THE ORDOVICIAN (CHATFIELDIAN) GUTTENBERG CARBON ISOTOPE EXCURSION. 2. SIGNIFICANCE FOR CLARIFYING BALTOSCANDIC AND TRANS-ATLANTIC BIOSTRATIGRAPHIC RELATIONS AND THE EQUIVALENCE OF THE NORTH AMERICAN MILLBRIG AND BALTOSCANDIC KINNEKULLE K-BENTONITES

The discovery of the apparently globally distributed Guttenberg d¹³C isotopic excursion in the Estonian and Swedish successions has important implications for detailed correlations within Baltoscandia as well with North American Midcontinent successions. The recognition of the excursion in the much-studied Fjäcka section in Dalarna, central Sweden is particularly significant because this section serves as a stratigraphic standard and is the reference succession for several index fossil zones. A similar excursion occurs in coeval strata in the Kullsberg Limestone mounds in Dalarna. The excursion suggests that equivalents of the Swedish Skagen and Moldå formations are represented in the Keila, Oandu, and lower Nabala Stages in Estonia which is in agreement with less conclusive faunal indications. Trans-Atlantic d¹³C curve comparisons show that the Baltoscandic interval mentioned is coeval with much of the Decorah Shale in the upper Mississippi Valley, with the lower Lexington in Kentucky, and with the upper Carters and Hermitage formations in Tennessee, and with the Salona Formation of Pennsylvania. Using the excursion as a reference level, the position of the base of the Amorphognathus superbus Conodont Zone, and the ranges of the index graptolites C. bicornis and C. spiniferus are similar in North America and Baltoscandia. Also, the d¹³C chemostratigraphy is consistent with the trans-Atlantic correlation of the Millbrig and Kinnekulle K-bentonites.