10. Site 12511

Shipboard Scientific Party2

INTRODUCTION

Site 1251 (proposed Site HR2alt) was drilled in ~1210 m of water, ~5.5 km east of the southern summit of Hydrate Ridge (see Fig. F1 in the "Leg 204 Summary" chapter). The site is located in a slope basin where well-stratified sediments were apparently deposited at a rapid rate. Seismic data record a history of deposition, tilting, folding, and depositional hiatuses in the basin that are probably related to the evolution of Hydrate Ridge. A strong bottom-simulating reflector (BSR) suggests that the base of the gas hydrate stability zone (GHSZ) is at ~200 meters below seafloor (mbsf) at this site.

The principal objectives at Site 1251 were the following:

  1. Determine the source of gases forming gas hydrates in a setting that is characterized by rapid deposition of hemipelagic sediments and mass-wasting deposits, in contrast to the uplifted sediments of the accretionary complex.
  2. Determine the distribution of gas hydrates in relation to the typical lithological parameters for the basin.
  3. Test general models for hydrate formation in regions of rapid sediment accumulation that were developed in the Blake Ridge area from results of Ocean Drilling Program (ODP) Leg 164.
  4. Provide age constraints on the geological history recorded by seismic stratigraphy.

Eight holes were drilled at Site 1251 (Fig. F1). In Hole 1251A, logging-while-drilling (LWD) measurements were made using a variable, but generally low, ROP in the upper 30 mbsf, followed by an increased ROP of ~50 m/hr from 30 mbsf to the bottom of the hole at 380 mbsf. Hole 1251B was cored with the advanced piston corer (APC) (Cores 204-1251B-1H through 24H) to 194.6 mbsf with average recovery of 80.6%. This interval included pressure core sampler (PCS) deployments at 105 and 154 mbsf and a Fugro Pressure Corer (FPC) deployment at 172 mbsf. At 194.6 mbsf, more lithified sediments significantly reduced the penetration of the bit. Coring continued using the extended core barrel (XCB) down to 445.1 mbsf (Cores 204-1251B-25X through 53X) with an average recovery of 85.5%. In addition to the XCB, we used all three pressure coring tools (PCS, FPC, and Hydrate Autoclave Coring Equipment [HYACE] Rotary Corer [HRC]). Hole 1251C was terminated after two cores. In Hole 1251D, which comprises 30 cores, 3 XCB cores were drilled to 26.9 mbsf, followed by APC cores to 173.4 mbsf and XCB cores to 226.5 mbsf. Holes 1251E and 1251F were each cored by APC to 9.5 mbsf for high-density geochemical and microbiological sampling. Hole 1251G was washed to 2.5 mbsf before one APC core was taken for special sampling of turbidite layers. The hole was then washed down to 20 mbsf before an additional PCS run (at 21 mbsf) was deployed. Hole 1251H was drilled for wireline logging and seismic profiling.

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-110

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