Site 1270 is the southernmost of all sites occupied during Leg 209, 16 nmi southeast of Site 1269. Originally planned as an alternate site, this location is on the ridge flank at the eastern limit of the axial valley. The site survey dive tapes reveal an exposed fault surface, and samples of mylonitic peridotite were recovered during the dive, together with less deformed peridotite and gabbroic rocks. We chose to occupy this site as a result of poor hole conditions and low recovery, including only basalt, at Site 1269.
On 28 May 2003, we arrived at the Global Positioning System (GPS) coordinates of a position along the dive track of Shinkai 6500 Dive 425 (Figs. F1, F2), just upslope of where serpentinized peridotite had been sampled from a large outcrop (sample R007). A six-collar bottom-hole assembly (BHA) (nominally 130 m long) was lowered with the drill string, and the subsea camera was deployed. After a ~3.5-hr survey (Fig. F1) during which we recognized several intervals of bare rock exposure, we selected a drilling target ~150 m west of our camera survey starting position.
Hole 1270A was initiated with slow rotation and low bit weight at 0500 hr on 29 May. The upper guide horn was removed to allow us to keep the subsea camera near the seafloor as we started the hole to ensure the bit did not track downslope. After 3 m of penetration, the camera was retrieved and coring continued through Core 4R (0.0–26.9 meters below seafloor [mbsf]) (Table T1). Core 4R contained fault gouge, and our attempt to deepen the hole was hampered by erratic torque on the drill string and wall collapse, which filled the borehole. After hole cleaning attempts failed, we pulled clear of the seafloor at 1600 hr on 29 May, ending Hole 1270A.
We returned to our original camera survey position, redeployed the subsea camera, and began a survey upslope in search of a new drilling location on a bare rock exposure. We moved 100 m east along an azimuth of ~120° over the path followed by Shinkai 6500 Dive 425 and then turned south to follow a depth contour where outcrop was evident in the dive videotape. After completing a rectangular box survey pattern, we located an extensive exposure of bare rock ~230 m southeast of Hole 1270A and initiated Hole 1270B.
Coring began in Hole 1270B as in Hole 1270A (at 0200 hr on 30 May) by advancing the bit with slow rotation and low bit weight to 3 mbsf while observing the operation with the subsea camera. After retrieving the camera, we cored to a depth of 45.9 mbsf (Cores 1R to 10M). As we began our third coring interval, the bit would not advance past 13.5 mbsf (1 m deeper than the top of Core 2R). At that depth, coring continued as if we had not already drilled the interval between 13.5 and 17.4 mbsf. Core 3M represents rock recovered in the same general interval as Core 2R (13.5–17.4 mbsf) but contains >1 m of cut core cylinders. When the same blockage was encountered at virtually the same depth on our next coring attempt (13.5 mbsf), we cored beyond the bottom depth of Cores 2R and 3M, to 22 mbsf. Since we redrilled the upper part of the interval, the recovered core was curated as Core 4M. Normal coring curation and nomenclature was resumed with Core 5R, when we were sure we were coring a new interval from the beginning of the cored interval. Coring progressed steadily with improving recovery until Core 9R. Hard fill in the bottom of the hole prevented advancement and caused high and erratic torque. Since we were still operating with a reduced-length BHA, we chose not to risk getting the pipe stuck and abandoned the hole at 1205 hr on 31 May.
We began our third camera survey (Fig. F1) at the GPS coordinates of sample R008 from Shinkai 6500 Dive 425 (Fig. F2) where mylonitinized peridotite had been sampled. Unlike outcrops at our two previous holes (bare rock free of talus), the dive videotape revealed this outcrop was littered with what appeared to be slabs of rock, albeit more or less in place. We selected this location upslope from our two previous holes, surmising that if the fault gouge encountered at depth in Hole 1270A was a result of incipient landsliding, this older, more weathered outcrop might yield better drilling conditions. After a 2.5-hr survey, we selected a drilling target ~240 m east of Hole 1270B.
Hole 1270C was initiated at 1735 hr on 31 May in a manner similar to our two previous holes. Coring only progressed to 18.5 mbsf (Cores 1R and 2R) before the hole began to fill, restricting circulation. No advancement was possible on our third coring interval, even after 2.5 hr of rotation, so the core barrel was recovered (curated as Core 3M) and the hole was abandoned at 0300 hr on 1 June.
The survey for Hole 1270D began over the position of Hole 1270C. We moved only a short distance (~30 m east) and selected a drilling target in bare rock. At 0730 hr on 1 June, Hole 1270D was initiated while we watched the operation with the subsea camera. We cored to 3 mbsf before the camera cable appeared to start wrapping around the drill string. While recovering the camera (without rotating the pipe) the driller became concerned when he was required to advance the pipe to maintain a minimum weight on bit. Upon lowering the camera back to the seafloor, the camera image showed that the bit had slid off the outcrop and embedded in sediment. A second hole initiation (at 0934 hr on 1 June) was successful to 3 mbsf, so the camera was retrieved and coring continued to 57.3 mbsf. After recovering Core 11R, hole conditions deteriorated to the point that no further advance was possible. The bit was pulled clear of the seafloor, ending operations at Site 1270 at 1445 hr on 2 June. Rate of penetration at Site 1270 varied from 3 to 7 m/hr.
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Ms 209IR-105