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Site 1206

Transit to Proposed Site HE-6A (DSDP Site 308) and Underway Survey over Koko Seamount

The position of Site 1206 was located nearly 400 nmi south-southeast of Site 1205, Nintoku Seamount, and situated on the southeastern sector of the lower summit platform of Koko Seamount. The site area was last visited during the DSDP Leg 32, in mid-September 1973, when drilling took place at Site 308.

The 377-nmi transit to the first survey waypoint was accomplished at an average speed of 11.6 kt. During the 32.5-hr voyage, the skies were mostly cloudy with good visibility. Seas were flat with a gentle 4-ft swell from the east-northeast. Sea temperature continued to rise from 15° to 24°C during the 1.5-day voyage.

Prior to spudding Hole 1206A and consonant with the routine established during Leg 197, a geophysical survey was conducted to augment the paucity of seismic reflection data available for the site area, to locate a suitable site, and to acoustically characterize its structural and stratigraphic setting. Crossing high-resolution seismic reflection profiles were obtained using a single 80-in3 water gun with a firing rate of 4 s. At 1430 hr on 13 August 2001, the vessel slowed to 6 kt to deploy the seismic equipment, and by 2115 hr the survey was completed. The 33.0-nmi survey was completed at an average speed of 5.5 kt.

Our arrival over Koko Seamount was greeted by Russian fishing trawlers whose crews were tending nets and long lines. We also encountered a small container vessel utilizing the great-circle route. The thrusters were lowered as the vessel, using the GPS, established the location for Site 1206 selected during the survey. Site 1206 was positioned ˜6.2 km south of Site 308 (proposed Site HE-6A identified in the Leg 197 prospectus). After the vessel was on station at 2116 hr, the hydrophones were lowered concurrent with making up a new BHA. A beacon was deployed at 2300 hr.

Hole 1206A

The corrected PDR depth referenced to the DES was obtained and indicated a water depth of 1544.1 m. The BHA was made up of a 9.875-in RCB, a hard-formation C-7 bit, a mechanical bit release, a modified head sub, an outer core barrel, a modified top sub, a modified head sub, seven 8.25-in drill collars, a tapered drill collar, six 5.5-in drill pipe sections, and one crossover sub.

Hole 1206A was spudded at 0215 hr on 14 August with the RCB and a wash barrel in place. The bit tagged the seafloor depth at 1557.0 m relative to the DES. The bit was drilled ahead to 57 mbsf when the penetration slowed, suggesting basement contact. The wash barrel contained a composite of sedimentary material, most of which was a dark, calcareous, sandy siltstone and silty sandstone. Volcanic basement coring began with Core 197-1206A-2R (57.0–58.5 mbsf) and proceeded to a depth of 79.8 mbsf, when operations were suspended to conduct a small wiper trip to 35.0 mbsf to reduce high erratic torque.

Coring resumed and continued slowly as frequent deplugger operations were required to free debris jamming the throat of the bit and the float valve. The recovery dropped off markedly below 104.4 mbsf (47 m into basement). After the hole was deepened an additional 23.8m (128.2 mbsf) with only 0.45 m obtained (recovery = 1.9%), it was clear that a round trip of the drill string was required to investigate the reason for poor core recovery.

A FFF was deployed at 0010 hr on 16 August. The bit cleared the top of the FFF at 0110 hr and was monitored via the VIT. The FFF was erect following the withdrawal of the bit. After the drill string was recovered, the bit and the area directly under the float valve in the mechanical bit release were found to be full of basalt fragments as well. Many of the fragments were obviously cored segments that apparently fell out of the core barrel during earlier operations. The reclaimed cored material added an additional 2.0 m to the 0.02-m recovery of the last core barrel (Core 197-1206A-14R). The bit was otherwise in excellent shape, but because it had accumulated 24 rotating hours in drilling 57 m of sediment and cored an additional 71.2 m of basement, it was retired from service. The average recovery for the cored interval of 71.2 m was 39.9%, corresponding to 28.39 m of cored material. The average ROP in basement coring was 3.3 m/hr.

Reentry (Third of Leg 197)

A new C-7 bit and a fresh mechanical bit release were made up to the BHA, and by 1015 hr on 16 August the bit was suspended above the open hole of Hole 1206A. Sometime in the interval between the extraction of the old bit and reentry positioning, the FFF had tipped over when the 2.5-m length of 13.375-in casing failed to sufficiently penetrate the hard surface of the seafloor. The dynamic positioning operator adroitly maneuvered the vessel and at 1021 hr, the bare hole was successfully reentered.

Basement coring resumed in Hole 1206A and by the evening of 19 August proceeded without incident from 128.2 to 323.2 mbsf. The hole was flushed with mud, and a wiper trip was made to 65 mbsf to prepare the hole for logging. The drill string stuck at two positions in the borehole, 101 and 171 mbsf. The driller had to apply 100,000 lb of overpull to free the pipe at a depth of 171 mbsf. The hole was washed and reamed from 65 to 323 mbsf after which 2 m of fill was found at the bottom of the hole. After considering the unstable nature at the top of the hole and factoring in the 11-ft heave of the vessel, the prospects for a successful logging endeavor appeared dim. The decision was made to extend coring for the few remaining hours until time expired on site. Coring resumed and deepened the hole to 335.2 mbsf when we decided to terminate operations and recover the drill string.

An additional 207.0 m of basaltic basement was cored with the second bit with an average recovery of 54.6%. The ROP varied from 1.2 m/hr (Core 197-1206A-23R [191.0–192.7 mbsf]) to 5.8 m/hr (Core 197-1206A-20R [162.2–171.8 mbsf]). The average ROP in basement with the second bit was 3.2 m/hr. The total cored interval in Hole 1206A was 278.2 m with 50.8% average recovery. The average ROP for the two bits was 3.2 m/hr.

The drill string was recovered and the BHA was inspected before being disassembled. The hydrophones and thrusters were retracted, the beacon was recovered, and all drilling equipment was secured by 2230 hr on 20 August, when the vessel left location for Yokohama.

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