Processes of Selective Grain Transport and the Formation of Placers on Beaches

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Abstract

Placers are found on many beaches of the Oregon coast. In the beach that is the focus of this study, the principal placer minerals are ilmenite (22.0%), epidote (20.0%), garnet (18.7%), augite (15.5%), hypersthene (8.1%), hornblende (5.8%) and zircon (2.0%), the "ilmenite" fraction actually being partly chromite. A series of beach-face sand samples was obtained along a profile when sand was being eroded from the upper beach and transported to offshore bars, since the processes of grain-selective sorting leading to placer formation were probably most active at such a time. The landward-most sample contained 96% heavy minerals, being part of the placer. This concentration systematically decreased offshore to a 6% heavy-mineral content in the seaward-most sample (54 m offshore). Determinations of individual heavy mineral concentrations revealed that, although they all tended to become concentrated within the placer with light quartz and feldspar dominating the offshore transport, the sorting processes were most effective in concentrating the ilmenite (concentration factor = 1403) and least efficient in concentrating the hornblende (factor = 5). All of the minerals form a pattern of increasing efficiency of concentration within the placer with an increase in grain density and decrease in mean grain size. Thus the sorting processes are most effective in concentrating the ilmenite, which is both the densest and finest-grained of the minerals present in the beach, least efficient for the hornblende, which has the lowest density and is the coarsest of the heavy minerals. Measurements of grain settling velocities indicate that this parameter cannot account for the observed selective grain sorting. Instead, evaluations of the selective entrainment and transport rates of the mineral grains yield trends which exactly parallel their observed concentration factors within the placer, indicating the probable dominance of these grain-sorting processes in placer formation. Shear sorting of the light and heavy minerals may also be an important process, but its role could not be determined in the present study.

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