Trace fossils, ichnofabrics and sedimentary facies in the shallow marine Lower Miocene Molasse of Upper Austria

The distribution of trace fossils in relation to sedimentary facies was studied in Lower Miocene (Ottnangian) siliciclastic sediments that accumulated in shallow subtidal to offshore environments. The sedimentary facies can be arranged according to their environmental energy level. The highenergy facies are characterized by evenly laminated and massive channel fill sands, slump deposits, and sand dunes with bundle structures. Trace fossils in this facies are restricted to isolated Rosselia and local biogenic disturbances of mud drapes. The moderate-energy facies consist of cross-bedded sand dunes at decimetre-scale, flaser bedding, wavy bedding as well as sandy and muddy beds, which are partly homogenized by bioturbation. Cylindrichnus and Macaronichnus occur in great abundance and form characteristic ichnofabrics. Echinoids, represented by trace fossils like Scolicia and Bichordites, ploughed sandy sediments in some localities. Also Planolithes, ?Alcyonidiopsis, Skolithos, Ophiomorpha, and Rosselia could be identified in the moderate energy facies. The relatively low energy facies are dominated by finely laminated mud with very thin sandy laminae and muddy layers of varying thickness, which are partly to totally homogenized by bioturbation. Generally these facies display an increased overall intensity of bioturbation and contain Teichichnus, Cylindrichnus, Planolithes, Macaronichnus, Rosselia, Ophiomorpha, Skolithos, Thalassinoides, and locally Scolicia. All identified ichnotaxa, except for Rosselia, are restricted to the moderate to relatively low-energy facies. Rosselia appears to be fairly independent of energy level and substrate. Generally all these trace fossil assemblages display most similarities to the Cruziana ichnofacies. Locally, the Skolithos ichnofacies occurs in moderate-energy facies. Most trace fossils indicate opportunistic, r-selected colonization of the sediment, lacking mature tiering. A reason for this might be environmental stress caused by the tidal regime, high depositional rates, and probable eutrophy. The bathymetric interpretation of microfossil assemblages suggests deep neritic environments, which differs drastically from the sedimentologic and ichnologic approach.