Abstract: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.
New investigations on Lower Cretaceous sedimentary rocks southeast of Bad Ischl (Upper Austria) focused especially on outcrops of the Ischl Breccia. The local term "Ischler Brekzie" describes mass flows of the Rossfeld Formation of Late Valanginian depositional age at Salzberg and Kolowratshöhe and of Late Hauterivian to Early Barremian depositional age in the Perneck valley. The polymictic component spectrum contains ophiolitic and siliciclastic material, derived from an ophiolitic nappe stack (Neotethyan Belt). Mixed siliciclastic-calcareous rock fragments and biogenic components originated from the Early Cretaceous shelf area. In addition, Upper Jurassic and Lower Cretaceous Oberalm and Schrambach formations occur as components. Upper Juvavic platform carbonates or Lower Juvavic hemipelagic rocks do not occur in the mass flows of the Rossfeld Formation. The evolution of the Rossfeld Formation was triggered by sea-level fluctuations and only minor tectonic activity. The alleged uniform, laterally continous Lower Cretaceous basin sequence of Bad Ischl in fact belongs to three blocks of different palaeogeographic position: The ReinfalzalmMitterberg Unit (distal Trattberg Rise), the Gschwandtalm-Perneck Unit (proximal Tauglboden Basin) and the Reiterndorf-Hubkogel Unit (distal Tauglboden Basin). The Alpine Haselgebirge Mélange of the Bad Ischl salt deposit was resedimented within the Oberalm Formation in the Late Tithonian corresponding to the well documented Gartenau and Weitenau occurrences. Today´s structural position and exposure of the Bad Ischl salt deposit was successively formed from mid-Cretaceous to Quaternary by both tectonism and erosion.Neue Untersuchungen der Unterkreide-Abfolge südöstlich von Bad Ischl (Oberösterreich) fokussierten sich vor allem auf die Vorkommen der sogenannten Ischler Brekzie. Diese lokale Gesteinsbezeichnung beschreibt olistostromatische Ablagerungen der Roßfeld-Formation. Diese zeigen im Bereich Salzberg und Kolowratshöhe ein spätvalangines, im Perneck Graben hingegen ein späthauterives bis frühbarremes Sedimentationsalter. Der gemischte Komponentenbestand beinhaltet ophiolithische und siliziklastische Resedimente eines ehemaligen ophiolitischen Deckenstapels (Neotethyan Belt). Gesteinsfragmente aus gemischt siliziklastischem-karbonatischem Ursprung sowie Reste von Organismen stammen aus dem ehemaligen unterkretazischen Schelfbereich. Resedimentierte Ober-Jura und Unter-Kreide Gesteine der Oberalm-und Schrambach-Formation stammen aus tieferliegenden erodierten Bereichen. Es treten weder hochjuvavische Plattformkarbonate noch tiefjuvavische, hemipelagische Gesteinsfragmente auf. Die Entwicklung der Roßfeld-Formation wurde durch Meeresspiegelschwankungen und geringe beziehungsweise abnehmende tektonische Aktivität beeinflusst. Die früher als einheitliche Beckensequenz dargestellte Unter-Kreide Schichtfolge von Bad Ischl kann drei Blöcken unterschiedlichen paläogeographischen Ursprungs zugeordnet werden: Der Reinfalzalm-Mitterberg Einheit (distale Trattberg-Schwelle), der...
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