Morphology and Internal Structure of a Recent Upper Bengal Fan-Valley Complex

Abstract

3D seismic and multibeam data show that the present seafloor morphology of the entire upper Bengal Fan-valley complex is broadly sinuous and is more than 20 km wide in places, and consists of a highly sinuous channel flanked by a series of several terraces or overbanks on either side all along its length. This morphology is but a surface expression of the underlying internal structure and evolution of several, vertically and laterally stacked valley fills and their flanking overbanks. Each of these valleys consists of underfit sinuous channel fills with development of scrolling, indicative of much lateral channel migration with downstream shifts in their courses in the initial stages of their evolution. The scrolls may be of high seismic amplitudes, sand-prone, or of low seismic amplitudes, mud-prone. In their later stages of evolution, the channels exhibit more aggradation. Cutoffs are more common in the initial stages of sinuous-channel evolution and less common in the latter stages. The highly sinuous channel on the present sea floor is also an underfit feature and represents the latest phase of the uppermost valley fill. The various stages of channel evolution are a function of the hydrodynamics of the flows in the channels and sediment grain size supplied. At the very base of the above mentioned main valley-fill complex, but frequently amalgamated to it, a fan-shaped network of straight to slightly sinuous channels with thin fills, fed by the same canyon as for the overlying valley complex, is present. This basal channel network reflects smaller flows in the very initial stages of avulsion from an older upper fan-valley complex to the east. However, the overlying main valley complex reflects large-volume flows when the avulsion became fully established later and the canyon was entirely feeding it. The innermost terraces on either side of the present sinuous channel on the seafloor resulted from its flanking overbanks over the abandoned channel fills within the uppermost valley of the complex. The more outer terraces formed from the overbanks of successively younger valleys when they abutted against the higher banks of the preceding older and larger valleys. The recent upper fan-valley complex may have originated during the last glacial stage and continued to evolve mainly until about 6000 years B.P. (Weber et al., 1997; Hübscher et al., 1997). Smaller turbidity flows that could not have generated overbanks may have continued subsequently. However, our cores from the latest upper-fan sinuous channel with brown oxidized muds at the tops show that there is little or no turbidity-current activity in it at present.

Key Future Directions For Research On Turbidity Currents and Their Deposits

“…Observations from cores in Monterey Canyon (Paull et al 2010a), the Congo Channel (Babonneau et al 2010), and Bengal Fan (Kolla et al 2012) also show sand deposits restricted to near the canyon floor, implying that caution is needed in relating sand transport in flows to observations of deposits. This appears inconsistent with data reported by Xu (2011) and Xu et al (2013), who found sand in traps suspended 70 m above the floor of Monterey Canyon.…”

Facies architecture of submarine channel deposits on the western Niger Delta slope: Implications for grain‐size and density stratification in turbidity currents

“…Modern submarine channels are another valuable data source for interpreting density stratification because of the preserved geomorphology and the opportunity to sample recent stratigraphy. Sediment coring in modern submarine channels indicates that grain size and bed thickness decrease markedly with increasing height above the thalweg [ Hiscott et al ., ; Pirmez and Imran , ; Dennielou et al ., ; Babonneau et al ., ; Paull et al ., ; Kolla et al ., ; Migeon et al ., ; Jobe et al ., ]. Some studies have linked these trends in bed thickness [ Dennielou et al ., ] and grain size [ Migeon et al ., ] to turbidity current flow properties, including concentration and velocity profiles.…”

Controls on submarine channel-modifying processes identified through morphometric scaling relationships

“…Although submarine channels can be an order of magnitude larger than rivers (Konsoer et al, 2013), their cross-sectional and planform morphologies bear striking similarities ( Fig. 1) with many submarine channels exhibiting levees (Hansen et al, 2015), erosional terraces (Babonneau et al, 2004), and/or migrating meander bends and cutoffs (Kolla et al, 2012;Maier et al, 2012). Given the morphological similarities between submarine channels and rivers, can techniques developed in fluvial geomorphology shed light on the processes that control submarine channel morphologies?…”