Downslope evolution of supercritical bedforms in a confined deep-sea fan lobe, Amantea Fan, Paola Basin (Southeastern Tyrrhenian Sea)

The sedimentology of upper flow regime bedforms represents an important research topic at the present. Deposits interpreted as those of supercritical flows are widely recognized in modern fan systems, but their recovery is challenging. Most of the sedimentological information has come from channel thalwegs but supercritical bedforms are also frequently downslope from the channel mouths. Such an environment has been identified in the Paola basin, where erosive and depositional cyclic steps have been imaged and identified in a sandy submarine lobe of the Amantea Fan. High-resolution sub-bottom profiles provide insight into the bedform internal architecture and their relationships with a frontally-confining ridge. For the first time, supercritical bedforms in a submarine lobe have been interpreted in two distinct positions: in the scour of an erosional cyclic step and in the stoss side of a depositional cyclic step. Coarse to medium-grained massive sand with flame structures, indicating rapid sediment fall-out and frequently associated with the occurrence of hydraulic jumps, has been identified in the scour and at the toe of the ridge. The latter represents an example of topographically induced hydraulic jumps driven by a frontal confinement. Top-cut-out medium to fine sands with tractive structures have been interpreted as the deposits related to the stoss side of a cyclic step or small-scale antidune superimposed on the cyclic step surface. The presented data broaden the understanding of the range of processes that are driven by the interaction between turbidity currents and seafloor topography and the dip of the slope. The recognition that topography influences the density structure and the degree of criticality of the flow and, consequently, the morphodynamics and facies of the relative deposits may help to explain sediment distribution and improve depositional models of fan lobes in confined settings.