A MID-SNOWBALL CAP CARBONATE? THE STURTIAN WARCOWIE DOLOMITE OF SOUTH AUSTRALIA

The early Cryogenian Sturtian glaciation is likely the most extreme ice age in Earth’s history. At over 55 million years long, this extensive glaciation poses a challenge for the survival of the life. The Sturtian succession of the Flinders Ranges, South Australia is exceptionally preserved and hosts up to several kilometres of glacial stratigraphy, providing a window into Cryogenian glacio-marine environments. In the Southern Flinders Ranges, newly measured sections record several intervals of glaciation and at least one significant unconformity. The lower Pualco Tillite records a dominantly ice-proximal sequence of dolomitic diamictite representing an early phase of Sturtian glaciation that is rarely preserved elsewhere in South Australia. This unit is transitionally overlain by the Holowilena Ironstone, which includes iron- and phosphorous-rich siltstones with rare dropstones. This unit can also host a network of unusual sedimentary dykes and sills filled with carbonate diamictite or laminated carbonate, interpreted to be a fracture system developed under an ice sheet. Above this, the Warcowie Dolomite Member of the Wilyerpa Formation is a laminated dolomite or dropstone-rich dolomite unit which may be up to 20m thick and strongly resembles a post-glacial ‘cap carbonate’. Where preserved, the sediments which overlie this unit may include several hundred metres of siltstones with no evidence for glaciation, which onlap antiformal structures in the Holowilena Ironstone. New work on these siltstones demonstrates a cyclic pattern in their geochemical composition, particularly in carbonate-content. Their onlapping relationship to the Holowilena paleo-high potentially indicates a significant time-gap, which may correlate with a regional unconformity. We interpret this Warcowie-Wilyerpa interval as an extensive interglacial episode within the Sturtian ice age, with areas of open ocean. Cyclicity may have been driven by distal ice volume changes due to orbital forcing prior to ice advance in this deep-marine setting. The remainder of the Wilyerpa Formation is characterised by ice-distal, sandy and shaley units with abundant dropstones. This research adds to growing evidence for a dynamic and complex Sturtian glaciation with a significant mid-Sturtian ice-free interval.