Erosion of a Jurassic ophiolitic nappe-stack as indicated by exotic components in the Lower Cretaceous Rossfeld Formation of the Northern Calcareous Alps (Austria)

The microfacies and biostratigraphy of components in mass-flow deposits from the Lower Cretaceous Rossfeld Formation of the Northern Calcareous Alps in Austria were analysed. The pebbles are classified into six groups: 1) Triassic carbonates (uppermost Werfen to basal Gutenstein Formations), 2) Upper Jurassic to lowermost Cretaceous carbonates (Oberalm Formation and Barmstein Limestone), 3) contemporaneous carbonate bioclasts (?Valanginian to ?Hauterivian), 4) siliceous pebbles (radiolarites, ophicalcites, siliceous deep-sea clays, cherts), 5) volcanic and ophiolitic rock fragments and 6) siliciclastics such as quartz-sandstones and siltstones. The radiolarites show three age groups: Ladinian to Early Carnian, Late Carnian/Norian and Late Bajocian to Callovian. The Middle Triassic radiolarites are interpreted as derived from the Meliata facies zone or from the Neotethys ocean floor, whereas the Late Triassic radiolarites give evidence of the sedimentary cover of the Neotethys ocean floor. During late Early to early Late Jurassic, the Triassic to Early/Middle Jurassic passive margin of the Neotethys attained a lower plate position and became obducted by the accreted ocean floor of the Neotethys Ocean. The accreted ocean floor was contemporaneously eroded and resedimented in different deep-water basins in front of the nappe-stack. These basin fills were subsequently incorporated in the orogen forming mélanges in this complex ophiolitic nappe-stack. The Middle Jurassic radiolarites are interpreted as the matrix of these mélanges. Together with the volcanic and ophiolitic material the siliceous rocks were eroded from this ophiolitic nappe-stack in Early Cretaceous times and brought by a fluvial system to the Rossfeld Basin within the Tirolic realm of the Northern Calcareous Alps. The different fining-upward sequences in the succession of the Lower Cretaceous Rossfeld Formation can be best explained by sea-level fluctuations and decreasing tectonic activity in the Jurassic orogen