Cambrian-Ordovician boundary age and duration of the lowest Ordovician Tremadoc Series based on U-Pb zircon dates from Avalonian Wales

Two thin volcaniclastic sandstone beds in the Bryn-llin-fawr road section i n North Wales overlie an apparent sequence boundary within the uppermost Ca mbrian Acerocare Zone and are overlain by lowest Ordovician (lower Tremadoc ) Rhabdinopora faunas. U-Pb geochronology of zircons from these sandstones yields a maximum Cambrian-Ordovician boundary age of 489+/-0.6 Ma. This age indicates both that the Tremadoc Series (lowest Ordovician) may be shorter in duration than was previously thought and that the duration of the Middl e and Late Cambrian (c. 22 Ma) was much less than that of the Early Cambria n (c. 33 Ma). Cambrian trilobite zones locally had an average duration as s hort as 1 Ma.