SITE SUMMARIES

Site 1173 Summary
We completed drilling at Site 1173 in the trench outer margin (Fig. 9) in order to provide a reference for the predeformation status of geological and geochemical characteristics of the incoming sedimentary section. We recognized five lithostratigraphic units (Figs. 8, 12): Unit I (0 to 102 mbsf) is Quaternary in age and composed of sandy to muddy turbidites of the outer Nankai trench—wedge facies; Unit II (102 to 344 mbsf) is Quaternary to Pliocene in age and made up of hemipelagic mud with abundant interbeds of volcanic that were ash probably derived from the Kyushu and/or Honshu volcanic arcs (upper Shikoku Basin facies; Fig. 13); Unit III (344 to 688 mbsf) consists of Pliocene to middle Miocene bioturbated silty claystone (lower Shikoku Basin facies); Unit IV (688 to 725 mbsf) is probably middle Miocene in age and composed of variegated siliceous claystone and silty claystone (volcaniclastic facies); Unit V (725 mbsf) is middle Miocene basalt. The boundary between Units II and III is controlled, in part, by diagenesis. There is an abrupt loss of unequivocal ash beds and replacement by siliceous claystone (Fig. 14).

Biostratigraphic age control provided by calcareous nannofossils identified a total of 23 biostratigraphic events. The continuous sedimentary section spans the time interval from the Pleistocene (Subzone NN21b) through the middle Miocene (Zone NN5). Magnetostratigraphy clearly identified the Bruhnes/Matuyama boundary (0.78 Ma), Matuyama/Gauss boundary (2.581 Ma), the Gauss/Gilbert boundary (3.58 Ma), and the termination of the Gilbert Chron (5.894 Ma). Paleomagnetic and biostratigraphic ages indicate high sedimentation rates (450—650 m/m.y.) for the turbidite deposits, decreasing rates for the upper Shikoku Basin section (72—77 m/m.y.), and lowest rates for the lower Shikoku Basin (27—37 m/m.y.).

Deformation structures in Hole 1173A are sparse oceanward of the prism, as expected from a reference site. The section above 375 mbsf is characterized by horizontal bedding with occasional microfaults between 250 and 275 mbsf. Bedding dips reaching 20°, perhaps due to lateral extension associated with normal faulting, occur abruptly at 375 mbsf and continue down to 550 mbsf and sporadically to the bottom of the hole. A 30-cm zone of foliated breccia indicates somewhat greater deformation around 440 mbsf. Lower cores contain rare mineralized veins; a few possible dewatering structures such as thin, sediment-filled veins reflect early compaction processes.

Variations in physical properties correlate well with the lithostratigraphic units. High variability characterizes the turbidites of the outer Nankai Trench wedge, and porosities decrease with depth. Porosity increases at the boundary between the outer Nankai Trench wedge and the upper Shikoku Basin facies and continues to increase slightly with depth. These elevated porosities deviate from a typical compaction profile. An increase in P-wave velocities within this interval of increasing porosity suggests that there may be slight cementation. At the boundary between the upper and lower Shikoku Basin facies (~340 mbsf), grain densities increase slightly and porosities decrease sharply. This porosity decrease is accompanied by increasing thermal conductivity, P-wave velocity, and resistivity. A gas-probe permeameter showed that ash bands in the upper Shikoku Basin sediments are significantly more permeable than the hemipelagites, although the contrast disappears in the lower Shikoku Basin section.

Seven reliable determinations of downhole temperatures were made at depths of 35 to 284 mbsf in Hole 1173A, using the advanced hydraulic piston corer (APC) temperature tool, water-sampling temperature probe (WSTP), and Davis-Villinger temperature probe (DVTP). The measured temperatures closely define a linear gradient of 0.183°C/m in the upper 300 m, where the average measured thermal conductivity is ~1.0 W/(m·K); this yields a conductive heat flow of ~180 mW/m² at Site 1173. Deeper than 300 m, thermal conductivities increase by 30%—50%, so the gradient should decrease proportionally, and in situ temperatures of ~110°C are estimated for the bottom of the hole‹ similar to the basement temperature estimated for Site 808. The heat flow value is somewhat higher than prior determinations of high heat flow near the site and greater than the predicted heat transfer for the 15-m.y. crustal age.

