New records of Oligocene diffuse hydrocarbon seeps, northern Cascadia margin

•Fossil methane seeps are described in the northern Cascadia margin.•These seeps are identified by lithologies, with and without macrofaunal indicators.•Seeps are characterized by diffuse seepage of fluids from various carbon sources.•These Oligocene seeps occurred within the actively deforming accretionary prism.

Fossil methane seeps are preserved in late Eocene through Pliocene marine sedimentary units of the Cascadia margin of the northeastern Pacific. All Cascadia fossil seeps are situated in the uplifted accretionary wedge that overlies extensive Middle Eocene coal-bearing units and the Paleocene–Early Eocene Siletzia large igneous province. This study describes eight previously unrecorded Oligocene seeps, all of which are identified by small, scattered carbonates with or without accompanying characteristic seep macrofauna, from both inner and outer shelf depths. We interpret these deposits as reflecting spatially and temporally diffuse seepage of hydrocarbon-enriched fluids, sourced from various carbon sources within the actively deforming accretionary prism, and term them diffuse seeps. δ13C and δ18O values from foraminiferal carbonate from the studied stratigraphic units have broad variability. Authigenic carbonates occurring as nodules, blebs, burrow fill and fracture fill indicated microbial and thermogenic methane, and multiple fluid flow pathways from within the actively deforming accretionary wedge. It is apparent that the complex fluid-flow regime observed in modern seeps of the Cascadia margin is a continuation of one established in the late Eocene.