Site 1255 is located ~0.4 km arcward from the deformation front at a water depth of 4311.6 m and in close vicinity to the holes of Site 1043 drilled during Leg 170 (Kimura, Silver, Blum, et al., 1997). Hole 1255A is ~20 m east of Hole 1043A and ~30 m northwest of Hole 1043B (Figs. F3, F29, F30). At Hole 1043A the complete section was cored to 282 mbsf in the underthrust sequence (Unit U3), whereas Hole 1043B was logged using LWD to 482 mbsf, to the top of the igneous basement. As both holes penetrated the décollement, their results were used to plan the drilling strategy and the installation of the CORK-II observatory.
The objective of Site 1255 was to identify the décollement with "real-time" geochemical analyses and penetration rate and to install a long-term observatory for monitoring of fluid flow, pressure, and temperature in the décollement. Because of time constraints, only four cores were taken from 123 to 157 mbsf. We recovered 7.2 m (21%) from the 34 m of cored section. Because of the limited recovery and whole-round sections taken for pore water analyses for locating the décollement, other studies on the cores were limited.
The seismic record (Fig. F30) in the vicinity of Hole 1255A (CMP number 3174) shows no coherent reflections above the décollement. This reflects the general chaotic sedimentary pattern observed already in cores from Site 1254 and in Leg 170 results. The confused seafloor reflection pattern, masking a clear seafloor identification, is probably due to side echoes, generated by local bathymetric relief (Fig. F3). The first prominent reflector relevant for drilling is at 5.96 s TWT that marks the boundary between margin sediments and the underthrust sequence, which was cored at 144 mbsf at Hole 1255A. The current seismic data do not show any evidence for fault zones above the décollement.
After setting the reentry cone at Hole 1255A, we drilled to a depth of 123 mbsf with a 143/4-in bit. We then installed 103/4-in casing to a depth of 117 mbsf and cemented it in. Coring started at 123 mbsf, after drilling out the cement shoe, and stopped at 157 mbsf when a sudden increase in penetration rate during cutting of the fourth core indicated that the underthrust sediments were reached. The installation of the CORK-II was successful and completed with the deployment of the remotely operated vehicle (ROV) platform. A postcruise Alvin dive showed the installation to be fully operational, and pressure data showed a return to hydrostatic conditions within the borehole.
The section recovered from Hole 1255A can be separated into lithostratigraphic Unit T1 (equivalent to Subunit P1B at Site 1254) and Subunit U1A just beneath the level of the décollement at 144.08 mbsf. The division marks a sharp junction between a series of structureless greenish gray claystones with silts and few or no diatoms above an underlying series of diatom-rich claystones with interbedded silts, sands, and occasional fine-grained conglomerates. The clastic sediments in the underthrust section (Subunit U1A), interpreted as near-trench turbidites during Leg 170, differ from the purely hemipelagic diatom ooze recovered at Site 1043 (Kimura, Silver, Blum, et al., 1997). The fact that these turbidites were not recovered at Site 1043 may indicate that this section is very thin and simply not recovered or not present at all because of lateral facies changes over short distances. Blocks of reworked carbonate in Unit T1 indicate that these sediments are mostly debris flows, in part derived originally from shallow, nearshore environments. The presence of a pelagic nannofossil chalk interval and the larger proportion of diatoms at Site 1255 suggests that this site experienced a larger amount of pelagic sedimentation than did Site 1254. The underthrust section represents a trench depositional setting, with turbidite silts and sand interbedded with hemipelagic mudstones. Unlike other sites of Leg 205, sediments at Site 1255 contain no primary ash layers in the short section recovered; however, fresh and altered volcanic glass shards do comprise a significant proportion (10%15%) of the prism sediments.
