In the Wynooche Valley quadrangle of southwest Washington, a sedimentary sequence of Tertiary rocks approximately 15,000 ft thick overlies the middle Eocene volcanic rocks of the Crescent Formation. These strata range in age from late Eocene to at least late Miocene and include unnamed beds of late Eocene age and the Lincoln Creek, Astoria(?), and Montesano Formations. At least three major periods of deformation are recorded: (1) post-Crescent and pre-late Eocene; (2) post-Astoria and pre-Montesano; and (3) following Montesano deposition. A few major strike-slip faults trending northeast and several associated normal faults trending northwest are the result of pre-Montesano deformation. Post-Montesano deformation is confined mainly to moderate folding and limited faulting

Potential source rock for petroleum is represented by the thick Lincoln Creek Formation and possibly by parts of the Astoria(?) Formation. Widespread sandstones of the Montesano Formation and some parts of the Astoria(?) Formation are possible reservoir rocks. Several major anticlines and several other smaller folds, together with conditions for stratigraphic as well as fault traps, are present in the Wynoochee Valley quadrangle.

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