Paper No. 19
Presentation Time: 9:00 AM-6:00 PM
THE EAST RIVER MOUNTAIN K-BENTONITE BED, A CENTRAL APPALACHIAN MARKER THAT CLOSELY APPROXIMATES THE MIDDLE-UPPER ORDOVICIAN (DARRIWILIAN-SANDBIAN) BOUNDARY
LESLIE, Stephen A., Department of Geology and Environmental Sciences, James Madison University, MSC 6903, Harrisonburg, VA 22807, SELL, Bryan K., Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, SALTZMAN, Matthew R., School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, REPETSKI, John E., U. S. Geological Survey, 926A National Center, Reston, VA 20192 and EDWARDS, Cole T., Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210, lesliesa@jmu.edu
A 10-15 cm-thick K-bentonite bed is present near the base of the Elway Limestone in the exposure on the northbound lane of I-77 in Mercer County, southern West Virginia, north of the East River Mountain Tunnel. We refer to this as the East River Mountain K-bentonite Bed (ERM K-b). Five concordant analyses of zircons from the ERM K-b yield a weighted mean
206Pb/
238U age of 458.76 ± 0.26 Ma (2σ analytical error). This age places the ERM K-b in the very uppermost Darriwilian, and it approximates for practical purposes the Darriwilian-Sandbian boundary (458.4 Ma, GTS 2012) in the central Appalachians. The ERM K-b is therefore an excellent tie-point for aligning biostratigraphic and geochemical data with the Middle-Upper Ordovician boundary, which is the Darriwilian-Sandbian global stage boundary.
The conodont fauna in the Elway Limestone immediately below the ERM K-b consists of Plectodina aculeata?, Pteracontiodus? sp., Phragmodus sp. cf. P. flexuous, Panderodus sp., and Erismodus sp. The conodont fauna in the Elway Formation immediately above the ERM K-b consists of Plectodina aculeata, Pteracontiodus? sp., Phragmodus flexuosus, Panderodus sp., Erismodus sp., Curtognathus sp., Drepanoistodus? sp., and Appalachignathus delicatulus. The ERM K-b helps to better constrain a C, Sr, and Nd isotope shift (UDIS, Upper Darriwilian Isotope Shift) that preliminary samples indicate is present in the same section, and that also is present in the Rocky Gap Section along I-77 in Bland County, Virginia, and at the Clear Spring section along I-70 in Washington County, western Maryland. Conodont apatite from just above the ERM-K-b horizon was analyzed for Sr isotopes. The measured value of 0.708341 is consistent with uppermost Darriwilian Sr isotope values. The new age data from the ERM K-b indicate that the UDIS begins in the very uppermost Darriwilian, and therefore, the UDIS also closely approximates the Middle-Upper Ordovician boundary.
