Grain density of four samples (185-1149A-18H-2, 65-67 cm; 18H-4, 75-77 cm; 19X-1, 40-42 cm; and 20X-1, 88-90 cm) are mostly the same, with values of 2.60, 2.60, 2.61, and 2.59 g/cm3, respectively. The average grain density of the four samples is 2.60 g/cm3. The porosity and void ratio calculated by this average grain density and the physical properties (Shipboard Scientific Party, 2000) are shown in Figure F3 and Table T1.
Although the magnetic susceptibility of the samples from Hole 1149A is a constant ~0.001 SI throughout Units I and II, it changes from 0.001 to 0.002 SI at three stratigraphic horizons: 120.76 mbsf (Sample 185-1149A-14H-2, 106-108 cm) in Subunit IIA, ~135 mbsf (Sample 15H-2, 116-118 cm; 130.36 mbsf, and Sample 15H-7, 36-38 cm; 137.06 mbsf) in Subunit IIB, and 170.68 mbsf (Sample 20X-1, 88-90 cm) in Subunit IIB (Fig. F3; Table T2). This change of magnetic susceptibility at these three horizons reflects the variation in content, kind, and amount of the magnetic minerals.
The magnetic minerals contributing to the magnetic susceptibility and their anisotropy are assumed to be ferrimagnetic minerals of detritic origin because of the following two reasons. The first is that the ratio of Khf/Klf, which reflects contribution percentage of paramagnetic minerals at low magnetic field, is <10% (Fig. F4A), suggesting that the AMS is >90% to the ferrimagnetic minerals. The second comes from a result of SEM-EDS analyses where ferrimagnetic minerals were observed as detritic shapes of ~10 µm diameter by SEM (Fig. F4B, F4C) and the chemical compositions of the minerals are predominantly O and Fe with small amounts of Ti by EDS (Fig. F4B, F4C).
The P´ value <1.02 of Unit I indicates the low anisotropy degree of the magnetic susceptibility ellipsoid (Fig. F3). Although the individual ferrimagnetic mineral grains in the sediments have their own shape and AMS, the results of the AMS measurements in Unit I show low degrees of anisotropy. This means that the ferrimagnetic mineral grains probably arrange in a low degree of preferred horizontal orientation, which is mostly random in the sediment.
On the other hand, Unit II is characterized by higher P´ and F values with steep K´ inclination, reflecting that the ferrimagnetic mineral grains are arranged into preferred horizontal orientation (Fig. F3). The degree of the horizontal arrangement becomes more distinct from ~118 mbsf, which is the boundary between Unit I and Subunit IIA. The P´ and F values in Unit II become maximum at 170.68 mbsf (Sample 185-1149A-20X-1, 88-90 cm) (Fig. F3).
The AMS of samples from Hole 1149B are characterized by low P´ and F values, high L value, and low angle of Kmin inclination (Fig. F3; Table T2). These are quite different than those from the same depths in mbsf from Hole 1149A. As the primary magnetic fabric is formed by sedimentation processes usually yielding ellipsoids with F > L and Kmin inclination >70° (Tarling and Hrouda, 1993), the results from Hole 1149B suggest that these sediments have suffered some deformation through secondary effects such as bioturbation, tectonic deformation, or drilling disturbance.
The grain-size distribution of each sample is shown in Figure F5. The patterns in Unit I are characterized by a peak at ~10 µm diameter and a wider range in size from ~0.1 to 100 µm than that in Unit II. Some of the patterns are polymodal, having more than two peaks at ~10 µm and 25-50 µm diameter, owing to the fine particles of clay minerals and the coarse particles of volcanic glass fragments and siliceous biogenic tests.
On the other hand, the size range of Unit II is finer than that of Unit I. The grain-size distribution patterns of Subunit IIA are characterized by better sorting, with the range in size being from 0.1 to 30 µm diameter and a sharp peak of ~10 µm. Those of Subunit IIB are characterized by further finer and acuter size range from 0.1 to 25 µm and a peak of ~5 µm diameter.
The above-mentioned tendency of fining and better sorting downward may reflect a more pelagic environment in the older section.
Results of the XRD analysis and clay composition are shown in Figures F6 and F7, respectively. The samples in Unit I are predominantly composed of illite, whereas those in Unit II are mainly composed of smectite and illite. Kaolinite decreases downward through Unit I to Unit II.
In the sample at 176.9 mbsf (Sample 185-1149B-3R-5, 60-62 cm), clinoptilolite (9 Å) was detected. The relative peak area of clinoptilolite (9 Å) to illite peak increases with burial depth, suggesting better crystallization in the deeper section. Opal-CT (4.04 Å) was detected in the sample at 179.51 mbsf (Sample 185-1149A-21X-1, 41-43 cm) close to the bottom of Unit II. According to correlation with results from DSDP Leg 20, Site 196, this horizon may be Cretaceous in age and corresponds to the transition zone from opal-A to opal-CT (Shipboard Scientific Party, 2000).
The microfabrics of Unit I are characterized by random arrangement of sedimentary particles (Fig. F8).
The coarse-grained particles, mainly volcanic glass fragments, are concentrated in many pipes, which are ~100 µm diameter and are considered to be burrows (Fig. F8C, F8D). The coarse-grained particles show preferred orientation along the extension direction of the pipe. As the extension direction of pipes are random in the sediment, the total orientation of the coarse-grained particles is also random.
