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The Bengal Fan: Some Preliminary Results from ODP Drilling

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The Bengal Fan: Some Preliminary Results from ODP Drilling View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Columbia University Academic Commons Geo-Marine Letters (1989) 9: I- 10 o-lVlarine Letters (~'~ 1989 Springcr-Vcrlag New York Inc, The Bengal Fan: Some Preliminary Results from ODP Drilling Dorrik A. V. Stow, 1 James R. Cochran, 2 and ODP Leg 116 Shipboard Scientific Party* ~Geology Department, University of Nottingham, Nottingham, NG7 2RD England; and 2Larnont-Doherty Geological Observatory, Palisades, NY, USA Abstract Introduction Leg 116 of the Ocean Drilling Program (ODP) pro- Three deep holes, with a maximum penetration of 960 m below vided one of the few opportunities to drill into one of Sea flOor, were drilled into the distal Bengal Fan just south of the equator during ODP Leg 116. The entire section recovered is dora- the world's largest submarine fans. Leg 96 of the Deep mated by sandy silt and mud turbidites derived from the Ganges Sea Drilling Program (DSDP) drilled a series of sites Delta and from the continental margin of the western Bay of Ben- on the mid- and lower Mississippi Fan (Bouma and gal, interbedded with thin pelagic clays and with biogenic turbi- others 1985, 1986) and several earlier legs have drilled dites probably from a local sea mount source. The effects of Hi- isolated holes on the Bengal (Von der Borsch and oth- malayan uplift, sea level fluctuations, local tectonics, and fan channel/lobe processes have closely interacted to produce the ob- ers 1974), Indus (Whitmarsh 1974), Amazon (Hayes served sedimentary record of the past 17 million years since the and others 1973), Nile (Ryan and others 1973), and early Miocene. Zambesi (Simpson and others 1974) Fans. These elongate submarine fans (Stow and others 1983) are large and complex and require much further study be- *Coauthors include: James R. Cochran, Lamont-Doherty Geo- fore a full picture of their characteristics can be pieced logical Observatory (Co-chief scientist); Dorrik A. V. Stow, Not- together. tingham University, England (Co-chief scientist); Christian A. Au- The Bengal Fan, together with its eastern lobe, the roux, Texas A & M University, USA (ODP staff scientist); Kazou Amano, Ibaraki University, Japan; Peter S. Balson, British Geo- Nicobar Fan, is the world's largest submarine fan (Von logical Survey, Keyworth, Notts, England; Jacques Boulegue, Pierre der Borsch and others 1974, Curray and Moore 1971, & Marie Curie University, Paris, France; Garrett W. Brass, Uni- Curray and others 1982, Emmel and Curray 1984) Versity of Miami, Florida, USA; Jeffrey Corrigan, University of covering an area of 3 × 106 km 2 (Fig. 1). It is 2800- Texas, Austin, USA; Stefan Gartner, Texas A & M University, 3000 km long, 830-1430 km wide, and over 16 km USA; Stuart A. Hall, University of Houston, Texas, USA; Silvia laccarino, University of Parma, Italy; Toshio lshizuka, University thick beneath the northern Bay of Bengal. Fewer than of Tokyo, Japan; Irena Kacmarska, Mount Allison University, New 100 piston cores have been recovered from the sur- Brunswick, Canada; Heidemarie Kassens, Kiel University, West face sediments, and only one spot-cored DSDP site Germany; Gregory Leger, Dalhousie University, Halifax, Nova (Leg 22, Site 218) has been drilled into the fan. This Scotia, Canada; Franca Proto Decima, University of Padova, Italy; hole terminated in mid-Miocene sediments at a depth C. V. Raman, Andhra University, Visakbapatnam, India; William W. Sager, Texas A & M University, USA; Kozo Takahashi, Woods of 772 m below the sea floor. Four main pulses of Hole Oceanographic Institution, USA; Thomas Thompson, 580 sandy turbidites were identified, one in the mid-Mio- Euclid Ave, Boulder, Colo, USA; Jean-Jacques Tiercelin, Uni- cene, two in the late Miocene-Pliocene and one in the versity of Bretagne, Brest, France; Mark Townsend, University of Pleistocene (Von der Borsch and others 1974). The Nottingham, England; Andreas Wetzel, Tubingen University, West Germany; N. P. Wijayananda, National Aquatatic Resource Agency, piston cores from the fan show that in most cases pe- Colombo, Sri Lanka; and Colin Williams, Lamont-Doherty Geo- lagic muds, 20-90 cm thick, are found overlying thin logical Observatory. turbidite sands, most less than 10 cm thick. Geo-Marine Letters I ! 