U-Pb zircon age of volcaniclastic layers in Middle Triassic platform carbonates of the Austroalpine Silvretta nappe (Switzerland)

Abstract

We present precise U-Pb age determinations from two volcaniclastic layers within Middle Triassic carbonates in the Upper Austroalpine Silvretta nappe near Davos (Switzerland). The two volcaniclastic layers were dated using annealing-leaching techniques and yielded ages of 240.91 ± 0.26 Ma (Prosanto Formation) and 239.89 ± 0.21 Ma (Altein Formation), respectively. The high resolution ages allow comparison of the Upper Austroalpine record of the Ducan with sections in the Southern Alps. The upper Prosanto Formation is, thus, equivalent to the middle part of the Buchenstein Formation (Middle Pietra Verde, Earliest Ladinian), and the Altein Formation is equivalent to the upper part of the Buchenstein Formation in the section with the Global boundary Stratotype Section and Point (GSSP) for the base of the Ladinian (Bagolino, northern Italy). This study demonstrates that we can use precise, accurate and carefully intercalibrated U-Pb zircon ages from volcaniclastic layers to infer the stratigraphic position of their host sediments on zone level. The older volcaniclastic layer (240.91 ± 0.26 Ma) allows a precise age determination (earliest Ladinian) for the marine vertebrate beds in the upper Prosanto Formation. © 2008 Birkhaueser.