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doi:10.2204/iodp.proc.335.103.2012 Site 12561Expedition 335 Scientists2IntroductionThis chapter presents descriptions and measurements on cores and other samples retrieved during Integrated Ocean Drilling Program (IODP) Expedition 335 to Hole 1256D. The background and objectives of Expedition 335 are described in detail in “Background and geological setting” and “Scientific objectives” both in the “Expedition 335 summary” chapter (Expedition 335 Scientists, 2012a), and expedition operations are documented in “Operations” in the “Expedition 335 summary” chapter (Expedition 335 Scientists, 2012a). The following sections report observations and measurements made on cores from the plutonic section recovered during IODP Expedition 312, as well as material recovered during Expedition 335 (Fig. F1). Here we summarize what was recovered as cores and as other nonstandard geological samples from the numerous hole-clearing and fishing runs during Expedition 335. These materials were described by the Shipboard Scientific Party and archived at the IODP Gulf Coast Repository. Redescription of Expedition 312 Cores 312-1256D-202R through 234RThe archive and working halves of Cores 312-1256D-202R through 234R (1372.8–1507.1 meters below seafloor [mbsf]) from the lower part of the granoblastic dikes to the bottom of the hole at 1507.1 mbsf, including the plutonic section of Hole 1256D (Fig. F2) (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006), were shipped to the R/V JOIDES Resolution prior to Expedition 335. These cores were made available to the science party for the purpose of familiarization with the recovered plutonic section of Hole 1256D and to establish and test description protocols during the transit to Site 1256. The scientists also developed the DESClogik interface for the capture of plutonic rock descriptions for the first time since the introduction of the LIMS database (see “Information architecture” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). The long duration of operations required to open Hole 1256D to its full depth at the beginning of Expedition 335 (~16 days; see “Operations” in the “Expedition 335 summary” chapter [Expedition 335 Scientists, 2012a]) provided sufficient time for the Shipboard Scientific Party to completely redescribe these cores. This was actually much more time than was available to the shipboard scientists at the end of Expedition 312. Discrete measurements (paleomagnetism and physical properties) were also performed. These observations and measurements were uploaded to the LIMS database and are presented together with descriptions and measurements of the new material recovered during Expedition 335. No Expedition 312 shipboard thin sections were available onboard the JOIDES Resolution to complete new descriptions, but Expedition 312 thin section descriptions were also uploaded to the LIMS database. Expedition 335 scientists supplemented their core descriptions through the use of a series of personal thin sections from Expedition 312 cores brought aboard by Expedition 335 science party members. Material recovered during Expedition 335Four cores were taken during Expedition 335 (Cores 335-1256D-235R through 238R) (see Table T4 in the “Expedition 335 summary” chapter [Expedition 335 Scientists, 2012a]) immediately following the ~16 day reopening of the 922 mbsf interval of Hole 1256D at the beginning of the expedition (see “Operations” in the “Expedition 335 summary” chapter [Expedition 335 Scientists, 2012a]). This period of coring lasted only 36 h before the C9 coring bit (Run 9) stopped making progress in the hole and was retrieved completely destroyed (Fig. F3), initiating the second long period of remediation for the remainder of Expedition 335. Thirteen subsequent drilling, milling, and fishing runs were required to retrieve metallic junk from the bottom of the hole. Twelve of these runs provided large amounts of cuttings (from fine-grained sand to several centimeter–sized pebbles and chips) and cobbles (Table T1). Expedition 335 activities in Hole 1256D concluded with a campaign of cementing operations that were directly preceded by the cutting of a final core (335-1256D-239R). During several fishing runs, large amounts (up to several hundreds of kilograms) of fine-grained cuttings were recovered, in particular in the bit sub of the fishing magnet (Run 10) and in the bottom-hole assembly (BHA) during the two reverse circulation junk basket Runs 12 and 13 (Table T1). Six shipboard grain-mount thin sections were prepared from the fine-grained cuttings from external junk baskets (EXJBs) on Runs 11 and 12 and from the sand-filled drilling collars of the Run 12 BHA. These grain-mounts were point counted and the grains identified to establish the origin of the fine-grained cuttings (Table T2; Fig. F4). The cuttings contain fragments of three main rock types recovered in Hole 1256D (basaltic lava, granoblastic basalt, and gabbro), indicating that cuttings from the entire hole are present. Basaltic lavas make up ~36% of the grains. These basaltic cuttings are likely to have been progressively accumulating in the hole since the beginning of drilling of Hole 1256D during Ocean Drilling Program (ODP) Leg 206. Granoblastic basalt makes up the largest proportion of the grains (49%), indicating also that coring (including several hours with a destroyed bit), subsequent drilling, and fishing operations during Expedition 335 near the bottom of the hole contributed significantly to the accumulation of these cuttings in the hole. Once the hole was clear of the basaltic sand that filled the BHA during Runs 12 and 13, most of the cuttings and cobbles recovered were granoblastic basalt and gabbroic rocks (Table T3). This material was described, measured, and analyzed following the same standard procedure (except for measurements in core reference frame) as those used for describing cores (see “Core handling and core flow” in the “Methods” chapter [Expedition 335 Scientists, 2012b]), and the results are reported in the following sections. To facilitate description and discussion of the crustal stratigraphy at Site 1256 and to assist in the interpretation of cores and junk basket materials recovered during Expedition 335, we present an updated lithostratigraphy of Hole 1256D showing the major divisions (Fig. F1; see Table T5 in the “Expedition 335 summary” chapter [Expedition 335 Scientists, 2012a]). The only major change to the lithostratigraphy presented at the end of IODP Expedition 309/312 is the differentiation of Dike Screens 1 and 2. Figure F5 shows the progress in Hole 1256D over four scientific ocean drilling expeditions. The lack of gradient in the Expedition 335 segment reflects the dominance of hole opening and remediation during the expedition. |
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