Deep Sea Fan Issues

Abstract

Deep sea fans occur along many continental margins. The Bengal Fan is the world's largest elongated submarine fan area, occupying over 3 x 106 km2 of seafloor in the Bay of Bengal. The Bay of Bengal is bordered by Sri Lanka, India, Bangladesh, Myanamar, the Andaman and Nicobar Islands, and Sumatra. The fan spans an area that is 2800-3000km in length and 830-1430 km in width. At the northern end of the Bay, the sediment cover is estimated to be more than 16 km in thickness (Curray and Moore, 1971, 1974, Moore et al., 1974). Recent drilling on the distal part of the fan just south of the equator during Ocean Drilling Program Leg 116 cored nearly 1 km of sediment without reaching hardrock basement (Cochran et al., 1990). The submarine feature of the Ninetyeast Ridge divides the fan into two major lobes, the main Bengal Fan and the eastern lobe, also known as the Nicobar Fan (Curray and Moore, 1974) (figure 19.1). The fan extends from 20°N latitude and, based on recent sedimentological and channel-system studies, to beyond 9°S latitude (Stow et al., 1990; Hübscher et al., 1997). The great size of the Bengal Fan is related to the history of the collision of the Indian tectonic plate with Eurasia and the subsequent uplift of the Himalayas. The first encounter of the northward-moving Indian Plate with the Asian mainland occurred around 50 million years (my) ago in the early Eocene Epoch (Haq, 1985). The first collision caused the initial uplift in the Himalayan region. Sedimentation in the bay is inferred to have started after this first collision, but extensive sedimentation probably did not begin until the early Miocene (ca. 17 my ago) after a major uplift in the Himalayas (Haq, 1985). Weathering and denudation of the Himalayas has furnished huge volumes of sediments that have built the Bengal Fan, supplied through the Ganges and Brahmaputra Rivers and their delta (figure 19.2). Sediments are transported largely by turbidity currents across the submerged continental terrace in the proximal part of the fan through a major delta-front canyon, also known as the Swatch-of-No-Ground. Currently, this canyon discharges its load into a single active channel that supplies sediment to the entire length of the fan.