Abstract:
A fossil methane seep community is preserved in marine strata of the Mio-Pliocene Quinault Formation, southwestern Washington. Quinault sediments were deposited unconformably on a middle Miocene accretionary complex, the Hoh rocks assemblage; both units are now accreted to the continental margin, uplifted, and exposed in coastal seacliffs along the western Olympic Peninsula. The Hoh accretionary complex was probably the source of methane-rich fluids to the proposed Quinault paleoseep, which comprises approximately 15 m vertically by 75 m horizontally of an extensive outcrop of mid-shelf depth deposited sandstone and siltstone. The paleoseep is characterized by the vent-type bivalves Solemya, Lucinoma and Modiolus, and by localized authigenic carbonates in a dark siltstone deposit that is finer-grained than the surrounding, non-seep strata. The stable carbon isotopic signatures from the localized carbonates suggest that their precipitation was influenced at least in part from the oxidation of thermogenic methane present while the paleoseep taxa flourished at the site. The Quinault paleoseep is similar to modern methane seep communities that thrive offshore of Oregon. However when compared to the present-day setting, ancient fluid expulsion was apparently relatively diffuse: it produced a fossil seep of limited areal extent with a localized distribution/abundance of seep invertebrates and associated carbonates. The tectonic, sedimentologic, isotopic and taxonomic evidence used to identify the Quinault paleoseep example can be combined with similar patterns observed worldwide for other fossil seeps of varying ages to predict the occurrence and distribution of new fossil cold seeps, particularly in ancient convergent margin settings. Specifically, offshore-deposited marine strata with anomalous sedimentologic (carbonates, sulfides) and isotopic (light stable carbon values) features as well as odd faunal associations (low diversity, high abundance) could be considered "seep-suspect," if found together with various preserved structural conduits for fluid flow (faults, veins, diapirs, shear zones).