SITE 1090
(Proposed Site TSO-3C)

The 127-nmi transit to Site 1090 on the south flank of the Agulhas Ridge began in fair weather with a light northerly breeze that strengthened as the vessel approached the new drill site. The average transit speed, therefore, was 11.0 kt. By the time the ship turned and maneuvered onto the site, however, the wind had reached gale force and large seas were building. A positioning beacon was dropped on 25 December at 1715 hr and the pipe trip began as soon as stable positioning had been achieved.

Hole 1090A
The winds continued to strengthen over the next few hours, with gusts reaching 50 kt and a large swell developing. At 2300 hr the pipe trip was stopped (with 3305 m suspended) as a result of violent pitch and heave motion that posed a hazard to personnel and the drill string. At midnight, the wind shifted from northerly to westerly but did not diminish in velocity. That caused both positioning and motion problems for the ship, with large swells at high angles to strong winds and seas. There was insufficient power to maintain station while on a minimum-roll heading, and the vessel was blown about 1.1-nmi off station to the east-northeast. Wind velocity and the northerly swell decreased slowly through the morning, and operations resumed after a weather delay of 13.25 hr. The pipe trip was completed in still marginal conditions, and the top drive and APC coring assembly were deployed.

Hole 1090A was spudded with an APC core shot from 3707 mbrf at 1515 hr on 26 December. Pump pressure indicated a mechanical shear, casting considerable doubt on the quality of the mudline core. The core barrel contained 7.0 m of sediment, indicating a seafloor depth of 3709.5 mbrf. Because of the amount of scientific interest in the interface core and because of its probable poor quality, a second seafloor core was requested.

Hole 1090B
The second spud attempt was positioned 2 m higher than for Core 1090A-1H. Core 1090B-1H gave the same pressure indication, however, and recovered 4.2 m of sediment. Because of the operating conditions, no attempt was made to obtain core orientation or temperature data from Hole 1090B. APC coring continued to 185 mbsf, where the coring mode was switched to XCB due to the stiff, chalky nature of the sediment and the increasing frequency and severity of liner failures.

Excellent XCB core recovery was achieved in slowly improving weather and motion conditions. At 295 mbsf, hard drilling was encountered, and it was necessary to pull a short core and to install a hard-formation coring shoe. The thin, hard stratum was penetrated successfully by Core 33H, but the hard material was not recovered, probably because of a total core liner failure that limited recovery to less than 3 m. Subsequent cores recovered isolated thin layers of Eocene porcellanite. Core 42X reached the coring target depth at 397.5 mbsf, and the bit was then pulled back above the seafloor to end operations in Hole 1090B.

Hole 1090C
The initial core interval was positioned 3 m deeper (per drill string measurement) than that of Hole 1090B. After the rig had been offset 10 m laterally, the first core recovered an unexpectedly short 2.8 m of sediment, setting the seafloor depth at 3714.7 mbrf by convention. Seven additional APC cores were taken to a depth of 69.3 mbsf to cover the interval of primary interest. Given the unfavorable weather prognosis at that time, it was decided to pull out and start a new hole in the critical upper section before the conditions could deteriorate.

Hole 1090D
The vessel was offset an additional 10 m in the direction of the positioning beacon. Again the core interval was adjusted 3 m deeper, and again a less-than-full core was recovered. The new seafloor depth was calculated to be 3713.1 mbrf. Continuous APC cores were taken to refusal, with orientation beginning with Core 4H. Significant overpull of about 45 kips was noted on the third core beyond the point where XCB coring had began in Hole 1090B. The next core, 24H, could not be freed with 100 kips overpull, and it was necessary to "drill over" with the main bit to free the APC barrel. Heave conditions allowed only about 6 m of drillover. Even then, 100 kips were required to free the core barrel, and APC refusal was acknowledged. Repeating the XCB-cored section from Hole 1090B was of secondary priority, and coring was terminated in Hole 1090D at 225.9 mbsf so that another APC section could be drilled.

Hole 1090E
Operational conditions and the available time allowed for a third hole to be drilled to the depth of APC refusal to provide complete stratigraphic coverage for that interval. The rig was offset back to the positioning coordinates of Hole 1090C, and the bit was positioned at 3714.5 mbrf for the initial core. The core recovered 8.7 m of sediment, placing the seafloor depth at 3715.3 mbrf by convention. Hole-to-hole correlation later indicated that the top of the core was taken from about 2 m below the seafloor (as planned) and that seafloor depth was near 3713 mbrf. Again, continuous APC cores were taken to refusal, with orientation beginning with Core 4H. Weather and vessel-motion conditions began to deteriorate as operations began on Hole 1090E, and the effect on coring results was evident as core recovery was somewhat reduced in the upper portion of the section. Conditions moderated as operations progressed, but high heave continued to be a factor. Winds increased to more than 40 kt as APC coring approached refusal, but Core 25H was taken before overpull reached 90 kips and vessel motion threatened to force suspension of operations. Coring was thus completed at Site 1090, and the drill string and beacon were recovered. The JOIDES Resolution again headed southward at 1230 hr on 31 December.

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