The major feature at Site 1173 is the Shikoku Basin section that extends from ~100 to 698 mbsf and is divided into an upper and lower facies (lithologic Units II and III). The two units are differentiated at least in part by a diagenetic front rather than purely depositional features. The log response is dominated by the relatively homogenous properties of this predominantly hemipelagic silty clay section.
LWD data collection at Site 1173 confirmed Hole 1173A wireline and coring results and extended them to greater depths and higher resolution, respectively. Comparison of LWD logging results with Leg 190 core-based physical properties and wireline data (Fig. F1) generally shows excellent agreement among the various measured parameters. The log units defined on the basis of visual inspection and statistical analysis coincide well for the most part with the lithologically defined units. Log Unit 1, the outer trench wedge, has a lower boundary that differs by ~20 m in depth from the equivalent boundary of lithologic Unit I. However, this boundary is gradational in the cores and defined on the basis of the deepest occurrence of clearly identified but volumetrically minor silty turbidites. Hence the log unit definition probably only differs in the placement of a sharp line through a gradational boundary. The Unit 2/3 boundary coincides within a few meters; it is chosen at 340 mbsf in the log units and 343.8 mbsf in the lithostratigraphic units on the basis of slightly different selection criteria (trough of deepest excursion vs. deepest unambiguous ash with unaltered glass). The Unit 4 boundary also coincides well (688 and 698 mbsf in cores and logs, respectively), with the difference being attributed to poor core recovery.
Structural features identified in the interpretation of the RAB images are consistent with those identified in cores in their nature, depth distribution, and orientation. Intervals of subhorizontal vs. significantly inclined bedding correspond between both data sets. Both also indicate steeply dipping fractures or minor faults, probably associated with compaction and/or the normal faulting imaged in the seismic reflection data. RAB imaging has certain trade-offs in comparison to FMS images collected during Leg 190: whereas FMS data have much higher spatial resolution, RAB images provide full 360° coverage of the borehole wall. Ash layers identified in Hole 1173A FMS images can be correlated to features imaged with the RAB tool, providing a sense of the relative response and resolution of the two instruments (Fig. F42). Maximum resolution within RAB images is thought to be ~3 cm, whereas resolution for FMS images is ~0.5 cm (Moore, Taira, Klaus, et al., 2001). Thinner ash layers can be clearly identified in the FMS images (e.g., at 336 and 338-339 mbsf) but are not easily distinguished in RAB images. Further correlation between RAB images, FMS images, and core descriptions indicates a discrepancy between visible ash thickness and extent of resistivity signature. For example, the ash layer that defines the base of Unit II is 3 cm thick in the recovered core but produces a significant resistivity anomaly over tens of centimeters in both RAB and FMS images (Fig. F42). The broader resistivity signature may in part result from reworking caused by bioturbation. Larger scale variations in resistivity (5-10 m scale) are easily correlated between the RAB and FMS images and can often be correlated with other physical properties. The high-resistivity interval at 333-338 mbsf (RAB and FMS; Fig. F42) corresponds to a peak in density (Fig. F29). The FMS image indicates a high frequency of thin ash layers, whereas the more homogeneous resistivity high in the RAB image suggests reworking of high-resistivity beds (ash layers) over this interval.
Here, we highlight the interpretation of several zones of special interest identified at Site 1173. These include the apparently diagenetically controlled boundary between log Units 2 and 3 and lithostratigraphic Units II and III, respectively; the reverse porosity vs. depth trend within log Unit 2; and the projected stratigraphic equivalent of the décollement zone at Sites 1174 and 808.
The boundary between the upper and lower Shikoku Basin facies was defined in Leg 190 Hole 1173A at the deepest occurrence of a fresh volcanic ash bed containing glass shards seen in smear slides, although this was chosen within a gradational change from ash to siliceous claystone. This boundary occurs at a mineral phase transition from cristobalite to quartz and where the pore water SiO2 content drops sharply (Moore, Taira, Klaus, et al., 2001). An added complexity is that clay content also increases at this boundary.
This transition also strongly affects the sediment physical properties. In core measurements, density increases, porosity decreases, and formation factor increases sharply across the boundary but P-wave velocity varies only weakly (Shipboard Scientific Party, 2001). The LWD data exhibit corresponding trends across the depth-equivalent log Unit 2/3 boundary. Resistivity and density exhibit a baseline shift to higher values below the boundary, as well as two excursions to similarly high values over an ~20-m-thick transitional zone between 320 and 344 mbsf. Porosity mirrors the density trend. The gamma ray log shows a pronounced increase across the transition, matching the clay mineral content (Fig. F1), and fluctuations in the transition zone are also observed in this log.
Taken together, these data are consistent with the previous interpretation of a largely diagenetically controlled boundary, with the cristobalite to quartz transition dominating the changes in physical properties (Moore, Taira, Klaus, et al., 2001). The sudden increase in density and resistivity associated with a porosity loss may indicate a loss of matrix strength due to cement dissolution (Isaacs et al., 1983). This alteration is variably developed over the ~20-m-thick transition interval, perhaps because of variations in silica content of the sediments over this range. The coincidental downhole transition to slightly higher clay content at this boundary may represent either a change in clay deposition or clay diagenesis, presumably also because of ash alteration. The temperature-controlled nature of this diagenetic front suggests that identification of the equivalent lithologic Unit II/III boundary at more landward sites must be approached with caution.
Above the lithologic Unit II/III boundary, the log trends deviate sharply from typical profiles of compacted deepwater sediments. Density and porosity exhibit normal compaction trends to ~60-80 mbsf but remain nearly constant below this depth to ~320 mbsf, actually reversing direction from the expected compaction trend (i.e., density decreases and porosity increases slightly). Resistivity wireline and LWD logs trend to lower values through this interval, paralleling the density. Because resistivity is primarily sensitive to saline pore fluid, we attribute this trend to the downhole increase in porosity over the same interval. As was interpreted on Leg 190 (Moore, Taira, Klaus, et al., 2001), these anomalies taken together are tentatively attributed to some combination of (1) the gradual downward transition from more terrigenous, coarser grained sediments of the trench-basin facies (Subunit 1b) to the more clay-rich sediments of the upper Shikoku Basin facies and (2) subtle cementation effects of silica diagenesis.
During Leg 190, a correlation was made via core magnetic susceptibility data of the known positions of the décollement zone at Sites 808 and 1174 to the equivalent interval at Site 1173; this interval can also be identified in the magnetic susceptibility data of reference Sites 1173 and 1177. The resulting projected equivalent of the Site 1174 décollement zone (Shipboard Scientific Party, 2001) is shown in Figures F1 and F43. Whereas there are excursions in several of the logs over this interval, it does not substantially stand out from the over- and underlying formation in any of the measured logs. There is an abrupt resistivity excursion to lower values at ~384 mbsf and back to higher values at ~412 mbsf, although the amplitude does not stand out strongly from other variations above and below. The same excursion can be weakly identified in the density and computed porosity logs. Overall, the specific interval projected as the stratigraphic equivalent of the décollement zone does not exhibit obviously unusual properties relative to the surrounding sediments.