Relation between natural radioactivity in sediment and potential heavy mineral enrichment on the Washington continental shelf Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/bn999935b

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  • Natural radionuclides may be indicators for dense mineral placers along marine shorelines. Relict beach and river deposits occur in continental shelf sediments. These deposits result from the reworking of beach sands by wave action during the Holocene Transgression. Some dense, resistant minerals associated with placer deposits are known to contain ²³⁸U and ²³²Th activities. Shelf sediments, enriched in heavy minerals, might be expected to be high in these natural radioactivities. The usefulness of natural radioactivity to locate and to map dense mineral deposits was therefore explored and relations between natural radioactivity in marine sediments and dense mineral content were established. High ⁴⁰K activity in sediments was positively correlated significantly with high mud content. High ²³⁰Th and ²³²Th activities were associated with fine, well-sorted sands and correlated very highly with the weight percent total heavy mineral sands in the sediment. These relations distinguish depositional environments as high in ⁴⁰K content and erosional environments as high in ²³⁰Th and ²³²Th content. Two areas of enrichment were found by radiometric mapping and by mineral analyses. Off Destruction Island, Washington, maximum ²³⁰Th and ²³²Th activities were found in 30 m water depth. Activity distribution and mineral enrichment were consistent with general northward transport of nearshore sediment. In 33 m water depth off Clatsop Spit, Oregon, the second enrichment area was found. The ²³²Th/²³⁰Th activity ratios are different for these two areas. Probably the heavy minerals in these two areas are from different sources.
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