Once the top drive was picked up and an XCB core barrel dropped, the driller slowly lowered the bit until he made contact with the seafloor at a depth of 1356.0 mbrf (based on a reduction in drill string weight). Cores 1052A-1X and 2X were taken to 13.2 mbsf to penetrate a 5-m-thick hard surficial crust. We then initiated APC coring and proceeded until 129.7 mbsf. Cores 1052A-4H through 15H were oriented using the Tensor tool. When Core 15H did not fully stroke, we switched to XCB coring. At 174.5 mbsf, we encountered a hard layer that effectively prevented further penetration with the XCB. To penetrate any deeper at this site, we would have to use the RCB. The drill string was then pulled out of the hole and the bit cleared the seafloor at 1200 hr on 31 January.
We offset the ship 30 m to the southwest and spudded Hole 1052B with the XCB at 1300 hr. Based on a reduction in drill string weight, the driller estimated the seafloor to be at 1356.5 mbrf. The driller did not observe the hard layer encountered at all the previous sites, so we retrieved the XCB barrel after penetrating only to 5 mbsf. We then took APC Cores 1052B-2H through 14H to 119.5 mbsf. The bit cleared the seafloor at 2045 hr on 31 January.
Because we did not observe a surficial hard layer at Hole 1052B, we felt it was possible to take a near mudline piston core without undue hazard to the drilling equipment. We also wanted to obtain slightly younger Eocene sediments that likely contained the tektites. Because there appeared to be local variability in the bottom topography on the PDR, we did not offset the ship. The driller observed a reduction in drill string weight at 1356.5 mbrf and attempted an APC core at this depth. It was a full stroke and recovered nearly 10 m so most likely was taken below the true seafloor. A second piston core was taken from 9.5 to 19.0 mbsf. The bit cleared the seafloor at 2200 hr on 31 January.
Without offsetting the vessel, we shot the first APC (at 2215 hr, 31 January) approximately 2 m above the inferred mudline. The first core was full, so we could not determine the seafloor depth by the recovery. We then took a second piston core from 9.5 to 19.0 mbsf. After recovering the second piston core, we tripped the bit to the surface to switch over to RCB coring to reach the deeper objectives at this site. The bit cleared the seafloor at 2330 hr on 31 January.
After the bit cleared the rig floor at 0200 hr on 1 February, we inspected the PDC XCB bit and found that it had lost most of its PDC cutting elements. It will be returned to ODP for refurbishing. The APC/XCB BHA was laid down and a new RCB bit (9 7/8 in RBI C-3) was assembled along with a mechanical bit release, so we could remotely release the bit to allow us to log this hole.
We spudded Hole 1052E at 0700 hr on 1 February and drilled without coring to 140 mbsf. After we retrieved the wash barrel and a core barrel was deployed, we started rotary coring at approximately 1100 hr on 1 February. The RCB had no trouble penetrating the hard formation that had stymied XCB coring in Hole 1052A. However, the recovery from 140 to 252 mbsf was poor (30%) due to the interbedded hard and soft sediments. Below this interval, core recovery improved as the sediments became more homogeneous and lithified. We recovered a nearly complete K/T boundary section in Core 1052E-19R (309.7 to 319.3 mbsf).
The recovery of the K/T boundary was due mainly to chance. When the core barrel for Core 19R was retrieved, it was empty. The driller, assuming that there was no recovery, dropped another core barrel to prepare for coring the next interval. While the core barrel was on the way down the pipe, a decision was made to drop a bit deplugger to make sure that the bit throat was clear. Before this could be done, we had to recover the core barrel that had just been dropped. When we recovered the core barrel, it contained approximately 1 m of core which coincidentally turned out to contain part of the K/T boundary sequence. We then decided to drop a second core barrel, which recovered an additional ~1 m of core. We suspect that both of these sections of core apparently fell out of the Core 19R core barrel. Finally, a bit deplugger was dropped, latched in, recovered, and we resumed RCB coring. Core recovery continued to improve with depth, and when we reached a total depth of 684.8 mbsf, the average recovery for the hole had improved to 60%.
In preparation for logging, we circulated 20 bbls of high viscosity mud and then made a wiper trip by pulling the drill pipe up to 213 mbsf and then lowering it back to the bottom of the hole. After washing and reaming with the drill bit from 674 to 685 mbsf, we circulated another 20 bbl of mud. The bit was then released and the bottom of the drill string raised to 222 mbsf for logging.
Once the Schlumberger equipment was rigged up, the first log Triple Combo (Temperature tool, DITE, HLDT, APS, HNGS) was run in the hole at 1045 hr on 5 February. The Triple Combo was able to log from the bottom of the hole (685 mbsf) up to 222 mbsf. A second logging pass was made over the section containing the K/T boundary (335 to 222 mbsf). The data appear to be of high quality. Caliper readings indicated good and very smooth borehole conditions with borehole diameters mostly around 10 in to 11 in. Vessel heave was approximately 1 m during logging (the wireline heave compensator was not available for this leg).
We then ran the Formation MicroScanner with the sonic tool. The FMS string was also able to log from the total depth up to 222 mbsf. As with the first log, a second pass was made over the K/T boundary. The deviation at the bottom of the hole was 1°. Our last logging run was the GHMT/NGT. The GHMT was also able to log the complete section and a second pass was made over the K/T boundary region.
After the logging equipment was rigged down, we pulled the drill string up to 100 mbsf and displaced the hole with 35 bbls of 10.5 ppg mud. After retrieving the drill string, securing the drilling equipment, retrieving the beacon, and retracting the thrusters and hydrophones, we began the transit to Site 1053 at 1930 hr on 6 February 1997.
To 171B Site 1053 (Proposed Site BN-4Alt)
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