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Migrating Sediment Waves Formed by Turbidity Currents Along the Indian North-Eastern Margin


Sediment waves are the dominant geomorphic elements in the entire lower slope and in some areas of the middle slope along the Indian north-eastern margin and their trends are parallel to the bathymetric contours. Four down-dip trending broad bands of sediment waves, adjoining to each other, are distinguished in the lower slope. The north-easternmost wave band is the most well developed one and the other wave bands to the southwest are less well developed. Upslope migration is common in all the wave bands because of more deposition on the stoss sides and less deposition (due to either bypassing and/or erosion) on the lee sides of wave crests. In the middle slope inter-canyon areas, very upslope migrating sediment waves, deposited since Late Miocene, characterize the lower subsurface sediment intervals whereas in the upper interval and on the seafloor, weakly up-dip migrating but highly aggrading wavy bedforms occur. Up-dip slope seismic geomorphology (canyon conduits, flow-lines and flow-scars trending downslope on the seafloor), combined with the absence of seafloor deformation and of contour-parallel currents along the Indian margin, show that the sediment gravity flows supplied fine-grained sediments from up-dip slope and generated the sediment waves down-dip. Numerical modeling results confirm that turbidity currents, flowing downdip over a rugose (cyclic stepped) seafloor and varying between super- to sub-critical velocities, can deposit upslope migrating wavy bedforms similar to those observed in the study area. Bathymetric trends and sediment source locales, guided the down-dip trends of turbidity currents and consequently, the different wave bands from up-dip to downdip. More clastic supplies (via effective turbidity currents) with some hemipelagic sediments and flatter seafloor gradients favored the most well-developed, upslope-migrating sediment waves in the northeast; whereas, less clastic and more hemipelagic supplies and/or steeper gradients led to the lesser development of wavy bed forms in the southwest. In the middle slopes of the inter canyon areas, active turbidity currents with more clastic supplies might have deposited the very upslope migrating bedforms in the lower subsurface sediment intervals whereas weak turbidity currents and more hemipelagic sedimentation might have generated the low upslope migrating but highly aggrading bedforms in the upper interval and on the seafloor.