TECTONIC SETTING OF THE WILLAMETTE VALLEY

The southern Willamette Valley is a strike valley underlain by erosionally-weak late Eocene fine-grained strata (Eugene Formation) overlain on the east by more resistant Western Cascade volcanic rocks and underlain on the west by more resistant Spencer and Tyee (Eocene) sandstones of the Coast Range. These form a gently east-dipping homocline, with folds and faults within the valley broadening the Eugene outcrop belt. North of Albany, the strike valley narrows, with its eastern edge an east-dipping cuesta of Columbia River Basalt Group (CRBG) forming the Salem and Eola Hills. The CRBG curves eastward to form the Waldo Hills, blocking the southern valley from the northern Willamette Basin. The northern basin is broadly synclinal, underlain by CRBG, with late Tertiary-Quaternary nonmarine strata in its core and Western Cascade volcanic rocks marking its eastern edge. The Stayton Basin on the south and the Tualatin Basin on the northwest are also synclinal, underlain by CRBG. The Coast Range rose as a north-south arch largely after CRBG emplacement. The Willamette River follows the strike valley northward and turns east, crossing the Salem Hills at the Salem water gap to flow northward through the northern Willamette Basin. The NE-striking Corvallis-Turner-Waldo Hills fault system is the largest structure in the mid-Willamette Valley. The northern basin is cut by the Portland Hills-Clackamas and Gales Creek-Mount Angel NW-SE right-lateral strike-slip faults. Other faults with diverse strikes are found in the subsurface, but their seismic potential is uncertain. Only the Portland Hills fault would be classified as active using California Alquist-Priolo standards.