Geology Reference
In-Depth Information
Miocene, and Pliocene reflect shallowing as the ocean
shoreline retreated northwestward. These marine
sediments were covered in turn by Columbia River
lavas that poured through the gorge from eastern
Oregon during the middle and late Miocene to invade
as far south as Salem.
Uplift and tilting of the Coast Range block
and Western Cascades brought about the trough-like
configuration of the Willamette Valley and the forma-
tion of a number of closed basins on the continental
shelf. During the Pliocene and Pleistocene in the
northern part of the province, a large lake received
silts, muds, and gravels from the Willamette and
Columbia rivers. The eruption of the Boring lavas from
over 100 small volcanoes near Oregon City as well as
east and west of Portland covered these earlier lake
sediments, and today the vents project as small buttes.
The dominant signature on Willamette Valley
geology resulted from a number of large-scale Pleisto-
cene floods that scoured eastern Washington and the
Columbia River gorge leaving deposits throughout the
province. Enormous glacial lakes formed in Montana
when the Clark Fork River was dammed by ice and
debris. Once the ice blockage was breached, rushing
flood waters carrying icebergs cascaded across Idaho,
southeastern Washington, and down the Columbia
gorge. Water backed up into the Willamette Valley
creating temporary lakes and strewing a field of boul-
ders in its wake. An unknown number of floods took
place during a 2,500 year interval until the climate
warmed, and glaciers retreated northward.
Because of its position close to the offshore
subduction zone between Pacific Northwest plates,
Oregon experiences a continual number of seismic
events, and in the future the state could be the site of
catastrophic earthquakes although details of time and
place are uncertain. The coastal regions and Willamette
Valley would be particularly vulnerable should a strong
quake occur.
Geology
Geologically part of the eastern margin of the
Coast Range block, foundation rocks of the Willamette
Valley have played something of a passive role against
the backdrop of moving tectonic plates. In Eocene time
an undersea chain of volcanoes atop the Kula and
Farallon plates collided with the westward moving
North American plate where they were accreted. With
a thickness of more than 2 miles, the volcanic rocks of
the island chain form the basement of the Coast Range
and Willamette Valley. After docking or making initial
contact with North America, the island archipelago was
rotated clockwise beginning in the early Eocene. With
accretion, the old subduction zone east of the volcanic
