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 them are included in slide/slump horizons as scattered blocks. The occurrence of seep-carbonates is clear evidence of 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.