EARLY DEVONIAN (LOCHKOVIAN) CHITINOZOAN BIOSTRATIGRAPHY OF THE LOWER HELDERBERG GROUP, APPALACHIAN BASIN, NEW YORK STATE AND THE AGE OF THE “KALKBERG” K-BENTONITE
The New Scotland Formation contains several K-bentonites, including a U-Pb-dated ash (reported as “Kalkberg Bentonite”) which has been used to set the age of the Silurian/Devonian boundary at 418 Ma (Tucker, et. al. 1998). The biostratigraphic position of this ash was designated as lower Lochkovian based on preliminary conodont work which assigned these strata to the woschmidti zone. More recent work (Kleffner, et al. 2009) identified conodonts from these strata as assignable to the postwoschmidti group, suggesting that the ash might be positioned later in the Lochkovian Stage.
Samples from the ash-bearing interval of the New Scotland Formation have yielded the biostratigraphically significant Lochkovian chitinozoans Eisenackitina bohemica and Margachitina catenaria. In addition, samples from the base of the ash-bearing interval have yielded Pterochitina megavelata and Cingulochitina ervensis which suggests a middle Lochkovian age for the K-bentonites in the New Scotland Formation. Samples from the lower Becraft Formation, above the New Scotland, have yielded a late Lochkovian assemblage which includes Eisenackitina bohemica and Fungochitina lata. In aggregate, these data suggest that the “Kalkberg” K-bentonite in the New Scotland Formation is middle Lochkovian in age.
Thus far, our preliminary chitinozoan data and published conodont studies have failed to locate the Silurian/Devonian boundary in the Appalachian Standard Succession of New York State. However, our determination of a middle Lochkovian age for the New Scotland Formation suggests that 418 Ma may be too young for the Silurian/Devonian boundary, perhaps by as much as 2 Ma, making the Devonian Period longer than previously thought.