Authigenic carbonates in the upper Miocene sediments of the Tertiary Piedmont Basin (NW Italy): vestiges of an ancient gas hydrate stability zone ?

A wide array of carbonate-rich rocks has been recognized in the Tertiary Piedmont Basin (NW Italy), hosted in lower Messinian slope deposits. Carbonate cements show negative 13C values and positive 18O values, suggesting that carbonate precipitation was induced by microbial degradation of methane produced from gas hydrate destabili za tion. Two groups of rocks have been distinguished: (1) Lucina-bearing mud breccias, representing the seafl oor product of an ancient seepage site; and (2) Lucina-free concretions, originating below the sedimentwater interface. Within this group, two subtypes have been further distinguished: stratiform concretions and cylindrical concretions. Stratiform concretions result from precipitation of dolomite in the pores of muddy sediments. Some of them display a brecciated structure; others show a network of septarian-like cracks that are empty, fi lled with sediments, or zoned carbonate cements. Their internal features are related to the formation of gas hydrates within the sediments and to their destabilization. Thus, these rocks mark a portion of the sedimentary column located within a (paleo) gas hydrate stability zone. Cylindrical concretions represent ancient fl uid conduits related to the upward migration of CH4-rich fl uids subsequent to gas hydrate destabilization. The carbonate-rich rocks of the Tertiary Piedmont Basin stand as one of the fi rst examples of methane-derived rocks that record successive episodes of dissociation and reformation of gas hydrates, and they provide precious elements to model the general evolution of a portion of the sedimentary column located within the hydrate stability zone.