THE ORDOVICIAN (CHATFIELDIAN) GUTTENBERG CARBON ISOTOPE EXCURSION. 1. NEW DATA FROM THE EASTERN NORTH AMERICAN MIDCONTINENT AND BALTOSCANDIA

Recent studies have shown that conspicuous positive shifts in the d¹³C curve through marine successions are highly significant for establishment of local and regional event stratigraphy. Two major d¹³C excursions of probably global extent are presently known in the Ordovician, one in the uppermost part of the system (Hirnantian Stage) and one in the upper Mohawkian (Chatfieldian Stage). The Hirnantian excursion, which is the largest one, is coeval with the latest Ordovician glaciation and has been the subject of much research in Europe and North America. The latter excursion, first reported from the Guttenberg Member of the Decorah Formation in Iowa, has been studied mainly in the Upper Mississippi Valley although it has recently been observed in Pennsylvania and the East Baltic (Estonia). We here present evidence for the Guttenberg excursion in the Logana Member of the Lexington Limestone in central Kentucky, in the Hermitage Formation of central Tennessee, and in the Skagen and Moldå formations in central Sweden. As is the case in the Upper Mississippi Valley, the d¹³C curve shows a rapid positive shift in the basal Logana in Kentucky and in the Hermitage in Tennessee from a relatively even shape in the underlying Tyrone-Curdsville and Carters, respectively. The d¹³C chemostratigraphy strongly supports the idea that equivalents of the Decorah Formation are present in the lower Lexington Limestone (Logana) and in the Hermitage although developed in a different biofacies and lithofacies. The d¹³C curve in the Swedish succession shows close similarity to the North American and Estonian ones with a marked positive shift a few m above the Kinnekulle K-bentonite which suggests that equivalents to he lower-middle Decorah and lower Lexington are present in the East Baltic Keila, Oandu, and lower Nabala Stages.