The Late Eocene ‘Whiskey Creek’ methane-seep deposit (western Washington State)

Abstract

Large limestone boulders are eroding from a landslide west of the mouth of Whiskey Creek, Clallam County, Washington State. These boulders are composed of micrite, carbonate cement, and densely-packed fossil bivalves. Siltstone in the landslide, and on the surfaces of the boulders, indicates that these limestones are derived from the lower part of the Pysht Formation. The molluscan taxa and their localised occurrence within limestone are typical features of ancient chemosymbiotic cold-seep communities. Formainiferans from both the siltstone and the limestone indicate that deposition occurred during Late Eocene time, at water depths of between 500 to 1,500 m. Lipid biomarkers, particularly isoprenoid hydrocarbons and fatty acids, with δ13C values as low as −101‰ PDB, reveal that the anaerobic oxidation of biogenic methane was an important component in the biogeochemical cycling of carbon in the ancient seep environment.