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doi:10.2204/iodp.proc.303306.108.2006

Operations

We departed Site U1307 for Site U1308 (proposed Site IRD1A) at 1300 h on 30 October 2004 using an indirect route to minimize the effects of a passing low-pressure system. After a 103.8 h, 1111 nmi transit, we arrived at Site U1308 at 2230 on 3 November.

Hole U1308A

Hole U1308A was spudded with the advanced piston corer (APC) at 1005 h on 4 November 2004. Recovery of a 8.62 m mudline core suggested a seafloor depth of 3871 meters below sea level (mbsl) (3882.3 meters below rig floor [mbrf]). Piston coring advanced the hole to 341.1 mbsf with an average recovery of 94.9% (Table T1). Because of difficulties with three of the four Tensor tool battery packs, the last four piston cores were obtained without orientation so repairs could be made in preparation for Hole U1308B coring. Cores 32H and 36H were obtained with partial strokes of the APC. Heave conditions (maximum of ~6 m) resulted in generally poor core quality in Hole U1308A below Core 18H. Coring was terminated at 341.1 mbsf because extending APC coring depth by drillover was not possible with the existing sea state. The drill string was pulled clear of the seafloor at 1825 h on 6 November, concluding operations in Hole U1308A.

Hole U1308B

The ship was offset 30 m east of Hole U1308A. Because the swell responsible for the large heave was expected to increase during the evening (eventually reaching a maximum of ~7 m), coring operations were suspended to wait until conditions improved. The heave gradually abated to <3 m by 1030 h on 7 November 2004, and coring operations continued when Hole U1308B was spudded at 1320 h on 7 November. Recovery of the first core indicated a seafloor depth of 3871.1 mbsl (3882.4 mbrf). Piston coring advanced the hole to a total depth of 198.3 mbsf, with an average recovery of 93.9%. Four of the top sixteen cores were recovered with crushed core liners, resulting in pervasive drilling-related disturbance and deformation (Table T1). The last three cores (Cores 20H through 22H) returned with an average recovery of only 30%. All three were partial strokes with overpulls of 20,000 to 40,000 lb required to extract the core barrels. Suspecting problems in the base of the hole, we terminated coring, ending Hole U1308B operations when the bit cleared the seafloor at 2315 h on 8 November.

Hole U1308C

The ship was offset 60 m east of Hole U1308B. Incomplete firing of the APC resulted in two unsuccessful attempts to spud Hole U1308C, which also included offsetting the ship 30 m north of Hole U1308A in case the problem was anomalous seafloor conditions. The bit was positioned 15 m above the seafloor for a firing test, which resulted in another incomplete stroke, suggesting the bit was plugged with detritus accumulated during coring at the end of Hole U1308B. The core barrel was recovered and a bit deplugger was dropped to clear the blockage at the bit throat and inside the seal bore drill collar. Following the recovery of the deplugger, Hole U1308C was successfully spudded with the APC at 0730 h on 9 November 2004. Piston coring advanced to 279.9 mbsf, with an average recovery of 96.8%. Six of the top nineteen cores were recovered with crushed core liners (Table T1). Six core barrels (Cores 22H through 27H, 29H, and 30H) were obtained by drillover. The final two coring runs resulted in bent core barrels. The last barrel was difficult to retrieve and extract from the drill string. Thus, coring was terminated to avoid a potential reoccurrence or a need to trip the entire drill string back to the rig floor. The bit was pulled clear of the seafloor at 0045 h on 11 November ending Hole U1308C.

Hole U1308D

The ship was offset 30 m north of Hole U1308C, and Hole U1308D was spudded with the APC at 0230 h on 11 November 2004. The core was recovered with a crushed liner. To ensure that a gap in the splice near the top of the section was filled, we decided to make another attempt at a mudline core.

Hole U1308E

Without offset from Hole U1308D, Hole U1308E was spudded with the APC at 0340 h on 11 November 2004, recovering a full core. Piston coring and drilling advanced the hole to 200.5 mbsf, with an average recovery for the cored interval of 89.4%. Three intervals were drilled (57.0–60.0, 83.0–84.0, and 93.5–97.0 mbsf) to maintain stratigraphic offset with previous holes. Core 9H was fired after picking up 5.5 m from the base of Core 8H to position Core 9H for optimal stratigraphic overlap. However, the completeness of the composite section (i.e., covering coring gaps from previous holes) was made difficult by poor recovery in Cores 3H through 11H. The last two cores (Cores 20H and 21H) were obtained by partial stokes of the APC, indicating we were approaching the interval requiring drillover in previous holes. Rather than expend coring time on this slower process, we terminated coring in Hole U1308E in favor of ensuring complete stratigraphic coverage for the upper sedimentary succession by coring in Hole U1308F. The bit cleared the seafloor at 0730 h on 12 November, concluding operations in Hole U1308E.

Hole U1308F

The ship was offset 30 m west of Hole U1308E. Before initiating coring, an XCB fitted with a center bit was dropped to the bit. We suspected that accumulated debris was causing the abnormally high frequency of crushed core liners and affecting core recovery, so we applied high pump rates to clear the bit nozzles and the internal surfaces of the seal bore drill collar. Following recovery of the XCB, Hole U1308F was spudded with the APC at 1055 h on 12 November 2004. Piston coring continued until available time expired, reaching a total depth of 227.0 mbsf, with an average recovery of 100.7% (Table T1). Although crushed liners were still a problem (in 13 of the top 17 cores), improved recovery in Hole U1308F allowed completion of a composite section to ~239 meters composite depth (mcd) (see “Composite section”). The last three core barrels (Cores 22 through 24H) required drillover. The bit was pulled free of the seafloor at 1545 h, ending operations in Hole U1308F.

Transit from Site U1308 to Ponta Delgada

After recovering the drill string and preparing the aft coring line for maintenance to be performed during the transit, we departed for Ponta Delgada (Portugal) at 0030 h on 14 November 2004. The 742 nmi transit took 79.3 h at an average speed of 9.4 kt. At 0817 h on 17 November 2004, the first line was ashore, officially concluding Expedition 303.

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