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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Petrology
Vol. 56 (1986)No. 1. (January), Pages 67-77

Distribution, Geometry, and Origin of Heavy Mineral Placer Deposits on Oregon Beaches

Curt D. Peterson, Paul D. Komar, Kenneth F. Scheidegger

ABSTRACT

Beach segments (0.5-3 km in length) enriched in heavy Previous HitmineralsNext Hit (20-100% by weight abundance) are located south of six prominent headlands that interrupt the 175-km coastline between Cape Lookout and Cape Perpetua, Oregon. Beach-face retreat in the winter of 1982-83 exposed underlying placers containing the economic Previous HitmineralsNext Hit garnet, zircon, ilmenite and chromite. In each case, the highest concentration of placer Previous HitmineralsNext Hit occurs in an area of maximum shoreline curvature at a distance south of the north bounding headland that is proportional to seaward extent of the headland.

Cross-sections and magnetic surveys of one 8-km coastal section, Otter Rock to Yaquina Head, define a small placer body (heavy Previous HitmineralsNext Hit > 50%) some 0.75 km in length and 80 ^times 103 m3 in volume that comprises ^sim 10% of the total beach sand between the bounding headlands. Mineral grain-size analyses of winter beach samples along this coastal section confirm earlier models of mineral segregation by entrainment processes; the light Previous HitmineralsNext Hit are more susceptible to fluid shear stress than the heavy Previous HitmineralsNext Hit. Maximum segregation and heavy mineral enrichment occur in the backshore and at shoreline inflection points where decreasing flow velocities produce gradients of fl id shear stress, allowing light Previous HitmineralsNext Hit to be winnowed from more stable, heavy Previous HitmineralsNext Hit.

Development of placer deposits is "controlled" by shoreline configuration as the first-order factor and seasonal variations of wave climate, and longshore currents as second-order factors that delineate shape and concentration of heavy Previous HitmineralsTop in placer bodies.


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