The Bengal Fan: morphology, geometry, stratigraphy, history and processes
Introduction
The Bengal Fan was apparently first recognized as a submarine fan by Dietz (1953). He had seen echograms collected in 1948 by the Swedish deep sea expedition from the research vessel ALBATROSS, which showed small depressions or notches in the otherwise flat topography in a transect running southeast from Sri Lanka. Scientists on the expedition had speculated that these depressions might be tectonic features or grabens in a smooth lava flow on the sea floor. Dietz drew on observations by Buffington, 1952, Menard and Ludwick, 1951 of natural levees bordering turbidity current channels across sediment-covered sea floor off southern California, and he speculated that these depressions were also turbidity current channels. He pointed out that the bathymetry in the Bay of Bengal (op. cit., 1953, Fig. 1) is a smooth plain gently sloping away from a submarine canyon located off the Mouths of the Ganges River. “The uniform gradient suggests that this gradient is determined by the profile of equilibrium of some sea-floor process. Could not this process be turbidity currents?” (Op. cit., 1953, p. 377).
This region was subsequently illustrated in the early physiographic maps of Heezen and Tharp (1964), and the fan was named the Ganges Cone. The first real survey and delineation of the fan was by Curray and Moore (1971), who proposed the name Bengal Fan. This name has subsequently totally replaced the earlier name of Ganges Cone. These early physiographic maps diagrammatically showed fans as having a distributary pattern of turbidity current channels. Subsequent surveys have shown that this was a misinterpretation, and that most of the channels shown in this pattern are abandoned channels.
The western margin of the Bengal Fan is the continental slope of eastern India (Fig. 1, Fig. 2); the northern proximal or upper fan lies off the Bangladesh continental slope; and the eastern margin is the northern end of the Sunda trench and the accretionary prism of the Sunda Subduction Zone, extending from Myanmar (Burma), through the Andaman–Nicobar Ridge, into the Mentawai Islands off Sumatra. Much Bengal Fan sediment has been subducted and/or uplifted into this accretionary prism.
The Bay of Bengal was created by the initial Paleocene–Eocene collision of India with the subduction zone of the north side of the Tethys Ocean (Alam et al., 2003, Curray et al., 1982, Curray and Moore, 1974, Lee and Lawver, 1995). Prior to collision, a thick continental rise prism of sediment had formed off the eastern margin of India. Rapid clastic sedimentation in the Bengal Basin and fan formation on top of this continental rise commenced during continuing collision in the Eocene, and prograded southward during the Tertiary.
The Ganges River flows through the Rajmahal–Shillong gap (Fig. 1, Fig. 3), and the Brahmaputra River flows through a deep gap in the eastern Himalayas, down the Assam Valley, and between the Shillong Hills and the accretionary prism of the Indoburman Ranges. These rivers, confluent in the Bengal Delta, drain the south and north slopes of the Himalayas, respectively, and deliver their huge sediment load to the Bengal Basin and the Bay of Bengal. The Bengal Delta has filled the Bengal Basin, and the sediment which has passed on through has been distributed across the entire Bay of Bengal to form the largest submarine fan in the world. Two distinct fans, separated by the Ninetyeast Ridge, are recognized; the eastern subfan is called the Nicobar Fan, and the western fan is the Bengal Fan proper (Fig. 1) (Curray and Moore, 1971, Curray and Moore, 1974). The Nicobar Fan has been inactive since mid-Pleistocene time because the channel system which fed it was cut off by convergence of the Ninetyeast Ridge with the Sunda Trench (Curray & Moore, 1974). Most of our discussion will, therefore, be of the Bengal Fan. The most distal sediments which can be attributed to the Bengal and/or Nicobar Fan are Pliocene silt and sand turbidites in DSDP Site 211, south of eastern Sumatra, 3800 km from the present fan apex (Fig. 1).
The Bengal Fan has been largely unaffected by tectonic disturbances; contour currents are probably a minor influence (Kolla, Moore, & Curray, 1976); and while several very large rivers enter the western side of the Bay of Bengal from eastern India, the overall morphology is dominated by the massive sediment supply from the present and past canyons fronting the Bengal Delta of the Ganges and Brahmaputra Rivers. Because it is a very large fan, it is probably quite representative for studies of processes acting in the deposition of extensive turbidite sections. An ancient sediment body like the Bengal Fan preserved in the geological record would probably not be recognized as a submarine fan. It would instead probably be called a sedimentary basin, or in earlier terminology, be labeled a ‘geosyncline’. Total volume of the Bengal Fan is comparable to the volume of the ‘Appalachian Geosyncline’ (Curray, 1991, Drake et al., 1959).
