Sea ice-free conditions during the Sturtian glaciation (early Cryogenian), South Australia

In the Neoproterozoic snowball Earth hypothesis, shutdown of the planet's hydrological system has been attributed to a global ice cover during one or more extreme glaciations. In the central Flinders Ranges, South Australia, the Yudnamutana Subgroup of Sturtian age includes diamictite, sandstone, and siltstone units of glaciomarine origin as much as 5000 m thick, and is overlain by postglacial transgressive siltstone and shale of the Tindelpina Shale Member, Tapley Hill Formation. In the central Flinders Ranges, the Yudnamutana Subgroup consists of (1) the Pualco Tillite (gravity resedimented glacial deposits), (2) the Holowilena Ironstone (glacioturbidites), (3) poorly stratified pebbly diamictite of the Warcowie Dolomite Member, lowermost Wilyerpa Formation (gravity resedimented glacial deposits), succeeded by (4) siltstones and sandstones with abundant hummocky cross-stratification (HCS: storm deposits), and (5) a lonestone-bearing succession with cobble-sized clasts in the upper Wilyerpa Formation (ice-rafted debris interpreted to record a glacial re-advance) in which HCS is absent. Because the action of oscillating waves is required to produce HCS on the seafloor, its presence indicates an interval of significant meltback prior to glacial re-advance. Given that the HCS occurs approximately 2 km beneath the Tindelpina Shale Member, it signifies a major ice-free interval during the Sturtian glaciation.