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Description
Actively migrating barchan dunes near the southwestern shore of the Salton Sea formed within the lost three-hundred years on a hard, nearly level, lightly vegetated substratum of Quaternary lacustrine sediments under the influence of a prevailing west wind. Comparative mineralogy between sand from a dune and from upwind areas indicates that the principle sand source for the dunes is the poorly consolidated Upper Cenozoic sedimentary rocks that crop out west of the dune field. A representative dune, termed the Salton barchan, was examined to determine the grain-size characteristics of surface sediments and interior sediments. Contour patterns of the statistical grain-size parameters show separate distributions between surface sediments and interior sediments, suggesting that the controlling factors are different in the two regimes. The surface sediments of barchan dunes represent material transported by saltation and surface creep and ac cumulated into sand ripples. The ripple sediments are traction deposits that are dependent on the action of the prevailing wind and are not representative of the underlying dune material. The mean grain-size distribution of surface sediments across a barchan is due to the selective action of wind acting on the grains in surface creep. The standard deviation, or sorting, is due to the winnowing effect of prevailing winds that remove fine grains from the windward side and deposit them to the lee. The variation in skewness of surface sediments depends on the relative increase in fine sediments resulting from capture in hollows between coarser grains on a rough surface. The variation in kurtosis is apparently due to the sorting action of wind affecting grains in surface creep opposed to grains in saltation. The interior sand of barchan dunes is composed of· material that has been reworked from the surface and incorporated into the dune during migration. The characteristics of the interior sediments are dependent on the "conveyor belt" turnover of material selected from the surface. Interior dune sand is more commonly preserved in the geologic record; therefore, the interior grain-size parameters are the most useful statistical measures to be compared to ancient dune deposits. The mean grain size in the interior of the Salton barchan ranges from 2.35 to 3.200. The distribution of mean grain size in the interior of the dune is inversely graded with coarser grains near the summit becoming progressively finer toward the base. The variation in standard deviation or sorting within the barchans is the result of inheritance of sorted material from the surface and protection in selected areas determined by the dune morphology. Likewise, the variation in skewness is dependent on the morphology of a migrating barchan in selectively favoring the retention of fine grains. The distribution of interior kurtosis values does not provide discrete trends, but the general variation seems to be due to the addition or removal of well sorted fines during the turnover of sand as the dune migrates.
