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doi:10.2204/iodp.pr.322.2009

Preliminary scientific assessment

When viewed in its entirety, Expedition 322 must be regarded as a success. Recovery of core from Site C0012, including successful penetration of igneous basement, was a major achievement, as was the acquisition of LWD logs from Site C0011 prior to coring. Based on preliminary analysis of the sandstone petrology, we now know that two sand-rich turbidite systems developed within the Shikoku Basin during the middle and late Miocene epochs. The younger system, which is characterized by a volcanic source of detritus, was sampled for the first time during this expedition. Another major success story involves acquisition of reliable pore water geochemistry data at Site C0012. But the expedition also suffered several failures and disappointments. Below, we provide candid assessments of each major objective in the science plan.

Objective 1: Core and log the lowermost sedimentary strata of the Shikoku Basin and the top of igneous basement.

When viewed within the context of the entire NanTroSEIZE science plan, this objective was the most important for Expedition 322 to achieve. This high priority is rooted in the fact that lowermost Shikoku Basin strata and uppermost basement host the plate boundary fault and the deep megasplay fault at seismogenic depths. Prior to drilling, the estimated depth to basement at Site C0011 was set at 1050 m SSF. LWD logging operations in Hole C0011A were terminated short of basement at 950 m LSF because of time limitations at the end of Expedition 319. Coring operations in Hole C0011B were terminated at 876 m CSF after exceedingly slow ROPs, followed by destruction of the drill bit. Consequently, our most important objective remained unfulfilled at the primary site.

Our decision to occupy contingency Site C0012 (proposed Site NT1-01) was driven by the overwhelming need to achieve Objective 1. Prior to drilling, the estimated depth to sediment/basalt interface at the basement high was placed at 515 m SSF using refined velocity models and constraints from LWD logs at Site C0011. We did achieve this coring goal, intersecting the top of basement at 537.81 m CSF and continuing ~20 m into basalt. TD for Hole C0012A was 560.74 m CSF.

Much of the science party's postexpedition research will focus on analyses of samples from the lower stratigraphic units of the Shikoku Basin. Those studies will provide much-needed information about the pristine physical, chemical, compositional, and hydrological properties of the subduction inputs prior to the effects of subduction. On the negative side, attempts to obtain wireline logs at Site C0012 met with failure, which limits our ability to extend core-log-seismic integration beyond the immediate vicinity of that borehole.

Objective 2: Core and log the sandy turbidite facies of the lower Shikoku Basin, and assess its potential for focused lateral transport of fluids via geochemical analyses of pore water and gases.

Cores and LWD logs both show that the Shikoku Basin contains two prominent intervals of sand-rich turbidite deposits. The hydrogeologic objective, however, met with limited success in Hole C0011B because of poor core recovery and an extensive interval of wash down within the inferred turbidites of the lower Shikoku Basin. LWD logs help constrain the 2-D geometry of the turbidite intervals, and core recovery was just enough to characterize the sandstone petrofacies and detrital provenance. The overall hydrogeologic properties and 3-D stratigraphic architecture of the sand bodies remain largely unknown. On the other hand, indirect evidence for focused fluid flow can be extracted from pore water geochemistry and hydrocarbon concentrations. Correlative facies were also recovered at Site C0012, which allows us to extend the characterization of fluid composition and fluid transport from the flank of the basement high to the updip extension of correlative facies units. These data are indicative of two fluid regimes: a more seawater-like fluid moving upward from igneous basement, and a deeper seated sedimentary source coupled with dehydration and focused flow toward the Shikoku Basin up to where the turbidites lap onto the basement high. This discovery is one of the more significant contributions of Expedition 322.

Objective 3: Complete detailed profiles of pore water geochemistry within the lowermost sedimentary strata of the Shikoku Basin as an indicator of fluid chemistry within upper basement.

