We determined the physical properties of whole-core sections from Hole 1141A, where sediments and basement rocks were recovered, and from Hole 1142A, where only basement was cored. Measurements obtained from the MST included magnetic susceptibility, gamma-ray attenuation porosity evaluator (GRAPE) bulk density, and natural gamma radiation (NGR). We determined compressional wave velocities (Vp) from the split cores in transverse directions (x) for soft sediments in liners and for hard-rock pieces without the liner. Measurements in the longitudinal (z) and transverse (x and y) directions on cut samples of consolidated sediment and hard rock allowed us to investigate velocity anisotropy. Index properties determinations included bulk density, water content, porosity, and grain density. We calculated index properties from wet and dry sample weights and dry volumes. We also determined thermal conductivity for sediment and basalt.
We determined index properties by using gravimetric methods on discrete samples from Sites 1141 and 1142 (Table T11). The general trends exhibited by the index properties data at both sites reflect downhole variations in lithology (Figs. F70, F71, F72).
Between ~20 and ~51 mbsf in Unit I, bulk densities change little, varying from 1.6 to 1.7 g/cm3, averaging 1.7 g/cm3. Porosity ranges from 57% to 65% with a mean of 62%. Grain densities also exhibit small scatter in this interval, with values between 2.6 and 2.7 g/cm3. Sediments in this interval consist of foraminifer nannofossil ooze (see "Lithostratigraphy").
From ~67 to ~86 mbsf, still within Unit I, bulk density increases slightly, from 1.7 to 1.8 g/cm3. The grain density changes between 2.5 and 2.7 g/cm3. Porosity decreases from 62% to 54% with a mean of 58% (Fig. F70C). Sediments in this depth interval also change little, from generally fine-grained white ooze with abundant nannofossils grading downward into light pale brown ooze composed mostly of sand-sized foraminifers.
Below the boundary between Units I and II (~114 mbsf), index properties change abruptly (Figs. F70, F71). From 115.8 to 116.7 mbsf in the upper part of basement Unit 2, which consists of reddish, loosely consolidated altered basalt (see "Physical Volcanology"), bulk densities increase to a mean value ~2.0 g/cm3, grain densities increase to 2.9 g/cm3, and porosities decrease to 48%. In this interval, the degree of alteration is moderate to high, and the mafic minerals are almost completely replaced by fine-grained carbonate, clay, and Fe-oxides (see "Igneous Petrology").
From 123.2 to 130.0 mbsf, in the middle to lower parts of basement Unit 2, bulk density slightly decreases to 1.8 g/cm3, grain density also decreases to a mean 2.7 g/cm3, and porosity increases to a mean of 49%.
From 132.9 to 135.8 mbsf, all index properties change markedly near the boundary between basement Units 2 and 3 because the base of Unit 2 and top of Unit 3 are relatively less altered (see Fig. F38). As will be discussed in the later sections, natural gamma ray and compressional wave velocities also change significantly at this interval. Bulk density and grain density increase downhole with mean values of 2.0 and 2.8 g/cm3, respectively, and porosity decreases to a mean value of 44%.
Between 158.1 and 169.7 mbsf in basement Units 4 and 5, recovered rocks are basalts with relatively uniform mineralogical compositions and textures. Index properties in this interval reflect this change in lithology—bulk density varies from 2.5 to 2.7 g/cm3, grain density increases from 2.8 to 2.9 g/cm3, and porosity decreases to 16%.
In the lowermost basement unit (Unit 6, 174.4 to 184.5 mbsf), bulk densities vary from 2.6 to 2.9 g/cm3 with a mean of 2.7 g/cm3, grain density approaches a mean of 2.8 g/cm3, and porosity varies from 16% to 3% (Fig. F71). The lithologies in this interval are relatively fresh basalts.
Because of limited time, we were only able to determine index properties for five representative samples from basement Units 1, 2, 3, and 6 (Table T11; Fig. F72). The index properties show that massive basalt in Sample 183-1142A-2R-1, 28-30 cm (91.28 mbsf) from basement Unit 1 has a bulk density of 2.87 g/cm3, grain density of 2.92 g/cm3, and porosity of 2.6%. In Sample 183-1142A-3R-1, 39-41 cm, which is from the basaltic breccia in basement Unit 2, the bulk density is 2.60 g/cm3, grain density is 2.89 g/cm3, and porosity is 15%. The index properties in the massive basalt Sample 183-1142A-5R-1, 107-109 cm, of basement Unit 3 are distinctly different, with bulk density of 2.53 g/cm3, grain density of 3.13 g/cm3, and porosity of 28%. Two samples from the possible pillow basalt in basement Unit 6 show that from 135.6 to 137.5 mbsf, bulk densities are 2.62 to 2.70 g/cm3; grain density, 2.75 to 2.87 g/cm3; and porosity, 7% to 9%. These basalts are relatively altered and contain large patches of carbonate in the groundmass (see "Alteration and Weathering").
Bulk density was measured by the GRAPE every 4 cm on whole sections of cores recovered from Holes 1141A and 1142A. For Hole 1141A, the maximum (right side of the bulk densities profile) (Fig. F73) values of GRAPE densities correspond very well with wet bulk densities determined from discrete samples in the sedimentary section (Unit I; 0 to 103.8 mbsf). Below ~103.8 mbsf, bulk densities are much more scattered than in overlying sediments. As previously noted, the larger scatter in the GRAPE bulk density data for the deeper units results from empty space between pieces of core and the core's fractured nature, whereas the generally lower maximum GRAPE values are because of the smaller diameters of the cores.
