The distribution of phosphatic facies in the Georgina, Wiso and Daly River Basins, Northern Australia

The lithological logging and chemical testing of cuttings from some 600 water bores and Bureau of Mineral Resources stratigraphic holes, together with the modelling of aeromagnetic, gravity and elevation data, have defined a carbonate-siltstone-chert phosphatic lithofacies of Middle Cambrian age. This in turn has led to the recognition of two new phosphorite deposits, the deposition of which was related to the basement configuration and its depth. The phosphatic lithofacies, which have been followed entirely in the subsurface, occur as belts peripheral to and within the Georgina, Wiso and Daly River Basins. These phosphatic belts have an average width of 32 km, a thickness of 10 to 190 m, and have been traced over a distance of 2,100 km. Of the newly discovered phosphorite deposits, the Lady Judith in the Wiso Basin rests on volcanic rocks and interdigitates with carbonates of the Montejinni Limestone, while the Ammaroo belt in the southwestern portion of the Georgina Basin is contained within a depression bounded by limestones of the Arthur Creek Formation. The phosphatic sediments are believed to have been deposited primarily as an Ordian Middle Cambrian event in the west with a 'younging' transition through Ordian and/or Early Templetonian to Late Templetonian in the southeast. The basins are extensional, exhibiting a series of broad downwarps crossed by peripheral aulacogens, grabens, half grabens formed in the Late Proterozoic and modified subsequently by the development of plateaux, narrow horst blocks and adjacent deeps during the Middle Cambrian along basin-dividing arches. The basement to the shallow-water phosphatic lithofacies consists of Proterozoic sediment or Early Cambrian volcanic plateaux or peripheral sloping platforms which in the Brunette Sub-basin have present elevations of 0-300 m ASL.