Channels, echo character mapping and tectonics from 3.5 kHz profiles, distal Bengal Fan

Abstract

The distal parts of the Bengal Fan are spectacularly affected by tectonic deformation related to the diffuse plate boundary between the Indian and Australian plates. Here we use 3.5 kHz and 12 kHz echosounder profiles, seismic reflection profiles and piston core results to examine sedimentary processes and their relationships to tectonism within an area (78°–82°E, 0°–6°S) just to the south of Ocean Drilling Program Leg 116 sites. Echo character mapping was completed using echosounder data in conjunction with results from piston coring, and a total of five different echo types have been recognised. Four of these fall into the echo character classification scheme developed by Damuth (1980a), whilst the fifth is believed to represent hemiturbidite deposits. Several types of submarine channel were also identified from echosounder data and a correlation between echo type and channel location can be seen. Their abundance, erosional and/or depositional nature together with a complex meandering and bifurcation pattern across a wide region of average gradient around 1°/km, are all features characteristic of a broad channel termination zone on a large elongate fan. It is clear, therefore, that the Bengal Fan extends beyond 6°S. Active faulting in the area has led to the development of an irregular topography of low rounded hillocks that interfere with incoming turbidity currents. This has resulted in ponding between highs rather than lobe construction, thinning and pinching out of turbidites against the flanks of local relief, flow lofting and hemiturbidite drape, and common small-scale slumping. In some cases uplifted channel segments can be seen abandoned and partially filled.