Systems supplying sediment to canyon heads (SSSCHs) in the Tyrrhenian Sea: The past and the present as a key to understanding deep-sea stratigraphy

Systems Supplying Sediment to Canyon or Slope Channel Heads (SSSCHs) initiate sediment transfer from the shallow water areas to the deep sea. SSSCHs are very sensitive to sea-level variations, to perturbations in sediment production and redistribution and to variations in accommodation in the shelf. Through the analysis of bathymetric and subbottom data, I review the variability in time and space of SSSCHs in the eastern Sardinian and northeastern Sicilian margins of the Tyrrhenian Sea. I distinguish three main types of SSSCHs: type I corresponds to shelf-edge deep-water coastal systems; type II corresponds to continental shelf, shoal-water coastal systems; type III is coast-disconnected. Further subdivisions involve the degree of confinement and the process-regime at the shelf-edge for Type I SSSCHs, and the distance from the canyon head and the process-regime for type II SSSCHs. Coast-disconnected type III SSSCHs consist of shelf-wide depositional belts, both un-channalised and channalised, as well as of landslide complexes. Each of the different SSSCHs has its distinct process-regime, which regulates its capability of sediment transfer to the canyons or channels and eventually to the deep-sea. SSSCHs of type IA and IB develop coastal bodies at the shelf-edge, which store part of the sediment budget. Type IC SSSCHs exhibit a direct connection between a river and a canyon, leading to shelf-edge sediment bypass; direct hyperpycnal discharge and landslides result in the largest efficiency in transferring sediment to the deep sea. The efficiency of type II SSSCHs depends mainly on their distance from the canyon head and on the ability of hyperpycnal flows, both from plunging river-floods and from wave resuspension, and storm waves, in transporting sediment offshore. Type IIC SSSCHs, consisting of tide-dominated shorelines, play a minor role in the study area due to the microtidal regime of the Mediterranean Sea. Type IIIA SSSCHs are also rare, since they correspond to shelf-wide depositional belts, that in the study area only seldom are able to reach the canyon heads. Type IIIB SSSCHs, corresponding to landslide complexes, also have a restricted frequency but can contribute a large amount of sediment to the deep-sea. Sea-level variations have the largest influence on SSSCH evolution along the two margins of the study area. Type I prevails during the falling-stage and the lowstansd of sea level. They are however, also active during the present sea-level highstand in the tectonically active Sicilian margin. Type II SSSCHs dominate the TST but their activation is diachronous, being dependent also on inherited margin physiography. The present review highlights the large spectrum of processes that contribute to sediment supply to canyon or slope channels heads. As a result, the complexity of the initial part of the marine tract of the source to sink system that transports sediment from the shallow-to the deep-sea is uncovered. While stressing the role of the previous evolution of a margin, of tectonic activity, and of climate variations, the present review acknowledges the importance of sea-level variations as a control on SSSCH development. It however shows that the complexity of the sedimentary processes that rules the transfer of sediment to the deep-sea results in times of activity during a cycle of sea-level variations, and consequent depositional histories, which do not conform with those predicted by the sequence stratigraphic models.