Along-coast variations of Oregon beach-sand compositions produced by the mixing of sediments from multiple sources under a transgressing sea

Graduation date: 1987Heavy mineral compositions of sands from Oregon beaches,\ud rivers and sea cliffs have been determined in order to examine the\ud causes of marked along-coast variations in the beach-sand\ud mineralogy. The study area extends southward from the Columbia\ud River to the Coquille River in southern Oregon. The heavy-mineral\ud compositions were determined by standard microscopic\ud identification with additional verification by X-ray diffraction\ud analyses. Initially the beach-sand samples were collected as single\ud grab samples from the mid-beachface, but significant selective\ud sorting of the important heavy minerals prevented reasonable\ud interpretations of the results. Factor analysis of multiple samples\ud from the same beach yielded distinct factors which correspond with\ud known mineral sorting patterns. The effects of local sorting were\ud reduced by the subsequent use of large composite samples,\ud permitting interpretations of along-coast variations in sand\ud compositions. Four principal beach-sand sources are identified by\ud factor analysis: the Columbia River on the north, a Coastal Range\ud volcanic source, sands from the Umpqua River on the south-Oregon\ud coast, and a metamorphic source from the Klamath Mountains of\ud southern Oregon and northern California. The end members identified\ud by factor analysis of the beach sands correspond closely to\ud river-source compositions, the proportions in a specific beach-sand\ud sample depending on its north to south location with respect to those\ud sources. During lowered sea levels of the Late Pleistocene, the\ud Columbia River supplied sand which was dispersed both to the north\ud and south, its content decreasing southward as it mixed with sands\ud from other sources. The distributions of minerals originating in the\ud Klamath Mountains indicate that the net littoral drift was to the\ud north during lowered sea levels. With a rise in sea level the\ud longshore movement of sand was interrupted by headlands such that\ud the Columbia River presently supplies beach sand southward only to\ud the first headland, Tillamook Head. At that headland there is a\ud marked change in mineralogy and in grain rounding with angular,\ud recently-supplied sands to the north and rounded sands to the south.\ud The results of this study indicate that the present-day central\ud Oregon coast Consists of a series of beaches separated by headlands,\ud the beach-sand compositions in part being relict, reflecting the\ud along-coast mixing at lower sea levels and subsequent isolation by\ud onshore migration of the beaches under the Holocene sea-level transgression. This pattern of relict compositions has been modified\ud during the past several thousand years by some addition of sand to\ud the beaches by sea-cliff erosion and contributions from the rivers\ud draining the nearby Coastal Range