The Cryogenian record of glaciation and deglaciation in South Australia

This paper reviews the state of our current knowledge of the Cryogenian glacial and deglacial sedimentary records of South Australia. Two, distinct, glacial intervals are recognised within rocks of Cryogenian (850–630 Ma) age, separated by an interglacial succession that by definition lacks glacial facies. The glacial successions were laid down during the Sturt and Elatina glaciations. Each of these glaciations is arguably of global extent under the Snowball Earth hypothesis (Hoffman and Schrag, 2002). Evidence for glaciation has been known for over one hundred years from the Sturt Tillite (Howchin, 1900). When compared like-for-like in a systematic facies analysis, both glacial successions show some similarities, notably the occurrence of diamictites representing glacial deposition and slope reflux, and dropstone-bearing intervals, but also show some dramatic differences. Notably, deposition of the Sturt succession was strongly influenced by extensional tectonics, for which evidence is provided by dramatic thickness variations either side of major faults and palaeocurrent data. Sub-basins formed during regional extension contain diamictites 4–6000 m thick (e.g. the Yudnamutana and Baratta “troughs” in the northern and central Flinders Ranges respectively). Deposition of the younger, Elatina glacial deposits was locally influenced by diapirism in the central Flinders Ranges, but otherwise these younger deposits seem to maintain a relatively constant thickness of < 100 m, although exceptionally, thicknesses of up to 1500 m occur in basinal sections (Williams et al., 2008). The emergence of South Australia from the Sturt and Elatina glaciations culminated in black, silty shale and rather complex “cap-dolostones” respectively- two contrasting styles of deglaciation.