80" 0 km 500 L, 1 i I 1 J GROUND UPPER i FAN : MIDDLE FAN 1 LOWER Y I APPROXIMAT| i FAN / BOUNDARY / NINETY EAS RIDGE LEG Figure 1. Map of the Bengal Fan 116 (Modified after Emmel and Curray SITES i'll (1984) and location of Leg 116 NICOBAR sites. Indian margin and tan chan- O FAN- LOBE nels shown by dashed lines: cur- AFANASY rcntly active fan channel shown by NIKITIN solid line. Seamounts and topo- SEAMOUNTS graphic highs indicated by a heavy I I I ~ I shading. Sediments are funnelled to the fan via a delta-front tions are published elsewhere (The Leg 116 Ship- trough, the "Swatch of No Ground ~ (Fig. 1). This board Scientific Party 1987a, 1987b). trough is presently connected to only one active fan These deal more specifically with the other main channel, but has been effectively cut off from its prin- scientific objectives of the Leg, namely the history of cipal sediment supply and the Ganges/Brahmaputra Himalayan uplift as recorded in the accumulation of Delta, since the most recent rise in sea level, probably their erosion products on the Bengal Fan, and the na- about 7,000-10,000 BP. Many other channels mark ture and timing of intraplate deformation in the cen- the fan surface and extend for various distances along tral Indian Ocean (Weissel, Anderson, and Geller its length. Most of these are thought to have been 1980). It is clear from this preliminary' work that the abandoned by channel switching in the past (Emmel history of the Bengal Fan and hence of Himalayan and Curray 1984). Several canyons and channels also uplift goes back to at least 17 (Ma). From a consid- cut across the east Indian margin, extending into the eration of seismic records in the area it seems likely western part of the fan (Fig. 1). that a major uplift event occurred around 18-20 Ma In this article, we present some preliminary ship- in the early Miocene. Intraplate deformation began to board results fi'om Leg 116 drilling regarding fan affect this part of the Indian Ocean about 7 Ma. Mo- growth and sediment characteristics. Other observa- tion along high-angle faults between blocks has been Vol. 9, No. l, 1989 3 gradual since that time at an average rate of approx- solution-resistant, mainly nannofossil biota, the imately 50 m per million years (m.y.). downhole biostratigraphy is consistant and adequately constrained. Results Sediment Facies Three closely-spaced sites were drilled on the extreme distal portion of the fan, some 800 km south of Sri The entire section penetrated is dominated by turbi- Lanka and 200 km northwest of the main Afanasy dites. The most common of these are gray micaceous Nikitin Seamount group (Fig. 1). The most recently silt and silt-to-mud turbidites that range up to 2.5 m active fan channel appears to terminate some 50 km in thickness (Facies 1, Fig. 3). Some turbidite beds to the north and the sea floor in the area of drilling have 1.0 to 1.5 m of graded silt at the base, whereas is smooth and almost flat (<0.5 m/km gradient). It others have no more than a few thin silt laminae, is not yet clear, pending further study of 3.5 kHz seis- overlain by up to 2.0 m of graded mud. Indistinct mic and depth-profile lines, whether the area shows parallel lamination is common in the thicker beds, morphological characteristics of a terminal lobe or a whereas the thinner beds (1-10 cm thick) may show Smooth lower fan. graded laminated units and, more rarely, cross and However, beneath this generally smooth fan sur- convolute lamination. Wood fragments (up to 2 cm face, the ocean crust and overlying sediments have long) are locally abundant in the silts, and bioturba- been deformed into long-wavelength (100-300 km) tion is rare at the tops of the mud interval. The max- Undulations (Weissel and others 1980, Geller and oth- imum quartz grain size at the base of the thicker silt ers 1983) that locally protrude through the younger turbidites is coarse sand grade, although the mean grain sediment cover giving a sea floor relief of up to 800 size is coarse silt to find sand. m. Superimposed on these undulations are faulted and Dark gray mud turbidites are common in certain rotated blocks spaced 5-20 km apart; these features intervals (particularly Units Ill and IV), ranging in result in a few tens of meters of sea floor relief where thickness from a few cm to 1.5 m (Facies 2, Fig. 3). the top of the fault block is exposed (Fig. 2(a)). These turbidites typically consist of silty mud which All three sites penetrated a similar sequence of sed- grades up into dark gray clay that is bioturbated to- iments but showed significant differences in thickness ward the top. Locally, the base is characterized by due to their relative positions on two adjacent fault thin (0.5-5.0 cm) silt-laminated muds. Very fine- blocks (Fig. 2). At each site, five major lithologic grained dispersed woody debris is ubiquitous, resedi- units can be recognized, from top to bottom: mented nannofossils are common, and iron sulphides ° Unit 1 -- the topmost 2-6 m of pelagic clayey oozes are locally abundant as finely dispersed framboids, thin and mud turbidites mycelia, and larger pyritized burrows, microfossils, • Unit II -- 100-150 m of sandy silt, silt-to-mud and pyritic silt, and concretions.
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