The numerical age of the Upper Frasnian (Upper Devonian) Kellwasser horizons: A new U-Pb zircon date from Steinbruch Schmidt (Kellerwald, Germany)

Steinbruch Schmidt (Kellerwald, Germany) is probably the world's most famous locality that exposes the late Frasnian anoxic events (Kellwasser Crisis). A 3-cm-thick bentonite layer ( Bed 36) is intercalated between the two Kellwasser horizons and can be precisely assigned to the Upper rhenana conodont Zone. Twenty-four U-Pb ID-TIMS ( isotope dilution-thermal ionization mass spectrometry) analyses of single zircons extracted from this layer yielded 17 concordant results, which form an elongated cluster along concordia. The oldest point of this cluster is assumed to represent the lowest amount of Pb loss, and its Pb-206/U-238 age of 377.2 +/- 1.7 Ma is regarded as the eruption age of the bentonite. This dating allows the geochronological fixing of the anoxic Kellwasser horizons, which characterize one of the most important biotic crises of the Palaeozoic era, immediately prior to the Frasnian-Famennian boundary. A biochronometric timescale of the mid-Frasnian to late Famennian interval is established based on all recently published Late Devonian U-Pb ID-TIMS ages. The age of the Frasnian-Famennian boundary can be estimated by interpolation to 376.1 +/- 1.6 Ma.