A high-resolution pore fluid concentration-depth profile shows that the pore fluid chemistry has been extensively modified from seawater by both microbially mediated reactions and by abiological, inorganic fluid-rock reactions. The chemical modifications from the microbially mediated reactions provide crucial independent information on the depth range, intensity, and nature of microbial activity in the deep subsurface. Each inorganically controlled dissolved species analyzed (i.e., Cl, Ca, Mg, SiO₂, K, and Na, shows a distinct to sharp discontinuity at 340 mbsf, which corresponds to the lithologic boundary between Units II and III. Furthermore, minima in Cl and Na concentrations and significant inflections in the Mg and Ca profiles occur at ~380—390 mbsf. These features suggest that this horizon may be hydrologically active. A broad ~350-m-thick low-Cl zone within Unit III, with ~9% dilution relative to seawater, requires a source of low-Cl fluid. The similarities between this low-Cl zone and that at ODP Site 808 are striking, except that at Site 808 the dilution relative to seawater is more than twice that observed at this site.

Total organic carbon values are low (average = 0.35 wt%) and decrease with depth (0.85 to 0.20 wt%). The C/N ratios indicate the presence of marine organic matter throughout the hole and show a slight increase in the lower ~200 m. The low sulfate and high methane concentrations in the upper section below the sulfate reduction zone are consistent with a bacterial origin. The increase in sulfate concentrations from ~400 to 700 mbsf coupled with the low concentrations of methane may indicate that sulfate is inhibiting production of hydrocarbons that were more abundant at Site 808. The presence of low concentrations of light hydrocarbons (ethane and propane) below 300 m to total depth may be due to some in situ thermal maturation of kerogen in the sediments. The low concentrations of methane at depth and the lack of evidence for any migration of hydrocarbons from above the facies transition (347.3 mbsf) support these conclusions. The microbes observed (~480 m) at temperatures above 90°C in the presence of elevated sulfate concentrations suggest that methanogenesis due to microbial activity is not completely inhibited, although at these temperatures, thermogenic hydrocarbons are likely being produced.

Samples taken for bacterial enumeration show that bacteria are present in all samples to 500 mbsf and thereafter are absent (the detection limit is 4.75 ± 10⁵). The population profile generally follows the average line obtained from other ODP sites for the upper 250 m of the hole with a rapid decrease in population size in the upper few meters as the sulfate was depleted. Between 43 and 80 mbsf, there is a significant (sevenfold) increase in bacterial numbers coincident with elevated methane concentrations. At 250 mbsf there is both a temperature boundary for bacteria (45°—50°C, the change from mesophilic to thermophilic populations) and significant differences in interstitial water (IW) chemistry, which complicates interpretation. Further changes occur in IW chemistry at a lithologic boundary at 343 mbsf. Between 250 and 460 mbsf, bacterial numbers are lower than average. Another microbiological temperature boundary (thermophilic to hyperthermophilic populations) occurs at 460 mbsf as temperatures exceed 80°C. One positive enumeration was made in this zone at 500 mbsf and ~85°C, where populations increase by a factor of 13. At this depth there were relatively high concentrations of organic carbon plus increasing concentrations of sulfate, methane, and hydrogen that could support a deep hyperthermophilic population of sulfate-reducing bacteria.

Tracer tests were successfully carried out on two APC cores and two extended core barrel (XCB) cores with both perfluorocarbon tracer (PFT) and fluorescent microspheres. PFT was detected in the center and midway between the center and the outside in some APC core sections; however, PFT was absent from the center of the XCB core sections. In contrast, microspheres were generally absent in samples taken midway between the center and the outside of the core in both APC cores and one of the XCB cores and were only present in the centers of some of the XCB core sections. These results suggest that intrusion of microspheres into the center of the cores was a result of postrecovery handling and not diffusion of drilling fluid during coring. This is the first time fluorescent microsphere tracers have been used during the collection of cores with the XCB.

Hole 1173A was logged with both the triple combination logging string (spectral gamma ray, dual-induction resistivity, lithodensity, and neutron porosity tools) and the Formation MicroScanner and dipole sonic imager (FMS-sonic) tools. The interval from 65 to 373 mbsf was logged in two passes, and high-quality compressional and shear travel time data and FMS images were acquired. During the second pass, a new low-frequency (<1 kHz) dipole source was used on the DSI and produced excellent shear waveforms despite the very low formation velocity. Logging results are generally consistent with the homogeneous hemipelagic core lithology, with few identifiable lithologic boundaries in the logged interval. Density is low from 97 to 336 mbsf, with a slight gradual decrease with depth, then sharply higher in the 358—440 mbsf interval. Compressional and shear wave velocities are nearly constant with depth to 225 mbsf, then increase with depth. Velocities decrease sharply in the short logged interval below 348 mbsf, corresponding to the Unit II/III boundary. Numerous ash layers and other sedimentologic and diagenetic features observed in the cores were well imaged by both FMS passes, which should permit high-resolution core-log integration.