Descending into the “snowball”: High resolution sedimentological and geochemical analysis across the Tonian to Cryogenian boundary in South Australia

Abstract The Tonian–Cryogenian transition (ca. 720 Ma) represents a period of significant environmental change in Earth history, involving variations in oceanic and atmospheric oxygenation, significant changes in the biosphere, tectonic reorganisation, and the onset of the global ‘Sturtian’ glaciation. South Australia has some of the thickest, continuous and best exposed sections of this unique interval globally. Here we present detailed palaeoenvironmental interpretations for a complete, ca. 3 km thick, pre- to post-glacial succession near Copley in the northern Flinders Ranges, South Australia. Elemental concentration data, complemented by screening for diagenesis, demonstrates the preservation of marine or primary REE signatures for studied carbonate samples and supports the proposed sedimentological and palaeoenvironmental interpretations. Multiple Tonian regressive-transgressive cycles are defined, which are recorded by deltaic rippled and cross-stratified sandstones (Copley Quartzite), through inner platform intraclastic magnesite and stromatolitic carbonates (Skillogalee Dolomite), to subtidal laminated siltstone and platform carbonates (Myrtle Springs Formation). The REE patterns from carbonate samples in the Skillogalee Dolomite and Myrtle Springs Formation indicate low Y/Ho, slight light rare earth element (LREE) depletion, weak negative Ce/Ce* and high Eu/Eu*. This suggests a nearshore, dysoxic setting fed by anoxic deep waters and more oxic shallow waters. In combination, the sedimentological and geochemical data build a picture of a partially restricted, shallow marine to lagoonal setting for the northern Flinders Ranges directly before the climate pivot to the Sturtian glaciation. These pre-glacial formations are unconformably overlain by Cryogenian subglacial to grounded ice-margin pebbly diamictites with immature, massive sand interbeds (Bolla Bollana Tillite). We suggest these facies reflect glacial grounding-line advance and retreat in a glaciomarine setting. These grade into turbiditic laminated sandstone and mudstone with dropstones (Wilyerpa Formation), which were likely deposited at the onset of deglaciation in a subaqueous proglacial environment. The post-glacial succession (Tapley Hill Formation) consists of subtidal laminated shales and carbonates, which are represented by increased Y/Ho, moderate LREE depletion, slight negative Ce/Ce* and low Eu/Eu*. This significant geochemical shift to a more open, oxic to suboxic subtidal setting coincides with widespread transgression and relative sea level rise after one of the most severe glaciations ever recorded.