T250 Geologic Timescale—Current Status, Future Enhancement, and Applications (Posters)

Wednesday, 7 November 2012
9:00 AM - 6:00 PM
Hall B

THE EAST RIVER MOUNTAIN K-BENTONITE BED, A CENTRAL APPALACHIAN MARKER THAT CLOSELY APPROXIMATES THE MIDDLE-UPPER ORDOVICIAN (DARRIWILIAN-SANDBIAN) BOUNDARY

Paper number: T250-19
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

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.