In November and December 1995, Ocean Drilling Program (ODP) Leg 164 targeted the Blake Ridge gas hydrate field and adjacent Carolina Rise (Paull, Matsumoto, Wallace et al., 1996). The Blake Ridge is a continental rise contourite deposit that has a crest that extends to the southeast perpendicular to the general trend of the North American Continental Margin (Mountain and Tucholke, 1985). The area is associated with an extensive and well-developed bottom simulating reflector (BSR) (Fig. 1), covering about 26,000 km2 (Dillon and Paull, 1983). Previous drilling by the Deep Sea Drilling Project had indicated that gas hydrates occurred in this region (e.g., Kvenvolden and Barnard, 1983).
The program consisted of drilling a transect of deep sites (Sites 994, 995, and 997) on the Blake Ridge, extending down through the zone where gas hydrates are stable and into the sedimentary section below. These sites provide an opportunity to study the in situ characteristics of gas hydrates and gas hydrate-bearing sediments and to document the nature of BSRs (Fig. 2). Because of the ephemeral nature of gas hydrates, emphasis was placed on proxy sampling and downhole tool measurements that allowed reconstruction of the in situ conditions associated with the gas hydrates. Thus, the decision was made to emphasize running in situ data-collecting tools in each hole, at the expense of increased core recovery.
Short holes (50-67 m) were also drilled at four sites (Sites 991, 992, 993, and 996) on the Carolina Rise where diapirism has disturbed the gas hydrate-bearing sediment section. The objective was to investigate physical properties and gas migration at these disturbed sites. Sites 991, 992, and 993 were located within the slide scar associated with a major Pleistocene age slide that surrounds a diapir (Popenoe et al., 1993). Site 996 was located along a fault trace on the crest of another diapir that is surrounded by a distinct BSR (Paull et al., 1995).