Abstract

Extensive black-sand placers are found on the beaches adjacent to the mouth of the Columbia River in the Northwest United States. Beachface sand samples were collected along 70 km of shoreline north and south from the river and were analyzed to determine the sorting patterns in order to understand the processes responsible for the formation of these placers. The heavy minerals are highly concentrated close to the river, reaching 60 to 70% of bulk samples from the summer beach. The concentrations decrease 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 the principal minerals generally become finer with longshore distance, but an away-from-source coarsening is found within 5 to 8 km from the river where the placers achieve their maximum development. Measurements of grain settling velocities show that sorting associated with this parameter may explain the bulk characteristics of the beach sediments but cannot account for the enrichment of the dense minerals and detailed sorting patterns responsible for the placer formation. Minerals requiring higher selective entrainment stresses and having lower bedload transport rates are those most concentrated in the placer deposits. This is the case for settling-equivalent fractions within the sands, as well as for the samples as a whole. We conclude that, as the sand is transported alongshore north and south away from the river, sorting is controlled initially by the differential settling velocities of the grains. However, close to the river mouth where the placer is most concentrated, the sorting patterns are best explained by selective entrainment and transport in the swash zone, with the highest density and finest grains being most difficult to mobilize and thus remaining within the placer. The transformation of dominant sorting modes is determined by the pattern of beach erosion which has been greatest adjacent to the jetties constructed at the mouth of the Columbia River.

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