U-Pb zircon dates from North American and British Avalonia bracket the Lower-Middle Cambrian boundary interval, with evaluation of the Miaolingian Series as a global unit

High-precision U-Pb zircon ages on SE Newfoundland tuffs now bracket the Avalonian Lower-Middle Cambrian boundary. Upper Lower Cambrian Brigus Formation tuffs yield depositional ages of 507.91±0.07Ma (Callavia broeggeri Zone) and 507.67±0.08Ma and 507.21±0.13Ma (Morocconus-Condylopyge eli Assemblage interval). Lower Middle Cambrian Chamberlain's Brook Formation tuffs have depositional ages of 506.34±0.21Ma (Kiskinella cristata Zone) and 506.25±0.07Ma (Eccaparadoxides bennetti Zone). The composite unconformity separating the Brigus and Chamberlain's Brook formations is constrained between these ages. An Avalonian Lower-Middle Cambrian boundary between 507.2±0.1 and 506.3±0.2Ma is consistent with maximum depositional age constraints from southwest Laurentia, which indicate an age for the base of the Miaolingian Series, as locally interpreted, of ≤ 506.6±0.3Ma. The Miaolingian Series' base is interpreted as correlative within ≤ 0.3±0.3Ma between Cambrian palaeocontinents, although its exact synchrony is questionable due to taxonomic problems with a possible Oryctocephalus indicus-plexus, invariable dysoxic lithofacies control of O. indicus and diachronous occurrence of O. indicus in temporally distinct δ 13C chemozones in South China and SW Laurentia. The lowest occurrence of O. indicus assemblages is linked to onlap (epeirogenic or eustatic) of dysoxic facies. A united Avalonia is shown by late Early Cambrian volcanics in SW New Brunswick; Cape Breton Island; SE Newfoundland; and the Wrekin area, England. The new U-Pb ages revise Avalonian geological evolution as they show rapid epeirogenic changes through depositional sequences 4a-6.