Early Cambrian volcanic and palaeoenvironmental evolution of eastern Australia

Early Cambrian tectonics of eastern Australia was characterised by the transition from a passive margin to a convergent regime with associated development of a volcanic arc system. This interval coincided with the Cambrian Explosion—the geologically sudden appearance of all major animal body plans. In South Australia, lower Cambrian successions in the Stansbury and Arrowie basins are stratigraphic archives that preserve evidence for diverse fossil faunas that flourished along the eastern margin of Gondwana, and the dynamic palaeoenvironments they inhabited. Sandwiched within these marine and marginal marine successions are distal volcanics—key for mapping the tectonically-driven palaeoenvironmental and palaeogeographic evolution of this region.

Proximal and distal volcanics from South Australia (SA) and western New South Wales (NSW) have been CA-TIMS dated to establish precise marker horizons. These dates link distal volcanics with their likely proximal equivalents in South Australia and the Gnalta Shelf in western NSW. In SA, a tuff from the lower part of the Parara Limestone in the SYC 101 drill core in the western Stansbury Basin has been dated to 517.5±0.2 Ma (Castle-Jones et al., in review) which is within error of a CA-TIMS date of 517.41±0.15 Ma from the Marne River Volcanics in the eastern part of the basin (Curtis, in prep.). Tuffs from the Mernmerna Formation in the Arrowie Basin have been dated to 515.38 ± 0.13 Ma (Big Green Tuff), 514.56 ± 0.13 Ma (Third Plain Creek Member), and 514.46 ± 0.13 Ma (Paralana 1B DW1 drill core) (Betts et al., 2018). These ages correspond closely to the 514.96 ±0.14 Ma tuff from Cymbric Vale Formation, western NSW (Betts et al., 2024). The Billy Creek Formation tuff in the Arrowie Basin, dated to 511.87 ±0.14 Ma (Betts et al., 2018), is slightly younger than the Ma Mooracoochie Volcanics in the Warburton Basin to the north (Curtis, in prep.).

Changes in volcanic regime over time accompanied profound changes in basinal palaeogeography, sedimentation and faunal composition in eastern Australia during the early Cambrian. This study shows how geochronology, accompanied by rigorous petrographic, biostratigraphic and geochemical data are important for resolving how tectonic evolution impacted nascent ecosystems along the early Cambrian margin of eastern Australia.

References

Betts, M.J., et al. 2024. First multi-proxy chronostratigraphy of the lower Cambrian Byrd Group, Transantarctic Mountains and correlation within East Gondwana. Gondwana Research 136, 126-141.

Betts, M.J., et al. 2018. Early Cambrian chronostratigraphy and geochronology of South Australia. Earth-Science Reviews 185, 498-543.

Castle-Jones, J., et al. in review. Integrated biostratigraphy, chemostratigraphy and geochronology of the lower Cambrian succession in the western Stansbury Basin, South Australia. Australian Journal of Earth Sciences.

Curtis, S., in prep. The Delamerian Orogen: Insights into a rapidly evolving convergent continental margin from the timing and petrogenesis of igneous rocks. PhD thesis. University of South Australia