High-resolution correlation of Anglo-Welsh Middle to Upper Ordovician sequences and its relevance to international chronostratigraphy

Continuously fossiliferous sections in south Wales and Shropshire spanning the base of the Upper Ordovician permit precise correlation of the Nemagrap tus gracilis Biozone. a horizon currently under scrutiny for the selection of a Global Stratotype Section and Point (GSSP). Using graphic correlation we have compiled a new composite global standard for the Middle-Upper Ordov ician, which shows that the first occurrence of the key graptolite N. graci lis is not everywhere of the same age, appearing earlier in North American successions than it does in Britain. Species of Dicellograptus, by contrast . appear to be almost synchronous in their first appearance. The status and correlation of standard British biozones through the Llandeilian-Caradoc i nterval, based on graptolites and trilobites, are revised. Suitable section s for the definition of bases of biozones are proposed. If the base of the N. gracilis Biozone is accepted internationally as the base of the Upper Or dovician. this biozone being coincident with the base of the Caradoc Series . there are sections suitable for its precise definition in the British suc cession.