Age of the Acraman impact ejecta layer in the adelaide superbasin and Implications on clay-mineral provenance from the Rb–Sr systematics of middle Ediacaran shales

• First direct dating of Acraman bolide ejecta layer that punctuates the Middle Ediacaran of Australia. • Novel Rb/Sr in-situ laser ICP-MS/MS dating of argillaceous bed demonstrates feasibility of technique. • Shales yield coeval ca. 780 Ma dates that likely reflect Tonian terrestrial pedogenesis, later flushed into the Adelaide Superbasin. • New age constraint on the evolution of Ediacara in its type area, the Wonoka anomaly and the middle Ediacaran glaciation. The Acraman bolide hit the Gawler Craton of South Australia in the middle Ediacaran. Debris from this impact punctuates the Ediacaran sequences of the Adelaide Superbasin and the eastern Officer Basin. Here we date an argillaceous Acraman impactite layer sampled from Arkaba Creek, in the Bunyeroo Formation of the Adelaide Superbasin at 585 ± 15 Ma (MSWD = 1.8, 2σ error) using the in-situ Rb–Sr ICP-MS/MS technique that we interpret to date the impact event. The analyses have a radiogenic 87 Sr/ 86 Sr initial ratio of 0.7210 ± 0.0028 and elevated light rare earth element values. Shales from both above and below the impact horizon yielded ages significantly pre-dating deposition and are interpreted to reflect detrital input into the Bunyeroo Formation, which is consistent with previous petrographic studies. Intriguingly, all 10 shales analysed yielded Rb–Sr ages within error of each other, with a Tonian weighted mean age of 778 ± 17 Ma (MSWD = 0.83). This is interpreted to reflect a very similar clay mineral age provenance for these shales despite being found in samples over 600 km apart. A similar mix of detrital mineral ages cannot be discounted. However, considering the spatial distribution, an alternative interpretation that the age similarity reflects Tonian pedogenic clay formation on the Gawler Craton, is thought more likely.