Sediments of Yaquina Bay, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/0v8383459

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  • 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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