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doi:10.2204/iodp.proc.346.104.2015

Operations

Three holes were drilled at Site U1423 (proposed Site JB-2) (Table T1; see also Fig. F2 in the “Expedition 346 summary” chapter [Tada et al., 2015a]). Hole U1423A was cored using the advanced piston corer (APC) to 206.6 m core depth below seafloor (CSF-A) (see the “Methods” chapter [Tada et al., 2015b]). Similarly, Hole U1423B was cored with the APC to 249.1 m CSF-A. Hole U1423C was drilled without coring to 114 m CSF-A. From this depth to 180 m CSF-A, we cut and recovered six cores in a series of alternating drilled and cored intervals with the objective of filling gaps in APC cores recovered from the previous two holes. Real-time stratigraphic correlation indicated that the carefully orchestrated coring operation appeared to be successful. Downhole measurements were carried out in Hole U1423B using a modified version of the triple combination (triple combo) tool string (paleo combo) and the Formation MicroScanner (FMS)-sonic tool string. A total of 56 cores were required to obtain 502.6 m of sediment (105.2% recovery).

Transit from Site U1422

The transit to Site U1423 was very short, and the 125 nmi distance was covered uneventfully in 12 h at an average speed of 10.4 kt. The sea passage ended at 2342 h on 21 August 2013. The vessel was maneuvered over the location coordinates, control of the vessel was turned over to dynamic positioning at 0030 h on 22 August, and a seafloor positioning beacon was deployed at 0215 h the same day.

Hole U1423A

Operations in Hole U1423 started on 22 August 2013. The same three-stand APC/extended core barrel (XCB) bottom-hole assembly used at the previous site was deployed, and the drill string was lowered to 1774 meters below rig floor (mbrf). After picking up the top drive and spacing out the drill string, an APC core barrel was deployed at 0530 h on 22 August. The bit was positioned at 1784.4 mbrf for the mudline core; however, the driller was unable to pressure up the drill string. Suspecting that the shear pins had prematurely failed during deployment, the barrel was recovered to the rig floor, where it was discovered empty and with all shear pins sheared. The pins were replaced, and the bit was lowered 10 m to 1794.4 mbrf for a second attempt. Hole U1423A started at 0640 h on 22 August. Core 346-U1423A-1H was recovered with 7.26 m of core, establishing a seafloor depth of 1796.6 mbrf. Cores 1H through 22H were taken from 0 to 206.6 m CSF-A and recovered 212.9 m of core (103%). Four temperature measurements were taken using the advanced piston corer temperature tool (APCT-3) temperature shoe on Cores 4H, 7H, 10H, and 13H (35.8, 64.3, 92.8, and 121.3 m CSF-A, respectively). With the top drive left in place, the drill string was pulled clear of the seafloor at 2355 h on 22 August, ending Hole U1423A.

Hole U1423B

The ship was offset 15 m north of Hole U1423A and the bit was positioned at 1791.4 mbrf. Hole U1423B was spudded at 0050 h on 23 August 2013. Core 346-U1423B-1H was recovered with 4 m of core, establishing a seafloor depth of 1797 mbrf. Cores 1H through 28H extended to 249 m CSF-A and recovered 250 m of core (100%). The recovery amount was negatively impacted because of zero recovery for Core 18H (151.6–158.1 m CSF-A), the result of a space-out error on the rig floor, resulting in an inadvertent drilling of hole that was to be cored on the next core. APC overpull (using full-length core barrels) varied between 60,000 and 80,000 lb between 142 and 249 m CSF-A (Cores 17H through 28H), with the exception of Core 27H (230.1–239.6 mbrf), when overpull reached 100,000 lb. The last core on deck was recovered at 1530 h.

To prepare for downhole logging, the hole was circulated clean, the logging tools were moved forward to the rig floor, the top drive was set back, and the drill string was pulled to a logging depth of 80.0 meters below seafloor (mbsf). The paleo combo tool string was deployed and recorded spectral gamma ray, caliper, magnetic susceptibility, resistivity, and lithologic density logs. The second string consisted of the FMS-sonic tool string, which recorded resistivity images of the borehole, sonic velocity, caliper, and natural gamma radiation (NGR) data over the entire interval. Both logging tool strings were able to reach total hole depth. The logging tools were then rigged down, and the drill string was pulled clear of the seafloor at 0515 h on 24 August. This marked the end of Hole U1423B.

Hole U1423C

The ship was offset 15 m south of Hole U1423A, and Hole U1423C was spudded at 0655 h on 24 August 2013. The operations plan for this hole was altered to save time and allow for recovering a few gaps in the stratigraphic record remaining after coring Holes U1423A and U1423B. The hole was drilled without coring (using an XCB center bit) to 114 m CSF-A. At that depth, APC coring commenced, recovering six cores interspersed with appropriate drilled intervals to properly space out the core breaks and cover the areas of interest. Hole U1423C was terminated at 180.5 m CSF-A. After setting back the top drive, the bit cleared the seafloor at 1700 h and was back on the rig floor at 2040 h. The positioning beacon was recovered aboard at 1756 h while tripping the drill string. After breaking out the bit and nonmagnetic drill collar, the rig floor was secured for transit, thrusters/hydrophones were raised, and the sea passage to IODP Site U1424 began at 2136 h.

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