Chapter 67 Neoproterozoic glacial deposits of central Australia

Abstract There are two distinct stratigraphic levels of Neoproterozoic glacigenic deposits in central Australia, both Cryogenian in age, spread over an area greater than 2.5×10 6 km 2 . They were deposited in a once continuous intracratonic sag basin and are now preserved in four major structural basins: the Officer, Amadeus, Ngalia and Georgina Basins. In all four basins there are units that correlate with the (older) Sturt Tillite and equivalent glacial deposits in the Adelaide Rift Complex – the Sturt glaciation (Preiss et al. 2011) – and with the (younger) Elatina Formation (Fm.) and equivalents – the Elatina glaciation (Williams et al. 2008, 2011). The clearest evidence for glacial activity is the occurrence of diamictites that contain clasts of lithologically diverse origin which are often striated, faceted and polished, and the occurrence of dropstones. Most glacial deposits were deposited in shallow marine to fluvio-lacustrine palaeoenvironments. In all basins, the Elatina glacial deposits are overlain (at least locally) by dolostone units that mark the onset of post-glacial transgression and contain unique sedimentary and geochemical features. The cap dolomite units are distinct from dolomite beds within glaciogenic sediments, and those that occur near the top of Sturt glacial units in the Amadeus (Areyonga Fm.) and eastern Officer Basins (Chambers Bluff Tillite). None of the central Australian glacial units have direct geochronological constraints. There are, however, radiometric dates for a Sturt glacial unit in the Adelaide Rift Complex (Wilyerpa Fm.) and post-glacial shales in the Amadeus Basin (Aralka Fm.), Stuart Shelf (Tapley Hill Fm.) and Adelaide Rift Complex (Tapley Hill Fm.) that indicate a c. 660 Ma age for the Sturt glaciation in Australia (Kendall et al. 2006, 2007; Fanning & Link 2008). The age of the Elatina glaciation in Australia is constrained only by the age of the Sturt glaciation and the presence of the Ediacara fauna in overlying strata of all the basins except the Ngalia Basin. Consequently, correlations have been mainly established by means of lithostratigraphy, chemostratigraphy, palynology, and to a lesser extent, stromatolite biostratigraphy, mainly on the successions above and below the glacial units. Results from each of the above techniques show a remarkable consistency, and indicate that the two major Cryogenian glacial episodes are of similar age across Australia.