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Geologic Trip, Rogue Valley Area Klamath Mountains |
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The
Klamath Mountains extend along the Pacific coast from northwestern California
into southwestern Oregon and are continuous with Coast Ranges of California
and Oregon. They differ from the Coast Ranges, however, in that they are
formed from hard and complexly folded and faulted metamorphic and igneous
rocks, known as the Klamath Terranes. This combination of hard rocks
and the rainy coastal climate has resulted in high, rugged mountains and
steep river valleys with poor access and sparse population. The
Klamath Mountains are cut into segments by two major west-flowing rivers—the
Rogue River in Oregon and the Klamath River near the boundary of Oregon and
California. These rivers both have their headwaters in the Cascade Range and
flow into the Pacific through deep canyons that cut completely across the
Klamath Mountains. The Rogue River has
cut such a deep and inaccessible canyon that no road has been built to follow
the river in its journey through the mountains. Part of this remote area
along the Rogue River has been preserved as the Wild Rogue Wilderness. The
Siskiyou Mountains are a high rugged range mountains that lie between the
Klamath River and Rogue River. These mountains are part of the Klamath
Mountains but get their own name because they form a significant barrier to
north-south travel. Interstate 5 crosses these mountains at Siskiyou Pass,
the highest pass along the highway from San Diego to Seattle. Almost
all of Washington and Oregon and parts of California and Nevada are formed
from accreted terranes. Terranes are pieces of crustal material that have
broken off of one plate and added to another plate. Most of the terranes in
Oregon and Washington were accreted to the North American Plate over the last
200 million years when the North American Plate began to move west and
collide with oceanic plates to the west. The terranes include many different
types of rocks. Some were formed from volcanic island arcs and their
associated volcaniclastic sediments, some were islands of continental crust
that had broken away from other continents, some were sedimentary rocks
scraped off the ocean floor, and some terranes were large pieces of the
oceanic crust itself. These terranes were accreted in a series of subduction
zones that formed along the plate boundary as the North America Plate moved
west. As each terrane entered its subduction zone, it was carried to great
depths where the rocks were heated, squeezed, faulted, folded and altered
into metamorphic rocks. The
accreted terranes form the basement rocks for Washington and Oregon. However,
these basement rocks are mostly hidden by overlying sedimentary and volcanic
rocks. The Klamath Mountains are one of the few areas in Oregon where we can
get a good look at these complex and exotic rocks. Mt. Ashland (7532’) is a large pluton of
granitic rock that intruded the Klamath Terranes 161 million years ago. These
granitic rocks are well exposed in many road cuts along Mt. Ashland Ski Road
from Colestin Rd. to the Ski Lift area at the top of the Mountain. This
pluton was formed from the slow cooling of granitic magma that intruded the
Klamath Terranes at depths of several miles below the earth’s surface. Many
thousands of feet of rock must have been eroded from the top of Mt. Ashland
for these granitic rocks to now be exposed at the surface. Several
other large plutons of granitic rocks also occur in the Klamath Terranes,
including the Grayback Pluton east of Cave Junction, the Jacksonville Pluton
north of Jacksonville, the Grants Pass Pluton in Grants Pass, and the Gold
Hill Pluton south of lower Table Rock. Many of these plutons are associated
with deposits of gold, silver, and other minerals. These minerals were
typically injected into nearby metamorphic rocks during the late stages of
the granitic intrusion. Hellgate Canyon, on the Galice Road 15 miles
northwest of Grants Pass, is a good place to see some of the rocks of the
Klamath Terranes. This is a narrow canyon, 2100’ long, that that been cut by
the Rogue River through a patch of exceptionally hard metamorphosed volcanic
rocks in the Klamath Terranes. A
couple of hundred million years ago these volcanic rocks were part of a
volcanic island arc in the Pacific Ocean off the west coast of the North
American Plate. At Hog Creek, immediately east of Hellgate Canyon, there are
good exposures of black slate along the river and in nearby road cuts. These
black slates began their life as thick deposits of mud on the floor of the
Pacific ocean during Jurassic time. At the west end of Hellgate Canyon, just
beyond the bridge that crosses the Rogue River, there are road cuts of green
serpentine. The serpentine is part of the Jurassic oceanic crust that has
been altered into a green slippery rock. The serpentine occurs along the
contact between the black slates and the hard volcanic rocks and acted as a
lubricant to insert the volcanic rocks into the black slates. These rocks are
just a sampling the many different types of rocks that make up the very
complex Klamath Terranes. Applegate Dam is anchored on a small pluton of granitic
rock that intruded the Klamath Terranes. Immediately south of the dam is a
complex mess of rocks that were mixed together in the Jurassic subduction
zone. Collier Tunnel. The rocks at the south end of the
Collier Tunnel on Highway 199 are slightly metamorphosed black slates of the
Galice formation. These rocks were once mud on the Jurassic sea floor. Eight Dollar Mountain near Selma is formed from
serpentine and peridotite that were once part of the oceanic crust. Oregon Caves. The Oregon Caves are in a large block of
marble within the Galice Formation. This block of marble was a limestone reef
on a volcanic island before it entered the subduction zone during the
Jurassic. Smith River. Highway 199 follows the Smith River
from Patrick Creek to Gasquet. In road cuts in this area you can see pillow
basalt, diabase dikes, gabbro, serpentine, and peridotite that were part of
the earth’s oceanic crust 200 million years ago. These rocks, known as the
Josephine Ophiolite, are part of the Klamath Terranes, and are some of the
best and most complete exposures of the earth’s oceanic crust in North
America. Upper Applegate Road. Most of
the rocks along Upper Applegate Road from Ruch
to Applegate Dam are metamorphosed volcanic and sedimentary rocks that were
once part of a Jurassic volcanic island arc. External Websites USGS:
Plutons and Accretionary Episodes of
Klamath Mountains Wikipedia:
Klamath Mountains,
Pangaea,
Siskiyou Mountains,
Course of the Rogue River Return to:
Rogue Valley Area |
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Hellgate Canyon |
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Mt. Ashland |
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Terranes Accreted to North American Plate |