Abstract Two remarkable events in the history of life on the Earth occur during the Ordovician Period (486.9–443.1 Ma). The first is an exceptionally rapid and sustained radiation of marine life known as the ‘Great Ordovician Biodiversification Event’ (GOBE), and the second is a catastrophic Late Ordovician mass extinction (LOME). Understanding the duration, rate and magnitude of these events requires an increasingly precise global correlation framework. In this chapter we review the major subdivisions of the Ordovician System, their Global Stratotype Section and Points, and the chronostratigraphic levels that define their bases. We also present a detailed set of correlation charts that illustrate the relationships between most of the regional graptolite, conodont and chitinozoan successions across the world.

Explosive eruptions from volcanoes are recorded in the stratigraphic record throughout the Phanerozoic, but evidence of these eruptions in the form of preserved tephra layers appears to be concentrated at times of active plate collision and concomitant high stands of sea level. The products of volcanic eruptions are lavas, tephra, and gases. Basaltic magmas (low-silica content) are usually erupted in the form of lava flows, whereas rhyolitic magmas (high-silica content) are commonly explosively erupted as plinian and ultraplinian plumes, and associated pyroclastic flows. Fallout tephras are preserved in ancient sedimentary sequences as tonsteins, bentonites, and K-bentonites. Middle Ordovician K-bentonites represent some of the largest known fallout ash deposits in the Phanerozoic Era. They cover minimally 2.2 × 10 6 km 2 in eastern North America and 6.9 × 10 5 km 2 in central and northwestern Europe as a result of explosive volcanism, which affected both Laurentia and Baltica during the closure of the Iapetus Ocean. The three most widespread beds are the Deicke and Millbrig K-bentonites in North America and the Kinnekulle K-bentonite in northwestern Europe. Similar successions are well known in South America and China. Sedimentation rates of volcanic ejecta range from meters per year locally to ~1 mm/1000 yr in the deep sea. Volcanogenic sediments react with seawater to produce secondary phases such as zeolites and clay minerals. Studies of recent ashfall behavior suggest that the preservation potential in the stratigraphic record can be viewed as somewhat remarkable in that such sudden events are preserved at all, much less produce such a wealth of valuable geologic information.

Two prominent, and apparently globally distributed, δ 13 C excursions have been documented from the Upper Ordovician, namely the early Katian Guttenberg isotope carbon excursion (GICE) and the latest Ordovician Hirnantian isotope carbon excursion (HICE). The former excursion, which has lower δ 13 C values than the HICE, is now recorded from dozens of localities in North America and Baltoscandia, and it appears to be present also in China. In North America the GICE ranges from the uppermost Phragmodus undatus Midcontinent Conodont Zone to near the top of the Plectodina tenuis Midcontinent Conodont Zone, an interval corresponding to the lower part of the Diplacanthograptus caudatus Global Graptolite Zone. The base of the GICE lies somewhat above the Millbrig K-bentonite. In Baltoscandia the GICE occurs in the upper Diplograptus foliaceus through the lower Dicranograptus clingani Graptolite Zones, and in the upper Amorphognathus tvaerensis Conodont Zone. Its base is a few meters above the widespread Kinnekulle K-bentonite. In Baltoscandia and in Oklahoma the GICE ranges through a part of the Spinachitina cervicornis Chitinozoan Zone. In North America the GICE is regionally in a transgressive-regressive succession. The bathymetric conditions in the GICE interval in Baltoscandia were somewhat complex and have been the subject of different interpretations, but there is no obvious correlation between the GICE and apparent sea level changes. A review of the relations between the GICE and potential climatic and water temperature indicators, such as lithofacies, faunas, and 18 O geochemistry, does not suggest a close correlation to specific environmental conditions. The cause of formation of the GICE is enigmatic, but there is no direct evidence that it was coeval with a period of extensive glaciation in the Gondwana. The GICE is a powerful chemostratigraphic tool that is useful for detailed local and even transatlantic correlations.

Statistical comparisons of conodont faunas from many parts of the world were carried out in an effort to shed light on one of the most discussed, and most controversial, problems in the lower Paleozoic geology of South America, namely, the geographic origin of the exotic terrane in western Argentina known as the Precordillera. The similarity between the conodont faunas from the Precordilleran La Silla, San Juan, Gualcamayo, and Yerba Loca Formations and many coeval faunas from Laurentia, as well as from other parts of the world, was assessed using the Jaccard Index. The analysis of faunas from six biostratigraphic intervals in the Lower and Middle Ordovician shows that the earliest Ordovician (Tremadocian) faunas cluster with those of Laurentia, whereas slightly younger faunas show less obvious provincialism. The conodont faunas of the Middle Ordovician (early Darriwilian) of the Precordillera again show dominantly Laurentian affinities. The hypothesis that the Precordillera rifted from the Ouachita embayment and moved across part of the Iapetus Ocean to dock with western Gondwana in Ordovician time is not clearly supported by the conodonts (and other non-conodont phosphatic microfossils). The similarity with faunas in southern Laurentia (mainly from the El Paso area of Texas and southern New Mexico) is high in the Tremadocian. The expected similarity decrease with presumed increase in distance from Laurentia later in the Early Ordovician is not evident in the conodont faunas. Similarity between the two regions (mainly the Precordillera and the Marathon area of Texas) remains about the same through the early Middle Ordovician. It is concluded that the conodonts, the best known and most widespread fossil group in the study areas, do not provide conclusive evidence of the geographic origin of the Precordillera.

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