Gravel fabric in a sub-Himalayan braided stream

Abstract

Data on the fabric properties of gravels have been collected at 10 locations over a distance of 27 km from the Chakki River - a braided channel of the Beas River system. The imbrication of AB-planes reveals a close correspondence with the channel direction; the deviations seldom exceed 30^°. Increase in sinuosity and the existence of subsidiary channels contribute, at some locations, to larger deviations. Imbrication directions tend to be modified because of the influence of local channels which develop temporarily on the bar surfaces. A current-normal mode exists for the A-axis orientation data. Bimodal distributions are recognised and attributed to shifting flow conditions. Variance values are low for both the imbrication and A-axis data. The range in mean values of the dip of AB-planes is between 22.6^° and 37.6^° with s = 8^° to 14^°. The mean values of plunge of the A-axis vary from 2.9^° to 13.2^° with s = 3.4^° to 12.69^°. There is a marked decrease in plunge of the A-axis in the downstream direction in the Chakki River. A particle size versus orientation approach indicates that, irrespective of size, there is a strong upcurrent imbrication. Longitudinal alignment of particles in the smaller size intervals is not supported by the data obtained in this study. A particle shape versus orientation approach reveals that the A-axis orientation is not dependent on particle shape. The A-axis fabric patterns of both tabular and bladed clasts show a peripheral circular arrangement of the maxima, the bisectrix of which points in a current-normal direction. Orientation patterns do not reflect any control in terms of the lithic composition of clasts.