New uppermost Cambrian U-Pb date from Avalonian Wales and age of the Cambrian-Ordovician boundary

A crystal-rich volcaniclastic sandstone in the lower Peltura scarabaeoides Zone at Ogof-ddu near Criccieth, North Wales, yields a U-Pb zircon age of 491 +/- 1 Ma. This late Late Cambrian date indicates a remarkably young age for the Cambrian-Ordovician boundary whose age must be less than 491 Ma. Hence the revised duration of the post-Placentian (trilobite-bearing) Cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparable to that provided by Ordovician graptolites and Mesozoic ammonites.