Mineralized conduits are part of the uppermost plumbing system of Oligocene methane-seep deposits, Washington State (USA)

Plumbing systems of marine seeps are complex pathways along which hydrocarbon-rich fluids move upward through the sedimentary column. In the shallow subsurface, seep plumbing systems may become successively filled by methane-derived authigenic carbonates resulting in mineralized conduits. This work investigates mineralized conduits in the uppermost plumbing network of Oligocene seep deposits from the Lincoln Creek Formation in Washington State that consist of a paragenetic sequence of authigenic mineral phases. At two study sites the pipes grade into the base of seep deposits. The studied samples of concentrically-mineralized conduits are cylindrical in shape, measure 2e3 cm in diameter, and are up to 15 cm in length; the mineralized conduits are referred to as pipes hereafter. The earliest mineral phase in the paragenetic sequence is matrix micrite, making up the outer part of the pipes. Toward their centers the pipes are filled by two intercalated cement phases, (1) clear, banded and botryoidal aragonite and (2) micro-to cryptocrystalline yellow aragonite. The innermost portions of the pipes are filled by brownish calcite, microspar, or pipe-filling micrite. The observed paragenetic sequences archive successions of biogeochemical processes. The d13C values of clear and yellow aragonite cements range from 50.6 to 41.2‰, suggesting that their main carbonate source was provided by anaerobic oxidation of methane. In contrast, the later phases (brownish calcite, microspar, pipe-filling micrite) are relatively enriched in 13C (d13C values: 2.3 to 3.9‰), pointing to carbonate precipitation from fluids affected by methanogenesis. The size and morphology of the pipes suggest that initial excavation was caused by burrowing organisms. The burrows subsequently acted as preferred fluid pathways, suggesting that the transport of fluids within near-surface sediments at the Oligocene seeps was strongly influenced by the activity of the local fauna. Possible producers of the burrows were sediment-dwelling crustaceans, specifically callianassid decapods, as well as bivalves.