MORPHOLOGY AND INTERNAL STRUCTURE OF A RECENT UPPER BENGAL FAN-VALLEY COMPLEX 347 MORPHOLOGY AND INTERNAL STRUCTURE OF A RECENT UPPER BENGAL FAN-VALLEY COMPLEX VENKATARATHNAM KOLLA Consulting Services, 6907 La Puente Dr., Houston, Texas 77083 e-mail: ratha.kolla@gmail.com ANINDYA BANDYOPADHYAY, PANKAJ GUPTA, BRUNTI MUKHERJEE, AND DEVULAPALLI V.RAMANA Petroleum Division, Reliance Industries, Navi Mumbai, India 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 WORDS: Bengal Fan, fan valley, sinuous channel, morphology, internal structure, lateral migration, aggradation, cutoffs Application of the Principles of Seismic Geomorphology to Continental-Slope and Base-of-Slope Systems: Case Studies from Seafloor and Near-Seafloor Analogues SEPM Special Publication No. 99, Copyright © 2012 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-304-3, p. 347–369. INTRODUCTION In recent years, many hydrocarbon discoveries have been made in deep-water sinuous-channel reservoir systems from Cenozoic stratigraphic intervals along several passive margins around the world ocean (e.g., Kolla et al., 2001; Abreu et al., 2003; Bastia, 2004; Mayall et al., 2006; Ardill et al., 2005; Kolla et al., 2007). Several of these discoveries have been developed into producing fields. For efficient field production, it is critical to have a detailed and precise understanding of the reservoir litho- logical distributions and architectures of deep-water sinuous- channel systems at exploration depths and their evolution. Such detailed understanding can be obtained from as many of the following studies as possible and their integration: (1) high- resolution 3D seismic imaging of the stratigraphic targets within the hydrocarbon fields, supplemented by cores, logs, and pro- duction histories (e.g., Porter et al. 2006; Abreu et al., 2003; Kolla et al., 2001); (2) very high-resolution 3D seismic studies of shallow subsurface sinuous channel systems in settings similar to those of the discoveries, supplemented by cores (e.g., Posamentier and Kolla, 2003; Kolla et al., 2007; Deptuck et al., 2003; Deptuck et al., 2007); (3) outcrop studies in analog settings (e.g., Abreu et al., 2003; Wynn et al., 2007, and the references therein); (4) experi- mental, numerical, and theoretical studies on deep-water sinu- ous channel systems (e.g., Wynn et al., 2007; Peakall et al., 2007; Islam et al., 2008); and (5) comparative studies of fluvial and deep- water sinuous channel systems (Kolla et al., 2007; Peakall et al., 2007; Islam et al., 2008 ). More studies of this type help to increase understanding of architectures of sinuous-channel reservoirs. The present study deals mainly with a high-resolution 3D seismic and multibeam data sets of the most recent, what Curray et al. (2003) referred to as, upper Bengal Fan “valley” (or “channel”), from the sea floor to about 400 m subsurface (Figs. 1A, 1B). Curray et al. (2003) used both the terms “valley” and “channel” inter- changeably to describe this geomorphic feature. In other fans, previous authors used either “valley” or “channel” or both for the same upper-fan feature (e.g., Damuth and Kumar, 1975; Normark, 1978; Kolla and Coumes, 1987). In our paper, we make a distinc- tion between “valley” and “channel”, although qualitatively (Wescott, 1997); we use the term “valley” for large and wide Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/4260065/9781565763043_ch18.pdf by guest on 12 August 2019