AN IOWA-BALTOSCANDIA-CHINA CONNECTION: PRECISE LONG-RANGE CHEMOSTRATIGRAPHIC CORRELATION OF UPPER ORDOVICIAN STRATA USING THE GUTTENBERG δ13C EXCURSION (GICE)

Among the seven Ordovician positive δ¹³C excursions currently recognized and named, two have proved to be particularly useful for international correlations, namely the latest Ordovician Hirnantian excursion (HICE), which is the most prominent among the Ordovician excursions, and the early Late Ordovician (early Katian) Guttenberg excursion (GICE), which was first recognized in the Decorah Shale of Iowa. Although being of smaller magnitude than the HICE, the GICE has now been documented from more than 20 localities across North America, ranging from Oklahoma to Ontario, and from New York State to Nevada. Stratigraphically, this excursion occurs slightly above the Deicke and Millbrig K-bentonites and somewhat above the base of the North American Chatfieldian Stage. Documented GICE occurrences include, for instance, the Viola Springs Formation in Oklahoma, the Lexington Limestone of Kentucky, the Hermitage Formation of Tennessee, the lower Decorah Shale of the Upper Mississippi Valley, the Napanee and Kings Falls formations of New York State, and the uppermost Copenhagen Formation of Nevada. In Europe, the GICE is now recorded from more than a dozen localities in Estonia and Sweden where it occurs above the widespread Kinnekulle K-bentonite in the upper Keila and Oandu stages. Recently, the GICE has been discovered at several localities in southern and northwestern China, where it is present in the Pagoda Formation. Being biostratigraphically dated by means of conodonts and graptolites, the GICE is a globally useful chemostratigraphic tool for clarifying the precise relationships between geographically widely separated sections that previously were difficult, or impossible, to correlate. This is illustrated by the fact that it offers a correlation precision of ± a couple of meters between strata in Iowa and southern China.