At Site 1255, structural deformation, with brecciation and polished clast surfaces as an indication of incipient scaly fabric, increases within the recovered section from 132.7 mbsf to the base of the décollement at 144.08 mbsf. The top of the décollement zone could not be defined as a result of limited coring and recovery. The base of the décollement is sharp and well defined and coincides with the division between Unit T1 and Subunit U1A. Only one measurement of bedding dip was possible in the décollement zone, giving a dip of 44°. The hemipelagic layers in the underthrust section below 144.08 mbsf show some medium-scale brecciation (1- to 3-cm fragments) with unpolished surfaces, whereas the sandy layers are undeformed.
Volatile hydrocarbon gases were sampled using the vacutainer and headspace technique. As no gas voids were apparent at Site 1255 in contrast to Site 1254, vacutainer samples show large air contamination. Headspace methane concentrations drop rapidly from >3000 ppmv in Core 205-1255A-2R to 5 ppmv below the décollement (145 mbsf). Propane as an indicator for deeply sourced fluids is low in the prism section (~1 ppmv) and absent below the décollement.
Only three whole rounds (one per core) could be taken for geochemical analyses of pore waters because of low recovery, and all conclusions are therefore somewhat speculative. As observed at Site 1254, the chemical composition of the pore fluids at Site 1255 also is distinctly different in the wedge and underthrust section, with a less sharp transition at the base of the décollement zone at ~144 mbsf. Fluid flow is indicated by salinity, sodium and calcium concentration minima, and lithium concentration maxima within the décollement sample at 134.2 mbsf. These concentration minima were observed at Site 1043 along with a magnesium concentration maxima within the same interval. Across the décollement, changes in Ca and Mg concentrations are in the opposite direction to those seen at Site 1254. In general, the pore fluids within the upper fault zone and in the décollement at Sites 1254 and 1040 are characterized by having a significantly stronger signature of a deeply sourced fluid than the pore fluids from Sites 1255 and 1043. At Sites 1255 and 1043, the pore fluid chemistry in the wedge and the uppermost underthrust sediments appears to reflect some mixing between the lower wedge and uppermost hemipelagic pore fluids, thereby partially obscuring the deeply sourced fluid signature observed at Site 1254. This pore water mixing could be achieved by advection of fluid from the underthrust section across the décollement and into the lower wedge. Furthermore, leakage of a portion of the deeply sourced fluid sampled at Site 1254 may have occurred through faults, fractures, and more permeable conduits between Site 1254 and Sites 1255 and 1043.
Two samples for microbiological investigations were taken and either frozen or fixed for postcruise ATP quantification, DNA assessment, or cell counts. Samples of drilling water were frozen as well to evaluate contamination of cores. The chemical tracer for quantifying microbiological contamination was not deployed during coring at Hole 1255A because of concern that the trace element chemistry of the PFT may affect postcruise pore fluid geochemical analyses. Particulate tracer tests yield fluorescent microsphere counts suggesting very low to no particulate contamination in the interior of the microbiology whole rounds.
Paleomagnetic declination and inclination, measured on discrete samples and archive halves, disagree in the upper part (132.76 to 134.84 mbsf) of the cored interval, making any magnetostratigraphic interpretation questionable. Across the décollement, a clear polarity change can be seen in both archive halves and discrete samples. However, the polarity changes cannot be assigned to a particular chron and are not usable for dating purposes. Magnetic intensities are generally low in the upper part of the section but increase substantially in the sandy layers of the underthrust sequence.
Sample porosities and LWD porosities from Leg 170 show a clear increase of porosities at the base of the décollement from values of ~55% in the prism section to values of ~70% in the underthrust. Sample porosities from Hole 1255A clearly confirm the values in the prism, but the few data points below the décollement are not representative for the porosity of the underthrust sequence but show rather the influence of sampling clayey or sandy material. Only the magnetic susceptibility shows a marked increase below the décollement, which reflects a presumably higher magnetite content in the turbidites of the underthrust.
A CORK-II observatory was successfully installed as shown in Figure F31. The center of the packer is at 129 mbsf and the center of the screen at 140 mbsf, in the middle of the geochemical anomaly as determined from Site 1255 data and Site 1043 results. The second pressure port inside a small screen was installed just above the upper packer.
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