CONODONTS, GRAPTOLITES, AND d13C CHEMOSTRATIGRAPHY IN THE LATEST ORDOVICIAN (GAMACHIAN, HIRNANTIAN): A GLOBAL REVIEW

The base of the Silurian, fixed internationally at the base of the acuminatus Graptolite Zone, has been difficult to recognize precisely in many cratonic carbonate successions lacking zonal graptolites. The geographically widespread Hirnantia shelly fauna is typical of latest Ordovician cold-water deposits and useful biostratigraphically, but it is rarely present in tropical strata such as those in most of North America. The composition of Hirnantian conodont faunas shows marked differences between different regions and their biostratigraphic use is also complicated by the fact that conodonts of Silurian aspect appeared before the end of the Ordovician. However, recently developed d¹³C chemostratigraphy offers great promise to solve these correlation problems. A conspicuous positive d¹³C excursion in the Hirnantian correlates closely with the range of the extraordinarius Graptolite Zone (the next youngest Ordovician zone) and has been recorded in Baltoscandia, Scotland, China, Canada (Anticosti Island), and the Great Basin (Nevada). It serves not only as a powerful regional correlation tool but also, it provides evidence of the magnitude of the stratigraphic gap(s) that are common in latest Ordovician cratonic successions. Interestingly, recently collected spot samples from the Girardeau Limestone and the overlying Leemon Formation in southeastern Missouri show d¹³C values of near 4, which are similar to those seen in the Anticosti Island successions. This provides the first indication in the Midcontinent of the latest Ordovician d¹³C excursion and confirms that Hirnantian strata are present there, which has been a matter of much controversy in the past. It also confirms the Hirnantian age of the enigmatic Noixodontus conodont fauna, which is recorded from the Girardeau Limestone, Leemon Formation, and equivalent strata in the Mississippi Valley.