DESCENDING INTO THE “SNOWBALL”: IMPROVING INTERPRETATIONS OF TONIAN AND CRYOGENIAN PALAEOENVIRONMENTS WITH DETAILED SEDIMENTOLOGY AND GEOCHEMISTRY

The Tonian–Cryogenian transition represents a period of significant physiochemical change in Earth history. It involved variations in oceanic and atmospheric oxygenation, significant changes in the biosphere, tectonic reorganisation, and the onset of the global ‘Sturtian’ glaciation. Tonian and Cryogenian sedimentary rocks in the Adelaide Superbasin, South Australia (SA), represent some of the most significant and well-exposed sections of this interval globally, recording major environmental shifts through distinct variations in lithology and isotope chemistry. Although this transition is geologically significant, it remains enigmatic due to a distinct lack of comprehensive, contemporary Tonian–Cryogenian research in South Australia.

We present robust palaeoenvironmental interpretations for a complete pre- to post- glacial succession near Copley in the northern Flinders Ranges, SA. During fieldwork, a ~3km sedimentary log was measured for facies and sequence stratigraphic analyses, and 350 samples were collected for elemental and isotopic geochemical analyses. Our study reveals multiple regressive–transgressive cycles, recorded by deltaic rippled and cross-stratified sandstones, through lagoonal intraclastic magnesite and stromatolitic carbonates, to subtidal laminated siltstone and platform carbonates. These pre-glacial formations are unconformably overlain by subglacial to ice contact pebbly diamictites with quarzitic and dolomitic interbeds, which grade into proglacial laminated mudstone and sandstone with dropstones. We suggest that these facies reflect glaciomarine conditions. The post-glacial formation consists of subtidal laminated shales and carbonates, reflecting widespread transgression after the glaciation. Elemental chemistry, along with C and Sr isotope analyses support preservation of primary basin water chemistries for significant parts of the preglacial sequence. Furthermore, radiogenic (87/86) and stable (88/86) Sr data are interpreted to reflect a restricted setting analogous to the modern Coorong lagoon. This high-resolution study presents new palaeoenvironmental insights into a key Tonian–Cryogenian succession, which sheds light in our understanding of how the world descended into one of the most severe glaciations ever recorded.