The sites of Leg 164 were all drilled at the eastern end of the Blake Ridge and southernmost Carolina Continental Rise. Previous investigations of Deep Sea Drilling Project (DSDP) Sites 102, 103, 104 (Shipboard Scientific Party, 1972), and 533 (Sheridan, Gradstein, et al., 1983) revealed that the Blake Ridge is a major Neogene and Quaternary sediment drift that consists of hemipelagic silt- and clay-rich contourite deposits. The objectives of Leg 164 were to increase the understanding of all the aspects of generation and occurrence of gas hydrates and their impact on the environment in an area where their existence had already been suggested by observations on DSDP Leg 11 (Claypool et al., 1973) and verified by sampling on DSDP Leg 76 (Kvenvolden and Barnard, 1983).
We have characterized the amount and composition of the organic matter in the drilled sections to explain its possible contribution to the formation of gas hydrates. The investigations were conducted on whole-round cores from Holes 994C, 995A, 997A, and 997B. One of the targets was to look for the occurrence of thermogenic gases. Only the deep holes of Leg 164 were sampled to determine the extent of in situ formation of gases during maturation of the sedimentary organic matter with increasing depth.
Sites 994, 995, and 997 make up a transect of holes that have penetrated below the base of gas hydrate stability within the same stratigraphic interval over a short distance of 9.6 km. Even within this distance, small variations in the composition of the organic matter (Rock-Eval data, maceral data) occurred. The ages of the sediments drilled at the three locations are roughly the same and in the range of late Miocene to Pleistocene
Three lithostratigraphic units were identified in all three holes based on sediment composition and primary variations in the nannofossil and total carbonate contents. The sediments consist mostly of green to gray nannofossil-bearing clays, claystones, and marls of late Miocene to Holocene age (Paull, Matsumoto, Wallace, et al., 1996). Pyrite is a common accessory mineral and occurs throughout the profile, being somewhat less abundant in deeper sections. Microfossils (diatoms, foraminifer, sponge spicules, and chitinous-phosphatic faunal relics) constitute the coarsest fraction of the sediments.