4. Site 12451

Shipboard Scientific Party2

INTRODUCTION

Site 1245 (proposed Site HR3a) is located in 870 m of water on the western flank of Hydrate Ridge, ~3 km northwest of the southern summit (see Fig. F1 in the "Leg 204 Summary" chapter). The three-dimensional (3-D) seismic data show that the bottom-simulating reflector (BSR) is at a depth of ~134 meters below seafloor (mbsf) at this site. As at all sites drilled during Leg 204, the temperature and pressure at the seafloor at Site 1245 are well within the gas hydrate stability zone (GHSZ), indicating that gas hydrates can exist within the entire stratigraphic section above the BSR if hydrate-forming gases are available in concentrations that exceed their in situ solubility. This site also samples seismic Horizon A. Horizon A can be mapped from the northern boundary of the seismic survey, where it clearly follows stratigraphic boundaries, to the summit, where Horizon A appears as a "bright spot" beneath the BSR. On its downdip edge, Horizon A appears to lap onto the boundary between coherent folded strata and the seismically incoherent facies interpreted to represent highly deformed sediments of the accretionary complex. Horizon A has been interpreted to be a "conduit" that transports fluids from the accretionary complex to the summit. Several unconformities, referred to as Horizons Y and Y´, overlie Horizon A and appear to represent discontinuities in sediment accumulation in a slope basin that was formed during the growth of an underlying accretionary anticline.

Primary objectives at Site 1245 were the following:

  1. Determine the distribution, composition, and concentration of gas hydrate in the sediments on the western flank of Hydrate Ridge and contrast these parameters with those on the eastern flank of the ridge and in the adjacent slope basin, where the sub-BSR fluid migration pathways inferred from seismic data are distinctly different.
  2. Sample sediments and fluids from seismic Horizon A.
  3. Sample the sedimentary section of the western flank of Hydrate Ridge below the BSR to provide constraints for interpreting variations in BSR strength across the western flank.
  4. Be a reference site tying a north-south-trending transect that extends from Site 1245 to the summit (including Sites 1247-1250) with an east-west transect comprising Sites 1244-1246 and 1252.

Five holes were drilled at Site 1245 (Fig. F1). Hole 1245A was drilled to a depth of 380 mbsf (without coring) to obtain the initial logging-while-drilling (LWD) data for this site. Hole 1245B was cored to 473.7 mbsf using the advanced piston corer (APC) and extended core barrel (XCB). Holes 1245C and 1245D were cored to 201.7 and 24 mbsf, respectively, for extensive high-resolution geochemical and microbiological sampling. Hole 1245E was drilled to 473.7 mbsf and then cored to 540.3 mbsf using the rotary core barrel (RCB). Coring in Hole 1245E stopped short of the originally planned depth of 700 mbsf because the primary science objectives had been met and hole conditions were deteriorating. Eleven whole-round samples of sediment thought to contain gas hydrate were preserved in liquid nitrogen or in pressure vessels for postcruise studies.

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-104

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