The architecture and evolution of the Middle Bengal Fan in vicinity of the active channel–levee system imaged by high-resolution seismic data

Abstract

High-resolution seismic data from the Middle Bengal Fan were analysed to study the architecture of the fan in the vicinity of the active channel–levee system and to evaluate channel behaviours. The upper 600 m column of the fan in the study area reveal a pattern of different sized channel–levee systems and high amplitude reflection packets (HARPS) of varying thicknesses, no mass-flow deposits or hemipelagic drapes are found. Within the channel–levee systems, units of chaotic high amplitude reflectors (CHARS), which represent aggraded and migrated channel axes, as well as abandoned channel-fill deposits appear. All channels are characterised by erosional incision into underlying deposits. Within the active channel–levee system, numerous cut-off loops are found showing a broad spectrum of complex behaviours with different ratios of vertical aggradation and lateral migration. As a difference to the Amazon Fan, the floors of all cut off loops and the active channel are not elevated above the surrounding seafloor, which may lead to such a long lifetime resulting in the large number of cut-off loops. In contrast, a detailed studied buried system shows no cut-off loops and is characterised by a more simple behaviour: a first phase of lateral migration is followed by a second phase of simultaneous vertical aggradation and lateral migration, which finally elevated the channel floor above the level of the surrounding seafloor. These architectural differences between the systems indicate that different loaded turbidity currents have built up both systems. The aggrading and migrating channel floors likely have reservoir potential, but a high-resolution 3D seismic survey and drilling are necessary for final proof.