The sixth site of Leg 187 is 50 nmi southwest of Site 1156. Our transit took 4.3 hr at an average speed of 11.6 kt. At 1315 hr on 12 December, we slowed to 5 kt to conduct a single-channel seismic (SCS) survey.
We deployed a positioning beacon about 1.6 km east of the prospectus Global Positioning System (GPS) coordinates where our survey indicated a 200-m sediment thickness. Water depth at this site is 5080.4 m below the rig floor, determined by the precision depth recorder (PDR). A new C-7 four-cone rotary bit was installed on the same nine-collar bottom-hole assembly used at our previous sites. We started washing through the sediment column at Hole 1157A at 2300 hr on 12 December and continued to 200.0 meters below seafloor (mbsf), where drilling conditions indicated we had reached basement. A single wash barrel was recovered, and we cored from 200.0 to 216.4 mbsf before we decided, based on poor drilling conditions and recovery (Table T1), to abandon the hole. We deployed fluorescent microspheres as a tracer for microbiological infiltration analysis on Core 1157A-2R. The drill string cleared the seafloor at 1300 hr on 13 December; we offset 200 m west to drill Hole 1157B.
We initiated Hole 1157B by washing down to 130.6 mbsf before hitting basement. Coring continued from 130.6 to 171.0 mbsf with an average recovery of 29%. Microbiological tracers were deployed on Cores 1157B-2R and 9R. After ~34 m of penetration and 10 m of recovered basalt, we concluded operations at this site. The bit cleared the seafloor at 0225 hr and the rotary table at 1045 hr on 15 December.