Regional recognition of the Neoproterozoic Sturtian–Marinoan boundary, Northern and Central Adelaide Geosyncline, South Australia

Abstract

Stratigraphic and sedimentological investigation of the interglacial succession within the Cryogenian-aged Umberatana Group of the Northern and Central Flinders Ranges reveals a complex array of sedimentary successions lying between the Sturtian and Marinoan glacial deposits. The Sturtian–Marinoan Series boundary was first defined from the Adelaide area at the uppermost contact of the Brighton Limestone. In the Northern Flinders Ranges the Sturtian–Marinoan Series boundary has been positioned at the uppermost contact of the Balcanoona Formation, which is thought to correlate with the Brighton Limestone. In the Northern Flinders Ranges a major unconformity separates the Sturtian and Marinoan-aged sedimentary successions (Nepouie–Upalinna Subgroups). In moderately deep marine depositional settings, this submarine unconformity is located at the base of the Yankaninna Formation where erosion has deeply incised (up to 300 m) into the underlying Tapley Hill Formation. In shallower marine settings the unconformity is found at the base of the Weetootla Dolomite. In very deep water depositional settings this unconformity is not recognised, and the Yankaninna Formation appears to conformably overlie the Tapley Hill Formation suggesting that this erosional feature is restricted to shallow and moderately deep depositional settings. This unconformity presents a regionally persistent chronostratigraphic marker horizon, which reliably marks the Sturtian–Marinoan Series boundary at the base of the Yankaninna Formation from shallow shelfal to deep-water basinal settings throughout the Northern Flinders Ranges. In the Central Flinders Ranges the post-Sturtian glacial stratigraphy records a very similar depositional record to that observed in the Northern Flinders Ranges. In the Central regions the Tapley Hill Formation is overlain by deep-marine carbonates and calcareous shales of the Wockerawirra Dolomite and Sunderland Formations, respectively. The base of the Wockerawirra Dolomite is found to be in erosional contact with the underlying Tapley Hill Formation. This stratigraphic relationship, together with lithological similarities, indicates the Wockerawirra Dolomite and Sunderland Formation of the Central Flinders Ranges are lateral correlatives of the Yankaninna Formation of the Northern Flinders Ranges. The regional nature of the Sturtian–Marinoan unconformity in the Adelaide Geosyncline suggest the possible existence of a glacio-eustatic event that may correlate with glacials/glaciation elsewhere on the Earth during the Cryogenian.