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 3.5kHz and 12kHz 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, 2 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.

Patterns of deposition from experimental turbidity currents with reversing buoyancy

“…In distal settings where only the wake may penetrate (the body having deposited a coarse bed in more proximal settings), lofting would result in sedimentation of thin silt from the lowermost portion of the wake while the remainder, predominantly clay‐grade mud, would loft into the water column and rain out over a longer period of time (Gladstone & Sparks, 2002). This process may not only account for the clay‐grade hemipelagites observed in Ocean Drilling Program core sections on the Bengal Fan (Stow & Wetzel, 1990; Redbourn et al. , 1993) and attributed to the process of buoyancy reversal by Sparks et al.…”

Sinuous deep-water channels: Genesis, geometry and architecture

“…HARPs occur beneath and between channel-levee systems, but large MTDs are apparently absent, although detailed surveying of the uppermost fan is required to confirm this (Schwenk et al, , 2005. On the lower fan, slope angles drop to o0.061 and channel widths are o3.5 km, but incision depths remain at a few 10s of metres (Emmel and Curray, 1981a, b, 1984Redbourn et al, 1993;Curray et al, 2003). Right-hand levees (looking downstream) typically show enhanced growth on the upper and middle fan due to the effects of Coriolis force, but are equal in size on the lower fan due to proximity to the Equator (Emmel and Curray, 1981b, 1984.…”

“…Deeper drilling of the distal-most Bengal Fan, during ODP Leg 116, recovered a 2-6 m thick sequence of pelagic and turbiditic mud, overlying 100s of metres of interbedded silt-mud turbidites (Stow et al, 1989). Individual turbidite beds are up to 2.5 m thick and contain occasional coarse (sand-sized) quartz grains; this suggests that, even 2500 km from source, turbidity currents were still capable of carrying significant sediment loads, and were most likely initiated by largescale slumping of pro-delta sediments (Stow et al, 1989;Redbourn et al, 1993). Estimates of turbidity current flow velocities through the active channel (during periods of lowered sea level) are in the order of 10 ms À1 on the upper fan to 4 ms À1 on the lower fan (Curray et al, 2003).…”

“…Lateral channel migration on Bengal Fan was first recognised in cross-sectional profiles by Curray and Moore (1971), while low continuity of channels in closely spaced profiles on the lower fan suggested a high degree of sinuosity (Redbourn et al, 1993). However, it was not until the use of Hydrosweep multibeam bathymetry that the true nature of channel sinuosity on Bengal Fan was revealed Schwenk et al, 2003Schwenk et al, , 2005.…”

Morphology, structure, composition and build-up processes of the active channel-mouth lobe complex of the Congo deep-sea fan with inputs from remotely operated underwater vehicle (ROV) multibeam and video surveys

“…Low penetration facies are commonly interpreted as coarse--grained deposits as observed along the channel floor (e.g. Damuth, 1980;Droz et al, 2001;Klaucke et al, 2004;Kolla et al, 1980 ;Redbourn et al, 1993), or on channel--mouth lobes (e.g. Jegou et al, 2008;Klaucke et al, 2004;Migeon et al, 2010).…”