Sedimentary structures exposed in bar tops in the Brahmaputra River, Bangladesh

Abstract

The Brahmaputra River is one of the world’s largest sand-bed braided rivers with a channel belt up to 15 km wide, a mean channel depth of 5 m and maximum scour depths of up to 40 m. The recorded discharge varies by around 60 000 cumecs every year following the annual monsoon, producing dramatic stage fluctuations with the water level falling between 7 and 8 m from bankfull to low flow stage. In the dry season large areas of bar top and channel bed are exposed and low flow channels cut natural sections through bar tops exposing the internal stratification. Elements of upstream, downstream and lateral accretion are identified, although the high width/depth ratio of channels (up to 500:1) results in extremely low depositional dips on bed bounding surfaces. Within the accretionary elements vertical and lateral changes in sedimentary structures are ubiquitous and varied but some patterns can be discerned. Channel deposits are dominated by sinuous-crested dunes and trough cross-stratification. Upper stage plane bed lamination, often with a very low angle depositional dip, has been observed in beds up to 4 m thick and is generally found at the base of the exposed bar top sections. Trough cross-stratification in bar tops occurs in sets from 0.1 m to 3 m thick with rapid vertical and lateral changes in set thickness. Very large sets of trough cross-stratification, >3 m thick, do not occur in the bar top sections. Current ripple lamination is most abundant on the tops of bars with very high rates of climb associated with high rates of sediment deposition. From these observations a generalised vertical sequence of bar top origin might include trough cross-stratification overlain by upper stage plane bed lamination, isolated sets of trough cross-stratification truncated and capped by current ripple lamination. This sequence, which is considered to represent the deposits of a single flood, may occur within the upstream, downstream or lateral accretion elements. However, it should be noted that vertical and lateral changes in facies are extremely abundant with many changes in sedimentary structures and reactivation surfaces within each depositional episode.