Stratigraphic framework for the Leichhardt and Calvert Superbasins: review and correlations of the pre-1730 Ma successions between Mt Isa and McArthur River
Jackson, MJ and Scott, DL and Rawlings, DJ, Stratigraphic framework for the Leichhardt and Calvert Superbasins: review and correlations of the pre-1730 Ma successions between Mt Isa and McArthur River, Australian Journal of Earth Sciences, 47, (3) pp. 381-403. ISSN 0812-0099 (2000) [Refereed Article]
New stratigraphic, geochemical and palaeomagnetic data from the Peters Creek Volcanics are used to revise the correlations of part of the Palaeoproterozoic of northern Australia. The revised geological history for these cover rocks of the Murphy Inlier is extrapolated into the 1800-1700 Ma successions of the McArthur Basin and Mt Isa regions. New stratigraphic subdivisions and relationships are contrasted with the established lithostratigraphic schemes and also with conflicting published tectono-stratigraphic interpretations. For the first time, a pletora of stratigraphic units can be rationalised into two major superbasins, the Leichhardt and Calvert Superbasins, and into eight pseudo-chronostratigraphic basin phases (Associations A-H). There are few absolute age constraints, but lateral correlations of the units in these eight basin phases are proposed. Results from the overlying Isa Superbasin (<1670 Ma) suggest that these eight associations probably represent second-order supersequences. Mixed non-marine and marine coarse clastics, deposited between about 1790 and 1780 Ma dominate Associations A and B. In the Mt Isa region these were deposited in an initial rift then a thermal relaxation or sag phase. To the northwest, however, the succession is dominated by rift facies. Association C is a widespread flood basalt and immature clastic suite that was deposited in clearly defined, north-trending half-grabens in the Mt Isa region. Along the southern edge of the Murphy Inlier, however, geophysically defined half-grabens, filled with magnetic rocks (basalt), trend orthogonal to those at Mt Isa. North of the inlier Association C is much thinner, and little can be deduced about its palaeogeography. Association D is only present in the Mt Isa region as the Myally Subgroup. Differing views on its tectonic setting and environments of deposition, as presented in recent papers, are reviewed. Association E, deposited around 1755 Ma, is a regional sag phase with mixed clastic-carbonate, shallow-marine lithofacies in all areas. There is a major gap in the rock record between about 1750 and 1735 Ma which is probably related to widespread basin inversion. The Mid-Tawallah Compressional Event (McArthur River area) and the Wonga Extension Event (Eastern Succession, Mt Isa) are both about this age. The overlying Association F is a thin, laterally uniform, upward-fining succession that commences with shallow-marine clastics and evolves through deeper marine clastics and ends in evaporitic facies. There are broad similarities between Associations F and E so interpretation as a third regional sag is favoured. The absence of Association F at Mt Isa may indicate that basin inversion was longer lived in the southeast. The youngest associations, G and H, are complex interstratified mixtures of felsic-mafic igneous rocks and immature clastics. U-Pb zircon SHRIMP ages appear to cluster around 1725 Ma and 1710 Ma, but they may all be part of one thermal event. These eight associations may represent the tectono-magmatic response of the lithosphere during and after the Strangways Orogeny (1780-1730 Ma).