Site 1248 (proposed Site HR6) was drilled in ~830 m of water ~300 m northwest of the southern Hydrate Ridge summit (see Fig. F1 in the "Leg 204 Summary" chapter). This site is located in the middle of a small (~150 m diameter) high-reflectivity spot on the seafloor imaged by a deep-tow side-scan sonar survey (see Fig. F7 in the "Leg 204 Summary" chapter) (Johnson et al., in press). The small spot is located 300 m north of a larger, circular high-reflectivity patch around the Pinnacle, a well-known active carbonate chemoherm. These are the only two locations on southern Hydrate Ridge where high backscatter reflectivity, interpreted to be the seafloor manifestation of fluid venting, is observed. Television-sled surveys revealed some evidence for the presence of scattered authigenic carbonate fragments within the small high-reflectivity patch, which might be responsible for the higher backscatter signal observed in the side-scan sonar data. The three-dimensional (3-D) seismic data show attenuation of the underlying stratigraphic reflectivity, similar to what is observed beneath the Pinnacle (see Fig. F7 in the "Leg 204 Summary" chapter). Both areas of high backscatter overlie the intersection of seismic Horizon A and the bottom-simulating reflector (BSR).
The principal objectives at Site 1248 were to investigate (1) whether the sediments below the high-reflectivity seafloor spot contain evidence of active fluid advection and (2) if these fluids are supplied by Horizon A.
Three holes were drilled at Site 1248 (Fig. F1). Logging-while-drilling (LWD) measurements were made in Hole 1248A down to 194 meters below seafloor (mbsf). Hole 1248B was abandoned at 17 mbsf after three cores. In Hole 1248C, 5 extended core barrel (XCB) cores were drilled to 48 mbsf followed by 11 advanced piston corer (APC) cores, and 1 XCB core to 149 mbsf. After poor core recovery (23%) in Cores 204-128C-1X through 5X, recovery increased to 90% and remained high to total depth.
1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.
Ms 204IR-107