SPATIAL AND STRATIGRAPHIC DISTRIBUTION OF FOSSILS FROM DIFFUSE SEEPS IN A PLIOCENE SHELF SETTING, CASCADIA CONVERGENT MARGIN

Methane seep signatures are evident in Pliocene sedimentary strata of the mid- and inner-shelf of an active forearc basin on the Cascadia convergent margin (Quinault Formation, Washington State). Persistent but diffuse authigenic carbonate occurs as nodules (blebs), burrow and shell infilling, and along horizontal sand-rich layers. These detrital-rich micrites are surrounded by volumetrically extensive (~1,000 m), thin-bedded sandstone and siltstone of the Pratt Cliff area. Seep-carbonate accumulations, together with characteristic macrofauna, show spatial and stratigraphic heterogeneity that record episodic methane-rich fluid flow, preserved even in the high energy environment that was characterized by Pacific storms and ancestral Quinault River flood. Body and trace fossils and patterns of carbonate deposits typify a spatial gradient of syndepositional fluid flow from the center of seepage outwards. Unlike other fossil and recent seep sites, Calyptogena is rare in the Quinault Formation. Instead, the seeps are defined by chemosymbiotic bivalve assemblages of Acharax ventricosa, (5-15/m³) with occasional Modiolis modiolis, occur with high densities of vertical burrows that are filled with a mud-swirled pattern of sand and carbonates. The burrows (attributed to sulphide mining activities of Acharax) are restricted to areas of high carbonate volume. Stratigraphically surrounding the area of concentrated methane seepage is a zone defined by few carbonate blebs and other bivalve taxa, usually Lucinoma and Yoldia, with a diversity of scavenging gastropods. In turn, this zone is overlapped by a gradational margin of well-formed glendonite crystals. In other sections of the Pratt Cliff outcrop, occurrences of carbonate-filled Rosselia (terebellid polychaete traces) indicate that these burrows served as conduits for even more diffuse hydrocarbon-rich fluid flow. Depleted carbonate δ13C values range from –11 to –34 ⁰/₀₀ PDB, suggest sporadic migration of thermogenic methane, passing through tortuous paths in shelf sediments, and variably mixing with seawater in burrows and sand-rich horizons. Syndepositional, active subsurface faulting, and episodic tectonic squeezing channeled the methane-rich pore-fluids to the Quinault seafloor seep sites.