Abstract

An attempt to use heavy minerals to evaluate sediment sources in the Middle Rio Grande Valley showed the inadequacy of commonly used methods of representing heavy-mineral compositions. To develop a usable method, six samples from a transverse channel section were studied by quantitative methods. The size-distribution curves (by weight) of 12 heavy minerals were similar in general form to those of the light minerals, but displaced toward the finer sizes. Among samples the heavy-mineral composition varied systematically with differences in average size and sorting of the light minerals.

The size distribution of a heavy mineral in a fluvial deposit appears to depend on (1) its relative availability in each size grade in the stream load, (2) its equivalent hydraulic size, (3) the hydraulic conditions at the time and place of deposition, and (4) some factor or factors now unknown. In the Rio Grande, the equivalent hydraulic sizes of 16 heavy minerals vary from 1.0ϕ to .2ϕ.

The hydraulic ratio is recommended as a genetically sound method of representing the heavy-mineral composition of sediments. This ratio is the same in the stream load and bed deposits. Changes in the hydraulic ratio caused by wear, selective transportation, and other factors are discussed.

Other methods of representing heavy-mineral compositions are discussed and their limitations outlined. All methods are of limited applicability, but ratios between heavies of about the same specific gravity may be used most extensively. If the limitations are not observed, invalid conclusions may be reached.

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