CARBONATE PIPES AS EVIDENCE OF FLUID EXPULSION ALONG THE FLANKS OF A MIOCENE MUD VOLCANO IN NW ITALY

The association of cold seep carbonates (blocks or chimneys) with submerged mud volcanoes has been observed in many present day accretionary complexes. In Monferrato (NW Italy) several carbonate masses interpreted as cold seep carbonates are present within the Miocene terrigenous succession; they show strongly depleted d ¹³C values (down to - 40‰PDB) because of the incorporation into the carbonates of light CO₂ deriving from the oxidation of CH₄, typically depleted in ¹³C . At Verrua Savoia, quite regular cylindrical carbonate concretions occurr in a chaotic body made of mud breccias resulting from the activity of a mud volcano. Some pipes can be seen in their original position obliquely crossing the sedimentary layers of mud breccias. They reach lengths of 1,5 m and diameters of 12 cm, show rounded, smoothed edges and a lighter-colored homogeneous, dolomite-cemented, rim surrounding a darker, sediment-filled, core. Calcitic cements and some crystals of barite fill thin fractures that crosscut some of the pipes. Isotopic analyses performed on different portions of the pipes display d ¹³C values ranging from –9 to –24‰ and d ¹⁸O values around 7‰. The petrographical and geochemical characteristics, together with the comparison with chimneys reported in present day settings, suggest that the pipes result of the precipitation of carbonate cement within a terrigenous sediment, below sediment-water interface, around preferential pathways of fluids migration towards the sea floor. The cementation started in the outer rim of the column of sediments involved in the migration of fluids; the so-formed pipes worked as conduits for fluids and unconsolidated fine-grained sediments squeezed towards the sea floor, until the flux stopped and the central conduit was plugged by sediments. These pipes represent a peculiar form of carbonate-cemented fluid expulsion structures that developed along the flanks of a submerged mud volcano during slow seeping of methane-rich and sediment-charged fluids.