Rudist formations in mixed siliciclastic-carbonate depositional environments, Upper Cretaceous, Austria: stratigraphy, sedimentology, and models of development

The upper Turonian to lower Campanian succession (Lower Gosau Subgroup) of the Northern Calcareous Alps, Austria, provides a model for the development of rudist formations on wave-dominated, mixed siliciclastic-carbonate shelves that were situated on top of an accretionary wedge. The rudist formations are present either within parasequences or in non-cyclic transgressive successions, and include (a) skeletal mounds up to about 20 m thick that are composed of corals, rudists and, locally, skeletal sponges; and (b) rudist biostromes of less than a metre to several meters in thickness. In the coral–rudist mounds hippuritids and radiolitids thrived together with scleractinians, skeletal sponges and diverse encrusters. The mounds are topped by a hippuritid biostrome. We distinguish hippuritid biostromes, radiolitid biostromes and `composite biostromesʹ with a vertical succession from hippuritids to radiolitids. The biostromes are less than a metre to a few meters thick and show autochthonous rudist fabrics, parautochthonous rudist fabrics, and vertical combinations thereof. Autochthonous rudist fabrics had a simple trophic structure composed of epifaunal suspension feeders, whereas parautochthonous rudist fabrics had a more complex trophic structure. The rudist biostromes were either terminated by frequent toppling and fragmentation of rudists, by burial with sediment, or by environmental restriction. Hippuritids and radiolitids could both colonize and persist in siliciclastic depositional environments. Overall, however, the presence/abundance of rudists and of intervals of shallow-water limestones correlate. Because the Late Cretaceous rudists thrived together with hermatypic corals as well as in environments precluded to the latter, a larger number of rudist formations and a smaller number of coral–rudist formations results. In siliciclastic-dominated depositional sequences, both in the transgressive and in the highstand systems tract, coral–rudist mounds and rudist biostromes locally accumulated in areas of intermittently reduced siliciclastic input in open lagoons and in the shoreface to inner shelf environment. Transgression of rocky coasts or of gravelly carbonate beaches is recorded by a basal interval either of cliff talus breccia or of beachface and shoreface conglomerates, overlain by a coral–rudist mound and/or a rudist biostrome with associated bioclastic limestones. Where siliciclastic input was persistently low, the highstand systems tract developed as a regressive carbonate shelf succession that consists, in its lower part, of coral–rudist mounds, hippuritid biostromes and bioclastic packstones to grainstones deposited from bioclastic sand bodies, whereas the upper part consists of radiolitid biostromes and bioclastic wackestones to packstones. The regressive carbonate shelves were narrow and included, on a reconstructed internal-to-external transect, micro-tidal flats, an open lagoon with radiolitid biostromes, a dissipative shore zone with bioclastic sand bodies, and an inner shelf facies belt with coral–rudist mounds and hippuritid biostromes. In the contemporaneous mid- to outer shelf environment, siliciclastics were deposited.