TRANS-ATLANTIC CORRELATION OF UPPER ORDOVICIAN δ¹³C EXCURSIONS

Several positive δ¹³C excursions have been recorded in the global Upper Ordovician Series of North America and northern Europe. The most prominent of these is the latest Ordovician (Hirnantian) excursion (HICE), which has been documented from several areas of the North American Midcontinent as well as from Nevada, Anticosti Island (S.E. Canada), the Yukon Territory, and the Canadian Arctic. In Europe the HICE has been recorded from Estonia, Norway, Scotland, and Sweden. δ¹³C values of the HICE are in the +4‰ to +8‰ range, and this excursion is extraordinarily useful for recognition and correlation of Hirnantian strata globally. The second most prominent and widely documented δ¹³C excursion, the Guttenberg excursion (GICE), occurs in early to middle Chatfieldian strata in North America and in the Keila and Oandu stages in Baltoscandia. Its heaviest δ¹³C values are lighter than those of the HICE, ranging from +2‰ to +3.5‰ at most sites. Being recorded from more than 30 localities in North America and Baltoscandia and apparently global in its distribution, the GICE is quite useful for both local and long-distance correlations. In the Upper Ordovician succession in Estonia, Kaljo et al. (2004) recognized three other significant periods of elevated δ¹³C values between the GICE and the HICE that were referred to as the 1^st and 2^nd Late Caradoc and Early Ashgill ‘excursions'. In North America, the Early Ashgill ‘excursion' can be correlated with elevated δ¹³C values in the upper 75 m of the Viola Springs Formation in Oklahoma and the 1^st Late Caradoc ‘excursion' may correspond to another elevated δ¹³C interval in the lower 50 m of the same formation in its exposure along I-35 near Ardmore. A comparison between North American and Baltoscandian Upper Ordovician δ¹³C curves shows many similarities indicating that δ¹³C chemostratigraphy will be an important new tool for trans-Atlantic correlations of Upper Ordovician rocks.