Geology Reference
In-Depth Information
In contrast to the mud seafloor below,
the banks often have a rocky substrate
where dense populations of inverte-
brates and fish dwell. Seaward from
these banks, the continental slope is
interrupted by prominent north-nor-
thwest ridges and intervening troughs.
The longest ridge between Cascade
Head and Tillamook Bay extends up
to 2,000 feet while a similar ridge
south of Cape Sebastian is a north-
ward continuation of the Klamath
plateau.
Cutting completely across the
continental shelf and slope, submarine
canyons and channels are important
avenues for dispersing sediments by
turbidity currents that originate in
shallow water. Of these, Astoria Can-
yon, a major submarine feature off the
Oregon coast, heads 10 miles west of
the mouth of the Columbia River and
extends across the Astoria submarine
fan. The canyon eventually reaches a
depth of over 9,000 feet where its
identity is lost in the abyssal plain.
Another noteworthy feature, the
Rogue Canyon, with its head near the
edge of the southern Oregon shelf,
cuts across the continental slope to
connect with the abyssal plain off the
Rogue River. The longest deep-sea
channel known in the Pacific Basin is
the Cascadia channel. About 1,200
miles in length, the channel connects
to the edge of the continental shelf
northwest of the mouth of the Colum-
bia River before angling in a southerly direction to
breach the Cape Blanco fracture zone. The great
volume of deep sea sediments carried from the Wash-
ington continental slope have pushed this channel
farther and farther out onto the abyssal plain since the
Pleistocene.
The focal point of Oregon offshore geology is
the subduction zone at the base of the continental
slope. Throughout the collision and accretion of the
Coast Range island archipelago, the two main slabs,
the Farallon and North American plates, continued to
grind slowly toward each other. With the collision
event, the subduction zone separating the two was
abandoned and a new zone established west of the
island chain. Today the subduction zone extends north
and south along the foot of the Oregon continental
slope, but the trench itself is largely obscured because
of a rapid sedimentation rate. For this reason Oregon
does not display the pronounced offshore subduction
Block diagram of the Coast Range, continental
shelf, and slope at Cape Arago.
trench or depression typical of much of the Pacific
basin. Off southern Oregon, upper slope deposits are
accumulating at a rate of 4 to 6 inches per 1,000 years,
whereas lower slope deposits both to the north and
south are accumulating at a phenomenal 8 to 26 inches
every 1,000 years. The distinctive Crater Lake ash and
pumice layer, dating back almost 7,000 years, is easily
recognized in the deep marine environment off Oregon
and provides a useful milepost for estimating rates of
sedimentation.
The great influx of sediment from the Colum-
bia River piles up at the base of the slope in huge
submarine deposits called fans. The Astoria fan extends
from 6,000 feet along the continental slope to the
abyssal plain below 9,000 feet and covers more than
3,500 square miles. Only gently sloping, the Astoria fan
