Application of the Principles of Seismic Geomorphology to Continental Slope and Base-of-Slope Systems: Case Studies from SeaFloor and Near-Sea Floor Analogues

The study of near-seafloor deepwater landscapes and the processes that form them are as important to the understanding of deeply buried marine depositional systems as the study of modern fluvial environments is to our understanding of ancient terrestrial depositional systems. In fact, these near-seafloor studies follow in the great tradition established by earlier clastic sedimentologists in the use of modern systems to understand ancient environments. The acquisition and mapping of exploration 3D seismic surveys over the last few decades allows for the study of seafloor geomorphology with a spatial resolution comparable to most deepwater multibeam bathymetric tools, and represents a significant advancement that can be used to push forward general understanding of slope and base-of-slope depositional systems through the application of the emerging science of seismic geomorphology. The papers assembled for this volume demonstrate the utility of seafloor-to-shallow subsurface data sets in studying the development of submarine landscapes and their affiliated sedimentary deposits. These contributions highlight the controls of slope morphology on patterns of both sedimentation and erosion. Many of the papers also highlight the influence of pre-existing seafloor relief on confining sediment-gravity flows specific transport pathways, thereby affecting subsequent evolution of the seafloor. The understanding of depositional processes that comes from studying deepwater analogue systems remains the best way take to knowledge from one basin or system and apply confidently to another for prediction and characterization of reservoirs for exploration and production of hydrocarbons.
Morphology and Internal Structure of a Recent Upper Bengal Fan-Valley Complex
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Published:January 01, 2012
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CiteCitation
Venkatarathnam Kolla, Anindya Bandyopadhyay, Pankaj Gupta, Brunti Mukherjee, Devulapalli V. Ramana, 2012. "Morphology and Internal Structure of a Recent Upper Bengal Fan-Valley Complex", Application of the Principles of Seismic Geomorphology to Continental Slope and Base-of-Slope Systems: Case Studies from SeaFloor and Near-Sea Floor Analogues, Bradford E. Prather, Mark E. Deptuck, David Mohrig, Berend Van Hoorn, Russell B. Wynn
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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.