Messinan chaotics: distribution and significance of large-scale mass wasting bodies in the Mediterranean area

Large scale submarine mass wasting deposits, exceeding hundreds of meters of length and width and a ten of meters of thickness, are a recurrent character of sedimentary successions. Their distribution in the Mediterranean mainly follows the circum-Mediterranean chains and spans in age from Precambrian to Quaternary (Camerlenghi and Pini, 2005). They can be of both intrabasinal and extrabasinal composition, made up of rocks and non-consolidated sediments, and are related to various processes, such as block slides, slumping and debris avalanches and flows. These deposits have been defined, therefore, with various terms in the geological literature, such as slide, (mega)slump, olistostrome, olistoliths accumulation and field, olistolithic flysch, wildflysch and sedimentary melange. Some of these terms point out the general stratally disrupted to chaotic aspect of these bodies at the outcrop observation. Recent works show that submarine instability played an important role in defining the present-day sedimentological characters and the structural setting of the evaporitic succession cropping out in the Northern Apennines, Sicily and Spain (Roveri et al., 2003 and in progress). Moreover, the majority of the chaotic bodies by mass wasting accumulation buried beneath the Po Plain occurs into seismic units of Messinian age and involves evaporitic rocks (Argnani et al., 2003; Artoni et al., 2004 ). Finally, the classic olistostromes in Sicily are associated to Messinian successions. Mass wasting bodies seem, therefore, to be of particularly importance in the Messinian, in association with the evaporites. This has been confirmed by a detailed study of the distribution of large-scale mass wasting bodies during the Miocene in the Apennines and Sicily: their distribution is not continuous through time, but appears to be concentrated in well defined stratigraphic intervals. In these intervals, large mass wasting deposits occur almost contemporaneously in different parts of the foreland basins, suggesting regional scale instability of the entire accretionary wedge-foredeep system (Lucente and Pini, 2003). As far as the Messinan is concerned, large mass wasting episodes are concentrated in the post-evaporitic interval, even if in some cases they are associated to the continuation of instability processes onset in the lower Messinian (Artoni et al., 2004). Messinian examples derive from a wide range of mass wasting processes, spanning from gliding of blocks to slumping of beds and bed packages of clastic gypsum, avalanches of gypsum blocks and to muddy debris flows (olistostromes). This and the unusually widespread occurrence of Messinian chaotic bodies are probably related to the activation of thrust and wedge fronts during an important tectonic phase (intra-Messinian phase), associated with a chronic instability of submarine slopes due to the presence of already consolidated rocks (gypsum) resting on non-consolidated and wet, fine-grained pre-evaporitic deposits. Other triggering effects can be related to variation of sea level and to destabilization of gas-hydrates. The contribution of mud volcanism and diapirism has been recently suggested (Festa et al., 2005) and might be a concomitant triggering mechanism. The distribution and characters of mass wasting deposits and the different genetic hypotheses will be discussed in this communication.