Geological setting

The Bengal Fan covers the floor of the entire Bay of Bengal (Fig. F1), from the continental margins of India and Bangladesh to the sediment-filled Sunda Trench off Myanmar and the Andaman Islands and along the west side of the Ninetyeast Ridge. It spills out south of the Bay of Bengal at its distal end to ~7°S. Another lobe of the fan, called the Nicobar Fan, lies east of the Ninetyeast Ridge, but its primary source of turbidites from the head of the Bay of Bengal apparently was cut off during the Pleistocene by convergence between the northern end of the Ninetyeast Ridge and the Sunda Trench. The northeastern edges of the fans have been subducted, and some of the Tertiary turbidites cropping out in the Indo-Burman Ranges of Myanmar, the Andaman and Nicobar Islands, and in the outerarc ridge off Sumatra have been interpreted as old Bengal and Nicobar Fan sediments.

The Bengal and Nicobar Fans were delineated and named by Curray and Moore (1974), who also noted two horizons in reflection and refraction seismic data that pass into unconformities over the exposed and buried hills of folded sediments in the southern part of the fan and over the Ninetyeast Ridge. They concluded that these two horizons are regional and used them to divide the sedimentary section into three units in the Bay of Bengal. The ages of these unconformities were tentatively determined to be uppermost Miocene and upper Paleocene to middle Eocene during DSDP Leg 22 (Moore et al., 1974; Shipboard Scientific Party, 1974) at Sites 218 and 217, respectively (Fig. F1). They interpreted the older unconformity as dating the India-Asia collision, with the pre-Eocene sedimentary unit consisting of pelagic sediment and terrigenous material derived from India before the collision. Hence, the upper two sedimentary units define the Bengal Fan sensu stricto. They associated the upper Miocene unconformity with intraplate deformation, probably correlated with a plate edge event. These tentative age assignments were confirmed and refined by later drilling during ODP Legs 116 and 121 (Cochran, Stow, et al., 1990; Peirce, Weissel, et al., 1991), although the interpretation and significance of the older unconformity and the time of initiation of Bengal Fan deposition and progradation remain very controversial.

The older unconformity was drilled and sampled only on the Ninetyeast Ridge. Correlation of this unconformity off the ridge into the fan section is possible along some but not all seismic lines (e.g., Gopala Rao et al., 1994, 1997; Krishna et al., 1998; Schwenk and Spieß, 2009). DSDP sites from Leg 22 sampled the older unconformity. DSDP Site 215 showed a hiatus from early Eocene to late Miocene (Fig. F3). Site 211, located at the eastern distal edge of the Nicobar Fan, showed a hiatus from some time after the Maastrichtian until the Pliocene. The overlying younger sections have been interpreted as distal fan.

An Eocene initiation of Bengal Fan deposition is also suggested by the geology of Bangladesh, the Indo-Burman Ranges of India and Myanmar, and the Andaman-Nicobar Ridge. Hydrocarbon exploration on and offshore from southeastern Bangladesh (e.g., Kingston, 1986) shows total sediment thicknesses calculated from gravity to be >20 km, apparently with the Eocene and Oligocene Disang Series of deepwater shales and turbidites overlying oceanic crust. These are in turn overlain by Neogene prograding deposits of the Bengal Delta and the Ganga-Brahmaputra (Jamuna), Meghna, and their ancestral rivers. Some of this rock has been mildly metamorphosed and uplifted into the accretionary prism in the Indo-Burman Ranges, with some sparsely fossiliferous flysch units correlated with the Disang Series (e.g., Brunnschweiller, 1966; Bender, 1983; Najman et al., 2008). Similar turbidites, the Andaman Flysch or Port Blair Formation, are found in the Andaman and Nicobar Islands, again usually assigned Eocene and Oligocene ages (e.g., Karunakaran et al., 1975; Acharyya et al., 1991) and interpreted to represent parts of the early Bengal Fan, some of which have been incorporated into the Sunda arc accretionary complex.

The initiation of deposition and progradation of the Bengal Fan followed the collision of India with Asia and the formation of a large proto-Bay of Bengal. Continued convergence of the Indian and Australian plates with the Southeast Asian plate reduced the size of the bay and focused the source of turbidites into the present Bengal Basin, Bangladesh shelf, and the shelf canyon Swatch-of-no-Ground (Curray et al., 2003).

