A short study of the modification of stream sediments during transportation, Elk Creek, Black Hills, South Dakota



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Elk Creek originates on the Precambrian metamorphic rocks of the Central Black Hills. Between the last outcrop of Precambrian rock and the Hogback Eidge, the outer boundary of the Black Hills, the stream flows eastward across the regional strike, and through 15 miles of Paleosoic and Mesozoic carbonates, shales, and fine-grained quartz sandstones. Both the Precambrian and Paleozoic rocks within the Hills are cut by shallow, quartz-bearing intrusions of Tertiary age. From the Hogback Ridge to its junction with the Cheyenne River, a distance of 85 stream miles, the stream flows eastward across the Plains and through Cretaceous shales and Cenozoic terrace gravels. Rock fragronts dominate the sand»sized sediment released from the crystalline source rocks, which include fine-grained from schists, amphibolites, and shallow intrusives ("volcanics"). Only a small amount of quartz coarser than silt-size is being released by these rocks. Elk Creek can be divided into 3 segments on the basis of its competences (1) from the Central Black Hills to the Hogback Ridge (15 stream miles); (2) from the Hogback Ridge to the "knickpoint" (40 stream miles) 1 (3) from the "knickpoint" to the Cheyenne River (45 stream miles). The stream is eroding its valley and transporting sediment in stream segments (1) and (3); however, aggradation is the dominant process in segment (2). Because little sediment presently being eroded from the Black Hills is being transported to the Plains, the composition of the sand-sized non-shale strew sediment on the Plains,is controlled by the terrace gravels. The terrace gravels are similar in compositlffin, but not identical, to the Recent stream sediment west of the Hogback Ridge. At the time of the deposition of some of the terrace sediment the following conditions existed; (1) the headwaters of Elk Creek drainage basin extended farther to the north in the Black Hills than at present, and contained a larger proportion of "volcanic" source rocks; (2) the eastern portion of the drainage basin wvs receiving arkosic sediment from the southern Black Hills; and, (3) the competence of the stream was greater than at present. A detailed examination of sand»sised sediment along the length of the stream suggest the following; (1) most fine-grained schist and amphibolite frayaenta are mechanically destroyed with less than 15 miles of transport in high gradient streams in this climatic region; (2) volcanic rock fragsyants are relatively durable and only a small percentage is lost due to mechanical abrasion during the first 100 miles of stream transport; (3) rowading of volcanic rock fragments does not occur in sediment finer than very coarse sand during the first 100 miles of stream transport. Hornblende decreases from 95 percent to 3 percent of the very fine sand-size non-opaque heavy mineral suite during the first 100 miles of transport in Elk Creek. This loss is due to dilution, Little if eny hornblende is lost due to chemical weathering or abrasion. If in the distant future a petrographer examined a sand-sample from near the present mouth of Elk Creek, he proably could recognize all the major source rock types with the exception of the terrace gravel. He would not, however, correctly estimate their relative percentages, Few, if any, petrographers would suspect that most of the non-shale sediment was derived from a mountainous darnel uplift, with mo»o than 8000 feet of structural relief, lass than 100 miles upstream.