All cores from Holes 1204A and 1204B were run through the multisensor track (MST). For Core 197-1204A-1R through Section 6R-4 and Sections 197-1204B-1R-1 and 1R-2, measurements were made of magnetic susceptibility, gamma ray attenuation (GRA) bulk density, and natural gamma radiation (NGR) on unsplit core sections. For the remaining cores, all of which contained basement material, only NGR measurements were made on the MST. Compressional wave velocities were measured on split cores in the transverse x-direction for sediment cores (Cores 197-1204A-1R through 6R, and Sections 197-1204B-1R-1 and 1R-2) and in the x- and z-directions for discrete basement samples. Index properties, including bulk density, water content, porosity, and grain density, were determined on discrete samples at a frequency of at least one per core. Thermal conductivity was determined for basement samples also at a frequency of at least one per core.
Volume-normalized magnetic susceptibility was determined on Cores 197-1204A-1R through 6R and Sections 197-1204B-1R-1 and 1R-2 at 5-cm intervals (Table T11; Figs. F54B, F55B). In the upper part of Hole 1204A, values generally range between ~10 x 10-6 and 150 x 10-6 SI. However, a slight increase in susceptibility can be seen at ~776 mbsf, close to the boundary between sedimentary Units I and II. Magnetic susceptibilities are somewhat elevated in the upper parts of Unit II relative to the other sedimentary units, ranging up to 1500 x 10-6 SI; this may be due to increased amounts of volcanic and Fe-rich material (see "Lithostratigraphy"). Values decrease below ~790 mbsf and then remain relatively constant to the base of Unit II and throughout Unit III. Magnetic susceptibility values in Subunit IVA appear to increase slightly with depth. The boundary between Subunits IVA and IVB is marked by a decrease in magnetic susceptibility.
For Hole 1204B, magnetic susceptibility ranges from ~68 x 10-6 to 850 x 10-6 SI (mean = 357 x 10-6 SI). In the upper 0.5 m of the core, values are <200 x 10-6 SI. An increase occurs at 811.0 mbsf, corresponding to the boundary between sedimentary Subunits IVA and IVB (Fig. F55B); this appears to be the reverse of the trend seen in the Hole 1204A data. From 811.0 to 811.9 mbsf, values remain relatively constant at ~420 x 10-6 SI. The interval from 811.9 to ~813.0 mbsf is marked by widely varying magnetic susceptibilities, ranging between 68 x 10-6 and 850 x 10-6 SI; this corresponds to a large variation in lithology at this depth. Below 813.0 mbsf, magnetic susceptibility again remains relatively constant to the base of the sediments.
Bulk density was measured by the GRA densitometer every 5 cm on whole sections of Cores 197-1204A-1R through 6R and Sections 197-1204B-1R-1 and 1R-2 (Table T12). Downhole in Hole 1204A, GRA bulk densities remain approximately constant at ~2 g/cm3 until ~795 mbsf, where a rather abrupt density increase to ~2.5 g/cm3 was measured (Fig. F54A). This is close to the boundary between sedimentary Units II and III. Density then remains high in Unit III until ~803 mbsf, where a sharp decrease in bulk density to ~2 g/cm3 was noted. The boundary between sedimentary Units III and IV, at ~811 mbsf, is marked by a further small decrease in bulk density. Bulk densities in Unit IV have a mean value of ~1.65 g/cm3 and appear to increase slightly with increasing depth.
Only two full sediment core sections were recovered from Hole 1204B; in these sections GRA bulk density generally ranges from 1.08 to 2.15 g/cm3 (mean = 1.70 g/cm3) (Fig. F55A). In sedimentary Subunit IVA, density shows an overall increase with depth. In Subunit IVB, density varies little with depth until ~812 mbsf. Below this depth a wider variation in GRA bulk densities can be seen.
A number of sample points from Holes 1204A and 1204B show very low GRA bulk density values (<<1 g/cm3). These density values are probably artifacts produced by drilling disturbance.
NGR was measured every 10 cm on both unsplit sediment and basalt cores from Holes 1204A and 1204B (Table T13). Total counts are reported here because the corrected counts (which are less by ~16 counts per second [cps]) include negative values, which are physically unreasonable. In the sediment section of Hole 1204A, gamma ray values are fairly constant between 760 and 810 mbsf (Figs. F54C, F56A), with most values between ~12 and 22 cps. However, higher counts were measured below 810 mbsf, in the lowest few meters of sediment above the basement. In Unit IV, NGR measurements mostly range between 20 and 30 cps and show an overall increase with depth. A particularly large peak of ~40 cps is present at the boundary between sedimentary Units III and IV at 810.7 mbsf.
In the sedimentary section of Hole 1204B (Figs. F55C, F56B), which consists of Unit IV only, total NGR values mostly range between 20 and 32 cps (mean = 27 cps). Subunit IVA is characterized by a total NGR value of ~20 cps; the boundary between this unit and Subunit IVB is marked by an increase in NGR.
