RECALIBRATING THE SILURIAN-DEVONIAN BOUNDARY: COMBINING A NEW HIGH-PRECISION CA-ID-TIMS U-PB AGE WITH CHITINOZOAN BIOSTRATIGRAPHY

The Geologic Time Scale 2012 (Gradstein, et al.) lists the age of the Silurian-Devonian (S-D) boundary at 419.2 +/- 3.2 Ma. Other calibrations of Devonian time placed the boundary at 418 Ma (Tucker et al. 1998), 418.1 +/- 3.0 Ma (Kaufmann 2006) or 418.8 +/- 2.7 Ma (De Vleeschouwer and Parnell 2014). The average of these boundary ages is 418.5 Ma with error bars that span the range from 415.1 to 422.4 Ma. All of these time-scale calibrations are based on U-Pb dating of the Judds Falls K-bentonite in the New Scotland Fm. (erroneously reported previously as “Kalkberg” K-bentonite) from Cherry Valley, NY. Zircons from the Judds Falls ash were dated at 417.6 +/-1.0 Ma (Tucker et al. 1998) and recalculated in GTS 2012 to 415.48 +/- 2.71 Ma. A new U-Pb CA-ID-TIMS analysis of zircons from the Judds Falls ash (Boise State University lab) utilized the EARTHTIME U-Pb tracer (ET535 spike) and yielded a high-precision date of 417.61 +/- 0.12 Ma, in close agreement with the age reported by Tucker et al. (1998; see also McAdams et al. 2014a,b). This age contravenes the younger range of previously reported error bars for the S-D boundary.

Within the Helderberg Grp., the S-D boundary falls in the upper portion of the Green Vedder Mbr. (GVM) of the Manlius Fm., based on the co-occurrence of scyphocrinitid loboliths and a positive δ¹³C excursion (Matteson and Ebert 2011). Previous workers regarded this ash as early Lochkovian from poor conodont data, with taxa that are not biostratigraphically diagnostic. However, a chitinozoan fauna comprising Eisenackitina bohemica, Pterochitina megavelata, Cingulochitina ervensis, and Margachitina catenaria (Bevington, Ebert and Dufka 2010) indicates that the dated ash is middle Lochkovian in age. The Judds Falls ash occurs in the New Scotland Fm. approximately 23-25 meters above the GVM/S-D boundary, with three disconformities between the boundary and the dated ash, further strengthening a middle Lochkovian placement for the Judds Falls ash.

When the Judds Falls K-bentonite is re-plotted as middle Lochkovian on linear calibrations of Devonian time (Tucker et al. 1998; Kaufmann 2006), projection of the time line results in placement of the Silurian-Devonian boundary at 420.5 Ma, lengthening Devonian time by as much as 2.5 Ma. We anticipate a similar result with cubic spline fitting and Bayesian Bchron methods.