During Ocean Drilling Program (ODP) Leg 172, drift deposit sediments were drilled along the Blake-Bahama Outer Ridge and Bermuda Rise. Drilling at Sohm Abyssal Plain was attempted but was interrupted. Contour currents, such as the Deep Western Boundary Current associated with the production of the North Atlantic Deep Water and Antarctic Bottom Water, supply sediments to the Blake-Bahama Outer Ridge and Bermuda Rise, resulting in high sedimentation rates. The maximum average sedimentation rate reached 47.12 cm/k.y. at Site 1061 during marine isotope Stages 2-4 (see Keigwin, Rio, Acton, et al., 1998, chap. 5, fig. 64). We are interested in discovering what kind of sedimentary processes can result in such amazingly high sedimentation rates in deep-sea environments. Although there have been studies of geochemistry and grain-size variations (e.g., Keigwin and Jones, 1989; Johnson et al., 1988; Haskell et al., 1991), sedimentary structures themselves have been paid less attention in this region.
Distinct sedimentary structures have been observed at Leg 172 sites. Research by the high-energy benthic boundary layer experiment (HEBBLE) (e.g., Nowell and Hollister, 1985) showed that fairly intense storms occur in the deep-sea environment. In light of these benthic storms, the sedimentary structures observed our sites could become a key to understanding the sedimentary processes associated with contour currents in the deep-sea environment. This paper will introduce and give detailed descriptions of some examples of sedimentary structures observed at these sites.