RECALIBRATING THE SILURIAN–DEVONIAN BOUNDARY: A NEW, HIGH-PRECISION CA-ID-TIMS DATE FOR THE ‘KALKBERG’ K-BENTONITE

The ‘Kalkberg’ K-bentonite (KKB), which was originally reported from the Kalkberg Formation of the Helderberg Group, in Cherry Valley, New York State, provides the radioisotopic date that is stratigraphically closest to the Silurian–Devonian boundary. It was dated at 415.48 ± 2.71 Ma, and said to occur in the early Lochkovian Icriodus (now Caudicriodus) woschmidti conodont zone. However, the reported age of the bentonite, the formation it is assigned to, and the biostratigraphic zonation of the surrounding strata are problematic in light of new, more precise dating techniques and improved litho- and biostratigraphy. New lithostratigraphic information indicates that the KKB falls in the stratigraphically overlying New Scotland Formation of the Helderberg Group. New conodont samples bracketing the KKB returned very poor yields, which included no biostratigraphically diagnostic taxa. However, chitinozoan biostratigraphy shows that the New Scotland Formation is middle Lochkovian in age. We have analyzed the KKB using chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) and the EARTHTIME U-Pb tracer (ET535 spike), and utilized new biostratigraphic information to more precisely correlate its age. The new date, returned from the Boise State University lab, will be presented in this talk. This information will ultimately help us to quantify uncertainties in the calibration of the Silurian–Devonian boundary. Studies of coeval bentonite-bearing strata in Pennsylvania (Mandata Formation) and Quebec (Chaleurs Formation), may further refine the biostratigraphic context of the KKB. At present the KKB provides the closest date to the base-Devonian GSSP, and while increased radioisotopic precision provides a better numerical age, its utility for calibrating the base of the Devonian is compromised without substantially improved biostratigraphy. This serves to demonstrate the interdependence of high-precision geochronology and high-resolution chronostratigraphy which is crucial to the calibration of global stage boundaries.