The Montandone Melange in the Northern Apennines: evidente of mud diapirism.

The Montardone melange (MM) is a chaotic unit consisting of polygenic breccias in a shale matrix (olistostrome texture) outcropping in the core of the E-W trending Montebaranzone syncline in the northern Apennines. The MM occurs mainly at the top of the Termina Formation, a Serravallian-Tortonian unit belonging to the Epiligurian succession; minor, lens-shaped bodies of MM occur at lower stratigraphic positions. The Termina Formation consists mainly of marlstones and includes lens-shaped sandstones and bodies of authigenic seep-carbonates (13C-depleted). Reworked blocks and slabs of the Termina marls are found within the MM.The succession is interpreted here as the filling of a tectonically active slope basin characterized by topographic constrains (lateral confinement) as testified by pinch-out, onlapping, and lateral facies changes to sandstone lobes.The main body of the MM covers the entire syncline with strong lateral thickness changes, making it possible to distinguish two different sectors of the syncline. In the eastern sector the MM achieves the maximum thickness of more than 200 meters. In the western sector it reduces to 40 meters, becoming progressively thinner westwards. In the eastern sector the MM shows a vertical contact with the hosting Termina marls. Bodies of authigenic carbonates occurs close to the contact within the hosting marls, and at the base of the melange. These carbonates, located in the southern flank of the Montebaranzone syncline, have systematic polygenic breccias at the base, with the same composition as the MM. Seep-carbonates occurring in the northern flank do not have these basal polygenic breccias. Previous interpretations suggested a gravity-driven emplacement for the MM, recognized as a large olistostrome. We propose here an alternative hypothesis interpreting the main body of the MM as a mud diapir on the basis of the following evidence: 1) distribution and geometry of the melange; 2) the vertical, abrupt contact with the hosting marls; and 3) the occurrence of basal polygenic breccias characterizing the seepage-carbonates close to the main diapiric body, along the southern flank of the syncline.Most likely, the surface expression was a mud volcano or a mud ridge affected by diffuse slope-instability with the emplacement of coalescent debris flows (olistostromes) also involving blocks of authigenic carbonates. Alternatively, the minor lens-like bodies of MM at different stratigraphic levels could be explained as sill-like intrusions of diapiric material. Mud diapirism and the formation of a mud volcano/mud ridge are attributed here to tectonic loading by overthrusting and the generation of a large amount of methane-rich fluid in the source rocks, i.e. a large older olistostromal body. The occurrence of methane-rich fluids, testified by authigenic carbonates, contributed to generate overpressured shales ascending along sin-sedimentary faults.