- The Tertiary geologic evolution of the Oregon and Washington continental margin was molded by episodic periods of convergence between the Pacific oceanic plates and the North American plate. This margin is the site of a deep basin that is floored by Paleocene to lower Eocene oceanic crust and contains more than 7000 m of sedimentary and volcanic rocks. A middle Eocene turbidite sequence overlaps this oceanic basalt and pre-Tertiary rocks of the Klamath Mountains, thus indicating that suturing of the Coast Range-Olympic terrane to North America occurred about 50 Ma.
Oblique convergence between the Farallon and North American plates occurred during most of the middle Eocene to middle Miocene; and sedimentation, punctuated by episodes of volcanism, was essentially continuous in a fore-arc basin whose axis lay along the present inner continental shelf. Two periods of more head-on convergence occurred during the late Eocene and middle Miocene, and thick accretionary wedges of melange and broken formation were formed during these strongly compressive episodes. Geochemical analyses indicate that the melange, which crops out along the west side of the Olympic Peninsula and beneath the adjacent shelf, has the highest potential for oil and gas generation. These tectonically complex strata are undoubtedly the source rocks for gas seeps and oil and gas shows in exploratory wells in this area. Potential exploration targets exist where Eocene and/or Oligo-Miocene melanges are underplated beneath the Eocene oceanic basalt. Hydrocarbons generated in the melanges could migrate upward into structures in strata that overlie the basalt in the upper plate.
- From: Scholl, D. W., Grantz, A., Vedder, J. G., & Geological Survey (U.S.). (1987). Geology and resource potential of the continental margin of western North America and adjacent ocean basins--Beaufort Sea to Baja California. Houston, Tex., U.S.A: Circum-Pacific Council for Energy and Mineral Resources.