VOLCANIC ASH BEDS DISCOVERED IN THE UPPER BROMIDE FORMATION AND WOMBLE SHALE (ORDOVICIAN) IN OKLAHOMA: THE WESTERNMOST OCCURRENCES OF THE MILLBRIG AND DEICKE K-BENTONITES?

The Mohawkian Deicke and Millbrig K-bentonites occur in a vast region from the Appalachians to the Upper Mississippi Valley and from Alabama to Ontario, and they are excellent marker beds for both local and regional correlations. The western limit of their distribution area is incompletely known. A report by Decker in 1933 of a thin clay bed at the Bromide/Viola Springs formational contact may be the first published indication that volcanic ash layers of this age are present as far west as eastern Oklahoma. Recent geochemical data show that Decker's clay bed is a K-bentonite. In 2004, we discovered two additional, 1 cm-thick, K-bentonites at 2.7 m and 11.5 m, respectively below the top of the Bromide in the OK Hwy 99 exposure about 5.6 km (3.5 mi.) south of Fittstown, which is one of Decker's clay-bed sections. Decker's K-bentonite occupies the same position in terms of biostratigraphy and sequence stratigraphy as the Millbrig in the eastern Midcontinent, therefore we think that it is likely the Millbrig. Further geochemical studies are required to clarify this and to determine if one of the two underlying K-bentonites is the Deicke. We also found a single, 4 cm-thick, K-bentonite 4.3 m below the top of the Womble Shale in the GSSP section for the second stage of the global Upper Ordovician Series at Black Knob Ridge north-east of Atoka, OK. Although it is uncertain if this bed is the Millbrig, the Deicke, or some other ash bed, its discovery is significant because of the potential to provide an isotopic age for the base of this global stage, and the high precision of correlation that it may allow with other sections that contain the same K-bentonite bed.