Paleocurrent patterns in braid-bar and point-bar deposits; examples from the Ganga River, India

Abstract

ABSTRACT A braid bar at Bithur and a point bar at Saidpur, both in the Ganga River, were studied for their paleocurrent patterns by making trenches in a grid system and documenting the azimuthal direction of planar and trough cross-bedding. The bars are made up of fine sand and subjected to annual monsoon discharge fluctuations. The paleocurrent trends in the Bithur braid bar show changes from the upstream to the downstream parts, viz., the upcurrent part has a unimodal pattern with high variance, the middle part shows a bimodal pattern with very high variance, and the downstream part shows a unimodal pattern with high variance. There is large variance between the paleocurrent directions of planar and trough cross-bedding and a very high variance in the overall paleocurrent trend. The Saidpur point bar shows a polymodal pattern with very high variance in the upstream part, unimodal distribution with low variance in the middle part, and unimodal distribution with high variance in the downstream part. There is no difference in the paleocurrent direction of planar and trough cross-bedding; the variance value for the overall paleocurrent is slightly lower than in the Bithur braid bar. Contrary to the existing models, the value of azimuthal dispersion is higher for the Bithur braid bar and lower for the Saidpur point bar. Reconstruction of bedform movement on both Bithur and Saidpur bars shows a prominent movement direction of bedforms obliquely into the channel in the marginal parts of the bars. This trend of bedform movement is in contrast to the trends of the theoretical models, where in the marginal parts of bars bedforms move obliquely on the bars.