We then resumed XCB coring and proceeded with excellent recovery until Core 1051A-41X advanced only 2.3 m when it encountered a hard chert layer at 381.6 mbsf. The only practical manner of advancing beyond this depth with the XCB was to drop a core barrel with a center bit and drill ahead 8.3 m to 389.9 mbsf. Although the chert layer was approximately 1 m thick, the extra advance was necessary to verify that we had completely penetrated the chert layer and because we wanted to push the tungsten carbide inserts (TCI) that had been stripped from the XCB cutting shoe while attempting Core 1051A-41X into the borehole wall. The extreme hardness of the TCIs could damage the main PDC bit.
We resumed XCB coring at 389.9 mbsf and advanced past the original objective of 600 mbsf to a total depth of 644.6 mbsf, where coring operations were terminated due to a very slow rate of penetration (ROP on the last core was 2.0 m/hr). The K/T boundary was assumed to lie perhaps another 30-40 m beneath the TD. To attain this depth with the present ROP would have taken another 12 hr of rotation.
At 0815 hr on 25 January, the bit was raised back to 105 mbsf for logging. Raising the drill pipe to logging depth was complicated by the discovery of over 400 m of monofilament fishing line that was wrapped around eight joints of drill pipe. As each joint was lifted past the dual elevator stool, the rig crew would cut off small sections of the fishing line. This process was made more difficult due to the presence of large sharp fishing hooks. Because there were no fishing boats in the area, we assumed the line may have drifted in the Gulf Stream for some distance before adhering to the drill string.
By 2030 hr on 25 January, the logging equipment was being rigged up. This took longer than the normal half hour, due to the extra rigging required to work around the absence of the wireline heave compensator. The first log in the hole was the Triple Combo, which logged the entire hole. The data was of good quality and indicated that the hole was in excellent condition with a smooth bore that ranged in diameter from just under 10 in to a maximum of 14 in. The second log was the Formation MicroScanner (FMS), which required extensive troubleshooting before it was run successfully without the sonic tool. The heave of the ship and the lack of the wireline heave compensator will require considerable reprocessing to remove the effect of the tools vertical motion in the hole. The last tool run was the Geological High-Sensitivity Magnetometer (GHMT), which provided good quality magnetic susceptibility data.
Once logging was finished and the tools recovered, the borehole was displaced with 35 bbls of 10.5 ppg mud. At 0200 hr on 27 January, the bit cleared the mudline and the vessel offset 30 m northwest to start Hole 1051B.
Hole 1051B
The driller tagged the seafloor at 1992.0 mbrf and spudded Hole 1051B with the XCB at 0315 hr on 27 January. After passing through a 2-m-thick hard crust and advancing to 4.8 mbsf, the XCB barrel was recovered and APC coring initiated. We APC-cored to 135.0 mbsf, which was just above the depth of the last APC core on the first hole. This allowed us to avoid having to consume an extra hour drilling over another stuck core barrel. APC Cores 4H through 15H were oriented with the Tensor tool, and Adara temperature tool measurements were obtained at 33, 62, and 87 mbsf (Cores 4H, 7H, and 11H).
We advanced XCB coring to 374.1 mbsf with excellent recovery. At this depth, we drilled through the chert layer with a center bit. After 70 min of rotation and advancing 2 m, we finally penetrated the chert layer and the center bit was retrieved. We resumed XCB coring to a total depth of 526.6 mbsf.
After we displaced the hole with 35 bbls of 10.5 ppg mud, we retrieved the drill string. The PDC XCB bit was missing 8 of 13 cutting elements. Although the bit body was in gauge, the bit was too worn for service and was retired. At 1315 hr on 31 January, the drilling equipment was secured and the vessel began the transit to Site 1052.
To 171B Site 1052 (Proposed Site BN-5Alt)