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doi:10.2204/iodp.proc.330.106.2012

Operations

The 322 nmi voyage to Site U1375 on Achernar Guyot was accomplished at an average speed of 10.2 kt. During the transit the vessel pitched and rolled moderately in rough sea conditions, and waves occasionally broke across the deck. As the transit progressed, improving weather and course adjustments made the transit more comfortable. The vessel was on station above Hole U1375A at 0330 h on 25 January 2011 (all times are New Zealand Daylight Time, Universal Time Coordinated [UTC] + 13 h).

In accordance with the established routine of the expedition, the vibration-isolated television frame was deployed prior to spudding the hole. The bit tagged the seafloor at 1269.0 meters below rig floor (mbrf; 1258 meters below sea level [mbsl]), 0.9 m shallower than the corrected precision depth recorder depth. Hole U1375A was spudded with the rotary core barrel assembly outfitted with a Type C-4 bit at 1345 h on 25 January (Table T1). Almost immediately, the driller experienced erratic high torque, indicating that we were attempting to core through loose rocks and boulders. Frequent overpulls of up to 90,000 lb were required to keep the drill string rotating freely. At 2130 h, coring of Hole U1375A was terminated at 11.5 meters below seafloor (mbsf) because of unstable hole conditions. The average recovery for just two cores was 13%.

The vessel was offset 300 m at 315°, and a second attempt at coring this site was initiated when Hole U1375B was spudded at 2345 h. The water depth was again 1269.0 mbrf, corresponding to 1258 mbsl. After penetrating 8.5 m with increasing difficulty and constantly fighting unstable hole conditions, operations in this hole were terminated as well. The lone core retrieved had an average recovery of 7%. It was concluded that the formation on the top of this seamount consisted of sedimentary breccia loosely held together in a “soft” carbonate matrix that quickly disintegrated during drilling, leaving mainly loose pebbles behind. The vessel departed at 1945 h on 26 January for an approved alternate site (LOUI-7A), located 91 nmi north-northwest of Site U1375 on Burton Guyot. The total time spent at Site U1375 was 34.3 h (1.4 days).

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