Slope breaks and turbidity currents interaction: process understanding from plunge pool analysis in the Tyrrhenian Sea

In the submarine environment, plunge or impact pools are depressions, which form through perturbations in the behaviour of submarine gravity-flows, at places of abrupt gradient reductions. In this paper, we examine a large number of plunge pools in the Tyrrhenian Sea, a back-arc basin characterized by large, complex slope sectors often with alternating higher- and lower-gradient areas.  In the present analysis, we target the morphologic parameters, the physiographic setting and the upslope and downslope surroundings of the slope breaks and the associated plunge pools. Canyon-mouth plunge pools are located where turbidity currents, originally confined within steep canyons, experience an abrupt slope reduction and a loss of confinement. This setting, occurring at the base of both the continental slope and intra-slope steps, results in enhanced erosion and in relatively large and deep plunge pools with long-axis perpendicular to the slope. Lateral bulges, which fade gradually away, laterally and downslope, flank some of the plunge pools. They resemble levees and are thus an indication of depositional processes associated with the spill-over of the highest portion of flows. These constructional features are not present in the frontal part of the plunge pools, which rather connects downslope to channels. In other cases, canyon-mouth plunge pools connect downslope to relatively large radial bulges suggesting deposition in fan bodies from rapid flow deceleration; concentric bedforms show that flow instabilities formed in the plunge pool area propagate in large part of the fan bodies. In some cases, the central deeper part of the plunge pools connects laterally to erosional moats parallel to the inbound slope, showing that flows spreading laterally away from the canyon-mouth have increased erosional power along the tectonic structure. Gully-mouth and slope-embayment plunge pools are mainly sub-circular and often surrounded by a rampart, evidence of rapid deposition at the border of the structure. Open-slope-plunge pool form at the base of featureless slope sectors and are likely due to mostly unconfined currents flowing down the slope of seamounts. Fault-controlled plunge pools occur in grabens, where unconfined flows cross an escarpment formed by a transverse fault. They form at the base of the structure as continuous depressions parallel to the structure or as an array of isolated, laterally discontinuous, circular structures. Landslide-plunge pool are located downslope from slope sectors characterized by extensive landslide scars; we interpret them as resulting from turbidity currents formed by the transformation of repeated landslides. Our analysis details the wide range of seafloor topography and turbidity current character that are conducive to plunge pool formation. It shows that plunge pools display large morphologic variability and a multiplicity of genesis, thus widening our process understanding of slope-break settings. Furthermore, our analysis show that plunge pools and their impact on sedimentary processes further downslope are important elements to be considered in environmental and facies models of topographically complex slopes. As such, it can contribute to submarine geo-hazard evaluations and to hydrocarbon reservoir assessment in areas characterized by slope breaks.