Cascade Mountains - Geology of Oregon - page 191

upright position. Subsidance of water-saturated coastal
sediments causes the mud and silt to liquify and flow.
In some localities these liquified sediments burst out as
a "sand volcano" or sand boil. In Oregon, 15 locations
between the Coquille River and Seaside record the
distinctive pattern of land subsidance, buried forests,
and bogs with a mud covering. At South Slough in
Coos Bay, marshes with at least 8 burial events over
the last 5,000 years demonstrate clear evidence for
large-scale coastal earthquakes.
An analogy can be made between this rapid
subsidance along the coastal region and a deck of
cards. As the Juan de Fuca plate is subducted beneath
the North American plate, the lower descending plate
may bind and hang up. As plate movement proceeds,
the plate begins to arch up like a bent playing card,
causing the coastal area to rise slowly. Once the slab
slips, the uplifted section snaps down bringing about
wide-spread and rapid dropping of the land accompany-
ing catastrophic earthquakes. These large-scale earth-
quakes are also recorded as displaced submarine
sediments deep on the shelf and slope. With the
vigorous shaking implicit in a strong quake, piles of
unconsolidated sediment perched at the edge of the
continental shelf are loosened and sent further into
deep water as a roiling, muddy density current or
turbidity flow to spread out over the slope and abyssal
plain. Over 14 of these separate turbidity flows that
correlate with depressed coastal swamps have been
identified. Each turbidity flow shows up as an unmis-
takable sequence of coarse sand at the base grading
toward the top into fine-grained, deep sea clays.

waves lasted up to 2 days. One of the highest waves to
reach Oregon in the early 1960s was measured at 95
feet high by an offshore wave-monitoring program
concerned primarily with oil exploration. On March 27
and 28, 1964, the large seismic waves that struck the
Oregon coast were generated by an earthquake in the
Prince William Sound of Alaska. When the enormous
waves reached Oregon, they tossed logs and debris onto
beaches, across highways, and into nearby buildings.
One wave caused the water level to drop exposing vast
areas of the upper shelf before seawater cascaded over
it. Although property damage in Oregon was light,
tragically, 4 children sleeping at Beverly Beach State
Park were drowned. A seismograph in operation at
Newport since 1971 measured waves as high as 21 feet
in the winter of 1972 to 1973.

Mining and Mineral Resources

Black Sands
Along the southern Oregon coast, heavy black
sands in marine placers contain an assortment of
economic minerals. During the Pleistocene minerals
from the interior were transported by streams draining
the Coast Range to the ocean where they were sorted
by winnowing action of the waves. Heavier mineral
grains such as gold, platinum, chromite, magnetite,
garnet, and zircon were left along the beach, whereas
the the lighter fragments such as quartz, feldspar, and
mica were carried out onto the continental shelf. Many
of the heavy minerals are black, and the deposits are
referred to as "black sands". The original source for this
placer gold is the Jurassic Galice Formation drained by
the South and Middle forks of the Sixes River. Shales,
sandstones, and greenstones of the Galice have been
intruded by small grantitic dikes that precipitated the
minerals into the host rock.
Heavy mineral concentrations are found from
Cape Blanco north to Cape Arago with the major
economic deposits located at the mouths of rivers and
on elevated terraces. Early day prospectors were mainly
interested in gold, which was discovered by Indians in
1852 near the mouth of Whisky Run north of the
Coquille River. Shortly after news of discovery had
spread, miners erected a network of long sluice boxes
in the small creeks and along the beach. Sand, shov-
elled into the sluices, was washed down the trough with
a stream of water so that the heavier gold particles
were caught in the riffles. On higher terraces miners
tunnelled deep into the basal black sand lenses. The 50
to 60 foot covering of loose, barren sands above the
placer layer and the extremely fine-grain of the miner-
als made recovery difficult. Near Whisky Run, Randol-
ph, a boom town of tents, stores, saloons, and houses,

Tsunamis
Earthquakes from fault activity on the ocean
floor can trigger enormous seismic waves called tsuna-
mis that cause considerable damage once they reach
the coast. Travelling up to 600 miles per hour, tsuna-
mis appear as low waves in the open ocean but pile up
as high as 100 feet when the wave energy is concentrat-
ed in shallower water near the coast. Evidence for
prehistoric tsunamis is preserved in estuaries and
coastal marshes where careful excavation shows traces
of wave damage far beyond that caused by storms.
Sediments record periodic scouring of bays and coastal
areas that are covered by a distinctive deposit identify-
ing the tsunami event.
Oceanic seismic events typically occur as a
series of destructive waves over the span of several
hours or longer. Since its installation in 1967, the
instrument to record tides at the Oregon State Univer-
sity Marine Science Center at Newport has measured
only three small tsunamis, even though one group of