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doi:10.2204/iodp.proc.339.104.2013

Operations

After a 150 nmi transit from Site U1385, the R/V JOIDES Resolution arrived at Site U1386 (proposed Site GC-01A; 560.4 mbsl) at 0415 h on 30 November 2011. We deployed the camera system and conducted a ~2 h survey of the seabed on a 30 m grid pattern to ensure that the seafloor was free of man-made obstructions. No obstructions were observed on the seafloor, but many shallow linear furrows were seen that were assumed to be the result of fishing trawls.

Three holes were drilled at Site U1386 (Table T1). Hole U1386A was cored using the advanced piston corer (APC) system to 183.1 meters below seafloor (mbsf) and then cored with the extended core barrel (XCB) system to 349.3 mbsf. Similarly, Hole U1386B was cored with the APC to 162.3 mbsf and then with the XCB to 464.9 mbsf. Hole U1386C was drilled to 405 mbsf and then cored using the rotary core barrel (RCB) system to the target depth of 526 mbsf. Two cores were taken in the initial drilled interval (Core 339-U1386C-2R, 165–174.6 mbsf, 40% recovery; Core 4R, 205–214.6 mbsf, 93% recovery) to try to fill in short gaps in APC cores recovered from Holes U1386A and U1386B. Downhole logging was carried out in Hole U1386C using the triple combination (triple combo), Formation MicroScanner (FMS)-sonic, and Versatile Sonic Imager (VSI) tool strings (see “Downhole logging at Site U1386”). Overall recovery at Site U1386 was 351 m (101.62%) with the APC, 417.63 m (89.1%) with the XCB, and 82 m (58.5%) with the RCB. The total cored interval for Site U1386 was 954.4 m, and total recovery was 850.64 m (89.1%).

Hole U1386A

Once the camera system was recovered at 1255 h, 30 November, we began piston coring in Hole U1386A. APC coring continued to refusal at 183.1 mbsf (Cores 1H through 21H). Recovery for this interval was 101%. Temperature measurements were obtained at 32.3 (Core 4H), 60.8 (Core 7H), 89.3 (Core 10H), 117.8 (Core 13H), 114.0 (Core 16H), and 167.2 (Core 19H) mbsf. Cores were oriented starting with Core 4H. The corer was advanced by recovery on Cores 16H to 19H and 21H. Nonmagnetic hardware was used on all cores. At 183.1 mbsf, the coring system was switched to the XCB and coring continued to a total depth of 349.3 mbsf (Cores 22X through 39X) by 0415 h, 2 December. The 166.2 m interval was cored with a recovery of 97%. The total recovery in Hole U1386A was 346.2 m, which represented 99% of the cored interval. The bit cleared the seafloor at 0620 h on 2 December, and the vessel offset 20 m east of Hole U1386A.

Hole U1386B

Piston coring in Hole U1386B began at 0825 h, 2 December, and advanced to an APC refusal depth of 162.3 mbsf (Cores 1H through 18H). Recovery for the APC interval was 100%. The calculated water depth from the recovery of the first core was 562 mbsl. Temperature measurements using the advanced piston corer temperature tool (APCT-3) were taken at 16.8 (Core 2H), 45.3 (Core 5H), 73.8 (Core 8H), 102.8 (Core 11H), 130.8 (Core 14H), and 157.6 (Core 17H) mbsf. The cores were oriented starting with Core 3H. There were partial strokes with Cores 15H, 16H, and 18H. Nonmagnetic hardware was used to obtain all cores. At 2200 h, 2 December, the coring system was switched to XCB and coring continued to 455.4 mbsf (Cores 19X through 49X), where XCB refusal was reached. Recovery for the XCB interval was 293.5 m (100%).

Hole U1386C

The third hole of Site U1386 was offset 20 m south of Hole U1386B and spudded with the RCB at 0345 h on 5 December. This hole was drilled with a wash barrel to 405 mbsf except for RCB coring of two intervals that eluded recovery in the two previous XCB holes (Cores 2R and 4R from 165.0–174.6 and 205.0–214.6 mbsf, respectively). Continuous RCB coring started at 405 mbsf at 0545 h, 6 December, and reached the depth objective of 526 mbsf by 0200 h, 7 December. RCB interval (140 m) recovery was 59%. The drilled interval was 386 m. The total cored interval for all holes at this site was 954 m with a recovery of 89% (104 cores).

Downhole logging at Site U1386

Following a wiper trip and hole conditioning, Hole U1386C was displaced with 172 bbl of 10.5 ppg mud to prepare the hole for downhole logging. The open end of the pipe was placed at a logging depth of 102.4 mbsf. During rig up of the wireline cable, the cable jumped from the lower left most sheave wheel in the wireline heave compensator and became jammed between the wheel and the frame. Because of the possibility of damage to the cable, it was cut above the crimped section.

The triple combo tool string descended through the seafloor at 2003 h and was successful in reaching the bottom of the hole (526 mbsf). The tool string was back on deck by 0100 h, 8 December. Subsequently, the FMS-sonic tool suite was run into the hole at 0410 h. The tool suite was blocked from further downhole progress by a bridge at 948 mbrf (375 mbsf), the lowermost of the tight sections observed in the triple combo run. Rig down of the FMS-sonic tool string was completed by 0930 h, 8 December.

The marine mammal watch for conducting the vertical seismic profile (VSP) experiment started at 0800 h, 8 December. The VSI tool started its descent in Hole U1386C at ~1030 h and reached a bridge at 940 mbrf (367 mbsf) at 1105 h. The slightly shallower penetration for this tool run indicated that the hole was closing with time. The seismic source (two-gun cluster; 7 mbsl on the port side) had been ramped up in soft start mode. It was difficult to get a good clamp with the VSI, and consequently noisy waveforms were obtained that were attributed to the rugose borehole and soft formation. Only a fraction of the shots produced clean first arrivals, but there were enough at most stations to stack the waveforms. The upper part of the hole was especially difficult. The tool was back on deck at 1440 h and rigged down by 1530 h, 8 December. The drill string was recovered and the vessel departed for the next site at 1730 h, 8 December. Total time at Site U1386 was 8.6 days.

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