Fine-grained particles, clay platelets, biogenic tests, and single grains are mechanically aggregated in ovoid forms of ~10-100 µm diameter as peds (Fig. F8A, F8B). Some of the biogenic tests and single grains are pressed mechanically into the ped. In the ped, some of the clay platelets are flocculated by low- to high-angle EF contact, although most of them are aggregated by low-angle EF and FF contacts. The peds are in contact with each other either directly or through bridges made of coarse-grained particles in low to high angles (Fig. F8A, F8B). The macropores at ~10-50 µm diameter could be seen between peds (Fig. F8A, F8B). The size of the macropores in the shallow depth (Sample 185-1149A-1H-2, 63-65 cm; 2.13 mbsf) is slightly larger than that in the deeper depth (Sample 7H-1, 79-81 cm; 52.49 mbsf) (Fig. F8A, F8B).
Radiolarian tests are well preserved in all microfabrics throughout Unit I, but there are different types between the inside and outside of the burrow. Inside the burrow, many radiolarian tests are filled with fine sediments (Figs. F2B, F8C), whereas those outside the burrows are vacant (Fig. F8C). This suggests that the radiolarian tests filled their own inside pore spaces with fine sediments as a result of activities of benthic animals during burrowing.
The microfabrics are characterized by dimensional preferred horizontal orientation of coarse-grained particles of ~10-100 µm length (Figs. F9C, F10B, F10C, F10D), although the compositions of coarse-grained particles are different in Subunits IIA and IIB. In Subunit IIA, such coarse-grained particles, mainly quartz and feldspar, form thin layers of ~0.5-1.0 mm thickness at sporadic depths (Fig. F9D). These laminations may result from episodic pulses of low density, low-velocity turbidity currents, and/or of volcanic ash falling. High P´ and F values in Subunit IIA probably result from these laminations. On the other hand, the coarse-grained particles in Subunit IIB are mainly composed of fragments of biogenic tests and fecal pellets showing preferred horizontal orientation (Fig. F10B, F10D) formed in association with mechanical compaction by overburden pressure.
Although the grains of preferred horizontal orientation obtained from AMS analysis in Subunits IIA and IIB are consistent with the orientation of the coarse-grained particles, the fine-grained particles show random fabric under high-magnification SEM (Figs. F9A, F9B, F10A, F10C). At 125.30 mbsf (Sample 185-1149A-14H-5, 110-112 cm) and 125.30 mbsf (Sample 14H-5, 110-112 cm) of Subunit IIA, clay platelets (mainly smectite and illite) contact at low angle in EF and FF fashion. Those clay platelets are aggregated to small spheres of ~3-30 µm diameter as peds (Fig. F9A, F9B). The small peds are connected by long chains of clay platelets at high-angle EF contact (Fig. F9C). Many macropores of ~5-10 µm diameter are present between the small peds behind the long connectors so that the microfabrics in Subunit IIA are porous.
On the other hand, the boundary of the small peds is unclear at 150.47 mbsf (Sample 185-1149A-17H-3, 77-79 cm), 164.8 mbsf (Sample 19X-1, 40-42 cm), and 170.68 mbsf (Sample 20X-1, 88-90 cm) of Subunit IIB (Fig. F10A). Elongated small peds of wedge shape are aligned along the bedding plane. The microfabrics are in direct contact with each other (Fig. F10A). Therefore, some of the sediments in Subunit IIB are not porous. In the small ped, several clay platelets contact in FF to form small units as paper-stacking structure at a smaller scale (10 µm) (Fig. F10A, F10C). The orientation of small units is random in the ped a mosaic fabric in larger scale (100 m) (Fig. F10B, F10C).
Palynomorphs are seen at 170.68 mbsf (Sample 185-1149A-20X-1, 88-90 cm) and 179.51 mbsf (Sample 21X-1, 41-43 cm), the bottom of Subunit IIB (Fig. F11). There is a growth ring as an internal microstructure (Figs. F10B, F11A), and the surface microstructure is composed of many micropores (Fig. F11D). Some of the palynomorphs are in a closed ellipsoidal form with a long axis parallel to the bedding plane (Fig. F11C). Because their original shapes are commonly spherical (O'Brien and Slatt, 1990), these ellipsoidal forms indicate the minimum amount of compaction that took place during burial. This further supports the assumption that this initially flocculated sediment has suffered vertical flattening and transformed to shale as explained by O'Brien and Slatt (1990).
The radiolarian tests in Subunit IIB are quite different in the degree of preservation from those in Unit I and Subunit IIA, although the latter are not well preserved. Those in Subunit IIB are not only poorly preserved but are also recrystallized by diagenesis, as marked by opal-CT fillings detected by XRD analysis (Fig. F10D). Furthermore, under the polarized microscope, the bright area surrounding the radiolarian tests can be recognized as pressure solution (Fig. F10D). The degree of brightness and width of the pressure solution increase with burial depth. Platy microcrystals are seen in the pressure solution under high magnification (1000x). The shape, optical properties, and XRD analysis of the platy grains indicate that they are clinoptilolite.
These platy microcrystals (clinoptilolite) of 10-50 µm in length are also observed in veins at 164.85 mbsf (Sample 185-1149A-19X-1, 40-42 cm), 170.68 mbsf (Sample 20X-1, 88-90 cm), and 179.51 mbsf (Sample 21X-1, 41-43 cm), Subunit IIB (Fig. F12A, F12B, F12C). The veins are recognized in two directions as a conjugate set that inclines ~45° against the bedding plane (Fig. F12A). Most of the veins cut laminations with a normal sense of displacement (Fig. F12A), and they are further cut by other larger normal faults (Fig. F12A).
Based on the above results, a schematic model of the microfabrics in Units I and II is illustrated in Figure F13.