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doi:10.2204/iodp.proc.345.109.2014 Hole U1415I1K.M. Gillis, J.E. Snow, A. Klaus, G. Guerin, N. Abe, N. Akizawa, G. Ceuleneer, M.J. Cheadle, Á. Adrião, K. Faak, T.J. Falloon, S.A. Friedman, M.M. Godard, Y. Harigane, A.J. Horst, T. Hoshide, B. Ildefonse, M.M. Jean, B.E. John, J.H. Koepke, S. Machi, J. Maeda, N.E. Marks, A.M. McCaig, R. Meyer, A. Morris, T. Nozaka, M. Python, A. Saha, and R.P. Wintsch2OperationsThe location for Integrated Ocean Drilling Program (IODP) rotary core barrel (RCB)-cored pilot Hole U1415I (see Fig. F8 in the “Expedition 345 summary” chapter [Gillis et al., 2014b]) was selected using 3.5 kHz data that indicated a thin sediment cover and bottom images that showed a flat-lying sedimented seafloor free of rock fragments. Hole operations are summarized in Table T1 and outlined below. All times are ship local time (UTC – 7 h). Near-bottom camera surveyAt the end of operations in Hole U1415H and while still at that location, we deployed the camera system without the 3.5 kHz pinger and moved the ship ~50 m northwest to the next potential site for a pilot hole. Once on location, a brief visual survey was conducted with the subsea camera confined to a ~5 m radius around the target coordinates (see Table T1 and Fig. F3 in the “Bench site survey” chapter [Gillis et al., 2014a]). The area appeared to be relatively free of boulders or any indications of rubble. Drilling operationsAfter the camera system was recovered, we started Hole U1415I at 1110 h on 28 December 2012. Drilling proceeded much the same as in Hole U1415H, albeit with a slower rate of penetration (ROP). After penetrating ~6 m, drilling was halted, the drill string was picked up a few meters off bottom, and the circulating pumps were reduced. This initial test indicated that the hole was remaining stable. The test was repeated the following morning at ~9 meters below seafloor (mbsf) with the same result, although once back on bottom, the drilling torque temporarily increased slightly as if something was being ground up below the bit. Associated with this increase in torque was an increase in ROP from 0.4 to 0.8 m/h. Slow penetration rates were to be expected, given that we were applying very light weight on bit (5000 lb), as the majority of the bottom-hole assembly (BHA) was still unsupported above seafloor. By mid-morning on 29 December, enough penetration had been achieved to allow the core barrel to be recovered and a drill pipe connection to be attempted. We deployed the sinker bars and recovered Core 345-U1415I-1R at 0945 h on 29 December. The core contained 0.16 m of gabbroic rock from 0 to 11.7 mbsf. A drill pipe connection was made, the bit was run back to bottom, and coring resumed after circulating ~3.0 m of fill out of the bottom of the hole. RCB coring continued in Hole U1415I to 35.2 mbsf under challenging borehole conditions. Cores 1R through 4R extended from 0 to 35.2 mbsf, with 7.12 m of recovered core. The overall recovery includes three sections of coarse sand–sized gabbro particles (Sections 345-U1415I-3R-1, 3R-2, and 3R-3). One interpretation of these sand-sized particles is that they are simply cuttings. Alternatively, later examination showed them to be more highly altered and fractured gabbroic material than the recovered core, suggesting that we might have been drilling through a zone of cataclasis. We spent many hours attempting to stabilize the hole with mud sweeps and wiper trips so that we could hang the drill pipe off at the rig floor to deploy a free-fall funnel (FFF), but we were unable to get the bit below 8 mbsf. We eventually abandoned the effort to deploy a FFF because of the excessive risk to the BHA. We pulled the bit out of the seafloor and retrieved a core barrel (Core 5G) that contained 0.21 m of gabbroic rock inferred to have come from no deeper than ~5 mbsf. The RCB coring assembly was changed out for a 14¾ inch tri-cone drilling assembly and deployed back to the seafloor. When the camera system reached bottom, we easily found the mound of cuttings around Hole U1415I; however, the hole appeared to have collapsed, precluding a bare-hole reentry. We decided to abandon Hole U1415I and drill a new hole nearby instead. |
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