BENTHIC FORAMINIFERAL CHARACTERISTICS OF A DIFFUSE PLIOCENE MID-SHELF HYDROCARBON SEEP, CASCADIA CONVERGENT MARGIN

Hydrocarbon seeps within the Pliocene Quinault Formation, western Washington State, show different characteristics from others described in the Cascadia accretionary wedge. Methane seeps in the Quinault sedimentary rocks do not contain the usual carbonate chimney structures, calcite hardgrounds or the dominant macrofauna suites that distinguish other fossil and recent seeps. Instead, diffuse seep signatures are found within the lithological facies that characterize this mid- to inner-shelf environment where both storms and floods were persistent perturbations to the background pattern of sediment accumulation. The purpose of this investigation was to characterize diffuse methane seeps using benthic foraminifera.

Foraminifera were extracted from strata that showed methane seep signatures and strata in which seep signatures were absent. Analytical methods included determining species diversity and population densities, and comparing δ13C values of seep and non-seep foraminiferal tests and authigenic carbonates. Foraminifera populations of the different samples are distinctive. Non-seep samples contain larger populations, averaging 45 individuals/gram of sediment. The fauna are dominated by epifaunal species, notably Haplophragmoides and Cibicides spp. Samples from the most intense seepage zones contain fewer individuals, averaging 15 individuals/gram of sediment. At one particular seep section, the population is dominated by the infaunal Globobulimina auriculata, with persistent occurrences of Uvigerina species. Studies of living foraminifera in Recent seeps have shown both of these species to be tolerant of low oxygen environments. Isotope results on the foraminifera are pending, however studies of the authigenic carbonate noducles, shell infillings and burrow fillings in this seep facies yielded values of -14 to -33.57⁰/₀₀ PDB (Campbell, 1992). These values indicate a thermogenic source of methane, and sedimentary features suggest syndepositional fluid flow regimes.