Authigenic seep-carbonates cementing coarse-grained deposits in a fan-delta depositional system (middle Miocene, Marnoso-arenacea Formation, central Italy)



Seep-carbonates (13C-depleted) are present at different levels within the Miocene terrigenous succession of Deruta (Marnoso-arenacea Formation, central Italy); they are associated with pebbly sandstones and conglomerates in a tectonically active fan-delta slope depositional system. Most of these seep-carbonates are included in slide/slump horizons as scattered blocks. The occurrence of seep-carbonates is clear evidence of the flow of methane-rich fluids pervading the sediments. Fluids, probably of biogenic origin, may have reached the sea-bottom through thrust faults and selectively infiltrated the more permeable coarse-grained horizons deposited along the slope. Different stages of fluid emissions are documented: slow flux stage, corresponding to the development of large carbonate bodies and dense chemosynthetic communities; and fast fluid flow associated with intense carbonate brecciation, pipes and veins. Large amounts of authigenic carbonates are reworked by slope failures triggered by tectonics and fluids reducing sediment strength; in situ cementation of slide blocks may also have occurred due to remobilization of methane-rich fluids by mass-wasting processes.