Steps toward a global standard for Ordovician stratigraph

This paper traces the development of the Ordovician concept from Charles LAPWORTH'S original proposal in 1879 to recent decisions of the International Subcommission on Ordovician Stratigraphy aimed at establishing a unified global nomenclature of Series and Stage divisions using key markers (graptolites and/or conodonts) with high global correlation potential. LAPWORTH incorporated the rocks with overlap relationships and common faunas of SEDGWICK'S Cambrian and MURCHISON'S Silurian classifications into his new System, so also providing a simple solution to the SEDGWICK-MURCHISON dispute. But adoption of LAPWORTH'S Ordovician System was resisted by British workers until the end of the 1890s, and then the international community took until 1960 to officially ratify the name. Greater interest for adopting standardized stratigraphic procedures and classification followed the establishment in 1974 of the IUGS/ICS-supported International Subcommission on Ordovician Stratigraphy, and relevant inter-system boundary working groups. The Cambrian-Ordovician Boundary Working Group agreed in 1982 to the base of the Ordovician System at or near the traditional British (and Scandinavian) base of the Tremadoc, but this remains to be ratified officially as it requires an accompanying boundary stratotype definition, a task for the newly reconstituted Subcommission Cambrian-Ordovician Boundary Working Group. The Ordovician-Silurian Boundary Working Group chose a boundary at the base of the graptolite Akidograptus acuminatus Zone at Dob's Linn, Scotland, i. e. at a level above the widespread and distinctive cool-water Hirnantia Fauna regarded as latest Ordovician. This was ratified officially by ICS and IUGS in 1985. In global terms, only minority support exists for a major bipartite subdivision of the Ordovician System. The majority prefer a division into three Series, and since 1995 there has been Subcommission agreement, a 90% majority of the voting membership, in favour of a tripartite division of the System named Lower, Middle and Upper Ordovician Series. Further, it has been agreed by an 82% majority of voting members that the base of the Tripodus laevis conodont Zone should be adopted as the base of the Middle Ordovician Series, and that the base of the graptolite Nemagraptus gracilis Zone should be recognized as the base of the Upper Ordovician Series. The votes provided a clear mandate for the Subcommission to focus its immediate programs of work towards establishing this tripartite subdivision. It has further been agreed that the three Series each be subdivided into two global Stages. Ballots held in 1992 and 1995 have identified the additional Stage boundaries for subdivision. A 95% majority of voting members agreed that the Zone of Tetragraptus approximatus would form the base of the upper Stage of the Lower Ordovician Series, and a 75% majority favoured the Zone of Dicellograptus complanatus as the base of the upper Stage of the Upper Ordovician Series. Furthermore, in a proposal supported in early 1996 by a 94% majority of voting members, a GSSP has been defined in the Huangnitang section, Zhejiang Province, China, at the base of the Undulograptus austrodentatus Zone. This defines the base of the upper Stage of the Middle Ordovician Series, and has been named the Darriwilian Stage (conforming to prior usage of an Australian regional stage). This decision was recently ratified by ICS and IUGS. The high level of biogeographic and ecologic differentiation of Ordovician biotas has made the development of really precise, unified global correlation a slow and difficult process. Indeed, it seems likely that in many areas of the world a dual nomenclature will be maintained for the immediate future - provincially based regional series and stage divisions as well as the global divisions - the former gradually giving way to the latter as higher resolution global correlation is achieved. An outline of current regional series usage is also presented.