Sedimentation in the Tethyan pelagic realm during the Cenomanian : monotonous settling or active redistribution?

Sea bottom processes of the pelagic realm are still not completely understood and represent an intriguing subject. This paper focuses on the relationships between “normal” settling processes, redistribution of sediments and oceanographic parameters in a pelagic setting, during the Cenomanian. Five key Tethyian localities in the Cenomanian Umbria-Marche and Belluno Basins have been studied in order to understand the interplay among sea bottom processes that acted on the sea floor. The dataset consists of the mm-scale sedimentological description of the sections complemented by microfacies analysis on selected samples. Different sedimentological indications, such as presence of intraclasts, lined forams, pervasive plane-parallel lamination, suggest a continuous reworking under action of bottom-currents with varying intensity and direction. All the identified facies are here illustrated in detail and organized in a comprehensive schematic facies framework, the “facies matrix”, that leads to recognize two depositional facies suites: the “settling dominated” and the “traction current dominated”, under different oxygenation conditions. Our results suggest that settling of biogenic and inorganic particles represents the main source of pelagic sediments, but not the unique depositional process: under the action of sea-bottom currents of different intensity, sediments are continuously redistributed on the sea floor. All the collected evidences contribute to the proposal of a comprehensive depositional model for these reworked and redistributed fine-grained sediments, that represent true calcareous pelagic contourites. The model suggests that the identified traction-related facies can be used as a proxy for bottom current intensity and, indirectly, as an indicator of changing ventilation regimes at the sea floor through time.