Palaeoenvironments of the Tonian to Cryogenian transition in the Adelaide Superbasin

Abstract

The Neoproterozoic (~1000 to 539 Ma) was one of the most dynamic eras in Earth history and was punctuated by significant tectonic, biological, atmospheric, and climatic events. This includes: the Neoproterozoic Oxygenation Event, the breakup of Rodinia and amalgamation of Gondwana, and extreme climatic shifts associated with an extreme icehouse (referred to as “Snowball Earth”). The transition from the Tonian (ca. 1000–720 Ma) to the Cryogenian (ca. 720–635 Ma) was marked by the onset of the Sturtian glaciation, which was the first, and arguably most intense, glaciation during this period. These global changes facilitated the development of distinct lithologies with geochemical signatures that correspond to a suite of depositional conditions, from warm shallow carbonate-rich seas to cold, ice-covered continents. The Neoproterozoic sedimentary rocks of the Adelaide Superbasin, in South Australia, record the variability and distribution of these environments through this unique time in Earth’s past. Despite the geological significance of this basin, it remains understudied. Historical research has established broad characterisations of Neoproterozoic successions across the basin. However, these regional studies lack resolution and contribution from modern analytical methods. More contemporary research of these Neoproterozoic rocks in South Australia commonly investigates particular formations or locations, which provides the detailed insights of stratigraphic horizons but lacks the temporal and/or spatial continuity provided from the aforementioned studies. Additionally, there has been considerable advances in research from other Neoproterozoic basins globally, including those in North America, Africa, Asia, and Europe. This further highlights the need for new, comprehensive research in South Australia to better constrain the temporal and spatial variability across the basin, and how this correlates with other coeval basins. This thesis presents sequence stratigraphic and geochemical analyses of the Tonian to Cryogenian across the northern Flinders Ranges in order to address this subject. Facies analyses reveal deposition in a carbonate platform setting for the pre-glacial succession, represented by carbonate and clastic lithologies. Deposition is controlled by basin geometry and proximity to uplifted source areas, where the topographic variability was largely fault-controlled and possibly constrained by salt diapirism. Carbonate samples in the pre-glacial succession record a shift from semi-restricted, dysoxic waters to a more restricted setting. This transition coincides with several negative carbon isotope excursions, demonstrating the fluctuating bioproducitvity that was governed by relative water depth and sediment supply. A glaciomarine setting was interpreted for the syn-glacial succession, characterised by two glacial advance-retreat phases and significant variations in thickness across the basin. This was likely the result of contemporaneous faulting and glacial erosion that occurred with climatically driven regression. At the end of the Sturtian glaciation, the basin experienced subsidence and widespread transgression into a more distal setting. The post-glacial succession records a geochemical pivot to more open, oxic to suboxic conditions that was stimulated from increased nutrient input associated with the widespread melting of continental glaciers. Consequently, this multifaceted approach provides a comprehensive framework for what controlled the distribution of depositional environments during this critical and unique time in Earth’s past.