CHARACTERISTICS OF METHANE-INFLUENCED FLUID-FLOW IN OLIGOCENE - MIOCENE SEDIMENTS OF THE CASCADIA MARGIN

Hydrocarbon seeps and vents are common features of modern continental margins, both passive and active. They feature a characteristic suite of organisms based on chemosynthesis, and stable isotope signatures typical of methane-rich fluids. The Cascadia margin of the eastern Pacific has hosted numerous cold methane seeps since the Late Eocene. This investigation used integrated stable isotope data from foraminiferal and inorganic carbonates to clarify the role of methane-influenced fluids in the sediments of the Late Oligocene/Early Miocene Pysht and Sooke Formations of northwest Washington State, U.S.A., and southwest Vancouver Island, Canada. Both of these units are part of the voluminious siliciclastic Cascadia accretionery wedge. The late Oligocene Pysht Formation was deposited in neritic to upper bathyal depths; most of the early Miocene Sooke Formation is intertidal, however a small embayment of neritic sediments occurs at Sombrio Beach on southern Vancouver Island. The presence of seep-specific bivalves such as Acharax dalli and Modiolus restorationensis, as well as anomalous carbonate nodules and pavements indicated former methane seepage.

As with modern methane seeps and fossil seeps from the Pliocene of western Washington, no foraminiferal species endemic to methane seeps were present. Carbon stable isotope values in the foraminiferal tests were highly variable and more depleted than those from normal marine waters. The carbonates of the nodules and pavements were considerably enriched in ¹³C. The combined evidence indicates the methanogenic source of the methane in these seeps was archaeal. Moreover, in the Pysht Formation, extremely negative ?¹⁸O values indicate the sediments have undergone early diagenetic alteration. Carbon isotopes, however, were enriched in ¹³C. Thus the early diagenetic fluids in these Late Oligocene sediments carried carbon derived from the oxidation of archaeal methane. The sediments of the Early Miocene Sooke Formation do not display the same diagenetic isotope signal.