A study of stream transport of sand sized sediment, Battle Creek, Black Hills, South Dakota



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Twenty-four samples of sand sized stream sediment were collected from Battle Creek, South Dakota, and studied petrographically in order to determine the effects of transportation on such grains. The headwaters of Battle Creek drain an area of Precambrian granite and metasedimentary rocks, the weathering of which furnishes newly derived, untransported sand grains into the stream; and it is these grains to which this study addresses itself. The results of the study include the following observations: 1) Battle Creek is transporting sand grains in certain segments only during flood; 2) the effect of stream transportation in modifying the character of sand sized monocrystalline quartz grains is masked by the influx of previously transported quartz grains into the stream sediment as a result of erosion of Paleozoic and Mesozoic sedimentary rocks over which Battle Creek flows east of the Precambrian source area; 3) the disporportionately low percentage of schist rock fragments in the stream, especially in the area of outcrop of schist source rocks, is suprising, and it is evident that schist, phyllite, and slate rock fragments are rapidly destroyed in the drainage basin of Battle Creek, both by fluvial transport and by weathering and erosion processes prior to their entry into the stream; 4) microcline and plagioclase grains are being lost from Battle Creek due to transport in the high gradient (74.5 feet/mile) segment, but microcline is apparently uneffected in the low gradient (11.9 feet/mile) portion of the stream; 5) the loss of polycrystailine quartz and quartz-feldspar rock fragments is inconclusively suggested by the data; 6) sandstone and limestone rock fragments are apparently being lost from the stream due to fluvial transport at about the same rate at which they are being resupplied; 7) Battle Creek has eroded its channel down through the terrace deposits east of the nick point and is actively eroding the underlying Pierre Shale, furnishing floods of very fine grained (shale) rock fragments to the stream; 8) conclusions regarding the absolute amount of a specific mineral or grain type based on the fluctuations of the occurrence of the species in a single size class are meaningless unless information about the original size distribution and size distribution of subsequent sources of the grains are considered. A serious problem in this study is the dilution of the original first cycle stream sediment by previously transported sediments from downstream sources which largely masks any effects of transportation on the original grains. Further complicating the study was the unknown effects of weathering of sand sized grains during periods of non-transportation.