MUD DIAPIRS ORIGINATED FROM MUD-RICH MASS TRANSPORT DEPOSITS: EXHUMED FIELD CASES AND MODERN ANALOGUES

This communication addresses the issue of post-sedimentary remobilization of mud-rich, block in matrix mass transport deposits, often defined as olistostromes. The open texture of the volumetrically predominant, muddy matrix (“edge to face” arrangement of clay platelets) predisposes the bodies to a high content of pore fluids. Internal pore pressure may match the lithostatic stress when the bodies are rapidly buried by either the accumulation of younger deposits, or the superposition of hanging wall of thrust sheets and nappes. In these cases, the high fluid pressure may trigger the en-mass remobilization of tens to hundreds of meters thick olistostromes, piercing the overlying sediments and developing mud diapirism, as suggested in various cases now exposed in the Ligurian Terranes of the Northern Apennines of Italy and in the Olympic Peninsula of West Coast of the USA. Mud diapirs originated from mud-rich debris flows are evident in submarine geophysical imagery as in the case of the giant Ruatoria landslide off the East Coast of New Zealand and repeatedly suggested to originating from regional scale olistostromes in the Gulf of Cadiz (Eastern Central Atlantic Ocean) and the Alboran Sea (Western Mediterranean). This communication will show the geometry and the internal fabric and structures of the observed cases in the Northern Apennines, discussing the diagnostic characters such as: 1) composition, age and thermal maturity of blocks, 2) shape, attitude and structures of contacts with surrounding “normal” deposits, and 3) the distribution and attitude of blocks, clasts and flow structures in the matrix. We will also point out that the ascent of muds deforms the overlaying strata, changing the geometry of sedimentary basins and, thus, controlling the deposition of contemporaneous sediments. Mud diapirs are in some cases associated to methane-rich, cold seepages at the sea floor developed during their ascent and emplacement, suggesting a genetic link with methane release and gas-charged fluid venting.