- Fh
= 1.87
-1.58
At Site 1177, laboratory measurements were made to provide a downhole profile of physical properties at a reference site seaward of the accretionary complex. Measurements from tectonically undeformed sediments at the reference site yield information about the initial state of sediments prior to deformation.
With the exception of short (<50 cm) or broken sections, all cores were initially passed through the multisensor track (MST) before being split. Gamma-ray attenuation (GRA) and magnetic susceptibility measurements were taken at 4-cm intervals with 2-s acquisition times for all cores. Natural gamma ray (NGR) was counted every 20 cm for 20-s intervals.
Moisture and density samples were selected from undisturbed core at a frequency of at least one per section. Measurements of dry volume and wet and dry mass were uploaded to the ODP (Janus) database and were used to calculate water content, bulk density, grain density, porosity, void ratio, and dry bulk density. P-wave velocities were measured on discrete sample cubes at a frequency of two to three per core when core conditions permitted. Electrical conductivity measurements were taken on the same cubes used for velocity measurements. Raw data and calculated physical properties are available from the Janus database for all MST, moisture and density, velocity, thermal conductivity, and shear strength measurements (see the "Related Leg Data" contents list). Because the electrical conductivity data are not currently available from the database, they are included in Table T19.
Sediment bulk density was determined by both the GRA method on unsplit cores and the mass/volume method ("index properties") on discrete samples (see "Physical Properties" in the "Explanatory Notes" chapter). The GRA density data and the bulk densities determined by the mass/volume method are in good agreement for most of the cored interval (Fig. F23A, F23B), despite the smaller diameter and the biscuited nature of RCB cores. In general, the GRA densities are 0.1-0.2 g/cm3 lower than the bulk densities. However, between 400 and 480 mbsf, the discrete densities are generally at the low end of the range of GRA measurements. The reason for this difference is unclear. The GRA densities for a ~30-m-thick low-density interval at 765-795 mbsf within Unit IV are consistently ~0.3 g/cm3 lower than those determined by the mass/volume method. The GRA densities exhibit significant scatter, especially within lithostratigraphic Units III and IV (Shikoku turbidite facies and volcaniclastic facies).
Grain densities determined from dry mass and volume measurements increase slightly from 2.58-2.69 g/cm3 at 300 mbsf to 2.67-2.75 g/cm3 at the base of Unit I (upper Shikoku Basin facies; 402 mbsf) (Fig. F23C). Grain densities remain nearly constant at ~2.68-2.76 g/cm3 throughout Units II (lower Shikoku Basin facies) and III (Shikoku turbidite facies). Within Unit IV, grain densities are similar to those within Unit III but decrease to values as low as 2.43 g/cm3 in a ~30-m-thick interval between 765 and 795 mbsf that exhibits low bulk density and high porosity values (Fig. F23A, F23D).
Porosities within lithostratigraphic Unit I (upper Shikoku Basin facies) remain nearly constant with depth and exhibit little scatter, ranging from 60% to 65%. These values are surprisingly high for a burial depth of 300 to 400 m. Porosities within Unit II (lower Shikoku Basin facies) decrease rapidly with depth from 60%-65% at 402 mbsf to 46%-54% by ~450 mbsf. Unit III porosities decrease slightly with depth, from 45%-53% at ~450 mbsf to 40%-45% at 748 mbsf. Between 475 and 510 mbsf, porosities are consistently 38%-40%, significantly lower than those directly above and below (Fig. F23D). Porosities within Unit III (Shikoku turbidite facies) exhibit more scatter than within Unit I, probably as a result of lithologic variations within and between turbidite sequences. Porosities within Unit IV show considerable scatter, ranging between ~36% and 53%, and exhibit no clear changes with depth. An excursion to higher porosity (49%-59%) between 765 and 792 mbsf coincides with a zone of low grain density and low GRA density.
Thermal conductivity was measured on split cores using the half-space method (Fig. F24). In Unit I, thermal conductivity values are relatively constant and average 1.1 W/(mˇ°C). Thermal conductivity increases within Unit II, corresponding to a decrease in porosity. In the portion of Unit III above 600 mbsf, thermal conductivity varies between 1.4 and 1.6 W/(mˇ°C). Values are scattered between 600 and 748 mbsf. Values <1.0 W/(mˇ°C) may reflect low-quality measurements caused by fracturing of core pieces. Within Unit IV, a drop in thermal conductivity occurs between 760 and 810 mbsf, corresponding to an interval of elevated porosity (Fig. F23D).
Measurements in all three directions were performed on sample cubes cut from cores. Acoustic velocity increase with depth follows a nearly constant gradient of 0.84 m/s per meter for the horizontal components (x- and y-axes) and 0.57 m/s per meter for the vertical component (z-axis) (Fig. F25B). The sharp decrease in porosity at the transition between lithostratigraphic Units I and II is not reflected in the acoustic velocity nor is the more progressive decrease in porosity within lithostratigraphic Unit II. The same behavior has been noted at the transition from the upper to lower Shikoku Basin facies at Site 1173.
A minor decrease in velocity of ~30 m/s at ~420 mbsf stratigraphically correlates with a somewhat more pronounced kink in the acoustic velocity profile at 390 mbsf at Site 1173 and with the décollement level as defined at Site 1174 (see "Paleomagnetism"). The zone of high porosity observed within lithostratigraphic Unit IV (between 765 and 790 mbsf) also displays an anomalous velocity profile. The velocity vs. porosity graph (Fig. F25B) shows that the velocity at a given porosity is generally lower at Site 1177 than at the eastern sites. This may be related to the lower heat flow and less advanced diagenesis at Site 1177 (Kinoshita and Yamano, 1986) (see "Lithostratigraphy"). The upper Shikoku Basin facies (lithostratigraphic Unit I) plots above the reference curve but appears less anomalous than at Site 1173 (see Fig. F38 in the "Site 1173" chapter); measurements plotting far away from the reference curve generally correspond to isolated samples. We note that a piece of carbonate-cemented mudstone yielded acoustic velocities of >4500 m/s in all directions and was not plotted on the graphs.
