The Cenozoic CH4 derived carbonates of Monferrato (NW Italy): a solid evidence of fluid circulation in the sedimentary column.

Two types of methane-derived carbonates, each with different lithological and paleontological signatures, have been distinguished in the Oligocene-Miocene sediments of Monferrato (NW Italy): 1) Irregularly-shaped, metre-sized rock bodies characterized by the widespread occurrence of remains of large bivalves (Lucina sp.). These bodies include two subtypes, differing in the lithological characteristics of the cemented sediments: a) Lucina-rich marly limestones, encased in Langhian outer shelf marls. They document a slow seepage of CH4-rich fluids at the paleo-sea floor; b) Lucina-bearing cemented mud breccias, found either as cemented masses in the original stratigraphic position or as redeposited blocks within Upper Messinian chaotic (block-in-matrix) sediments. These carbonate bodies document the activity of submarine mud volcanoes during the late Messinian. In particular, they document intermittent fluid emission ranging from violent eruptive phases, responsible for the extrusion of soft mud breccias at the sea floor, to quiet degassing phases, that promoted colonization by chemosymbiotic communities and localized cementation. 2) Very irregularly shaped strongly cemented, mainly coarse-grained, clastic sediments, reaching the lateral extent of few hundred meters and the thickness of few tens of meters. They occur within Oligocene to Messinian sediments, lack remains of chemosymbiotic taxa and are highly fractured, with fractures filled with sediments and cements. These carbonate-cemented masses, here referred to as macroconcretions, result from the precipitation of carbonate cements within buried sediments flushed by ascending CH4- rich fluids. They document a complex fluid circulation pattern in the sedimentary column, in which SO4- rich waters were drawn from surrounding sediments by the fast and prolonged flow of CH4-rich fluids and promoted anaerobic oxidation of methane and the precipitation of carbonate cements. The polyphase network of clastic dykes and veins that characterize macroconcretions is related to an intermittent fluid flow regime: dykes formed during fast upward flows of sediment-rich fluids whereas carbonate-filled veins formed during slower, degassing phases. The local lithostratigraphic and structural framework allow to correlate genetically and chronologically the macroconcretions to Lucina-bearing mud breccias. Consequently, macroconcretions are here regarded as the result of cementation along fluid conduits feeding Messinian mud volcanoes at the sea floor. The massive release of fluids could have played a role in triggering large-scale masswasting events responsible for the emplacement of sedimentary mélanges including CH4-derived rocks in the Messinian.