Mud Volcanoes, olistostromes and argile scagliose

Mud volcanoes are chaotic sedimentary bodies originated by the episodic extrusion of deep-seated over-pressurized sediments. Modern submarines mud volcanoes bear diagnostic subrounded shape, conical profile, often with a flat top and slopes marked by debris lobes. Below the extruded body, mud dikes and sills can be recognized. A conduit links the mud volcano to the source rock, that can be a mud diapiric mass. Mud volcanoes are composed of mud breccia with a high degree of variability including polymictic gravels containing hydrofractured clasts, matrix-supported mud debris flow deposits, pebbly mud. Submarine mud volcanoes of the Mediterranean basin occur primarily in spots, or belts, in areas of fold and thrust tectonics, often associated to strike slip motion in the Calabrian and Mediterranean ridge accretionary complexes. In the Apennine chain a belt of small mud volcanoes exists along the westward thrust fronts. Olistostromes are chaotic sedimentary bodies originated by subacqueous mass gravitational processes, such as debris-flows, and submarine slides. They are typically embedded in non-disrupted strata in on-land fossil sequences and are characterized by a chaotic assemblage of blocks in a mudstone-rich matrix. Olistostromes are precursors of the emplacement of Ligurian nappes of the Romagna –Umbria fold and thrust belt of the Northern Apennine in Italy. Along present-day submerged Mediterranean accretionary complexes, submarine slope instability is apparently not taking place. Mass gravitational processes are instead presently widespread on the extensional Mediterranean continental margin. Mud diapirism has been recently considered as the triggering factor for the onset of olistostromes and some of the classic olistostromes of Northern Apennines and Sicily have been recently reconsidered as the product of mud diapirs/volcanoes. Argille Scagliose (Scaly Shales) are a rock type recurrent in the Apennine chain that includes olistostromes and tectonosomes. Whether the olistostomes represent fossil, uplifted and at times deformed mass wasting deposits and/or mud volcanoes/diapirs, tectonosomes are strongly deformed up to stratally disrupted units, which retain their original stratigraphic coherence. The latter represent fossil, uplifted portions of the offscraping complexes of the Cretaceous-Eocene paleo-Apennine accretionary wedge. We review and discuss the occurrence of chaotic deposits in the context of the Mediterranean geological setting.