In Hole 1142A, the aphyric basalts in basement Unit 6 exhibited highest maximum values of GRAPE densities, followed by the mixed volcanic rocks in Unit 2 (Fig. F74). The maximum GRAPE bulk densities for the remaining units are quite uniform, although the data are highly scattered (Fig. F74).
We measured NGR every 12 cm on unsplit sections of cores from Sites 1141 and 1142. At Site 1141, the NGR count in Unit I (between the seafloor and ~103 mbsf) is uniform. Between ~116 and ~130 mbsf, within basement Units 1 and 2, NGR values are constantly greater than 7 counts per second (cps) (Fig. F71B), reflecting the relatively higher clay content in these intervals. Gamma-ray values decrease near the boundary between basement Units 2 and 3, with an associated increase in bulk and grain densities, as mentioned previously. The remaining basement Units have values of ~6 cps. Note that the downhole natural gamma-ray profile exhibit fluctuations associated with those of downhole magnetic susceptibility data (see Fig. F71A, F71B).
NGR measurements in basement sections of Hole 1142A also reflect lithologic changes (Fig. F74C). Between ~80 and ~119 mbsf in basement Units 1 through 3, gamma-ray values average 5 cps. NGR count increase distinctively at a depth of ~108 mbsf, with an average value 14 cps and corresponding to the clay-rich Subunit 3A (Section 183-1142A-3R-2) (Fig. 56B). The highest count (average >30 cps) is in the granule-bearing clay (basement Unit 4) at a depth of 119.3-119.7 mbsf. Between 132.3 and 140.7 mbsf, gamma-ray values reached an average value of 11 cps, corresponding to the altered breccia in basement Unit 5 (see "Igneous Petrology"). In basement Unit 6 (aphyric basalt; 132.3-140.7 mbsf), NGR values fluctuate between 4 and 16 cps, with a mean value of 10 cps.
We routinely determined magnetic susceptibility on all cores from Sites 1141 and 1142 (Figs. F71A, F73B, F74B). The results are discussed in "Paleomagnetism".
We determined compressional wave velocities from split-core sections from Sites 1141 and 1142, and on discrete samples at Site 1141 (Figs. F70, F71, F74).
The compressional wave velocity data for Unit I of Hole 1141A (20.4 to ~86 mbsf), which consist of foraminifer-bearing nannofossil ooze, show very little scatter, with a mean value of ~1860 m/s (Table T12; Fig. F70). Below the boundary between Unit I and basement at ~114 mbsf (Fig. F71), compressional wave velocities start to increase with depth. Velocity from 115.8 to 116.7 mbsf increases from 1723 to 2353 m/s, with a mean of 2018 m/s, corresponding to a change in lithology from coarse-grained sedimentary deposit (gabbro pebbles) in basement Unit 1 to the altered basalt in basement Unit 2. Compressional wave velocities are fairly uniform between 123.2 and 130.0 mbsf (middle part of basement Unit 2), ranging between 2013 and 2213 m/s and corresponding to the clay-rich altered basalt in this interval.
Velocities gradually increase between 132.9 and 135.8 mbsf, from 2099 to 3422 m/s. Core 183-1141A-16R in this interval is altered basalt at the boundary between basement Units 2 and 3.
Velocities in basement Units 4 and 5 (~144 to ~170 mbsf) range from 4550 to 5180 m/s, with a mean around 4900 m/s. Compressional wave velocities in basement Unit 6 also increase gradually with depth, from 4276 to 6902 m/s. Velocities above 6000 m/s seem high and may be a consequence of the calibration method (see "Physical Properties" in the "Explanatory Notes" chapter), but the general differences are probably valid. These changes correspond to a decrease in porosity and an increase in bulk and grain densities, as mentioned above. Changes in velocity, bulk density, and porosity in this unit correlate well.
Velocities for the basement units at Site 1142 vary considerably downhole (Table T13; Fig. F72). The highest velocities of >6000 m/s correspond to massive basalt forming basement Unit 1 and the top of basement Unit 2 (from 91.1 to 92.8 mbsf). Velocities in the altered volcanic rocks and breccia of basement Units 3 and 5 decrease significantly, ranging between 2840 and 3640 m/s. Compressional wave velocities for the aphyric basalts in basement Unit 6 (132.8-140.8 mbsf) vary from 3600 to 6370 m/s, with a mean value of 4725 m/s.
At Site 1141 thermal conductivity values for sediments from Unit I are commonly between 1.0 and 1.3 W/(m·K), with a mean value of 1.1 W/(m·K), and show little scatter (Fig. F75; Table T14). The mean value is similar to values in sedimentary units at other Leg 183 sites. For the basement unit, thermal conductivity values vary quite widely, from a low of 0.7 W/(m·K) to a high of 2.4 W/(m·K) between ~154 and ~184 mbsf, with a mean value of 1.6 W/(m·K).
At Site 1142, thermal conductivity values for basement Units 1-4 average 1.6 W/(m·K) (Fig. F76; Table T14), similar to the mean value in basement rocks at Site 1141 (Fig. F75). Between ~133 and ~138 mbsf (aphyric basalt of basement Unit 6), thermal conductivity averages 1.5 W/(m·K). The highest values (2.4 W/[m·K]) were in Section 183-1142A-6R-2 (~119 mbsf), which is the reddish clay and granule-bearing mudstone in basement Unit 4. The mudstone also yields the highest gamma-ray value in Hole 1142A, as described earlier.