The Global Stratotype Section and Point (GSSP) for the base of the Katian Stage of the Upper Ordovician Series at Black Knob Ridge, Southeastern Oklahoma, USA

The Global Boundary Stratotype Section and Point (GSSP) for the Katian Stage of the Upper Ordovician Series is defined as the 4.0 m-level above the base of the Bigfork Chert in the Black Knob Ridge section, southeastern Oklahoma. This point in this section is coincident with the first appearance of the graptolite Diplacanthograptus caudatus, which has proved to be a reliable datum for precise worldwide correlation. The FAD of D. caudatus occurs very near the first occurrences of the graptolites D. lanceolatus, Corynoides americanus, Orthograptus pageanus, O. quadrimucronatus, Dicranograptus hians, and Neurograptus margaritatus. This rapid succession of fossil species appearance events provides a secure basis for identification of the base of the Katian Stage of the Upper Ordovician Series and for its global chronostratigraphic correlation. In addition to graptolites, the Black Knob Ridge section also contains biostratigraphically important conodonts and chitinozoans. The conodonts and chitinozoans occur with graptolites on black shale bedding surfaces and allow for the precise correlation of the Katian Stage into regions where the succession is represented by carbonate facies. The base of the Katian Stage occurs high in the Amorphognathus tvaerensis North Atlantic Conodont Zone and just below the base of the Plectodina tenuis North American Midcontinent Conodont Zone. This stratigraphic level is also in close proximity to several important marker horizons - just above the Millbrig and Kinnekulle K-bentonite complexes in eastern North America and Scandinavia, respectively, and just below the beginning of the Upper Ordovician Guttenberg (GICE) δ13C excursion. These event and chemostratigraphic marker horizons provide an independent test of the global synchroneity of the base of the Katian Stage, and greatly increase the usefulness of that level for global chronostratigraphic correlation.