Subdivision and correlation of late Precambrian sequences in Australia

Abstract

The diversity of the Australian Precambrian record is used to empirically assess the application of all available techniques of correlation of late Precambrian rocks — lithofacies, isotopic ages, palaeontology, and palaeomagnetism — within the framework of latest lithostratigraphic/geochronologic correlation charts. Lithostratigraphy is the main tool for intrabasinal correlation and mapping, but isotopic ages will continue for some time yet to provide the main criteria for interregional correlation. Lithostratigraphic correlation is demonstrated over hundreds of kilometres, but is generally constrained by tectonic controls to individual major crustal blocks. Climatic markers, such as tillites, may be correlated interregionally over thousands of kilometres. The most exciting recent development in platform geochronology is the application of U—Pb zircon dating to fine-grained tuffs. Illite may provide reliable minimum estimates, but can be shown to be up to 150 Ma too young. Glauconite and Rb—Sr total-rock shale ages have provided useful broad age estimates of sequences for which better methods are unavailable, but only when based on cautious assessment and multiple sampling; their ages may be either minimum or maximum. Stromatolites continue to demonstrate their potential for broad correlation when based on rigorous taxonomic study, but the framework of time ranges of known taxa must be expanded for stromatolites to achieve their full potential, especially for rocks older than 1000 Ma. Acritarchs in black shales provide diverse and complex assemblages, with excellent correlation potential, but once again the Australian taxonomic framework is still inadequate. Black cherts contain an evolutionarily conservative biota, with only limited correlation potential. Recent palaeomagnetic studies, based on both APWP and magnetostratigraphy, demonstrate excellent potential for detailed correlations, down to at least 1700 Ma old. However, the present chronologically-calibrated data base is sparse and must be supplemented by many more detailed studies through thick continuous rock sequences, spanning substantial intervals of time. The value of multidisciplinary correlation is elegantly demonstrated by very detailed and unambiguous correlation of combined palaeomagnetic, isotopic age, and stromatolite data across the 1700 Ma McArthur Basin. Previous regional lithostratigraphic correlations have been conclusively and significantly revised, with major implications to regional palaeogeography.