2009: The new chronostratigraphic classification of the Ordovician System and its relations to major regional series and stages and to δ 13 C chemostratigraphy. Lethaia , Vol. 42, The extensive work carried out during more than a decade by the International Subcommission on Ordovician Stratigraphy has resulted in a new global classification of the Ordovician System into three series and seven stages. Formal Global Boundary Stratotype Section and Points (GSSPs) for all stages have been selected and these and the new stage names have been ratified by the International Commission on Stratigraphy. Based on a variety of biostratigraphic data, these new units are correlated with chronostratigraphic series and stages in the standard regional classifications used in the UK, North America, Baltoscandia, Australia, China, Siberia and the MediterraneanNorth Gondwana region. Furthermore, based mainly on graptolite and conodont zones, the Ordovician is subdivided into 20 stage slices (SS) that have potential for precise correlations in both carbonate and shale facies. The new chronostratigraphic scheme is also tied to a new composite δ 13 C curve through the entire Ordovician. ᮀ Chemostratigraphy , chronostratigraphy , classification , Ordovician System , stage slices . Stig M. Bergström [stig@geology.ohio-state.edu],

Detailed investigations of the stratigraphically remarkably complete Middle and Upper Ordovician succession inSweden have permitted recognition of a sequence of five main conodont zones and ten subzones, principally based on evolutionary changes in rapidly evolving stocks of multielement species. Zonal fossils include species of Amorphognathus, Eoplacognathus, Prioniodus, and Pygodus. The Pygodus serrus, the Pygodus anserinus, the Amorphognathus tvaerensis, and the lower part of the Amorphognathus superbus Zones are Middle Ordovician, the upper part of the Amorphognathus superbus and the Amorphognathus ordovicicus Zones are Upper Ordovician in age. The Pygodus serrus Zone is subdivided into five, the Pygodus anserinus Zone into two, and the Amorphognathus tvaerensis Zone into three subzones. The conodont zones and subzones established are recognizable not only in the Balto-Scandic area but also, to a large extent, in other parts of Europe and in eastern North America. Although the horizontal distribution of Ordovician conodonts in Europe and eastern North America is still incompletely known, it is evident that the lower Middle Ordovician deposits of this area contain elements of at least three major conodont faunal provinces, namely the American midcontinent province, the North Atlantic province, and the Australian province. Llanvirnian faunas of the Balto-Scandic area are unlike most contemporaneous conodont faunas from eastern North America, although a few common species permit some important correlations. Llandeilian and lower Caradocian faunas are similar in 83 on June 1, 2015 memoirs.gsapubs.org Downloaded from 84 SYMPOSIUM ON CONODONT BIOSTRATIGRAPHY the Balto-Scandic area and in the eastern part of the Appalachian Valley, due mainly to an invasion of Balto-Scandic species into the Appalachian Valley during the deposition of rocks corresponding to the Glyptograptus teretiusculus through lower Diplograptus multidens graptolite zones. Younger Ordovician faunas from eastern North America and Europe have, with some exceptions, little in common; the American faunas are of midcontinent type, whereas a Balto-Scandic type of fauna was distributed over much of Europe (

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Isotopic analysis of conodonts and their host limestones sampled between two regionally extensive, altered volcanic ash layers in eastern Laurentia shows that a 454 Ma epeiric sea maintained large lateral differences in Nd and C isotope compositions. This is consistent with inferred temperature-salinity-defined epicontinental water masses and restricted circulation between epicontinental and oceanic environments. Because the majority of old marine fossils and sedimentary rocks are known from epeiric seas, some isotope excursions in ancient marine strata may originate from expansion and contraction of geochemically distinct epicontinental water masses, rather than global-scale changes in the state of the earth-ocean system. Data Repository item 9861 contains additional material related to this article.

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The most prominent of the two major global δ13C excursions in the Ordovician, the Hirnantian δ13C excursion (HICE), which is previously recorded from the uppermost Ordovician in a few sections in Nevada, Quebec, Arctic Canada, Baltoscandia, Scotland and China, is documented for the first time from the North American Midcontinent. Samples through the Girardeau Limestone and Leemon Formation in Missouri and Illinois show elevated δ13C values of +4‰ to +5‰. Although not determined precisely, the beginning of the HICE is likely to be in the upper part of the Orchard Creek Shale, and it ends in the upper Leemon Formation. Being extraordinarily useful chronostratigraphically, the presence of the HICE makes it possible to provide a firm dating of the study interval, whose age has long been controversial. Comparison between the study sections and coeval HICE sequences in North America and Europe show striking similarities, especially in sea-level history, indicating that major local lowstands reflect eustatic sea-level changes. A comparison with Hirnantian diamictite successions in North and South Africa and Argentina suggests that these lowstands correspond to two major Gondwanan glacial episodes.

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