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SITE SUMMARIES (continued)

Site 808

We drilled Hole 808I (Table T1) to obtain LWD data through the frontal thrust zone and décollement zone at the deformation front of the Nankai Trough and to install an ACORK long-term subseafloor hydrological monitoring experiment (Fig. F6). This hole complements cores recovered at Site 808 during Leg 131. Coring, logging, and monitoring here are intended to document the physical and chemical state of the Nankai accretionary prism and underthrust sediments through the frontal thrust zone, the décollement zone, and into oceanic basement.

Log Quality

The overall quality of the LWD logs recorded in Hole 808I is variable. We recorded at least one sample per 15 cm over 99% of the total section. Sections of enlarged borehole estimated by the differential caliper yield unreliable density data and associated derived porosity that is confirmed by comparison to core data. This problem is primarily associated with the depth intervals of 725–776 and 967–1057 mbsf, in which there was a long duration between drilling and recording of the logs due to wiper trips or poor hole conditions. Density and density-derived porosity should be used cautiously until more complete corrections and editing are completed postcruise. Although the LWD ISONIC tool worked well, the processing of the waveforms was not straightforward and postcruise processing is required to yield reliable sonic data.

Logging Units and Lithology

A combination of visual interpretation and multivariate statistical analysis defined four logging units and six subunits.

Logging Unit 1 (156–268 mbsf) is characterized by the overall lowest mean values of gamma ray, density, and photoelectric effect and overall highest mean values of resistivity and neutron porosity. These values coincide with very fine grained sandstone, siltstone, and clayey siltstone/silty claystone observed in the cores.

Logging Unit 2 (268–530 mbsf) shows a constant value range of gamma ray and neutron porosity and a decreasing resistivity log. A high variability in differential caliper log and a large number of values >1 in reflect poor borehole conditions.

Logging Unit 3 (530–620 mbsf) is marked by a significant increase of the mean values of gamma ray, density, and photoelectric effect logs.

Logging Unit 4 (620–1035 mbsf) is characterized by the overall highest mean values of gamma ray, photoelectric effect, and density and the lowest mean values of resistivity and neutron porosity. Generally, a positive correlation between gamma ray and photoelectric effect is observed.

A continuous increase in gamma ray and photoelectric effect, which reflects an increase in clay and carbonate content, is observed from logging Unit 1 to logging Unit 4. A positive correlation of resistivity with density defines logging Units 3 and 4.

Structural Geology

RAB tools imaged fracture populations and borehole breakouts throughout much of the borehole. We identified both resistive and conductive fractures, respectively interpreted as compactively deformed fractures and open fractures. Fractures are concentrated in discrete deformation zones that correlate with core analysis of Leg 131 Site 808: the frontal thrust zone (389–414 mbsf), a fractured interval (559–574 mbsf), and the décollement zone (~940–960 mbsf). Only relatively sparse deformation occurs between these zones. Fractures are steeply dipping (majority >30°) and strike predominantly east-northeast–west-southwest, close to perpendicular to the convergence vector (~310°). Bedding dips are predominantly low angle (<50°) but are difficult to identify in the highly deformed zones, therefore biasing this result. Bedding strike is more random than fracture orientation, but, where a preferred orientation is recorded, beds strike subparallel to fractures and perpendicular to the estimated convergence vector.

The frontal thrust zone (389–414 mbsf) represents the most highly deformed zone in Hole 808I and contains predominantly S-dipping fractures (antithetic to the seismically imaged main thrust fault) and a few north-dipping east-northeast–west-southwest striking fractures. The highly fractured interval at 559–574 mbsf contains similar fracture patterns to the frontal thrust zone. Both deformation zones are characterized by high-conductivity (open?) fractures within a zone of overall high resistivity.

Deformation close to the décollement is more subdued and is represented by a series of discrete fracture zones. The décollement in the RAB images (937–965 mbsf) is defined by a general increase in fracture density and variability in physical properties.

Borehole breakouts are recorded throughout Hole 808I and are particularly strongly developed within logging Unit 2 (270–530 mbsf), suggesting lithologic control on sediment strength and breakout formation. Breakouts indicate a northeast-southwest orientation for the minimum horizontal compressive stress (2), consistent with a northwest-southeast convergence vector (310°, parallel to 1). Breakout orientation deviates slightly from the dominant strike of fractures (east-northeast–west-southwest), but this deviation may be within error of measurements.

