The 980-nmi voyage to Site 1236 (proposed Site NAZCA-10A) was accomplished in 86.0 hr at an average speed of 11.3 kt. The 3.5-kHz precision depth recorder (PDR) was used as a final check of site characteristics by comparison with precruise survey data. The vessel was on station at 1345 hr on 22 April 2002.
The drill string was deployed to a depth of 1309 meters below rig floor (mbrf), and Hole 1236A was initiated with the APC at 1815 hr on 22 April. The seafloor depth inferred from the recovery of the first core was 1334.9 mbrf. This value was 3.5 m shallower than the corrected PDR depth. Piston coring deepened the hole until Core 20H failed to achieve a full stroke at 173.1 mbsf. The recovery for the APC interval was 169.6 m (recovery = 98%) (Table T1). The nonmagnetic core barrel was used on all even-numbered cores (Table T1). Cores were oriented starting with Core 4H. Downhole temperature measurements were taken with the APC temperature (APCT) tool (Table T1). Coring in Hole 1236A continued with the extended core barrel (XCB) system to 207.7 mbsf, where the rate of penetration slowed markedly. XCB coring penetrated 34.6 m and recovered 27.4 m (recovery = 79%). A total of 207.7 m was cored and 197.2 m was recovered (Table T1) in Hole 1236A. The bit was pulled free of the seafloor at 1340 hr on 23 April.
The vessel was moved 10 m west relative to Hole 1236A. Prior to coring in Hole 1236B, a bottom water temperature measurement was obtained with the APCT tool and the bit just above the seafloor. To obtain the desired stratigraphic overlap with the first hole, the bit was then positioned at 1333 mbrf and Hole 1236B was initiated with the APC at 1456 hr. The seafloor depth inferred from the recovery of the initial core was 1333.7 mbrf. Thirteen piston cores advanced to 122.8 mbsf.
When the driller attempted to cut Core 14H, the pressure in the drill string could not be raised above 2000 psi. The barrel was retrieved and the liner was found to be deformed, indicating that a large differential pressure had been applied across the liner. After two more coring attempts and an attempt to force the presumed obstruction out of the bottom-hole assembly (BHA), the drill string was retrieved. The flapper in the float valve was found resting on the bottom of the float valve assembly. Both hinges on the flapper had failed. The broken flapper prevented the end of the core barrel from entering the throat of the bit, causing the core barrel to land high in the BHA. These prevented a proper piston seal, and the firing pressure of 2700-2800 psi could not be achieved.
A total of 122.8 m was cored and 125.1 m was recovered (Table T1) in the aborted hole. The nonmagnetic core barrel was deployed with the odd-numbered cores. Cores were oriented starting with Core 3H. Downhole temperature measurements were taken at depths summarized in Table T1. All temperature measurements combined yielded a thermal gradient of ~3.9°C/100 m for Site 1236.
The vessel was offset 10 m west relative to Hole 1236B. At 1303 hr on 24 April, Hole 1236C was initiated with the APC, and the bit was positioned at 1336.5 mbrf (2.8 mbsf relative to Hole 1236B). Because this hole was initiated below the seafloor, the seafloor depth calculated from the previous hole was used (1333.7 mbrf). Piston coring advanced to the APC refusal depth of 167.3 mbsf). The nonmagnetic core barrel was deployed on every even-numbered core. The cores were oriented starting with Core 3H. Core 16H (145.3-154.8 mbsf) had to be drilled over. Of the 164.5 m cored, 167.7 m was recovered (recovery = 102%) (Table T1).
The vessel was secured for transit and left location at 0500 hr on 25 April.