Geology Reference
In-Depth Information
the heavy fluid lavas forced them downward into the
Pleistocene
underlying soft watersaturated sediments. Once the
Subduction of the Juan de Fuca plate under
lavas invaded the unconsolidated sediments, they
North America and continued growth of the wedge of
intruded a variety of Eocene and Oligocene rocks in
accreted sediments on the leading edge of the North
the central and northern Coastal Range as dikes and
American plate at a rate of about 1 inch per year has
sills. After cutting across the incipient Coast Range,
pushed the Coast Range upward. Areas offshore closer
the lavas terminated as far south as Seal Rocks.
to the subduction trench have been subject to more
Geophysical transects across these coastal basalt
uplift than areas farther inland. In a regional sense, the
exposures reveal that the dikes and sills are "rootless",
varying uplift produces a tilting effect since the western
which further supports an invasive origin rather than a
edge of the coastal mountains are rising, while the
local volcanic source.
eastern margin as well as the Willamette Valley are
At Cape Blanco and Coos Bay sandstones of
either subsiding or only rising at a minimal rate. In the
the Empire Formation are late Miocene. Molluscs,
early 1980s a Cornell University team conducted very
echinoids, and barnacles suggest a shallow water
accurate east-west leveling surveys across Coast Range
temperate marine environment. Many of the shells
highways in Oregon and Washington repeating mea-
show wear and abrasion from action of the waves close
surements which had been taken over 50 years previ-
to shore.
ously. The striking results showed a seesaw effect in
every transect, where western coastal areas had risen
Pliocene
with respect to the eastern part of the range. Conse-
Few marine sediments record the Pliocene in
quently Cape Blanco, only 35 miles from the subduc-
western Oregon indicating this was a time of erosion
tion zone, has the fastest uplift anywhere on the coast
when the area was above sea level and the coastline lay
at 35 inches vertically every century or 1 inch every 3
approximately in its present position. Pliocene muds,
years. As Neah Bay along the Washington coast rises
sands, and silts were carried to the shelf and slope
at the rate of about 1 inch every 15 years, Seattle and
where they form vast sheets of poorly consolidated
Vancouver on the inland side are sinking at 1 inch
sediments. Up to 1,000 feet of gravels of the Troutdale
every 11 years and 1 inch every 40 years respectively. In
Formation accumulated along the Columbia River in
Oregon, Astoria rises at the rate of 1 inch in 36 years
the St. Helens area and intermittently all the way to
as Rainier to the east is proportionally depressed.
the mouth of the river.
Terrace levels in the vicinity of Bandon
(after Peterson, Gleeson, and Wetzel,
1987; Mclnelly and Kelsey, 1990)
Search WWH ::




Custom Search