Proterozoic and Early Cambrian trace fossils from the Amadeus and Georgina Basins, central Australia

Abstract

The latest Proterozoic to Early Cambrian sequences of the Amadeus and Georgina Basins of central Australia have yielded abundant and diverse trace fossils. Those of the Arumbera Sandstone are particularly significant because recent studies of that unit have shown that it is lithologically uniform and spans the Proterozoic-Cambrian boundary with no discernible break. In our studies of this and other units we have been able to distinguish four assemblages of trace fossils forming a succession that we interpret as due to the evolution of bioturbating animals. The lowest strata examined occur immediately above the upper of the two Proterozoic glacial units that are widespread in Australia and comprise the Grant Bluff and Elyuah Formations of the Georgina Basin and the lower Pertatataka Formation (Cyclops Member and below) of the Amadeus Basin. These units lack definite trace fossils, despite having lithofacies comparable with those containing traces higher in the sequence. The probable fossil medusa Bunyerichnus dalgarnoi occurs at this level in the Adelaide Geosyncline, but no trace fossils are known from equivalent strata anywhere in Australia. The lack of trace fossils is significant and we draw attention to it by defining this as Assemblage 0. Assemblage 1 consists only of Planolites ballandus from the lower Elkera Formation of the Georgina Basin. This ‘assemblage’ predates the Ediacara fauna. Assemblage 2 contains trace fossils that in the Adelaide Geosyncline occur with the Ediacara fauna. Vertical burrows are rare in this assemblage although depressions comparable to the lower ends of U-shaped burrows have been recorded from the Adelaide Geosyncline and vertical pit-shaped traces (Hormosiroidea arumbera ichnosp. nov.) are reported here from the Central Mount Stuart Formation, Georgina Basin. Simple horizontal trails and burrows (Planolites sp.), chains of faecal pellets (Neonereites sp.) and arthropod scratch marks (Monomorphichnus sp.) also occur in this assemblage. Assemblage 3 is distinguished by its abundance and diversity of taxa, many of which are large and conspicuous — 35 ichnofossil taxa are recognised. This assemblage is characterised by the presence of deep vertical and complex subhorizontal burrows. It includes the following traces: Curvolithus aequus ichnosp. nov., Curvolithus sp., Didymaulichnus lyelli, D. miettensis, Gordia arcuata, Gordia sp., Gyrochorte sp., Helminthopsis irregularis, Muensteria sp., Nereites sp., Palaeophycus alternatus, P. canalis ichnosp. nov., P. tubularis, Phycodes pedum, Plagiogmus arcuatus, Planolites ballandus, P. beverlyensis, Torrowangea rosei, Treptichnus sp., Diplocraterion parallelum, Diplocraterion sp., Hormosiroidea arumbera ichnosp. nov., Skolithos ramosus ichnosp. nov., S. verticalis, Diplichnites sp., Monomorphichnus bilinearis and M. lineatus. Assemblage 3 occurs in the Uratanna, Parachilna and Lintiss Vale Formations of the Adelaide Geosyncline, Arumbera Sandstone members 3 and 4 in the Amadeus Basin, and the Donkey Creek beds and Mt Baldwin Formation in the Georgina Basin, amongst other units. There is sufficient sedimentological information available to indicate that the succession of assemblages cannot be attributed to environmental changes or to the colonisation of new habitats, and is best interpreted as due to evolution. Very similar successions occur worldwide at this time. Comparisons with all major sections from which trace fossils have been described, including key sections in southern China, the Baltic region and Newfoundland, indicate that Assemblage 3 is Early Cambrian (Tommotian to early Atdabanian) and Assemblages 0–2 are latest Proterozoic (Ediacarian, Vendian).