Reassessment of the Pennsylvanian bio- and chronostratigraphy of the Saar-Lorraine Basin using high-precision UPb ages of volcanic ashes

Abstract

Chronostratigraphic calibration of the Westphalian and early Stephanian succession in the Saar-Lorraine Basin of Germany and France has been improved using high-precision UPb zircon dating of intercalated tonsteins (volcanic ashes) in coal seams. The clay mineralogy and elemental composition were determined to specify the source of these volcanic ashes and the post-depositional weathering processes. Samples selected for dating were tonsteins with homogeneous zircons without zoning to avoid the inherited ages from the volcanic chamber. Four of them provided concordant ages in agreement with their stratigraphic position. New radiometric ages constrain the hiatus below the Holz Conglomerate approximately between 307.9 and 306.0 Ma, that corresponds to the early and mid-Cantabrian substage as defined in its type area, the NW Spain. The age of T600 tonstein, which corresponds in Saar-Lorraine Basin to the Duckmantian/Bolsovian boundary is fully in agreement with ages of that boundary determined in the Ruhr Basin of the Variscan foreland and in continental basins of the Bohemian Massif. T10 tonstein on top of the Westphalian succession confirms its late Asturian age previously determined from macroflora. T00 tonstein in the Ottweiler Group provided a mid-Barruelian age, which allowed to re-define macrofloral biozones in the lower part of that group from Alethopteris zeilleri of Saberian substage to Crenulopteris lamuriana of Barruelian substage. New UPb ages further improved correlation of local lithostratigraphic units with other basins in Europe and also with global marine-based stages. These new ages allow us to compare radioisotopically constrained Bayesian age model to metronomic age models from Bolsovian to Barruelian and show that the high frequency sequences fall in the range of the precession and obliquity. The sources of the volcanic ashes were probably volcanoes in the Vosges and Black Forest massifs. Considered could be also volcanic centers at a larger distance, e.g., the Teplice-Altenberg volcanic complex for Westphalian, Gothard and Aar massifs in Alps and the Thüringian Forest in Saxony for Stephanian and Autunian tonsteins. The detrital zircons extracted from the sandstones of the Bolsovian deposits and Holz Conglomerate (late Cantabrian) show a wide range of ages from Precambrian to Ordovician that confirms the northward source of these zircons (Rhenohercynian zone) mixed with a local source from the Mid-German Crystalline High.