WHERE DOES THE DEVONIAN BEGIN IN THE APPALACHIAN STANDARD SUCCESSION? RECOGNITION OF THE SILURIAN – DEVONIAN BOUNDARY EVENT IN THE HELDERBERG GROUP OF NEW YORK STATE

Placement of the Silurian - Devonian (S/D) boundary in the Appalachian Standard Succession (NY) has a long history of ambiguity owing to 1) the absence of monograptids, 2) low yields of poorly preserved conodonts and 3) confusion over taxonomy and ranges of key conodonts.

The S/D boundary event comprises the FAD of Monograptus uniformis, transgressive/highstand conditions, a positive δ¹³C excursion and an epibole of pelagic scyphocrinitid crinoids with distinctive floats (loboliths). In the Barrandian (Czech Republic), scyphocrinitids first appear in the ultimus zone (Prídolí), peak in abundance at or just above the S/D boundary and disappear within the lower uniformis zone (basal Lochkovian). In the Central Appalachian Basin, the S/D event is expressed by the δ¹³C excursion in the upper Keyser Fm. (Saltzman, 2002) and loboliths of Camarocrinus sp. in the Big Mountain Shale/upper Keyser Fm. (Ebert and Matteson 2005). Kleffner et al. (2009) identified seven possible positions for the boundary in the Helderberg Grp. of NY based on conodont biostratigraphy and δ¹³C isotope geochemistry. Of these, two horizons are most likely: 1) at the base of the Kalkberg Fm., a candidate horizon awaiting resolution of icriodid biostratigraphy and 2) an isotope excursion in the Green Vedder Mbr. (GVM) of the Manlius Fm. that lacks biostratigraphic control.

In NY, the GVM is a thin (0-5.6 m), heterolithic, tempestitic unit bearing orthocones and a diverse carbonized biota. The GVM records highstand conditions within the predominantly peritidal Manlius Fm. Near Schoharie, we have collected plate loboliths of C. stellatus (Hall) exclusively from the upper GVM. Both plate and cirrus-type loboliths were found together in the GVM at Van Leuven Lake (VLL). This is the first report of cirrus-type loboliths (Scyphocrinites sp.) in the Appalachian Basin. The hardground-dominated GVM at VLL is also the first recognition of the GVM in the Hudson Valley. We interpret the GVM loboliths as representing the scyphocrinitid epibole near the S-D boundary.

The limited stratigraphic range of scyphocrinitid loboliths in the GVM, along with a positive δ¹³C excursion in a highstand systems tract, indicate that the S/D boundary occurs in the upper GVM in the Appalachian Standard Succession. Chitinozoan studies are underway to confirm this placement.