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

Abstract

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.

Ophiolitic detritus in Kimmeridgian resedimented limestones and its provenance from an eroded obducted ophiolitic nappe stack south of the Northern Calcareous Alps (Austria)

“…They become more common in the Lower Cretaceous Rossfeld Formation (Krische et al 2014) when the erosion cuts deeper into the obducted ophiolitic nappe stack including the subophiolitic mélange.…”

Age and microfacies of oceanic Upper Triassic radiolarite components from the Middle Jurassic ophiolitic mélange in the Zlatibor Mountains (Inner Dinarides, Serbia) and their provenance

“…Therefore, Late Anisian to early Late Ladinian radiolarites either derive from the distal shelf to continental slope region or the oceanic realm, as also expressed in a characteristic microfacies (e.g., Gawlick & Missoni 2015;Gawlick et al 2016 a, b). In contrast, latest Ladinian to Rhaetian ribbon radio larites were absent in continental-margin settings and clearly indicate deposition on the ocean floor (Krische et al 2014;Gawlick & Missoni 2015). The absence of ribbon radiolarites on the continental margin is due to the fact that supply from shallow-water carbonate ramps and platforms led to the accumulation of a thick pile of carbonate mud on the distal shelf and partly even in the proximal oceanic domain (Gawlick & Böhm 2000).…”

“…Detailed microfacies investigations and biostratigraphic data allow detailed information about the depositional history for both the carbonate (e.g., Flügel 2004) and the radiolarite sequences. Such combined investigations on radiolarites remain rare (e.g., Gawlick et al 2009Gawlick et al a, 2016Krische et al 2014;Gawlick & Missoni 2015), but the reconstruction of the depositional environment of radiolarites provides a number of answers for the open questions.…”

“…This is valid for the late Middle and Late Triassic except the Julian stage. Accordingly, radiolarites of this age can only be expected in more distal oceanic areas (Gawlick et al 2008;Krische et al 2014).…”

“…One classical approach is provenance analysis, the reconstruction of the source area from the clast spectrum of the re-sedimented rocks (Blatt 1967;Zuffa 1980Zuffa , 1985Lewis 1984). Whereas the detailed provenance analyses of siliciclastic material is common, provenance analyses of carbonate or radiolarite clasts in conglomerates or breccias remain rare (Gawlick et al 2008(Gawlick et al , 2009(Gawlick et al a, 2015(Gawlick et al , 2016Krische et al 2014). For reliable results, a macroscopic description of the incorporated clasts has to be combined with microfacies analyses (Flügel 2004) and age dating.…”