Messinian chaotic sediments in the Tertiary Piedmont Basin (NW Italy).

Studies on present day continental margins have shown that chaotic deposits, made up of a fine grained matrix enveloping blocks of different size and composition, can be generated by different processes such as sea level oscillations, tectonic deformation, gas hydrate dissociation and mud diapirism. The recognition of role played by these factors in the fossil record is problematic, due to the strong facies convergence of their products. The Messinian succession of the Tertiary Piedmont Basin consists, in large sectors, of a post evaporitic chaotic sedimentary body (the Complesso Caotico della Valle Versa = CTV) that has been mapped both in the Monferrato and in the Torino Hill domains and at the N-margin of the Langhe basin. The CTV is sandwiched, through two regional discontinuity surfaces, between marine sediments spanning in age from the Oligocene to the Early Messinian, and uppermost Messinian "Lago Mare" continental deposits or Early Pliocene deep water marine facies. It consists of a poorly exposed fine-grained matrix, locally consisting of mud breccias, and of blocks of different size and composition including both evaporitic sediments and a wide range of carbonate facies. The latters are made up of bioclastic sediments of Early Messinian age, of "evaporitic" carbonates deposited during the evaporitic phase and of methane -derived carbonates that are related to the emission of methane- rich fluids on the basin floor. Slumped intervals of the Messinian succession, containing scattered masses of methane-derived carbonates, also occur. The geometry and the internal characteristics of the CTV point to an origin related to large scale gravity-driven phenomena that involved both sulphate and carbonate facies deposited during the pre-evaporitic and evaporitic phase of the Messinian. However, the association between chaotic deposits and methane-derived carbonates suggests that also mud diaprism and gas hydrate dissociation could have played a role in the genesis of the chaotic sediments.The occurrence of a sharp angular unconformity at the base of the CTV suggests that intra-messinian tectonics, related to the activation of the Padane thrust front, was the driving factors for both fluid expulsion and gravity-induced phenomena. Both these processes that induced the dismemberment of the studied succession, faded out in the uppermost Messinian with the unconformable deposition of the continental "Lago Mare " sediments.