Physical Properties

The Hole 808I LWD density log shows a good fit to the core bulk density, slightly under-estimating core values in the upper 550 mbsf and slightly overestimating core values between 550 and 970 mbsf. Below 156 mbsf the LWD density log shows a steady increase from ~1.7 to ~1.95 g/cm3 at 389 mbsf. Between 390 and 415 mbsf density varies greatly, corresponding to the frontal thrust zone. Below the frontal thrust zone density decreases sharply to ~1.85 g/cm3. Below 530 mbsf it increases to ~2.1 g/cm3. Between 725 and 776 mbsf density drops sharply to ~1.75 g/cm3, corresponding to a period of borehole wiper trips. Density increases more rapidly from ~1.95 g/cm3 at 776 mbsf to 2.25 g/cm3 at 930 mbsf, decreasing steadily to ~2.15 g/cm3 before stepping down to 1.4 g/cm3 at 965 mbsf. This corresponds to the base of the décollement zone; below, density increases steadily from ~1.7 g/cm3 at 975 mbsf to ~2.0 g/cm3 at 1034.79 mbsf.

The downhole variation of LWD resistivity measurements shows identical trends in all five Hole 808I resistivity logs; all logging unit boundaries are clearly identified. Logging Unit 1 has an average resistivity of ~0.8–0.9 m. After a sharp decrease in resistivity from 1.3 to 0.5 m between 156 and 168 mbsf, the signal shows an increasing trend downward to the boundary between logging Units 1 and 2. Logging Unit 2 is characterized by an overall decreasing trend in resistivity from ~0.9 to ~0.6 m. However, this trend is sharply offset from 389 to 415 mbsf (logging Subunit 2b), where the resistivity values are ~0.3 m higher. This zone seems to correspond to the frontal thrust; the higher resistivity here may reflect compactive deformation in the frontal thrust zone. Logging Unit 3 shows a higher degree in variability of the resistivity signal. Resistivity exhibits less variation again in logging Unit 4, where it averages ~0.6 m. As observed in Hole 1173B, shallow resistivity is systematically higher than both medium and deep resistivity.

Logs and Seismic Reflection Data

Beneath the depth of the casing (~150 mbsf), correlations between the synthetic seismogram and seismic reflection data are only broadly consistent and the defined lithologic boundaries or units correlate at some depth intervals. The details of amplitude and waveform throughout most of the section do not match the seismic data. Amplitudes of the intervals between ~200 and ~400 mbsf, between ~750 and ~850 mbsf, and below ~925 mbsf are significantly higher in the synthetic seismogram than in the seismic data and are probably generated by numerous anomalously low velocity and density values, which are a product of poor hole conditions.

ACORK Installation

In Hole 808I we assembled a 964-m-long ACORK casing string incorporating two packers and six screens for long-term observations of pressures in three principal zones, as follows:

  1. The décollement and overlying section of the Lower Shikoku Basin (lithologic Unit IV). A screen was placed immediately above the casing shoe, with a packer immediately above the screen. The hole was opened with the intent of emplacing the screen just into the décollement, with the packer positioned in a competent zone immediately above the décollement. Three other screens were configured above the packer, to span the upper section of the Lower Shikoku formation to study the variation of physical properties and the propagation of any pressure signals away from the décollement.
  2. A fractured interval at 560–574 mbsf in the Upper Shikoku Basin formation, as identified in RAB logs (see "Structural Geology"). A single screen was intended to be deployed in this zone.
  3. The frontal thrust, centered at ~400 mbsf. A single screen was intended to be deployed in this zone.
Drilling conditions during installation of the ACORK steadily worsened, starting ~200 m above the intended depth. Despite all efforts, progress stopped 37 m short of the intended installation depth. This left the screen sections offset above the intended zones (Fig. F6), not an ideal installation but still viable in terms of scientific objectives. In addition, this left the ACORK head 42 m above seafloor, unable to support its own weight once we pulled out. Fortunately, when the ACORK head fell over, it landed on the seafloor such that all critical components remained in good condition. This includes the critical hydraulic umbilical, data logger, and the underwater-mateable connector, which remains easily accessible by a remotely operated vehicle (ROV) or by submersible for data download.

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