THE BLACK KNOB RIDGE SECTION, SOUTHEASTERN OKLAHOMA, USA: A POSSIBLE GLOBAL STRATOTYPE-SECTION AND POINT (GSSP) FOR THE BASE OF THE DIPLACANTHOGRAPTUS CAUDATUS BIOZONE AND THE MIDDLE STAGE OF THE UPPER ORDOVICIAN SERIES

At the 2003 meeting of the International Symposium on the Ordovician System in San Juan, Argentina, the Ordovician Subcommission recommended that the base of the second (middle) stage of the Upper Ordovician Series be placed at the first appearance datum (FAD) of the graptolite species Diplacanthograptus caudatus. D. caudatus is an easily recognizable, cosmopolitan taxon with a consistent FAD within a succession of first appearances of several other graptolite taxa. The rapid succession of FADs provides a secure basis for identification of the D. caudatus Zone and for its global chronostratigraphic correlation. Additionally, the FAD of D. caudatus is in close proximity to several important marker horizons - just above the Millbrig and Kinnekulle K-bentonite complexes in Eastern North America and Scandinavia, respectively; just below the base of the Plectodina tenuis conodont zone; and just below the beginning of the Upper Ordovician Guttenberg δ13C excursion (GICE). These event and chemostratigraphic marker horizons provide an independent test on the global synchroneity of the FAD of D. caudatus, and greatly increase our confidence in the usefulness of that zone for chronostratigraphic correlation. An excellent Global Stratotype-Section and Point (GSSP) for the base of the D. caudatus Zone is an exposure along Black Knob Ridge at the western end of the Ouachita Mountains, Atoka County, southeastern Oklahoma. This exposure extends for several hundred meters, is readily accessible, contains a continuous graptolite succession across the C. bicornis – D. caudatus zonal boundary, and yields biostratigraphically important conodonts and chitinozoans. The boundary interval is abundantly fossiliferous and the FAD of D. caudatus is precisely located at 4 meters above the base of the Bigfork Chert. Additionally, the shale above and below the graptolite zonal boundary contains biostratigraphically important conodonts. These conodonts can be correlated with nearby sections of the Viola Springs Fm. that contain a more complete conodont zonation, and are part of Sweet’s (1984, 1995) graphic correlation framework. Thus, the biostratigraphic level of the FAD of D. caudatus, and hence the base of the middle Upper Ordovician Stage, can be precisely correlated into both graptolitic shale and shallower platform sections.