The JOIDES Resolution arrived at Pier 6 in Balboa, Republic of Panama, with the first line ashore at 1030 hr on 6 November 2002, marking the end of Leg 205 and the start of Leg 206. All times are reported in local ship time, which is Universal Time Coordinated (UTC) (a list of operation acronyms is given in Table T2) minus 5 hr in Panama and UTC minus 6 hr at Site 1256. A summary of operations completed during Leg 206 is given in Table T3.
The pilot boarded the vessel at 1404 hr on 11 November, which corresponded to low tide. This time was selected to ensure safe clearance of the derrick under the Bridge of Americas. At 1534 hr, the pilot disembarked and the vessel headed to Site 1256 (proposed Site GUATB-03C in the Leg 206 Scientific Prospectus) in the Guatemala Basin. The 821-nmi transit to Site 1256 was accomplished at an average speed of 11.0 kt and was without incident. The vessel was on station using the Global Positioning System data by 1630 hr on 14 November.
Hole 1256A was spudded with the advanced piston corer (APC) at 0640 hr on 14 November. The seafloor depth was estimated at 3634.5 m (3645.2 meters below rig floor [mbrf]) based on core recovery. The single mudline core from this hole was obtained to ensure that a good mudline was recovered and to provide additional material for microbiological and geochemical sampling.
Before starting Hole 1256B, the advanced piston corer temperature (APCT; formerly known as the Adara temperature tool) shoe was held just above seafloor from 0753 to 0813 hr to obtain a temperature measurement of the bottom water. Hole 1256B was then spudded with the APC at 0815 hr on 14 November. The seafloor depth inferred from the recovery was 3634.7 m (3645.4 mbrf).
Piston coring deepened the hole until Core 18H failed to achieve a full stroke of the corer at a depth of 160.1 mbsf. The recovery for the piston-cored interval was 163.91 m (average recovery = 102.4%). Cores were oriented using the Tensor tool starting with Core 3H. In addition to the bottom-water temperature measurement, downhole temperature measurements were attempted with the APCT at 34.6 mbsf (Core 4H), 53.6 mbsf (Core 6H), 82.1 mbsf (Core 9H), 120.1 mbsf (Core 13H), and 158.1 mbsf (Core 17H). The first attempt (Core 4H) did not record a valid temperature because of battery failure in the instrument, but other measurements were successful.
Coring in Hole 1256B continued with the extended core barrel (XCB) system to 251.7 mbsf, where the last core penetrated basaltic basement and recovered 6 cm of basalt. The XCB cored 91.6 m and recovered 60.23 m (average recovery = 65.8%). Recovery below 210 mbsf dropped considerably because chert nodules were getting jammed in the throat of the cutting shoe.
The drill string was pulled free of the seafloor at 1635 hr on 16 November, ending Hole 1256B. The total results for the hole were 251.7 m cored and 224.14 recovered, giving an average recovery of 89.1% (Tables T4, T5).
The vessel offset 10 m south of Hole 1256B to conduct a jet-in test. This test was performed in order to establish the length of 20-in surface casing to deploy with the reentry cone in Hole 1256D. The test penetrated 100 m with a maximum pump of 34 strokes/min (spm) and pump pressure of 200 psi. The drill string was then recovered to change the bottom-hole assembly (BHA) for rotary core barrel (RCB) coring in Hole 1256C. The APC/XCB bit was at the rotary table at 0630 hr on 17 November.
After assembling the RCB BHA, the pipe was tripped to the seafloor and Hole 1256C was spudded at 1530 on 16 November. The hole was drilled down to 220.1 mbsf before coring began. The first core (1R) contained only a few small pieces of chert in the core catcher. The presence of chert plagued recovery on the next two sedimentary cores, although 4.67 m of nannofossil ooze was recovered in Core 3R. Basement was tagged in Core 4R at a depth of 251.8 mbsf.
Subsequent coring into basement resulted in some spectacular recovery percentages for RCB coring in hard rock, with 10.07 m recovered (recovery = 109.5%) in Core 9R and 10.24 m recovered (recovery = 107.6%) in Core 11R. Recovery from Cores 5R through 11R (252.4312.8 mbsf) averaged 81.8%. The last two cores (13R and 14R) penetrated 18.3 m and recovered 0.16 m, or 0.9% of the cored interval. This reduced the average recovery in basement to 61.3%. The average rate of penetration (ROP) for RCB coring in basement was 1.4 m/hr. Recovery for the hole, including sediment and basement, averaged 50.8% (Tables T4, T5). The total depth of Hole 1256C is 340.3 mbsf, with the lower 88.5 m in basement.
At the conclusion of coring, Hole 1256C was prepared for logging with a wiper trip to 83 mbsf. After the bit was released, the hole was displaced with 92 bbl of sepiolite mud and the end of the pipe (EOP) was set at the logging depth of 125.8 mbsf.
The triple combination (triple combo) tool string was deployed first but was unable to pass a bridge at ~203 mbsf (only 77 m below the EOP). Following recovery of the logging tool and de-rigging the Schlumberger equipment, the drill string was lowered to within 20 m of the bottom of the hole to clear obstructions. A second attempt to deploy the triple combo tool string was made with the EOP placed at 231 mbsf. This time the tool string successfully logged across the sediment/basement interface. The FMS/sonic tool was then deployed but was unable to advance deeper than ~257 mbsf (~5 m into basement). Given the time it would have taken to clear the obstruction and the relatively short interval of basement that could have been logged, we terminated the logging program. The drill string was recovered with the EOP clearing the rotary table at 0345 hr on 23 November.
Next Section | Table of Contents