Geological Magazine

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

E. LANDING a1c1, S. A. BOWRING a2, K. L. DAVIDEK a2, A. W. A. RUSHTON a3a4, R. A. FORTEY a3 and W. A. P. WIMBLEDON a5
a1 Center for Stratigraphy and Paleontology, New York State Museum, The State Education Department, Albany, NY 12230, USA
a2 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
a3 Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK
a4 Formerly British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
a5 Countryside Council for Wales, 4 Castleton Court, Fortran Road, St. Mellons, Cardiff CF3 0LT, UK


Two thin volcaniclastic sandstone beds in the Bryn-llin-fawr road section in North Wales overlie an apparent sequence boundary within the uppermost Cambrian 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 Middle and Late Cambrian (c. 22 Ma) was much less than that of the Early Cambrian (c. 33 Ma). Cambrian trilobite zones locally had an average duration as short as 1 Ma.

(Received October 14 1999)
(Accepted May 10 2000)

c1 Author for correspondence: