Record of environmental changes in the Triassic–Jurassic boundary interval in the Zliechov Basin, Western Carpathians

Abstract

The Fatra and Kopieniec formations in the Zliechov Basin comprise a record of several environmental crises that may have contributed to the global Triassic–Jurassic (T–J) boundary events. The diversity of the benthic fauna decreases at the base of the ‘transition beds’, the uppermost member of the Fatra Formation. This assemblage comprises many species that do not appear in younger strata. In ‘cycle 13’ of the Fatra Formation, stable isotope excursions correspond to significant lithological changes in the sequence. The negative δ 13C excursion is correlated with a positive δ 18O peak and the beds containing the excursion are overlain by a thin layer with unusual lithological and mineral composition. This layer is composed of small calcitized microspheres showing complex alteration during diagenesis. The origin of this layer, traceable over tens of kilometres, is problematic (impact ejecta, volcanic glass, or altered aragonitic particles). An increase in terrigenous sediment recorded by sandy beds in the uppermost part of the Fatra Formation indicates increased run-off. A sharp lithological boundary between the Fatra and the Kopieniec formations marks a sudden termination of carbonate sedimentation, followed by deposition of the ‘boundary clay’. The negative δ 13C anomaly recognized in this succession may correspond to the isotopic signature at the T–J boundary which has been described from many sections in the world.