Two Cryogenian glacial successions compared: Aspects of the Sturt and Elatina sediment records of South Australia

Abstract

Two Cryogenian glacial sequences, the Sturt and Elatina, occur within the Adelaide Rift Complex, S Australia, and are part of an ∼8km thick succession recording Rodinia fragmentation and subsequent passive margin development. We compare and contrast these glacial successions, presenting new data, including detailed logged sections, for both glacial sequences. Facies analysis reveals a total of six facies associations, recording a range of glacial and glacially related processes. These are a diamictite facies association (ice-proximal debris flows), a sheet sandstone facies association (fluvial deposits), a flaser and rhythmically bedded facies association (tidal flat), a hummocky cross-stratified sandstone facies association (shoreface to offshore transition zone deposits) and an underflow facies association (recording offshore deposition). A sixth facies association, a ferruginous lonestone-bearing siltstone, is restricted to the top of the Elatina Formation and records rapid environmental amelioration and emergence from glaciation. In the central Flinders Ranges, the Sturt succession commences with a glacial erosion surface (GES), which in agreement with previous workers, was probably enhanced by rifting. This GES is overlain, in ascending order, by the Pualco Tillite, Holowilena Ironstone and Wilyerpa Formation, with the Warcowie Dolomite Member at the base of this latter formation. A second GES is preserved within this latter unit, and is overlain by a stratigraphic motif that records an alternation of offshore transition zone and offshore deposits. Significant base level falls within the Wilyerpa Formation are identified by fluvial deposits sitting disconformably upon offshore deposits. In contrast, within the Elatina Formation, a single GES is proposed, which defines the base of the formation. The Elatina Formation is split into two informal members on the basis of analysis of sediment stacking patterns and the recognition of a sharp stratigraphic surface that divides the formation. Apart from a locally preserved basal diamictite, the lower member consists of fluvial deposits, whereas the upper member is dominated by a mixture of tidal flat sediments and debris flow deposits. The boundary between these units is therefore interpreted as a hybrid surface recording tidal ravinement in some areas and a glacial advance in others. Deglaciation left contrasting deposits: the Sturt deglaciation culminated in deposition of black shale of the Tindelpina Shale Member of the Tapley Hill Formation, seemingly as a sheet-like deposit over the Adelaide Rift Complex and beyond into the Amadeus Basin of central Australia. The Elatina deglaciation, meanwhile, began first with deposition of the sixth facies association (ferruginous dropstone-bearing siltstone), then proceeded with deposition of cap dolostone of the Nuccaleena Formation.