Longshore grain sorting and beach-placer formation adjacent to the Columbia River Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9593tz326

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  • The formation of beach placers primarily involves processes of waves and currents that selectively sort and concentrate the valuable minerals according to their densities, sizes and shapes. Black sand placers are found on the beaches adjacent to the mouth of the Columbia River. Reviews of historical shoreline changes show that jetty construction has caused rapid beach accretion immediately adjacent to the river mouth, and thus is important to the placer development. Beach-face sand samples were collected along 70 km of shoreline north and south from the river mouth, and were analyzed to determine the sorting processes responsible for the formation of this placer. It is found that heavy minerals are highly concentrated close to the Columbia River mouth, reaching 60% to 70% on the summer beach, and in excess of 90% during the winter. The concentration decreases systematically with longshore distance, being reduced to less than 2% after 20 km of longshore transport from the river mouth. The median grain sizes of principal minerals generally become finer with longshore distance, but an away-from-source coarsening is found within 5 to 8 km of the river mouth. These analyses indicate that the Columbia River is the major sediment source for these beaches. The sand is transported alongshore north and south away from the river mouth. Though normal grain sorting and sediment transport processes are important for most parts of the beaches, selective grain sorting and transport processes are dominant immediately adjacent to the river mouth. Calculations of hydraulic ratios for various mineral pairs show that the longshore transportability of a heavy mineral increases with its relative grain size and decreases with its density. This suggests that the heavy minerals of higher densities and finer grain sizes are less easily transported alongshore and are more concentrated close to the river mouth. Settling velocity measurements show that sorting due to contrasting settling rates could be responsible for the overall separation of the heavy minerals from the tight minerals, but cannot explain the separation of individual heavy minerals. Evaluations of selective entrainment stresses and bedload transport rates, and results of the flume experiments show that minerals requiring higher selective entrainment stresses and with resulting lower bedload transport rates are those most concentrated in the placer deposits. This suggests that selective entrainment and differential transport sorting processes have been most important in the formation of the placer deposits adjacent to the Columbia River.
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  • description.provenance : Approved for entry into archive by Patricia Black(patricia.black@oregonstate.edu) on 2011-09-23T19:03:17Z (GMT) No. of bitstreams: 1 LiChen-lin1991.pdf: 1907906 bytes, checksum: bd3ca4428ecb1fd6686c6ecc863372f2 (MD5)
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  • description.provenance : Approved for entry into archive by Patricia Black(patricia.black@oregonstate.edu) on 2011-09-27T19:24:51Z (GMT) No. of bitstreams: 1 LiChen-lin1991.pdf: 1907906 bytes, checksum: bd3ca4428ecb1fd6686c6ecc863372f2 (MD5)
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