POSSIBLE SEEP CARBONATES AS SUBSTRATES FOR EARLY OLIGOCENE CORAL AND CRINOID PALEOCOMMUNITIES, NORTHWESTERN OREGON

The unusual occurrence of a rich, concentrated zone of well-preserved fossil crinoids and other organisms in an otherwise barren outcrop of fine-grained sandstone and siltstone of the early Oligocene Keasey Formation at Mist, Oregon, has long puzzled paleontologists. Part of this puzzle could be solved if we consider the possibility that a methane seep once was active at this locality. Localized carbonate deposits generated by this proposed seep activity provided a preferred hardground substrate for a crinoid paleocommunity as well as an adjacent coral-dominated community. The localized nature of these hardgrounds restricted the lateral distribution of the associated paleocommunities. Macrofossils of animals with probable chemosymbiotic life strategies (Solemya, Thyasira, pogonophoran wormtubes) occur in the nearby Keasey siltstone. Moreover, low in the poorly exposed stratigraphic section are brecciated carbonate concretions with vertical calcite veins that indicate fluid migration. Additional field work is required to assess if the veins are syn-depositional or post-date the occurrence of the overlying crinoid- and coral-rich carbonates. Analogs for this proposed paleoenvironmental affiliation include deep-water coral/seep-carbonate associations in the modern Gulf of Mexico and North Sea, and the Miocene Hikurangi Forearc of New Zealand.