Upper Cretaceous oceanic red beds (CORB) in the Northern Calcareous Alps (Nierental Formation, Austria): slope topography and clastic input as primary controlling factors

In the Northern Calcareous Alps, Upper Cretaceous oceanic red beds are composed of the Nierental Formation, deposited in bathyal slope basins along the active margin of the Austroalpine microplate. The sedimentation of red shales and marls was highly diachronous, starting in the Late Santonian. Carbonate contents vary between 50 and 90%. Siliciclastic and carbonate-dominated turbidite and mass-flow and slump deposits are common. Planktic foraminifera dominate the foraminiferal assemblages. Sedimentation rates for red (hemi)pelagic intervals are generally in the order of a few mm/ka up to 26 mm/ka. Within the Gosau valley area red (hemi)pelagic intervals a few tens of metres thick are diachronous. This case study suggests that Late Cretaceous oceanic red-bed deposition was not a single event but distributed over a longer time span and can, therefore, be regarded as the normal background sedimentation in deep-sea settings with relatively low sedimentation rates. However, the colour and thus the oxidation state of the (hemi)pelagic as well as turbiditic sediments was controlled by various regional and local factors. Within the morphologically complex orogenic-wedge setting of the slope basins of the Gosau Group, basin topography and local clastic input strongly influenced the occurrence and facies of the oceanic red beds.