Preliminary scientific assessment

Expedition 315 started on 16 November 2007, one day earlier than originally planned. Before the start of coring, we established a core processing flow largely different from previous ODP and IODP expeditions, mainly because of extensive whole-round core sampling. Our basic strategy was to take whole-round core samples while avoiding disturbing structurally and sedimentologically important layers. We cut whole-round cores for interstitial water analyses at the core cutting area and took X-ray CT scan images immediately, as they had precedence over other core sections. After real-time screening of internal structures by a sedimentologist and a structural geologist, whole-round cores were squeezed for geochemical studies. Whole-round core samples for microbiology were taken just after X-ray CT scanning and before MSCL-W measurements. Other less time-critical whole-round core samples, such as those taken for geomechanical, structural, and physical property purposes, were cut after MSCL-W measurements. These samples were carefully chosen based on screening by X-ray CT scan images. This routine satisfied both good quality whole-round core sampling and protection of structurally and/or sedimentologically important horizons. The process resulted in a complex core flow and in fact caused some confusion in core processing at the beginning; however, it worked very effectively once it was well established.

HPCS coring in stiff formations often caused twisting or burst core liners; however, the cause is still unidentified. Because penetration of HPCS coring was poor, we conducted ESCS coring. Nevertheless, we stopped it for two cores because of severe biscuiting that prevented our taking any decent quality samples for physical properties, even though recovery rates were high. RCB coring was then performed in Hole C0001H; this first attempt on the Chikyu was successfully conducted from 240 to 458 m CSF, although recovery was generally as low as 30%–40%. We stopped coring just above the "sticky zone" intervals in which LWD operation during the previous expedition had met with difficult borehole conditions. We washed out this zone and attempted to restart coring at 600 m CSF. We could drill through the sticky zone without any major drilling problem; however, borehole conditions became worse with time and we finally gave up coring below the sticky zone. It was a disappointment not to obtain any samples from within and below the sticky zone because different features were presented in LWD data from the accretionary prisms above and below this zone. The nature of the sticky zone still remains unclear. Regarding the change in stress state between the hanging wall and footwall interpreted from resistivity images, it is likely to be a group of small faults or a fault swarm. Although we could not reach the original target depth, we obtained a complete data set of both slope basin sequences, including a thick basal sand layer and part of the underlying accretionary prism. High resistivity and low gamma ray horizons frequently intercalated with mud were interpreted as turbidites during Expedition 314; however, most of them were in fact volcanic ash layers.

The original plan after coring at Site C0001 was a casing operation for 3.5 km riser drilling during NanTroSEIZE Stage 2. The casing operation was postponed, however, because of the strong Kuroshio Current, which sometimes exceeded 5 kt. As a consequence, the 12 days planned for the casing operation were used for science. There were several contingency sites in the Expedition 315 Scientific Prospectus (Ashi et al., 2007), including input sites seaward of the trench axis (proposed Sites NT1-01 and NT1-07), slope basins to study slope instability caused by splay fault activities (proposed Sites NT2-05 and NT2-10), and the Kumano forearc basin and underlying accretionary prism (proposed Site NT3-01). We chose the last one for the following reasons:

    • The site is a future 6 km riser site (NanTroSEIZE Stage 3), and obtaining geologic and geotechnical information from the shallow part of the accretionary prism and the overlying sedimentary sequences fits the primary expedition goal of "riser pilot" study.

    • Paleostress reconstruction in the forearc basin and comparison with results from Site C0001 aid our understanding of the changes of stress fields associated with the splay fault system.

    • A complete data set of LWD data to 1400 m CSF were taken during Expedition 314 that enables core-log (and seismic) integration and provides more useful information than that gained by coring only.

    • Sedimentation history of the forearc basin is key information for elucidating the relationships between the accretionary prism growth and the evolution of the splay fault system.

We did not have enough time for coring of entire intervals to TD at Site C0002. Because our primary target was the shallow portion of the accretionary prism and the boundary to its overlying forearc basin, we started coring at 475 m CSF in order to reach TD within the expedition schedule. As was expected from LWD results, we encountered many thick sand layers in the forearc basin sequences. However, their actual occurrence was far less than expected from LWD data. The poor RCB core recovery rate (average = 35%) suggests selective omission of loose sand layers. Coring was successfully conducted through the unconformity between the forearc basin sequence and the accretionary prism at ~920 m CSF and continued to 1057 m CSF. Depositional ages were considerably well determined by micropaleontological and paleomagnetic investigations.

We had to stop coring in Hole C0002B at 1057 m CSF because of bad borehole conditions. As there were still several days remaining, we determined to conduct further coring in the shallow part of the forearc basin. Our main purpose was a geotechnical assessment of the riser tophole section to 70 mbsf. Coring and downhole temperature measurement were conducted in Hole C0002D to 204 m CSF. Before drilling Hole C0002D, a short (13.77 m) hole (C0002C) was cored. Two cores, mainly dedicated to fluid geochemistry and microbiology, were cut just beneath the seafloor to compensate dense whole-round sampling from Hole C0002D. Cores from Holes C0002C and C0002D were stored in the cold reefer without splitting after nondestructive whole-round measurements and sampling.

Expedition 315 was originally scheduled as a short expedition: 21 days for coring and 12 days for riser tophole casing. As riser hole casing was one of the primary objectives of the expedition, its postponement compelled a major revision of our scientific operations during the expedition. It allowed us to add coring at another planned riser site in the forearc basin (Site C0002). As a consequence, we could acquire geological and geothermal information in the shallow part of the accretionary prism and the overlying slope/forearc basin sequences at both planned riser sites (C0001 and C0002). These sites are located at vital positions and also elucidate the relationships between the growth of the accretionary prism and the evolution of the splay fault system. Coring to 458 m CSF at Site C0001 was not fully successful with regard to its intended TD of 1000 mbsf. As for Site C0002, we had to give up coring at 1057 m CSF although our intention was to core to 1400 mbsf. However, this was not because of any unexpected trouble or problem. We think that at that depth we almost reached the technical limitations of riserless coring in such a geologic setting; hence, we may have obtained possible maximum results for the accretionary prism for riserless coring. Note that Site C0002 results were additional and were not anticipated at the beginning of the expedition. Our expedition, therefore, could not fully have satisfied the original scientific objectives; however, we set up alternative targets in the course of the expedition and executed them successfully. Despite these difficulties, we believe that the overall achievement of the expedition as a "Megasplay Riser Pilot" study was satisfactory.