A volcanic ashfall bed from the New Scotland Formation, Helderberg Group, New York State, that was previously identified as the Kalkberg K-bentonite provides the stratigraphically closest numerical date to the Silurian-Devonian boundary. Although zircon grains from this bed have previously been dated, outdated methodologies and the lack of accurate lithostratigraphic and biostratigraphic data from the surrounding strata limit the value of the existing radioisotopic age determination for the calibration of the Silurian-Devonian boundary. This study presents a new, high-precision CA-ID-TIMS radioisotopic age determination of 417.61±0.12(0.23)[0.50] Ma, revised conodont and chitinozoan biostratigraphy, and revised lithostratigraphic context for the ash bed. The bed should be referred to as the Judds Falls Bentonite, because this name was established prior to “Kalkberg K-bentonite”. These results help to correlate the classic Lower Devonian of New York State into the global Devonian chronostratigraphic scheme, more precisely calibrate the Silurian-Devonian boundary, and better estimate the durations of both the Silurian and Devonian periods.
Silurian carbonate strata of Iowa (East-Central Iowa Basin) consist of the Mosalem, Tete des Morts, Blanding, Hopkinton, Scotch Grove, and Gower formations. The Knapp Creek Core from Johnson County, Iowa, penetrates the upper four of these formations, which are unconformably underlain by the Ordovician Maquoketa Fm. and unconformably overlain by the Devonian Wapsipinicon Formation. This study targeted these units because it was long known that they represent a thick package of Llandovery and early Wenlock strata that is uncommonly preserved as carbonates worldwide. Integrated high-resolution carbon isotope (δ13C) chemostratigraphy and conodont biostratigraphy of the core indicates that the late Aeronian, early Telychian ‘Valgu’, and early Sheinwoodian ‘Ireviken’ positive carbon isotope excursions are present in the core. The late Aeronian to early Sheinwoodian interval is therefore well represented in the core, which provides a globally important reference section for the study of these biogeochemical events.
Silurian strata from the Illinois Basin of southwestern Illinois and southeastern Missouri (ILMO) are assigned to the Sexton Creek Limestone, Seventy-Six Shale, St. Clair Limestone, and Moccasin Springs. The latter three units are variously considered members of the Bainbridge Formation or members within the Bainbridge Group. We advocate the formalization of the term Bainbridge at group level throughout its area of use, composed of (lowest to highest) the Seventy-Six Shale Fm., the St. Clair Limestone Fm. (expanded herein), and the Moccasin Springs Fm. with four members: the Greens Ferry, Sheppard Point, Lithium (proposed herein), and Randol members. The Bainbridge Group is unconformably underlain by the Silurian Sexton Creek Limestone Fm., and conformably overlain by the ?Pridoli–Devonian Bailey Formation. Integrated carbon isotope (δ13C) chemostratigraphy and conodont and graptolite biostratigraphy of three localities in the ILMO area indicates the presence of the early Sheinwoodian ‘Ireviken’, Homerian ‘Mulde’, and the Ludfordian ‘Lau’ positive carbon isotope excursions in the Bainbridge Group. This study provides the first detailed integrated biochemostratigraphic record of all three major Wenlock–Ludfordian excursions from this important Laurentian basin and enables precise correlation of the Bainbridge Group into the global Silurian chronostratigraphic scheme.