Soft Sediment Deformation Structures in Late Quaternary Abandoned Channel Fill Deposit of Yamuna River in NW Sub-Himalaya, India

Soft sediment deformation (SSD) structures are observed in a 26-25 ka old fluvio-lacustrine channel-fill deposit in tectonically active Dun valley in NW Sub- Himalaya. This fluvio-lacustrine channel-fill deposit experienced intermittent inundation during initial phase of erosional avulsion with the growth of a plug bar causing complete channel abandonment. The facies depict variation from an active channel to rhythmite and lacustrine deposition in an abandoned channel setting. In the studied section, three zones of SSD structures comprising load structures, contorted beds, slumps, folds and faults are observed. These zones are invariably overlain by undeformed beds suggesting recurrent deformation. The varying geometry of SSD structures suggest gravity-driven viscous-brittle deformation in the sediment column possibly due to differential liquidization in the beds with varying grain size, when the equilibrium was disturbed by a trigger mechanism. Since the region lies in the seismically active Himalayan belt, earthquakeinduced strong ground motion may be the most plausible trigger mechanism for the observed SSD in the abandoned channel deposits and slope, sediment overloading or groundwater change may not have played a major role.