The Permian–Triassic boundary in the continental series of Eurasia

Abstract

The most important Eurasian sections, where the transitional Permian/Triassic boundary beds are characterized palaeontologically, are found in the Germanic Basin (West European platform), Moscow Basin (East European platform), Tungusska Basin (Siberian platform), Jimsar Basin, Dalongkou area (Tianshan Mountains), and Noyan Somon depression (Mongolia). Usually the Permian and Triassic continental formations formed under distinct palaeogeographic (especially climatic) and palaeotectonic conditions, so the lithological and sedimentological differences between them are pronounced. Difficult problems appear in those regions where the sedimentation at this level proceeded under very similar conditions (Tungusska Basin, Jimsar Basin) across the Permo–Triassic boundary (PTB). Among palaeontological data, the most important for correlation of these continental beds is the tetrapod fauna. Broad interchange between the tetrapod fauna of the Old World continents resulted in the wide distribution during the Late Permian and Early Triassic of common or closely related forms, which enables distant correlations. The PTB is marked by the change of the Late Permian tetrapod communities, dominated by the large herbivore Dicynodon (or closely allied forms), to assemblages that include Lystrosaurus as the most common form. This is recorded in Lower Triassic beds of Eastern Europe, China, Mongolia, India and Siberia. The Upper Permian sporomorph associations (sa) are dominated by striate bissacate pollen, whereas the early Lower Triassic ones are distinguished by the very important role of spores, primarily by cavate trilete ( Lundbladispora, Densoisporites) and also by non-striate dissacate ( Klausipollenites) and teniate pollen ( Lunatisporites). In the oldest horizons of the Triassic, very distinctive species of conchostracans ( Falsisca eotriassica Kozur or the closely related form F. verhojanica Molin) appear. The PTB is within a normal polarity zone, formerly considered the lowest palaeomagnetic zone of the Triassic. The boundary beds of two sections (Moscow Basin and Jimsar Basin) may be equally worthy as candidates for the PTB GSSP in the continental series. The preference should be given to the section better characterized and internationally accessible to scientists. The PTB generally accepted for the continental series coincides more or less with the base of the Otoceras concavum ammonite zone of the Boreal province, and with the base of the boundary clay in Tethys. At the same time, it lies clearly below the traditionally adopted PTB in marine sequences at the base of the Otoceras woodwardi zone and also below the First Appearance Datum (FAD) of I. parva in the Meishan section, proposed now as the PTB GSSP (Yin, 1996, The Palaeozoic–Mesozoic Boundary Candidate of Global Stratotype Section and Point of the Permian–Triassic Boundary, China University Geoscience Press, Wuhan, 137 pp.). This demonstrates that the FAD of I. parva can be asynchronous (see Baud, 1996, Albertiana 18, pp. 6–9). That is why in the choice of the PTB GSSP in the marine series it is necessary to take into consideration the generally accepted point of view on the position of PTB in the continental series.