The chemostratigraphic (δ 13 C carb ) record of the Lexington Limestone in central Kentucky is a high-resolution record in Chatfieldian and lower Edenian strata. Parts of the Lexington Limestone in this region reflect an anomalous structurally controlled carbonate buildup with complex facies relationships. Chemostratigraphic analysis of buildup and off-buildup composite sections, which exemplify the Lexington facies mosaic, reveals an overall decreasing trend in 13 C compositions throughout the formation and into the overlying Clays Ferry Formation. Superimposed on this trend are four locally correlative excursions. The most significant excursion is found within the Logana Member. The Guttenberg excursion is locally expressed as two prominent δ 13 C peaks (maxima +2.58‰). The excursion is important because of its stratigraphic position above the regional and globally correlated Millbrig K-bentonite, which allows for correlation with chronostratigraphically equivalent successions. The ubiquity of the Guttenberg excursion has been recognized throughout the eastern and central United States and the Baltic region. Additional excursions have been detected and are common in other sections, showing that the chemostratigraphic record of the Lexington Limestone is similar in buildup and off-buildup sections, even across complex lithofacies boundaries. The similarity of the record across complex facies, moreover, suggests that these localized excursions are “events,” which can be used to constrain correlations and augment known facies relationships. Furthermore, the overall chemostratigraphic trend in the Lexington Limestone appears similar to published δ 13 C values from equivalent strata in Baltoscandia, emphasizing the global correlative value of chemostratigraphic trends in Upper Ordovician strata.

Two prominent, and apparently globally distributed, δ 13 C excursions have been documented from the Upper Ordovician, namely the early Katian Guttenberg isotope carbon excursion (GICE) and the latest Ordovician Hirnantian isotope carbon excursion (HICE). The former excursion, which has lower δ 13 C values than the HICE, is now recorded from dozens of localities in North America and Baltoscandia, and it appears to be present also in China. In North America the GICE ranges from the uppermost Phragmodus undatus Midcontinent Conodont Zone to near the top of the Plectodina tenuis Midcontinent Conodont Zone, an interval corresponding to the lower part of the Diplacanthograptus caudatus Global Graptolite Zone. The base of the GICE lies somewhat above the Millbrig K-bentonite. In Baltoscandia the GICE occurs in the upper Diplograptus foliaceus through the lower Dicranograptus clingani Graptolite Zones, and in the upper Amorphognathus tvaerensis Conodont Zone. Its base is a few meters above the widespread Kinnekulle K-bentonite. In Baltoscandia and in Oklahoma the GICE ranges through a part of the Spinachitina cervicornis Chitinozoan Zone. In North America the GICE is regionally in a transgressive-regressive succession. The bathymetric conditions in the GICE interval in Baltoscandia were somewhat complex and have been the subject of different interpretations, but there is no obvious correlation between the GICE and apparent sea level changes. A review of the relations between the GICE and potential climatic and water temperature indicators, such as lithofacies, faunas, and 18 O geochemistry, does not suggest a close correlation to specific environmental conditions. The cause of formation of the GICE is enigmatic, but there is no direct evidence that it was coeval with a period of extensive glaciation in the Gondwana. The GICE is a powerful chemostratigraphic tool that is useful for detailed local and even transatlantic correlations.