THE GUTTENBERG (CHATFIELDIAN, ORDOVICIAN) 13C EXCURSION (GICE): SIGNIFICANCE FOR NORTH AMERICAN AND TRANS-ATLANTIC CHRONOSTRATIGRAPHIC CORRELATIONS AND FOR ASSESSMENT OF THE AGE RELATIONS BETWEEN THE NORTH AMERICAN MILLBRIG AND THE BALTOSCANDIC KINNEKULLE K-BENTONITES

The Guttenberg ¹³C excursion (GICE), which was first recognized in Iowa and later found in Pennsylvania, is now known also from the coeval stratigraphic interval in Illinois, Kentucky, Tennessee, New York, the East Baltic states, and Sweden. Apart from the terminal Ordovician excursion, it is the most prominent ¹³C excursion currently recognized in the Ordovician and it is of major significance for local and regional correlation. Biostratigraphically, it occurs in the Midcontinent P. tenuis Conodont Zone and in the upper part of the Atlantic A. tvaerensis Conodont Zone, and in the North American C. americanus Graptolite Zone and the European D. clingani Graptolite Zone. The ¹³C curve shows a remarkable similarity between North American and Baltic sections suggesting a very high correlation resolution. Based on GICE , the Logana Mbr. of the Lexington Ls. of Kentucky can be correlated with the Guttenberg Mbr. of the Decorah Fm. in the Upper Mississippi Valley, the Hermitage Fm. in Tennessee, the Napanee-Kings Falls Ls. of New York and with upper Keilan and Oanduan units in Baltoscandia. These correlations are in good agreement with biostratigraphic data. However, the fact that the GICE occurs just a few m above both the Millbrig K-bentonite in North America and the Kinnekulle K-bentonite in Baltoscandia is not in agreement with the recently published Ar-Ar isotopic age difference of 4-5 Ma between these ash beds.