-
(
/)/(VP/VP) = ~0.4.
All cores from Core 197-1205A-4R through 45R were run through the multisensor track (MST). For Sections 197-1205A-4R-1 and 5R-1, which contained sedimentary material, measurements were made of magnetic susceptibility, gamma ray attenuation (GRA) bulk density, and natural gamma radiation (NGR) on unsplit core sections. For Cores 197-1205A-5R-2 to 45R, which contained basement material, only NGR measurements were made on the MST on unsplit core sections. Index properties (bulk density, grain density, and porosity) and thermal conductivity were determined on discrete samples at a sampling frequency of one per core. Compressional wave velocities were measured at a sampling frequency of approximately one per section.
Volume-normalized magnetic susceptibility was determined on core Sections 197-1205A-4R-1 and 5R-1 at 5-cm intervals (Table T11). Values generally range between ~300 x 10-6 and 1600 x 10-6 SI. A number of low values at 24.2, 24.4, 33.25, 33.5, and 33.85 mbsf appear to be due to voids in the liner, since there were also low GRA densities measured at the same depths (see "GRA Density"). The highest value, at 24.75 mbsf at the base of Section 197-1205A-4R-1, could be attributed to drilling contamination. The remaining values increase with depth from ~700 x 10-6 to ~1300 x 10-6 SI in Section 197-1205A-4R-1 but remain relatively constant at ~1000 x 10-6 to 1100 x 10-6 SI in Section 197-1205A-5R-1.
Bulk density was measured by the GRA densitometer every 5 cm on whole-core Sections 197-1205A-4R-1 and 5R-1 (Table T12). Low density values at 24.2, 24.4, 33.25, 33.5, and 33.85 mbsf probably reflect voids in the liner. A general increase with depth from ~1.5 to ~2 g/cm3 was observed in Section 197-1205A-4R-1, but values remain relatively constant at ~2 g/cm3 in Section 197-1205A-5R-1.
NGR was measured every 10 cm on both unsplit sediment and basalt cores from Hole 1205 (Table T13; Fig. F36A). Total counts are reported here because the corrected counts (which are less by ~16 counts per second [cps]) include negative values that are physically unreasonable. The sediment section in Hole 1205A consists of fossiliferous sandstones and a basalt conglomerate (see "Lithostratigraphy"). For the sandstones, NGR values between 17 and 25 cps were measured (mean = 20 cps). For the basalt conglomerate, NGR values between 17 and 32 cps were measured (mean = 22 cps).
In the basement, total NGR counts varied between 12 and 39 cps. The data points show considerable variation with depth. High NGR counts of ~31 cps in basement Units 1, 2, and 3 at depths >79.2 mbsf are followed by a rather abrupt decrease to ~16 cps at ~84 mbsf. These lower gamma ray values are present at the base of basement Subunit 3b (see "Physical Volcanology and Igneous Petrology"). Below 90 mbsf, NGR counts again increase to ~26 cps at ~110 mbsf then gradually decrease again to ~19 cps at ~180 mbsf. This is followed by a slight increase to ~22 cps at ~225 mbsf. At ~242 mbsf, total NGR counts decrease rapidly to ~14 cps; these low gamma ray values are present in basement Subunits 19a and 19b, a weathered flow top and moderately to highly olivine-phyric basalt (see "Physical Volcanology and Igneous Petrology"). At depths >250 mbsf, NGR counts are higher again and remain so to the base of the hole, ranging from ~20 to 30 cps.
Whole-round core images were taken of cylindrical pieces from the Hole 1205A core. Owing to the massive nature of many of the recovered core pieces, 153 m of images was recorded, representing ~93% of the recovered material. However, because of an obstruction, downhole logging was canceled. Hence, correlation of core image and logging data is not possible at this site.
Thermal conductivity was measured at a frequency of one per core from Cores 197-1205A-5R through 45R. Cores 197-1205A-1R through 4R did not contain suitable samples for thermal conductivity measurement. For the sample from Core 197-1205A-5R, a basalt cobble/boulder conglomerate, a value of 1.331 W/(m·K) was measured. All other measurements were made on basalt samples. For these, thermal conductivity values ranged from 1.515 to 1.985 W/(m·K) (average = 1.70 W/[m·K]) (Table T14; Fig. F37). Particularly high thermal conductivity was recorded in samples from basement Units 3, 10, 13, 16, and 22.
