| Abstract |
- Surface geology, seismic data, petroleum exploratory well data, and water well data
have been used to analyze the structural and tectonic history of the southern Willamette
Valley. Tertiary strata beneath the southern Willamette Valley appear to have had an early
Cascade or Clarno volcanic source to the east by the middle Eocene. The Tertiary strata
have been deformed into a series of broad north-northeast trending folds and northeast and
northwest trending faults which initially developed under east-northeast compression
during the middle Eocene and have since been rotated clockwise to their present positions.
The cross-cutting pattern of subsurface faults has been complicated by reactivation during
the clockwise rotation of Si to its present orientation of north-south. Uplift of the Coast
Range prior to emplacement of the Miocene Columbia River Basalt Group (CRBG)
produced the gentle east dip of strata beneath the western edge of the Valley and beneath the
CRBG in the Salem and Eola Hills.
The southern Willamette Valley is controlled by erosion of the relatively
incompetent Eugene Formation following emplacement of the CRBG. Neogene sediments
deposited after this degradational event suggest that during the late Miocene to Pliocene, the
proto-Willamette River flowed east of the Salem Hills before uplift along the Waldo Hills
forced its course to the west. This aggradation appears to have been caused by increased
uplift of the Coast Range and/or subsidence of the Willamette Valley over the slab bend in
the subducting Juan de Fuca plate. Degradational and aggradational periods during the
Pleistocene appear to have been caused by readjustment of the Willamette River system to
new base levels and changes in sediment supply to the valley.
Neotectonic features in the valley include: 1) the Owl Creek fault which is at least
Pleistocene in age and possibly younger, 2) the Harrisburg anticline, 3) the Turner fault,
and 4) deformation in the North Santiam River basin including the Mill Creek fault. With
the exception of the Owl Creek fault, the minimum age of these structures is poorly
constrained but is at least post-Miocene and possibly younger.
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| Additional Information |
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