PRECISE U-PB ZIRCON DATE SUCCESSION THROUGH FOSSILIFEROUS AVALONIAN LOWER–MIDDLE CAMBRIAN BOUNDARY INTERVAL AND EVALUATION OF THE MIAOLINGIAN AS A GLOBAL CAMBRIAN SERIES

Cambrian volcanic ashes are common in SE Newfoundland, and regional sampling has provided a zircon date sequence that brackets the Avalonian Lower–Middle Cambrian boundary interval faunal succession. The new U-Pb ages from the Brigus Fm. (upper–uppermost Lower Cambrian, Callavia broeggeri Zone–Morocconus-Condylopyge eli Assemblage) and lower Chamberlain’s Brook Fm. (lower Middle Cambrian, lower Kiskinella cristata–Eccaparadoxides bennetti? zones) show rapid epeirogenic changes through Avalonian depositional sequences 4a–lower 6. Rapid alternations of uplift–erosion–down-drop with marine onlap in narrow, NNE-elongated, fault-defined depocenters produced unconformities at the base of all formations and members and the abrupt biotic changes at the unconformable base of most trilobite zones. Reevaluation of the Miaolingian Series, its lower Wuliuan Stage, and the Oryctocephalus indicus Zone as global chronostratigraphic units is required. The Lowest Occurrence (LO, not “FAD”) of the taxonomically problematical form O. indicus and associated taxa of the O. indicus Zone is argued to be lithofacies-associated with replacement of older trilobite faunas with onlap of strongly dysoxic facies in SW Laurentia; Spiti, India; and, likely, South China. Chemostratigraphic (carbon isotope) SW Laurentia–South China correlation is reinterpreted as showing a late, and diachronous, appearance of O. indicus Zone taxa used to define the base of the Miaolingian in SW Laurentia. A ca. 509 Ma date on the Miaolingian base in the ICS Chart is too old based on earlier determined SW New Brunswick (Hanford Brook Fm.) and our new SE Newfoundland data. That date likely reflects reworking of “fresh appearing” zircons into the Quarry Ridge Grits at Nuneaton, England. Unity of the high temperate Avalonian ribbon continent is shown by regional appearance of late Early Cambrian volcanics (Ads 4b, c. 508 Ma) in SW New Brunswick (Waites Lane volcanic edifice and coeval Somerset Street Mbr. ashes); Cape Breton Island (Eskasoni Fm.); top Brigus Fm., SE Newfoundland; and Wrekin area, England, resulting from activity on the terminal Ediacaran–Ordovician Avalonian transform fault.