable reservoirs may occur within vuggy carbonates or among the extensive 
quartz arenites.