Index properties of sediments were determined on two discrete samples from Core 197-1205A-2R and one sample from each of Cores 197-1205A-4R and 5R. There were no suitable samples in Cores 197-1205A-1R and 3R. In the basement units from Cores 197-1205A-5R through 45R, index properties were usually determined on one sample per core. Uncommonly, more than one sample per core was taken if different lithologies were present in the recovered core. Values of wet mass, dry mass, and dry volume of the samples were measured and used to calculate moisture content, bulk density, grain density, and porosity (Table T15; Fig. F36B, F36C, F36D).
For all four sediment samples, grain densities between 2.78 and 2.83 g/cm3 were determined. However, porosity for the samples from Cores 197-1205A-4R and 5R is much higher (64% and 60%, respectively) than samples from Core 197-1205-2R (35% and 37%). Consequently, bulk density in Cores 197-1204A-4R and 5R is much lower (1.65 and 1.74 g/cm3, respectively) than in Core 197-1205A-2R (2.14 and 2.19 g/cm3). In Cores 197-1205A-4R and 5R, bulk density values are somewhat lower (~10% to 20%) than the GRA density at the respective depths. GRA density was not measured in Core 197-1205-2R.
In the basement, bulk density ranges from 2.52 to 3.06 g/cm3 for the basalt samples (mean = 2.83 g/cm3). Low bulk density (1.82 g/cm3) was recorded in a soil horizon/weathered flow top from basement Unit 12a (see "Physical Volcanology and Igneous Petrology"); this sample also has correspondingly high porosity (~60%). For the remainder of the basement samples, all of which were taken from basalt sections, bulk density shows very little systematic variation with depth. Grain density also varies very little downhole, ranging from 2.85 to 3.08 g/cm3 (mean = 2.97 g/cm3). Porosity in the basement section of Hole 1205A is relatively low, generally ranging from 0% to 22.6% (mean = 7.2%). It shows very little systematic variation with depth.
Compressional wave velocity was determined in the x-direction for two split-core sediment sections from Hole 1205A (Sections 197-1205A-4R-1 and 5R-1) and for two sediment pieces from Core 197-1205A-2R. In these four sediment samples, P-wave velocities range from ~1774 to 2710 m/s (mean = 2322 m/s).
For basement material, compressional wave velocity measurements were made on approximately one discrete sample per section. P-wave velocity was measured in the x- and z-directions for the basement samples. On selected samples, P-wave velocity was also measured in the y-direction. P-wave velocity ranges from ~3200 to 6000 m/s (mean = ~5100 m/s) (Table T16; Fig. F38). Velocities determined in the x-, y-, and z-directions differ by up to 300 m/s, and it is not immediately apparent whether this relates to anisotropy or to the difference in quality of coupling between sampling and transducers described in previous site chapters. In the upper part of the basement, from ~50 to 135 mbsf, velocity is relatively high. The decrease in velocity toward the center of basement Unit 3 may be related to an increase in devitrification (see "Physical Volcanology and Igneous Petrology"). Similarly, but less clearly, this tendency of velocity to decrease toward the center of a unit can also be seen in basement Units 6, 8, 10, and 18, whereas basement Unit 27 shows the opposite trend. Other basement units are thinner, and, hence, fewer samples were measured and no trends were apparent.
At ~135 mbsf, the base of Unit 9, velocity decreases slightly. Highly variable velocity was recorded in basement Unit 10, ranging from 3425 to 5228 m/s; this may reflect variable amounts of vesicles in the unit. Velocity then shows a slight increase with depth until ~225 mbsf, where a sharp decrease in velocity is observed. Particularly low velocity (<4000 m/s) was recorded in basement Unit 18 (predominantly aphyric to moderately olivine-phyric basalt), which is probably due to its high vesicularity (see "Physical Volcanology and Igneous Petrology"). From ~245 to 278 mbsf, an increase in P-wave velocity with depth is observed, the highest values occurring in basement Unit 22, a moderately olivine-plagioclase-phyric basalt; elevated thermal conductivity was also recorded in this unit (see above). Lower velocity was recorded in basement Units 24 to 26, which include aphyric basalt breccias (see "Physical Volcanology and Igneous Petrology"). Slightly higher velocity (~5760 m/s) is present throughout basement Unit 27, an aphyric to moderately plagioclase-phyric basalt.
At Site 1205, P-wave velocities were measured on all minicores drilled for paleomagnetic measurement and index properties were frequently determined on chips left after cutting of paleomagnetism minicore samples; therefore, it was possible to compare bulk densities and P-wave velocities at the same sampling depth (Fig. F39). P-wave velocity and bulk density data appear to be correlated:
/)/(VP/VP) = ~0.4.
A possible explanation for this relationship is that high vesicularity causes both low bulk density and low seismic velocity. This interpretation is further supported by the absence of any obvious relationship between grain density and P-wave velocity.
