The Elatina glaciation, late Cryogenian (Marinoan Epoch), South Australia: Sedimentary facies and palaeoenvironments

Abstract

The late Cryogenian Elatina glaciation in South Australia, of Marinoan age, is named after the Elatina Formation of the glaciogenic Yerelina Subgroup, which covers ∼200,000 km 2 in the Adelaide Geosyncline and on the cratonic Stuart Shelf and is up to ∼1500 m thick. The Elatina glaciation is marked by numerous facies that are like those of Phanerozoic glaciations: • Basal diamictite displaying glacitectonites with penetrative deformation of preglacial beds, indicating grounded ice or scouring by icebergs. • Glaciomarine diamictites containing numerous faceted and striated clasts up to several metres across of intrabasinal and extrabasinal origin. • Laminated siltstone and mudstone containing scattered, ice-rafted dropstones in outer marine-shelf environments. • Sandstones deposited in fluvial, deltaic and inner marine-shelf settings. • Tidalites deposited in estuaries and on tidal deltas during interstadial rise of sea level, with cyclic tidal rhythmites recording the annual oscillation of sea level and displaying wave-generated ripple marks, which together indicate long-lived and extensive open seas. • Siltstone containing acicular crystal pseudomorphs implying the formation of evaporite minerals in littoral deposits. • Permafrost regolith of frost-shattered quartzite breccia ≤20 m deep, displaying large-scale periglacial structures including primary sand wedges 3 m deep that indicate a frigid, strongly seasonal climate near sea level. • Periglacial–aeolian sandstone covering 25,000 km 2 and containing primary sand wedges near its base. These deposits record a spectrum of settings ranging from permafrost regolith and periglacial sand sheet on the Stuart Shelf in the west, through fluvial, deltaic and inner marine-shelf in the western and central parts of the Adelaide Geosyncline, to outer marine-shelf in the north and southeast. The widespread and persistent rainout of fine-grained sediment and ice-rafted debris indicates that the sea was not frozen over during the Elatina glaciation. No direct age determination is available for the Elatina glaciation, and only maximum and minimum age limits of ∼640 and ∼580 Ma, respectively, can at present be applied. High-quality palaeomagnetic data for red beds from the Elatina Formation, supported by positive fold tests on soft-sediment slump folds that demonstrate the early acquisition of magnetic remanence, indicate deposition within 10° of the palaeoequator. The Yerelina Subgroup in the Adelaide Geosyncline and on the Stuart Shelf is disconformably to unconformably overlain by the Nuccaleena Formation “cap carbonate” that marks the early Ediacaran post-glacial marine transgression. The presence in near-equatorial palaeolatitudes of glaciomarine deposition, grounded ice and permafrost near sea level, a strongly seasonal periglacial climate, and widespread open seas implies a paradoxical palaeoclimate and palaeogeographic setting for the Elatina glaciation. The strong evidence for a non-actualistic late Cryogenian glacial climate in South Australia has been a stimulus to worldwide multidisciplinary research on Cryogenian glaciogenic successions.