Middle Triassic radiolarite pebbles in the Middle Jurassic Hallstatt Melange of the Eastern Alps: implications for Triassic-Jurassic geodynamic and paleogeographic reconstructions of the western Tethyan realm

摘要

The microfacies and biostratigraphy of radiolarite and limestone components in mass-flow deposits from the upper Middle to lower Upper Jurassic Hallstatt Melange were analyzed. The radiolarite components are of Late Anisian to early Late Ladinian age, the various colored limestone components are Late Triassic Hallstatt pelagics. All components are interpreted as derived from the continental slope towards the Neotethys Ocean (Meliata facies zone). A comparison with preserved successions from the Carpathians, the Pannonian realm, and the Dinarides is presented to strengthen this interpretation. Reworked oceanic crust is missing in the mass-flow deposits. The reconstructed Middle-Upper Triassic sedimentary succession documents the existence of Triassic radiolarites also in the distal passive margin setting of the Eastern Alps. The occurrence of Middle Triassic radiolarites can therefore no longer be used to attribute tectonically isolated units in the whole Circum-Pannonian realm to indicate either Alpine-Carpathian or Dinaride-Hellenide provenance. In addition, the occurrence of Middle Triassic radiolarites in the Jurassic melanges is no proof for the existence of an ocean (e.g., Meliata Ocean). Middle Triassic radiolarites are a common sedimentary feature in the distal passive margin setting of the Neotethys. In contrast, Upper Triassic radiolarites are missing in the passive margin setting and were exclusively deposited on the ocean floor.