When the survey was concluded at 1514 hr, we dropped a positioning beacon on the GPS coordinates at the crossing of the two survey tracks, ~400 m north of the prospectus site. Water depth was estimated, with the PDR at 4878.4 mbrf. The nine-collar BHA employed on the previous sites was made up with a new C-7 four-cone rotary bit. We initiated Hole 1156A with a RCB at 2345 hr on 9 December and washed ahead to 118.2 mbsf before reaching basement. After recovering an empty wash barrel, we cored from 118.2 to 129.6 mbsf with a recovery of 55%. We deployed fluorescent microspheres as a tracer for microbiological infiltration analysis on Core 187-1156A-3R. Because of excessive heave when recovering the last core barrel (Core 3R), we could not reenter the hole into basement, so the hole was abandoned. The drill string cleared the seafloor at 1500 hr on 10 December. We offset 200 m north of Hole 1156A to a location where seismic data indicated a somewhat thicker sediment cover for our next hole.
We began washing down at Hole 1156B at 1555 hr on 10 December and drilled ahead without incident to 181.6 mbsf before contacting basement ~60 m deeper than at Hole 1156A. The wash barrel from this interval contained 2.32 m of sediment. We recovered five rotary core barrels from this hole, representing the interval 181.6-215.2 mbsf (Cores 187-1156B 2R to 6R), with about 30% recovery. When we retrieved the barrel containing Core 187-1156B-4R (196.8-205.9 mbsf) we discovered that the core liner had been jammed and severely twisted in the core barrel. To prevent a recurrence, we did not place plastic liners in the last two core barrels. We deployed another fluorescent microsphere tracer on Core 187 1156B-2R. Following the recovery of Core 187-1156B-6R, we decided that we had recovered enough material to reach our scientific objectives and to conclude operations. The bit cleared the seafloor at 0115 hr and the rotary table at 0845 hr on 12 December, completing operations at Site 1156.
Site 1157 | Table of Contents