A new, high-precision CA-ID-TIMS date for the ‘Kalkberg’ K-bentonite (Judds Falls Bentonite)

Abstract

The numerical calibration of the base of the Devonian is poorly constrained due to several factors. Few precise radioisotopic age determinations are available from the late Silurian and Early Devonian, and the limited published data carry large error bars from older analytical methodologies. Volcanic ashfalls suitable for dating occur in the Lower Devonian of the Appalachian Basin, but have not been precisely correlated into the global chronostratigraphical scheme because of limited bio‐ and lithostratigraphical information. Here, we report a new U‐Pb zircon radioisotopic age determination of 417.61 ± 0.12(0.23)[0.50] Ma and improved chronostratigraphical context, including revised biostratigraphy, for an ash bed in the New Scotland Formation, Helderberg Group, from the Lochkovian Stage that was previously identified as the Kalkberg K‐bentonite. This new information helps to integrate the classic New York Appalachian Basin succession into global Siluro‐Devonian stratigraphy, refine the calibration of the Silurian–Devonian boundary and more precisely estimate the duration of both time periods.

The Lac au Renard Tephra Cluster: a record of Lochkovian (Lower Devonian) volcanism in the Indian Point Formation, Gaspe Peninsula, Quebec, Canada

“…Kaufmann (2006 accepted this age but increased the error bars by 2 Myr. Zircon grains from the Judds Falls Bentonite were also dated by later workers with ages reported as 417.68 ± 0.52 Ma (Husson et al 2015; their sample H1-1) and 417.61± 0.12 Ma (McAdams et al 2017). If our correlations between Gaspe and New York are correct, then the 417.6 Ma age is better constrained biostratigraphically to the concurrent range zone of Monograptus aequabilis ssp., Monograptus praehercynicus and the conodont Icriodus woschmidti (However, see cautionary note of Carls et al 2007 regarding this conodont.).…”

“…These ages correspond to Late Silurian through Emsian activity. The mean age is late Pridoli, which predates the 417.61 ± 0.50 Ma age of the Judds Falls tephra in the Helderberg Group of New York (McAdams et al 2017). However, this range of ages does overlap with the age of the Lac au Renard tephras.…”

“…These authors also described Ludlow to Emsian volcanism in the Piscataquis belt and Ludlow to Lochkovian ages for the East Branch Group, the West Branch Volcanics, and the Fish River, Spider Lake, Grand Lake Seboesis and Allagash Lake formations (Bradley et al 2000). Although the Lincoln shonkinite is closest in age to the dated tephras from the Helderberg Group (Tucker et al 1998;Husson et al 2015;McAdams et al 2017), and poten- Scotland and Ireland. Similarly, Dewey et al (2015) reported that the Newer Granites of Scotland ranged from 447 Ma to 380 Ma, reflecting multiple tectonic events.…”

“…On the basis of brachiopod biostratigraphy, Johnson and Murphy (1969) showed that the New Scotland Formation in New York could be assigned to the praehercynicus zone. Bevington et al (2010) and McAdams et al (2017) reported the chitinozoans Eisenackitina bohemica, Margachitina catenaria, Pterochitina megavelata, and Cingulochitina ervensis indicating a middle Lochkovian age for the tephra-bearing portion of the New Scotland Formation at Cherry Valley, New York. The occurrence of Monograptus praehercynicus in the Indian Point Formation, coupled with the presence of Canadian dollar coin for scale.…”

“…Tephras commonly include minerals such as zircon and monazite that contain radiogenic isotopes that enable the determination of radiometric ages. Important opportunities for calibration of the geologic time scale are presented where tephras occur in strata that contain index fossils (e.g., Tucker et al 1998;McAdams et al 2017).…”