The Siluro-Devonian geological time scale: a critical review and interim revision
Agreement between biostratigraphically constrained SHRIMP U–Pb zircon isotopic dates from the Lachlan Orogen and international time scales for the Silurian and Devonian has been poor. While some of the disagreement has been resolved by the latest international Silurian and Devonian time scales, a critical review of the data points used indicates that at least one Silurian date (Laidlaw Volcanics, New South Wales, Australia) should be discarded on the grounds of inadequate biostratigraphic constraint and potential ambiguity in the interpretation of the isotopic results. The readmission of two other apparently sound dates for the late Silurian, in conjunction with a SHRIMP U–Pb zircon date of ~432 Ma for the early Wenlock Canowindra Volcanics in the Lachlan Orogen, requires significant revision of the Silurian Stage boundary ages estimated by A Geologic Time Scale 2004. Specifically, the base of the Lochkovian (Devonian) is shifted from 416 Ma to 418.1 Ma, the base of the Pridoli from 418.7 Ma to 420.0 Ma, the base of the Ludlow from 422.9 Ma to 427.0 Ma, and the base of the Wenlock from 428.2 Ma to 432.5 Ma, with the base of the Llandovery (Silurian) unchanged at 443.7 Ma.
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