Nature and significance of the Neoproterozoic Sturtian–Marinoan Boundary, Northern Adelaide Geosyncline, South Australia T. A. FROMHOLD* AND M. W. WALLACE School of Earth Sciences, University of Melbourne, Parkville Vic 3010, Australia The Cryogenian succession of the Northern Flinders Ranges reveals a complex sedimentary record between the Sturtian and Marinoan glacial deposits. A major unconformity separates the Sturtian and Marinoan-aged sedimentary successions in the area. This forms a subaerial erosion surface with terrestrial and marginal marine infill directly above the Angepena and Balcanoona Formations in their respective localities. This exposure surface is here correlated with the previously documented submarine unconformity between the Yankaninna Formation and the underlying deep marine Tapley Hill Formation. This erosional event provides a chronostratigraphic marker horizon that coincides approximately with the previously defined Sturtian–Marinoan Time Series boundary in the Northern Flinders Ranges. These stratigraphic relationships also constrain lateral facies relationships between the Oodnaminta Reef Complex (Balcanoona Formation) and the Angepena Formation. Similarly, the shallow-water Weetootla Dolomite is correlated with the deeper water carbonates of the Yankaninna Formation. KEY WORDS: Cryogenian, Sturtian, Marinoan, Umberatana Group, stratigraphy, Balcanoona Formation, Angepena Formation, unconformity. INTRODUCTION At least two very severe glaciations occurred during the Neoproterozoic (ca 750–630 Ma), and these are generally known as the Sturtian (older) and Marinoan (younger) glacial events. In addition, this general period of glacial activity (Cryogenian) almost certainly records the origin of metazoan animal life (e.g. Love et al. 2009; Maloof et al. 2010; Sperling et al. 2010). The Adelaide Geosyncline of South Australia contains a Neoproter- ozoic sedimentary succession that provides an out- standing record of the two major glacial events and the intervening interglacial sequence of the Cryogenian. However, there are few geochronological constraints on Cryogenian sediments of the Adelaide Geosyncline, where the Sturtian and Marinoan glacials were first recognised. Furthermore, there is now considerable uncertainty about how the Australian Sturtian and Marinoan glacial episodes relate to glacial succession on other continents (Calver et al. 2004; Allen & Etienne 2008). Usage of the Australian Sturtian and Marinoan terminology for Neoproterozoic glacial episodes world- wide appears now to be entrenched in the international literature. Such terminology is reasonable given the early development of knowledge on Neoproterozoic glacials was heavily influenced by the Adelaidean succession of South Australia (e.g. Chewings 1901; Howchin 1901; Mawson 1949). However, at present, the problems in global chronostratigraphic correlation of the Neoproterozoic glacials inhibit usage of these terms internationally. Geochronological and stratigraphic studies of the Sturtian and Marinoan sediments of South Australia will contribute to improved under- standing of this time period. In this study, we examine the litho- and chronostrati- graphic relationships of the interglacial succession between the Sturtian and Marinoan glacials. We also examine the stratigraphic basis of the Sturtian and Marinoan Time Series (Mawson & Sprigg 1950), terms that are now used globally to denote the two major Neoproterozoic glacial episodes. GEOLOGICAL SETTING AND AGE CONSTRAINTS The Adelaide Geosyncline contains a Neoproterozoic to Cambrian aged sedimentary succession (the term ‘geo- syncline’ is used as a non-genetic, historical term; see Williams et al. 2008). The basin consists of relatively undeformed sediments with a total stratigraphic thick- ness of over 12 km. This thick sedimentary succession has resulted from ongoing deposition beginning at ca 830 Ma and finishing in the Middle Cambrian (Coats & Blisset 1971; Preiss 2000). The basin was subsequently deformed in the Delamerian Orogeny, which has been dated between ca 514 and 490 Ma (Foden et al. 2006). Age constraints for the Cryogenian Umberatana Group still remain poor due to a lack of volcanically derived deposits in the Adelaide System that can *Corresponding author: tfromhold@gmail.com Australian Journal of Earth Sciences (2011) 58, (599–613) ISSN 0812-0099 print/ISSN 1440-0952 online Ó 2011 Geological Society of Australia DOI: 10.1080/08120099.2011.579624 Downloaded by [University Of Melbourne] at 00:22 19 September 2011