We failed to achieve this objective at Site C0011 because of premature termination of the coring hole (destruction of bit). Furthermore, geochemical profiles higher in the sedimentary section are ambiguous with respect to a possible basement fluid source because of the extensive amounts of contamination by drilling fluid. At Site C0012, however, pore fluid samples were successfully extracted from cores in close proximity to the sediment/basement interface. Initial evaluation of the data reveals several intriguing possibilities for fluid sources, migration pathways, and mixing, including a more seawater-like fluid emanating from the upper basement. Chlorinity increases above seawater values by 12% toward the basement contact, whereas all of the other comparable sites in the Nankai subduction zone show freshening of pore water with depth. In addition, the sulfate reduction zone is much deeper than anticipated at Site C0012, and sulfate depletion coincides with marked increases in methane and ethane concentrations. The sulfate profile is probably driven by AMO and hydrogen sulfide production. There is also widespread geochemical evidence for alteration of volcanic material at both sites; this alteration includes a component of dispersed ash, volcanic sand, and the basaltic basement. These hydration reactions result in silica, potassium, and magnesium depletion, together with high levels of dissolved calcium. As stated above, the pore water geochemistry program was a major success when viewed in its entirety.

Objective 4: Measure in situ pore pressure within the turbidite facies of the lower Shikoku Basin.

Deployment of the SET-P tool was aborted in Hole C0011B because of ambiguities in the location and properties of our primary targets for measurement. Although clearly defined by LWD data, failure to recover inferred sand(stone) beds immediately above the target intervals for probe insertion was a major drawback. Thus, we failed to achieve this objective.

Objective 5: Construct a complete depth profile of sediment physical properties and integrate those changes with trends in sediment composition and texture.

This objective was not achieved at Site C0011 because coring was interrupted by several extensive washdown intervals. We also failed to reach the anticipated TD at Site C0011. In addition, for the core that was recovered, measured values of porosity, P-wave velocity, V_P anisotropy, and electrical resistivity were compromised by pervasive damage to the core. Widespread occurrences of water-filled microcracks led to an unfortunate expansion of the scatter and artifacts in all types of physical property data. This noise makes it more difficult to compare results from Expedition 322 with other data sets in the Shikoku Basin and Nankai Trough. The physical property profiles for Site C0012 are more continuous than for Site C0011, but those data also suffer from the effects of widespread coring disturbance and poor recovery. This problem needs to be taken into consideration when attempting to interpret the results of shore-based laboratory tests of consolidation state, permeability, and the like.

Objective 6: Reconstruct the complete history of Shikoku Basin sedimentation in the vicinity of the Kashinosaki Knoll, including age, detrital provenance, dispersal paths, contributions of volcaniclastic/pyroclastic versus siliciclastic material, and physical mechanisms of transport and deposition.

In spite of the spotty recovery of turbidite sand beds, having core samples from both of the two reference sites in the Shikoku Basin will allow sedimentary petrologists, mineralogists, and geochemists to conduct a comprehensive program of shore-based analyses. The goal will be to determine the composition, age, and provenance of sedimentary sources throughout the last 20 m.y. of basin evolution. Examination of smear slides during the expedition revealed several lines of evidence for multiple detrital sources, but shore-based research will add quantitative details through age dating of such grain types as zircon and chemical fingerprinting of single pyroxene grains and volcanic glass shards. Scientists will also analyze dispersed volcanic ash in the hemipelagic mudstone and quantify the composition and alteration state of clay-mineral assemblages. When integrated with the age constraints provided by nannofossils, foraminifers, and paleomagnetic reversals, the compositional data will provide an unprecedented view of the basin's history in the region between the Kinan seamount chain and the Izu-Bonin volcanic arc. Ultimately, we will be able to combine that regional reconstruction with information already available from the Muroto and Ashizuri transect areas to the west, thereby demonstrating how different parts of the backarc basin responded to major tectonic and climate events during the margin's Miocene–Quaternary evolution.

Objective 7: Integrate and correlate among cores, logs, and seismic profiles from the seaward edge of the trench wedge to the crest of the Kashinosaki Knoll.

Overall, the core-log-seismic integration effort was marginally successful. Our ability to extend correlations for significant distances away from the two drill sites was limited by the small size of the mini-3-D seismic survey and the proprietary hold on most of the seismic data. Ties between logs and core were hampered by poor recovery and ubiquitous damage to the cores. Pervasive water-filled microcracks led to unreliable measurements of porosity, density, and P-wave velocity. The severity of this damage can be seen in the mismatch between reflections on the seismic line through Site C0011 and the synthetic seismogram, which was calculated using the values from discrete measurements of physical properties on core. In addition, the wireline logging program at Site C0011 was cancelled, and logging at Site C0012 failed because of operational difficulties (excessive deviation of the borehole) and an approaching typhoon. On the positive side, our success in making qualitative comparisons between logging units identified from LWD data and lithologic units identified during VCD improved the interpretations of basin stratigraphy and facies evolution.

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