Fans grow by progradation, and the first sediments are deposited at the mouth of a canyon and at the base of the slope, in this case the continental slope. With time the fan progrades farther from the original base of the slope. Our limited information suggests that the Bengal Fan has prograded, as shown in Figure F3. The oldest rocks interpreted as Bengal Fan in the Indo-Burman Ranges are early Eocene; the oldest such turbidites in the Andaman and Nicobar Islands are middle Eocene. DSDP Sites 215 and 211 revealed upper Miocene and Pliocene turbidites, respectively. DSDP Site 218 did not reach the base of the fan, nor did the ODP Leg 116 sites. The interpretation that the Eocene unconformity marks the base of the fan suggests that it may represent a hiatus of variable duration (Fig. F3).

Seismic studies/Site survey data

The original IODP proposal for this expedition (552) was suggested based on a single 500 km long multichannel seismic line at 8°N (GeoB97-020+027), which was acquired during R/V Sonne Cruise SO125 in 1997 (Spiess et al., 1998) to gain a better understanding of the buildup of the fan with respect to channel-levee geometries and stacking patterns (Fig. F2). For stratigraphic calibration, this seismic profile crossed DSDP Site 218 (Leg 22), where sediments were cored and dated to 773 meters below seafloor (mbsf) (middle Miocene).

To further support IODP Proposal 552, a dedicated presite survey during R/V Sonne Cruise SO188 was carried out in June–July 2006. This cruise collected multichannel seismic, swath bathymetric, and subbottom profile data on crossing lines through Proposed Sites MBF-1A to MBF-6A. These additional data provided further understanding of the spatial variation of sediment structures, particularly the underlying complex structures of buried channels (Figs. F2, F4). Shallow penetration sediment cores were taken at selected sites to complement the data already available from DSDP Site 218 (Site MBF-1A).

The long seismic reflection Profile GeoB97-020/027 (Fig. F2) has been interpreted with respect to the presence of channels, channel-levee systems, and seismic stratigraphy (Schwenk and Spieß, 2009). Several major reflectors and unconformities were traced across the drilling transect, revealing increased average sedimentation rates as a function of distance from the basement ridges at 85°E and 90°E. Age constraints provided by correlation to DSDP Site 218 suggest that our shallow-penetration sites (MBF-6A, MBF-5A, and MBF-4A) should extend to ~1.8 Ma (Pleistocene/Pliocene boundary) and our mid-depth penetration sites (MBF-2A and MBF-1A) should extend to at least 8 Ma. Our deepest penetration and easternmost site (MBF-3A) has been selected to provide prefan sediments by drilling through the regional Reflector P (Eocene unconformity).

High-resolution seismic data were collected during two different surveys, and the two seismic data sets were processed slightly differently because of the availability of software and computer power.

R/V Sonne Cruise SO125:

  • Streamer: 750 m long, active section 600 m, 48 channels.
  • Seismic source: generator-injector (GI) gun, 2 × 0.41 L.
  • Standard processing sequence:
    • Trace editing,
    • Spherical divergence correction,
    • Bandpass-filter (55/100–300/600),
    • Common midpoint (CMP)-stacking (20 m CMP-distance 97-020/027; 10 m 97-022), and
    • T-K migration.
  • Processing was carried out with Seismic Unix on a Linux-PC.

R/V Sonne Cruise SO188:

  • Streamer: 750 m long, active section 600 m, 96 channels.
  • Seismic source: GI gun, 4.1 L (generator) + 1.7 L (injector)
  • Standard processing sequence:
    • Trace editing,
    • Spherical divergence correction,
    • Bandpass-filter (55/100–300/600),
    • CMP-stacking with 10 m distance, and
    • F-K migration/FD migration.
  • Processing was carried out with VISTA Seismic Processing 2D/3D (GEDCO) on a Windows PC.

Swath bathymetric data were gathered during Cruise SO125 using a Hydrosweep DS System and during Cruise SO188 using a Simrad EM120 echo-sounder system. The total multibeam coverage width is now 19 km, increasing in those areas where crossing lines were shot. Although bathymetry reveals a clear picture of the recent or Quaternary channel systems in the area, this information only applies to the upper few tens of meters of the section, which is otherwise a mix of stacked older channels and interlevee sequences.

Digital Parasound subbottom profiler data (4 kHz) were also routinely collected during both cruises.

Supporting site survey data for Expedition 354 are archived at the IODP Site Survey Data Bank.