ABSTRACT

Heavy mineral compositions of sands from Oregon beaches north of Cape Blanco have been determined optically in order to examine the causes of marked along-coast variations in mineralogy. Effects of local sorting were reduced by the use of large composite samples. Sands from rivers and sea cliffs were also analyzed as potential sources. Four principal beach-sand sources are identified by factor analysis: the Columbia River on the north, a Coast Range volcanic source, sands from the Umpqua River on the south Oregon coast, and a metamorphic source from the Klamath Mountains of southern Oregon and northern California. Most beach sands consist of mixtures of these four sources in spite of the presence of headlands which prevent along-coast sand movements.

The results of the study demonstrate that many beach-sand compositions are in part relict, reflecting an along-coast mixing of mineralogies from the four sources during lowered sea levels when blockage by headlands was absent. The distributions of minerals originating in the Klamath Mountains indicate that the net littoral sand transport was to the north. Sand from the Columbia River is found in progressively decreasing amounts in the beach sands toward the south, a pattern which results from its southward diffusion in opposition to the net transport. With a rise in sea level and accompanying migrations of the beaches, headlands eventually interrupted the along-coast mixing of sand. The pattern of relict compositions inherited from mixing during lowered sea levels has been modified du ing the past several thousand years by additions of sand to the beaches from sea-cliff erosion and from rivers. Now the Columbia River supplies beach sand southward only to the first headland, Tillamook Head. At that headland there is a marked change in mineralogy and grain rounding with angular, recently supplied sand to the north and rounded relict sand to the south.