For basement rocks, NGR values show an overall decrease with depth to the base of the hole in Hole 1204A (Fig. F56A) and to ~900 mbsf in Hole 1204B (Fig. F56B). This may correspond to an increasing degree of reduction downhole in this zone (see "Alteration and Weathering"). The lowest NGR measurements in the basement of Hole 1204B are observed at the center of Subunit 2b (872-918 mbsf), an aphyric diabase (see "Physical Volcanology and Igneous Petrology"). From 900 to ~930 mbsf in Hole 1204B, the mean NGR value is somewhat higher, at ~22 cps. At depths >930 mbsf, increased natural gamma ray counts are observed in Subunit 2d, a hyaloclastite lapilli breccia, and the sediments of Unit 4, with values ranging up to 37 cps.
Whole-round core images were taken of cylindrical pieces from both the Hole 1204A and 1204B cores, representing 25% and 70% of the recovered material, respectively. Because of time restrictions at Site 1204, downhole logging measurements were not made; hence, correlation of core image and logging data is not possible at this site. However, core images were still acquired to provide a visual record of the recovered material.
Thermal conductivity was measured at a frequency of one to two measurements per core for basement sections from Holes 1204A and 1204B (Table T14). Only four measurements were made in Hole 1204A, due to the small amount of basement material recovered. Thermal conductivity values for these samples range from 1.61 to 1.91 W/(m·K), and values appear to decrease slightly with increasing depth.
In the volcanic basement of Hole 1204B, thermal conductivity generally ranges between 1.41 and 1.78 W/(m·K) (mean = ~1.60 W/[m·K]). There appears to be no systematic variation with depth (Fig. F57). Two samples have lower thermal conductivity values, 0.83 and 1.29 W/(m·K); these represent a hyaloclastite breccia and calcareous vitric sandstone, respectively.
Index properties were determined at a frequency of one sample per core for discrete samples from Holes 1204A and 1204B. Values of wet mass, dry mass, and dry volume of discrete samples were measured and used to calculate moisture content, bulk density, grain density, and porosity (Table T15; Figs. F58, F59).
In the sedimentary units from Hole 1204A (Cores 197-1204A-1R through 6R), bulk density remains approximately constant at 1.9 g/cm3 between 762 and 811 mbsf. Trends in this upper section of Hole 1204A show some limited correlation with the GRA density measured on the MST (Fig. F58A, F58B). However, the section from 795 to 811 mbsf with higher GRA densities is not observed in the index properties data; this may be simply the result of the lower sampling frequency represented by the discrete samples. Below 811 mbsf in Unit IV, bulk densities from Holes 1204A and 1204B range between 1.55 and 1.94 g/cm3 (apart from one outlier), corresponding to lower GRA densities in the same depth interval.
Grain density in sediment from Holes 1204A and 1204B varies very little downhole, with values of ~2.7-2.8 g/cm3. An exception is seen in Section 197-1204A-6R-4 at a depth of ~815 mbsf, for which both bulk and grain density <1 g/cm3 was determined. Porosity is also constant at ~50% between 762 and 811 mbsf. Below 811 mbsf in Unit IV, porosity ranges up to ~70%, with higher porosities corresponding to the lower bulk densities.
In the basement units of Holes 1204A and 1204B, the basaltic units have a bulk density between 2.41 and 2.91 g/cm3; the average for all basalt is 2.71 g/cm3. The grain density of basalt varies between 2.79 and 3.12 g/cm3 (average = 2.95 g/cm3), and porosity varies between 8.1% and 21.5% (average = 12.5%). No systematic downhole variation of density or porosity is apparent for the basaltic basement. Lower bulk densities of 1.99 and 2.14 g/cm3 and correspondingly higher porosities of 38.4% and 37% were found for a hyaloclastite breccia and a sandstone in the basement, respectively. For the breccia, a lower grain density of 2.6 g/cm3 was also determined.
Compressional wave velocity was determined from both split-core sections (Cores 197-1204A-1R through 6R and Sections 197-1204B-1R-1 through 1R-3) and discrete sample measurements (Table T16; Fig. F60). P-wave velocity was measured in only the x-direction for split sediment cores and ranges from ~1600 to ~2400 m/s. However, only 12 out of 32 samples has a P-wave velocity >1800 m/s; the average of all samples is 1860 m/s.
For discrete samples of basement material from Holes 1204A and 1204B, P-wave velocity was measured in x- and z-directions. For the basalt, the P-wave velocity ranges from 3600 to 5700 m/s, with velocities determined in x-direction generally lower than those determined in the z-direction. However, this is probably only an apparent anisotropy because to obtain a good signal on the unevenly cut minicores in the x-direction (along the axis of the minicores) larger amounts of water were needed to couple the PWS3 transducers than in the z-direction, where the surface is curved. In Hole 1204A, a smooth line through the data points suggests a decrease in P-wave velocity from the top of the basement section to the bottom of the hole in Subunit 2a (from ~5000 to 4500 m/s). However, this trend is uncertain because of the large spread of data points and the small basement penetration. In contrast, the data from Hole 1204B suggest an increase in P-wave velocity from the top of the basement section to a depth of ~872 mbsf, throughout the basalt in basement Unit 1 and Subunit 2a, from ~4500 to >5000 m/s. Throughout basement Subunit 2b (a diabase) (see "Physical Volcanology and Igneous Petrology"), P-wave velocities decrease again with depth to ~4500 m/s then remain approximately constant at 4600 m/s in basement Subunit 2c and Unit 3. In basement Subunit 2d and basement Unit 4, a hyaloclastite and a sediment, respectively, P-wave velocities of 2600-2900 and ~2900 m/s were recorded.