| Abstract |
- Three realms of deposition, Marine, Fluviatile, and Marine-
Fluviatile, are recognized in Yaquina Bay, Oregon, on the basis of
sediment texture and mineralogy. The Marine Realm extends 1.5
miles into the entrance of the estuary and is typified by normal
marine salinity and vigorous tidal action. Sediments of this realm
are similar to those of the adjacent beach and coastal dune sands
and consist of well-sorted, subangular to subrounded, fine to medium
sand. The immature arkosic sands in this realm are distinguished
by the marine suite of heavy minerals which include abundant
pyroxenes, primarily hypersthene and diopside, and such metamorphic
minerals as kyanite, sillimanite, and staurolite. The
Fluviatile Realm occurs at the fresh-water head of the estuary and
reaches to a point 6 miles from the entrance, where brackish water
conditions prevail. The poorly sorted, angular to subangular sediments
of this realm range in grain size from silt to coarse sand.
They are somewhat more arkosic than the sands of the Marine
Realm and are represented by the fluviatile suite of heavy minerals.
This assemblage includes such diagnostic minerals as biotite and
muscovite, and hematite and limonite. Diopside is absent, hypersthene
is restricted, and there is a marked decrease in the abundance
of garnet and the number of metamorphic species, compared
with the Marine Realm. The Marine-Fluviatile Realm lies between
the Fluviatile and Marine Realms and contains admixtures of sediments
of the other two realms.
The chief sources of Recent sediments in the Yaquina Bay
area are the Tertiary rocks of the central Oregon Coast Range, the
Pleistocene marine terrace sands and estuarine deposits near the
bay mouth, and the Recent transitory beach and dune sands that
flank the bay entrance.
Marine sand from the adjacent ocean beaches is transported
into the estuary by strong tidal currents to Oneatta Point 6 miles
from the entrance. Nearby coastal dune sands are blown into the
tidal channel near the mouth of the estuary and onto the southwestern
shore of Southbeach Tidal Flat by strong onshore winds. Suspended
sediments are contributed by the Yaquina River during periods
of high runoff.
The type of estuarine system is dependent upon seasonal and
annual climatic conditions. Generally, from June to October the
system is well-mixed, but it may alternate between a well-mixed to partly-mixed system from November to May. Precipitation recorded
at Newport apparently reflects the type of estuarine system
present during each month of the year for any given year.
Deposition in Yaquina Bay appears to be largely seasonal.
Maximum deposition probably occurs in the winter and early spring
when river runoff is highest, the littoral drift is from south to
north, and the highest velocity winds are from the southwest. At
this time, the partly-mixed estuarine system is effective in transporting
drifting beach sands into the entrance of the estuary. During
the summer, deposition is slight because of the low runoff,
southward littoral drift, and northwest winds. The well-mixed estuarine
system inhibits the transportation of sediments into the
estuary.
Known areas of shoaling occur on the bar, in the main channel,
and in the turning basin. The shoaled areas have maintained
a fairly constant position from 1950 to 1961. Estimated average rate
of deposition in the dredged channel is 9.1 inches per year. Marine
sand is the principal shoaling material. As a result of jetty construction
in 1888, and through subsequent additions, extensive
deposition has occurred on the southern ocean beach behind the
south jetty. An average estimate of Z74 cubic yards of material
accumulated annually during the past 73 years.
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