Origin and chronology of the Augias deposit in the Ionian Sea (Central Mediterranean Sea), based on new regional sedimentological data

In the Ionian Sea (Central Mediterranean Sea), several thick acoustically transparent layers are present including the Augias deposit. The Augias deposit is the most recent, thick layer covering the entire floor of the Ionian Abyssal Plain with an average thickness of 10–12 m and a maximum thickness of up to 24 m in the Sirte Abyssal Plain. This deposit has also been observed in several adjacent smaller basins in waters deeper than 3000 m. Its estimated volume is > 65 km3. There are multiple plausible hypotheses regarding its age and triggering event, which include the 1600 BC Santorini volcanic caldera collapse, the 365 AD Crete M 8.5 earthquake and other smaller earthquakes such as the 374 AD Calabria M 6.3 earthquake and the 361 AD Sicily M 6.6 earthquake. Understanding the cause of this mass-transport deposit is crucial for improving the natural hazard assessment of a vast area between southern Italy and western Greece. In this study, we propose a new interpretation of the Augias deposit in terms of sedimentary processes and origin, based on sediment cores collected in the NW Ionian Sea during the CIRCEE cruise onboard R/V Le Suroit in October 2013. The sedimentological analysis of seven piston cores reveals three sedimentary facies corresponding to the Augias deposit: 1) “homogenite”, 2) “megaturbidite” and 3) “thick sandy turbidite”. These sedimentary facies are distributed within specific morpho-tectonic regions as defined by newly acquired bathymetric data, indicating a strong control by transport and depositional processes. Sixteen radiocarbon dates obtained above and below the Augias deposit indicate a possible time window of about 500 years. This means that we cannot definitively prove a link to the 365 AD Crete earthquake, but we regard it as the most likely trigger. Other depositional models may be plausible, but here we propose a sequence of events as follows: (1) earthquake shock and possible triggering of submarine mass flow in the eastern part of the Ionian Sea (western Hellenic subduction zone); (2) tsunami waves amplified by the confined morphology of the Sicily and Calabria continental shelves inducing an intense re-suspension of fine-grained sediment and massive destabilization at the heads of submarine canyons triggering turbidity currents; (3) gravity driven downslope transport of suspended sediment towards the deep basin and decantation; and (4) a final stage of decantation from seiche waves forming the majority of the homogenite facies in the Ionian Abyssal Plain