Velocity anisotropy is near 0% in Unit I but increases progressively with compaction in lithostratigraphic Units II, III, and IV to reach 6%-12% near the oceanic basement. The zone of anomalously high porosity between 765 and 790 mbsf also appears as a zone of reduced anisotropy (Fig. F25C).
Electrical conductivity was measured on the same sample cubes as the P-wave measurements with a 30-kHz two-electrode system. Electrical conductivity and formation factor (see "Physical Properties" in the "Explanatory Notes" chapter) measured on the sample cubes are given in Table T19. The formation factor increases between 409 and 415 mbsf, from ~3.5 above to 4-5 below for measurements along the horizontal direction (x- and y-axes) and from 3.5-4 above to 5-6 below for measurements along the vertical direction (z-axis). This change in properties is also reflected in the conductivity anisotropy (Fig. F26B). Vertical plane anisotropy first decreases from 10%-15% to 0%-1% between 300 and 409 mbsf but then increases to >15% at 415 mbsf. This abrupt transition occurs within lithostratigraphic Unit II. At Site 1177, increases in bulk density, formation factor, and electrical conductivity anisotropy occur across the transition from the upper to lower Shikoku Basin facies. This transition also occurs at Site 1173 but apparently at a younger stratigraphic age (3.1-3.5 Ma) than at Site 1177 (4-5 Ma; see "Paleomagnetism"). Below 415 mbsf, the horizontal components increase slowly, reaching only 5-6 at the base of lithostratigraphic Unit III, whereas the vertical component increases steadily, reaching 10-16 at the base of lithostratigraphic Unit III. This leads to a very high anisotropy (60%-100%) in the lower part of lithostratigraphic Unit III, higher than at any depth at the other sites. The zone of anomalously high porosity and low P-wave velocity between 765 and 790 mbsf in lithostratigraphic Unit IV coincides with a zone of lower formation factor and anisotropy.
Formation factor measured along the z-axis and average formation factor measured along the x-and y-axes are plotted vs. porosity in Figure F26C and may be compared with results obtained at Site 1173 on cube samples (Fig. F26D). At Site 1173, the vertical and the horizontal formation factors were fitted independently using the generalized Archie's law. The best-fitting laws are
-1.58
for the horizontal formation factor and
-2.17
for the vertical formation
factor, where
is the porosity.
At Site 1177, data are compared with the same empirical relationships and plotted as reference curves (Fig. F26C). The horizontal component generally plots below the corresponding reference curve. The vertical component displays a high scatter and, for porosities of <50%, generally plots above the reference curve. Consequently, anisotropy at a given porosity is generally higher at Site 1177 than at Site 1173. The difference between the electrical conductivity-porosity relationships at these two sites may reflect differences in clay composition (see "Lithostratigraphy").
Volumetric magnetic susceptibility was measured on unsplit cores on the MST (Fig. F27). The uncorrected values of magnetic susceptibility from the Janus database were used. The magnetic susceptibility data show an increase from 20-60 × 10-5 SI at 300 mbsf to 80-150 × 10-5 SI at 450 mbsf. From below 450 mbsf to basement, the values are relatively low (0-50 × 10-5 SI) with several peaks as high as 200 × 10-5 SI. From 680 to 760 mbsf, there is a continuous increase in magnetic susceptibility values from 10-50 to 30-150 × 10-5 SI.
NGR results are presented in counts per second (cps) (Fig. F28). The background scatter, produced by Compton scattering, photoelectric absorption, and pair production, was measured at the beginning (6.39 cps) and subtracted from the measured gamma-ray values. In general, NGR counts are low and are consequently likely to be affected by the short counting interval and by porosity variations. The scatter of NGR data is high throughout the cored interval, with values between 10 and 60 cps. No overall downhole trend of NGR data is observed. NGR values show a slight increase from 15-40 cps at 400 mbsf to 22-50 cps at 500 mbsf. The interval between 500 and 550 mbsf is characterized by low gamma-ray values (10-40 cps). NGR values range between 10 and 60 cps below 550 mbsf, and the scatter of data increases below 600 mbsf.
Variations in physical properties at Site 1177 correlate well with lithostratigraphic units. Units I and II are both characterized by low scatter in porosity. Unit I maintains nearly constant porosity of 60%-65%. Within Unit II (402 mbsf), porosities decrease rapidly with depth, reaching 45%-53% by 450 mbsf. Unit III is characterized by a gradual decrease in porosity with depth and by increased scatter that may be caused by lithologic variations in this turbidite-rich sequence. An excursion to lower porosity values (~40%) occurs within Unit III at 475-510 mbsf and may represent a sandy layer. Unit IV exhibits significant scatter and shows no clear trend with depth. A major excursion to high porosity (~8%-15% higher than in surrounding sediments) within Unit IV occurs in a 30-m-thick zone between 765 and 795 mbsf. This zone is also characterized by low vertical P-wave velocities and formation factors.
The nearly constant and relatively high porosities within Unit I are similar but 5%-8% lower than those observed in the analogous sequence (upper Shikoku Basin facies) at Site 1173. The decrease in porosity with depth in Unit II is also similar to the pattern observed within the lower Shikoku Basin facies at Site 1173.
