Bedform migration and bar evolution in a late Quaternary monsoon-influenced river, Gujarat, western India

A study of a well-exposed late Quaternary calcrete-conglomerate sheet from Gujarat, western India provides some insights on bedform migration and bar evolution in sub-humid monsoon-influenced rivers. The architecture of the conglomerate sheet is defined by the presence of two elements, channel (CH) and gravel-bar (GB). This architectural simplicity is remarkable and may be typical of monsoon-influenced rivers that show dominance of 3D dunes. In the initial stage (1), the river was sand-load dominated as seen in the presence of cross-bedded sands at the base. Associated with this stage was a channel element (CH1) at the northern extremity of the outcrop. During stage (2), bank-excavated blocks were redistributed in the channel, which aided the appearance of gravel mesoforms within depressions on the channel bed and subsequent low stage sedimentation leading to the formation of channel element CH2. In stage (3), gravel dunes covered the entire channel bed and are separated from the preceding deposits by a channel lag. Falling stage dissection of the bar was accompanied by relocation of the flow, along a smaller channel (represented by channel element CH3) at the northern end of the outcrop and bar top dissection. This history of bar evolution compares well with the central-bar initiation and transverse bar braiding mechanisms described in flume studies except for change in bed-material. Palaeocurrents during the termination of stage-3 showed typical patterns of convergence and divergence of bedforms in the southern extremity of the outcrop whereas the palaeocurrent directions are more consistent in the central regions. This could be related to dune combination and splitting.