The Atlantic continental margin of the United States is a classic "passive" margin and is generally used as an example of a geologic feature developed during continental rifting (Bally, 1981). During rifting of North America from North Africa and subsequent subsidence, a thick wedge of Mesozoic and Cenozoic sediments built out onto the subsiding continental margin. Major deltaic systems prograded across the continental shelf, which occasionally were interrupted by more open-marine conditions. Carbonate reefs and micritic limestone buildups marked the shelf-slope break. The major sedimentary basins along the Atlantic Margin are generally elongated parallel to the present-day coast and most contain more than 10,000 m of sediment. The major basins include the Scotian, Georges Bank, Baltimore Canyon Trough, Carolina Trough, and Blake Plateau. North of the Carolinas, the continental margin above 2000-m bathymetric depth is less than 150 km wide, whereas to the south, into the Florida-Bahama region, the continental margin broadens to about 500 km. Beneath this broad continental shelf region are located the Carolina Trough and Blake Plateau Basin, which is bounded to the northeast by the Blake Ridge (Fig. 1). For a more complete description of the geology of the Blake Ridge see the review by Dillon and Popenoe (1988).
Seismic profiles along the Atlantic Margin are often marked by large-amplitude bottom simulating reflectors (BSRs) (Dillon et al., 1993; Lee et al., 1993), which in this region are believed to be caused by a large acoustic impedance contrast at the base of the gas-hydrate stability zone that juxtaposes sediments containing gas hydrate with sediments containing free gas. BSRs have been extensively mapped at two locations off the east coast of the United States—along the crest of the Blake Ridge and beneath the upper continental rise of New Jersey and Delaware (Tucholke et al., 1977; Dillon et al., 1993). The Blake Ridge is a positive topographic sedimentary feature on the continental slope and rise of the United States (Fig. 1). The crest of the ridge extends approximately perpendicular to the general trend of the continental rise for more than 500 km to the southwest from water depths of 2000 to 4800 m. The Blake Ridge is thought to be a large sediment drift that was built upon transitional continental to oceanic crust by the complex accretion of marine sediments deposited by longitudinal drift currents (Tucholke et al., 1977). The Blake Ridge consists of Tertiary to Quaternary sediments of hemipelagic muds and silty clay (Paull, Matsumoto, Wallace, et al., 1996). The thickness of the methane-hydrate stability zone in this region ranges from zero along the northwestern edge of the continental shelf to a maximum thickness of about 700 m along the eastern edge of the Blake Ridge (Collett, 1995). The first direct evidence that gas hydrate might be present along the Atlantic Margin was found during deep-sea drilling on the Blake Ridge in 1970 (Shipboard Scientific Party, 1972). Cores recovered from DSDP Sites 102, 103, and 104 contained large quantities of methane, which suggested the presence of gas hydrate. In addition, measured acoustic velocities (>2 km/s) within the suspected gas hydrate-bearing section exceeded normal marine sediment velocities, indicating the presence of gas hydrate. The occurrence of gas hydrate on the Blake Ridge was confirmed during Leg 76 of the DSDP when a sample of gas hydrate was recovered from a sub-bottom depth of 238 m at Site 533 (Shipboard Scientific Party, 1983).
Leg 164 of the ODP (Paull, Matsumoto, Wallace, et al., 1996) was the first drilling leg dedicated to the research of natural gas hydrate (Fig. 1). Sites 994, 995, and 997 compose a transect of holes that penetrate below the base of gas hydrate stability within the same stratigraphic interval over a relatively short distance (9.6 km). This transect of holes on the Blake Ridge extends from an area where a BSR is not detectable to an area where an extremely well-developed and distinct BSR exists (Fig. 2). A BSR is not observed at Site 994, a modest BSR occurs at 995, and a strong BSR is seen at Site 997. However, the geology and topography along this transect are relatively simple (Paull, Matsumoto, Wallace, et al., 1996), which provides an opportunity to assess the basic properties of gas hydrate-bearing sediments and to understand lateral variations caused by local lithologic, chemical, and hydrologic factors. Because drilling at all three sites on the Blake Ridge penetrated below the base of gas hydrate stability, they provide critical information on the amounts of gas and gas hydrate in the sediments as well as the nature of the BSR itself.