Corals from deep-water methane-seep deposits in Paleogene strata of Western Oregon and Washington, U.S.A.

In general, fossils of corals are rare within Eocene and Oligocene marine strata that accumulated in a deep-water, convergent-margin setting and are now exposed in western Oregon and Washington, northwestern USA. At some localities, however, specimens of a few coral taxa are relatively abundant and associated with authigenic limestone deposits. Recently, these highly-localized limestone deposits were recognized as having precipitated due to the microbial oxidation of methane at seeps, areas where hydrocarbon-rich fluids were vented to the sea floor because of the tectonic compression and faulting of underlying sediments. As at modern methane-seeps, the ancient seeps supported dense invertebrate communities, in most cases dominated by tube-dwelling worms and bivalve mollusks, but they can also include gastropods, polyplacophorans, sponges, and corals. A few methaneseep assemblages in Eocene and Oligocene rocks of the Lincoln Creek Formation include the corals Caryophyllia wynoocheensis Durham, and an undescribed Flabellum (Ulocyathus) species. A new species of Deltocyathus appears to be restricted to a single methane-seep site. The corals Stephanocyathus holcombensis Durham, and Flabellum hertleini Durham have been reported from what may be methane-seeps sites in the Lincoln Creek Formation near Holcomb, Washington, and associated with an unusual crinoid-rich bioherm in the Keasey Formation near Mist, Oregon. Other corals, Flabellum (Ulocyathus) n. sp., Archohelia? sp., Caryophyllia wynoocheensis, and Dendrophyllia hannibali Nomland, are reported from a Lincoln Creek Formation locality that includes methane-seep related assemblages near Knappton, Washington. Although widespread in the deep sea today, none of the genera found in the ancient seeps in Washington and Oregon have yet been reported from modern seeps, and corals have rarely been reported from pre-Tertiary methane-seep deposits. It is unlikely that any of these corals, like the bivalves and tubeworms found at methane-seeps, hosted and derived nutrients from endosymbiotic chemotrophic bacteria that were capable of metabolizing some of the reduced compounds in the seeping fluids. More likely, the corals probably were attracted to the greater amount of food at seeps relative to the surrounding deep sea, or to the attachment sites provided by hardgrounds of methane-derived carbonate deposits on the muddy seafloor. [ABSTRACT FROM AUTHOR]