The 27-nmi south-southeast transit from Site 1161 to Site 1162 required just over 3 hr at a speed of ~8 kt. Site 1162 is the second of three new sites surveyed on our transit between Sites 1158 and 1159 subsequently approved for drilling. Our seismic data indicated that a continuous sediment pond, >12 km across (north-south) and from 300 to >400 m deep, overlies strong reflectors interpreted to be volcanic basement. To optimize our drilling target, we conducted a 21-nmi, west-to-east single-channel seismic (SCS) survey across the Global Positioning System coordinates of our proposed site. A positioning beacon was dropped at 1525 hr 27 December where our seismic records indicated 200-300 m of sediment.
The precision depth recorder (PDR) depth referenced to the dual-elevator stool at Site 1162 is 5475.4 m. The nine-collar bottom-hole assembly (BHA) employed on the previous sites was reassembled with a new C-4 four-cone rotary bit made up to a mechanical bit release. We began Hole 1162A at 0015 on 28 December by washing down through 333.2 m of sediment and recovering a single wash barrel. Coring began when drilling conditions indicated a change from soft sediment to a harder formation. We advanced the hole by rotary coring from 333.2 to 364.6 meters below seafloor (mbsf) (Cores 187-1162A-2R to 5R; Table T1) with generally stable hole drilling conditions. After coring 31 m, we decided to abandon the hole because recovery was low (<10%) and the character of the recovered material indicated that we were coring down a fault zone. The predominantly cataclastic material is of little or no value in identifying the geochemical signature of the mantle source (our primary scientific objective), so the hole was terminated. As part of our continuing study on the infiltration of drilling fluid into the core, we deployed tracers of fluorescent microspheres on Cores 187-1162A-2R and 3R. The drill string cleared the seafloor at 0100 hr on 29 December, and we offset in dynamic positioning mode 200 m north.
We began drilling Hole 1162B with a wash barrel in place at 0215 hr on 29 December. After washing through 348.4 m of sediment and encountering a change in drilling conditions, we recovered the wash barrel and began rotary coring. Hole 1162B was advanced from 348.4 to 407.30 mbsf (Cores 187-1162B-2R to 11R) at an average penetration rate of 3 m/hr. We terminated coring when the time allocated to this site during our active planning process expired, even though we had not recovered material we could interpret as basaltic basement. Nevertheless, we expect to be able to use a few basalt fragments recovered from this site with shore-based analytical tools to reach our objectives. We deployed microsphere tracers on Cores 187-1162B-2R and 11R. The average recovery for Site 1162 was 14%.
The drill bit cleared the seafloor at 0245 hr on 31 December. During this pipe trip, the rig crew conducted the end-of-the-leg nondestructive examination of the BHA. The bit cleared the rotary table at 1345 hr on 31 December, concluding operations at this site.