THE GLOBAL STRATOTYPE-SECTION AND POINT (GSSP) FOR THE BASE OF THE MIDDLE STAGE OF THE UPPER ORDOVICIAN SERIES AT BLACK KNOB RIDGE, SOUTHEASTERN OKLAHOMA, USA

In 2005 the Subcommission on Ordovician Stratigraphy approved the Global Stratotype Section and Point (GSSP) for the middle stage of the Upper Ordovician Series to be a point 4.0 meters above the base of the Bigfork Chert in a quarried exposure along Black Knob Ridge in southeastern Oklahoma. This level is coincident with the first appearance of the graptolite Diplacanthograptus caudatus. The first appearance datum of D. caudatus is very near the first occurrences of D. lanceolatus, Corynoides americanus, Orthograptus pageanus, O. quadrimucronatus, Dicranograptus hians, and Neurograptus margaritatus. This rapid succession of biological events provides a secure basis for identification of the middle stage of the Upper Ordovician Series and for its global chronostratigraphic correlation. In addition to graptolites, the Black Knob Ridge section 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 middle stage into regions that are represented by carbonate facies. The base of the middle 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. It 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 Guttenberg (GICE) Upper Ordovician positive δ¹³C excursion. These event and chemostratigraphic marker horizons provide an independent test on the global synchroneity of the base of the middle stage, and greatly increase our confidence in the usefulness of that level for chronostratigraphic correlation. A name for the middle stage of the Upper Ordovician Series is not yet chosen. As of December 2005 we are waiting for final ratification of this GSSP by the International Commission on Stratigraphy and the International Union of Geological Sciences.