Many descriptions of modern submarine fans have been published during the last few decades, and many models for formation and growth have been proposed (see, for example: Bouma et al., 1985, Damuth and Flood, 1985, Mutti, 1974, Nelson and Nilsen, 1984, Normark, 1970, Normark, 1978; Normark et al., 1993, Piper and Normark, 2001, Shanmugam and Moiola, 1988, Stow et al., 1984, Walker, 1978; and many others). We have previously published several discussions of the Bengal Fan (see, for example: Curray et al., 1982, Curray and Moore, 1971, Curray and Moore, 1974, Emmel and Curray, 1985, Moore et al., 1974; and others). This paper is an update to those papers, with inclusion of new observations and interpretations. Our objective here is to review the geometry, stratigraphy, history and processes of the Bengal Fan. We will not attempt to fit this fan into previously published classifications of submarine fans, we will not apply previously published models of formation, nor will we cite many comparisons of our observations on the Bengal fan to observations on other large passive margin fans. We will, however, propose our model for processes of formation of this fan.
This study is based mainly on data collected during cruises of the Scripps Institution of Oceanography, between 1968 and 1986, utilizing 3.5 kHz bottom-penetrating (∼100 m maximum) echo sounder and airgun seismic reflection profiling, mainly analog, but with some multichannel digital seismic reflection data. For most of our seismic reflection surveys, data were collected with two different sweep times and filter settings: a slower sweep, generally 5 s, filtered to 20–60 Hz; and a faster sweep, generally 2 s, filtered to 50–150 Hz for higher resolution. We did not have swath-mapping capability until the 1986 cruise, so although we knew from some detailed surveys that all of the fan valleys meandered, we could not document that fact. Tracks of these expeditions are shown in Fig. 4. Our contour chart (Fig. 3) is based on many additional ship tracks. Because of the very large area of the fan and limitations of time, the survey over some of the flat area of the fan was conducted at speeds of 8–10 knots or more. Thus, our bathymetric and reflection profiles commonly exhibit a large vertical exaggeration averaging about 25 X.
Section snippets
Morphology
The Bengal Fan has an area of approximately 2.8–3.0×106 km2, not including the Nicobar Fan. The length of the Bengal Fan is between 2800 and 3000 km, extending from 20°l0′N to 7°S latitude. Its greatest width is 1430 km at 15°N, and its narrowest part is at 6°N, 830 km between Sri Lanka and the Ninetyeast Ridge. Depths at its apex and distal end are 1400 and almost 5000 m, respectively. The fan volume is about 12.5×106 km3, and the mass of the sediments and metasediments in the fan is about
Tertiary stratigraphy
Very early in our studies of the seismic stratigraphy of the Bengal Fan, we recognized two major unconformities in the localities of deformation in the southern or lower fan (Fig. 6) (Curray & Moore, 1971). We could trace these onlap unconformities from one deformed locality through the conformable section to the next. On the basis of these unconformities, we subdivided the section into the basement, an older sequence shown with velocities from about 4.4 to 6.6 km/s, and two younger sequences
The submarine drainage system of the Bay of Bengal
The present submarine drainage system of the Bay of Bengal and Bengal Fan consists of one active submarine canyon, the Swatch of No Ground, lying on the Bangladesh continental shelf off the Bengal Delta. It serves as a point source for one active fan valley or channel, AV (Fig. 5). At times in the past other submarine canyons cut into this shelf, which is a broad delta front. In this regard the Bengal Fan is different from some other large fans, which have had only one submarine canyon
One active canyon, one active fan valley
One of the important conclusions from this study and our previous publications is that there is only one active fan valley at the present time, issuing from the one active submarine canyon. As the fan valleys migrated, two active channels could have coexisted during the short time required to fill an old channel and stabilize a new one. In contrast, our investigations of the Quaternary stratigraphy (Fig. 15) suggests that during some periods of time, there may have been more that one active
Acknowledgements
The work at sea and initial analysis and publication of data for this project, many years ago, was funded by various grants from the Office of Naval Research and the National Science Foundation. Funding for the expenses of this long overdue summary publication have come from our own personal resources. During the cruises and the reduction of data, we had the assistance of many colleagues, some of whom have coauthored publications with us, as listed in the references below. We cannot list them
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