DEEP-SEA TSUNAMI DEPOSITS IN THE EASTERN MEDITERRANEAN - NEW EVIDENCEAND DEPOSITIONAL MODELS

The tsunami wave induced by the collapse of the Santorini caldera afte r the Bronze age (Minoan) eruption (3500 BP) produced turbidites and l arge volume mega-turbidites in the abyssal plains of the Ionian Sea as well as on the floor of small basins of the Mediterranean and Calabri an Ridges, characterized by the so-called 'Cobblestone Topography'. Si nce the first discovery in 1978, a Holocene mud layer which has been t ermed 'homogenite'' and which typically shows a graded interval at its base, has been identified and correlated in over 50 deep-sea cores re covered in the eastern Mediterranean. Four types of 'homogenite' can b e distinguished, each related to a particular depositional setting: (a ) 'Closed Cobblestone': these are from a few decimetres to several met res thick pelagic turbidites of local provenance, exclusively found at the bottom of small-sized ponded basins of the Mediterranean and Cala brian ridges. A debris flow may be present at the base of the turbidit e where the vertical relief of the basins is over 200 m. (b) 'Abyssal Plain': a 10 to 20-m thick megaturbidite recorded in the Ionian and Si rte abyssal plains. The source area is the African continental margin, possibly the continental shelf; it can be easily identified as a tran sparent acoustic layer that shows recent deformation across the deform ation front of the Mediterranean Ridge accretionary prism. The volume of the Minoan 'homogenite' in the Ionian Abyssal Plain has been calcul ated at a minimum of 11 km(3). (c) 'Open Cobblestone': this type is ex clusively found on the outer slope of the Mediterranean Ridge, near th e deformation front. Unlike types (a) and (b), the base of the turbidi te here is erosional instead of depositional, This 'homogenite' has be en deposited in small basins, and occasionally on topographic highs of the Mediterranean Ridge by the up-slope flow of turbidity currents of African provenance that formed the abyssal plain deposit (type b). (d ) A fourth depositional model has been identified: homogenites present in deep anoxic basins of the Mediterranean Ridge. This setting is sub stantially similar to type (a), but the vertical relief of the basin i s much higher (up to 800 m) and the deposition occurs in high-density anoxic brines which modify the settling rate and hence the resulting s edimentological characters.