Generation of Tsunamite Seismo-Turbidites in the Ionian Sea (Mediterranean Basin)

Abstract:

We are investigating the effects of earthquakes on the sedimentary record in the Ionian Sea through the analysis of turbidite deposits. A comparison between radiometric dating and historical earthquake catalogs suggests that turbidite emplacement triggered by great earthquakes represents over 90% of the deposits in this region. Although the average recurrence time of single events is about 500 yrs, age modelling indicates that their occurrence is not periodic, but rather varies between 100 and 700 yrs with clusters of higher frequency. Seismic reflection images show that some turbidite beds are very thick and marked by acoustic transparent homogenite mud layers at their top. Based on a high resolution study of the most recent of such megabeds, the Homogenite/Augias turbidite (HAT), we show that it was triggered by a catastrophic event, the AD 365 Crete earthquake. Radiometric dating support a scenario of synchronous deposition of the HAT in an area as wide as 100.000 km², suggesting basin-scale sediment remobilization processes. The HAT (up to 25 m thick) is made of classic stacked and graded sand/silt units with different compositions related to the Malta, Calabria and Sicilian margins. This composition suggests multiple synchronous slope failures typical of seismo-turbidites; however, the Crete earthquake source is too distant from the Italian margins to cause sediment failures by earthquake shaking. Consequently, we propose that the HAT is a deep-sea “tsunamite” deposit. This megabed, together with other turbidites triggered by italian tsunamigenic earthquakes (i.e. AD 1908 Messina, 1693 Catania and 1169 Sicily), is being studied to define differences and similarities with the better known deposits triggered by direct seismic shaking. Textural, micropaleontological, geochemical and mineralogical signatures reveal that such turbidites show cyclic ordered series of sediment units suggesting discontinuities in deposition processes. Utilizing the expanded stratigraphy of the HAT, and the heterogeneity of the sediment sources of the Ionian margins, we are trying to unravel the relative contribution of seismic shaking (sediment failures, MTDs, turbidity currents) and of tsunami wave loading (overwash surges, backwash flows, turbidity currents) for seismo-turbidite generation.