Abstract

Studies of the South Platte-Platte River in Colorado and Nebraska substantiate Ore's (1964) conclusion that braided patterns in streams are created mainly by accretion of longitudinal bars and dissection of transverse bars. Distribution of bars in the South Platte-Platte River depends on texture of the bed load. Coarse, poorly sorted sediment favors formation of longitudinal bars, and finer grained, better sorted materials form transverse bars. The relative proportion of transverse to longitudinal bars increases downstream, following the river's tendency to fractionate its load into finer sizes downstream. This is accompanied by an increase in the ratio of planar cross-stratification to horizontal stratification and a decrease in cross-channel topographic relief expressed as a bed-relief index.

Relative abundances of planar cross-stratification and horizontal stratification, as well as bed-relief indices were measured in sandstones and conglomerates of the Lower Silurian Shawangunk Conglomerate, Green Pond Conglomerate, and Tuscarora Sandstone in New Jersey and Pennsylvania. These formations display downslope trends similar to those of the South Platte and Platte Rivers, and, combined with paleocurrent, grain-size distribution, and other data, suggest that the coarse eastern facies (Green Pond, Shawangunk) represent proximal braided stream deposits with longitudinal bars that grade westward and northwestward into finer grained distal braided stream sediments (Tuscarora) characterized by transverse bars.

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