(z)
= o
e-kz,
Evaluation of physical properties at Site 1192 included nondestructive measurements of bulk density, bulk magnetic susceptibility (MS), natural gamma radiation (NGR), and P-wave velocity on whole cores using the multisensor track (MST). Transverse P-wave velocity (x-direction) and moisture and density (MAD) were measured on split cores and plug samples. Low recovery in Cores 194-1192B-11X, 13X, 14X, and 22X precluded the use of the MST. Thermal conductivity was measured on whole cores and semilithified half core samples.
Bulk density at Site 1192 was computed from gamma ray attenuation (GRA) using unsplit cores and from mass and volume measurements on plug samples. GRA bulk density decreases from 1.70 to 1.65 g/cm3 in core from the upper 20 m of Hole 1192A (Fig. F21). Below 20 mbsf, the general bulk density trend is an overall increase down to ~345 mbsf. Small-scale variations in density occur downhole. Between 85 and 160 mbsf, the bulk density is rather uniform. Below ~180 mbsf, the variability increases markedly. Composite profiles of these independently derived bulk density estimates indicate a similar trend in the two data sets (Fig. F21). With the exception of a few outliers, the GRA bulk density values exceed the MAD values between 0 and 245 mbsf by ~0.1 g/cm3. Cores from this interval were recovered using the APC. Such a discrepancy is not uncommon (e.g., see "Core Physical Properties" in the "Site 1109" and "Site 1115" chapters of the Leg 180 Initial Reports volume [Taylor, Huchon, Klaus, et al., 1997]). Sites 1109 and 1115 chapters, Leg 180) and is likely a function of the core diameter variations produced by the type of coring (e.g., Fig. F21; Cores 194-1192B-7H through 9X), problems with the GRA calibration, excessive drying of the core prior to sampling for MAD measurements, and/or mass loss during the sample drying and pycnometer measurement. Nevertheless, the repeatability of the MAD measurements suggests that the consistent difference between GRA and MAD densities arise because of either the variability in core diameter or problems with instrument calibration. Given these caveats, the GRA bulk densities otherwise agree well with the MAD measurements.
Grain density averages 2.77 g/cm3 and shows a distinct pattern of variability as a function of depth (Fig. F22). Three distinct zones can be recognized:
Values in Zone 2 are compatible with a high carbonate content (85-90 wt%) (see "Geochemistry"), whereas the higher variability in Zones 1 and 3 reflects the input and variability of a terrigenous clastic component. This terrigenous component is assumed to be the result of hemipelagic deposition across the region (see "Lithostratigraphy and Sedimentology"). Suspect grain densities, especially those >3.0 g/cm3, have been noted in Figure F22 (open circles). These densities are considered suspect because they are also associated with either anomalous bulk density or porosity. In turn, the bulk density and porosity outliers in the depth interval <100 mbsf do result in reasonable grain densities, suggesting that these values may be correct.
Porosity profiles generally reflect a combination of stress history and sedimentologic and diagenetic effects such as variability in compressibility, permeability, sorting, grain fabric, and cementation. Porosity is calculated from the pore water content, assuming complete saturation of the wet sediment sample (Blum, 1997) (see "Core Physical Properties" in the "Explanatory Notes" chapter). The porosity curve mirrors that of the bulk density curve, with minor differences caused by changes in grain density (Fig. F22). The variability in porosity at Site 1192 shows a general decrease with depth. Superimposed on this trend are shorter wavelength variations. No abrupt steps in porosity, a common characteristic of erosional unconformities, are observed.
Typically, seafloor porosities of abyssal plain marine oozes are high (85%-90%). For homogeneous sediments that are not overpressured, porosity may be approximated by an exponential function of depth (e.g., Athy, 1930). Porosities from Site 1192 show relatively low values at the seafloor (60%-70%) and decrease gradually to 40%-50% at a depth of 300-350 mbsf (Fig. F22). This low surface porosity may reflect surficial reworking, sorting, and efficient grain packing by oceanographic currents. Distinct zones of relatively higher porosity are found between 20 and 30 mbsf and between 210 and 240 mbsf. The general behavior of Site 1192 porosity as a function of depth is broadly consistent with Athy's relationship:
(z)
= o
e-kz,
where (z)
is the porosity as a function of depth z, o
is the surface porosity, and k
controls the rate of decay of porosity with depth (Athy, 1930). A least-squares
fit to this equation estimates a surface porosity of 66.2% and a compaction
decay constant of 0.001 m-1 (Fig. F22)
(correlation coefficient of 0.78). The inverse of the decay constant (1000 m)
can be physically interpreted as the depth over which porosity is halved with
respect to the surface or initial value. Porosity shows no obvious correlation
with relative clay content (see "Lithostratigraphy
and Sedimentology"), color reflectance, an indirect proxy for
clay content, or grain size (Fig. F2).
P-wave velocities were measured with the MST P-wave logger (PWL) on whole cores and the PWS3 contact probe system on both split cores (within the core liner) and ~9.5-cm3 cube samples of semilithified and lithified sediments. The cubes were used to measure velocities in the transverse (x and y) and longitudinal (z) directions. Insufficient x- and y-direction P-wave sensor (PWS) data precluded a study of velocity anisotropy. Extreme scatter and unreasonable velocities, likely a function of drilling disturbance, call into question the quality of the PWL, and thus these data were not used.
Initially, it appeared that sediment plasticity and induration prevented reliable use of the PWS1 and PWS2 probes. However, it was eventually determined that a software problem failed to set the minimum receiver voltage to recognize the seismic waveform. Once fixed, the PWS system operated within acceptable limits and was used to remeasure the velocity in Cores 194-1192-1H through 12H. These measurements show that most of the earlier velocities measured at Site 1192 have been compromised to systematically higher velocities (Fig. F23A). Originally, velocity measurements from ~40 to 345 mbsf ranged from 1940 to 2500 m/s. To salvage some of the older velocity data, a constant value of 346.5 m/s was subtracted to make the earlier velocities consistent with the newer, correctly measured velocity data (Fig. F23B). Corrected velocity values increase gradually from ~1580 to a maximum of 2097 m/s with an average of 1662 m/s.
Two features of the corrected velocity profile shown in Figure F23B are worth noting. First, an abrupt downcore increase is followed by a slower decrease in velocity over a depth of 80-105 mbsf. Below this interval, the velocities are characterized by low scatter over a depth range of 105-175 mbsf. This same depth range correlates with high carbonate content (see "Geochemistry") and relatively constant grain density and porosity (Fig. F22). Below ~180 mbsf, the velocities show high scatter, as do bulk density and porosity.
Thermal conductivity measurements at Site 1192 show an overall increase with depth, ranging from ~0.8 W/(m·K) near the seafloor to ~1.35 W/(m·K) at the base of Hole 1192B (Fig. F24). Large scatter is observed between 0 and 40 mbsf, which is broadly consistent with the porosity data (Fig. F22). A direct inverse relationship should exist between porosity and thermal conductivity as a result of the power law dependence of bulk thermal conductivity, Kbulk, on the solid matrix grain thermal conductivity, Kgrain, and the thermal conductivity of the interstitial fluid, Kw, namely (e.g., Keen and Beaumont, 1990):
· Kgrain(1-).
The relationship between the observed thermal conductivities, porosity, and sediment facies (i.e., degree of mixing of siliciclastic, clay, and carbonate components) can be investigated by calculating the variation in bulk thermal conductivity for the observed porosity range obtained at Site 1192 (Fig. F22).
The result (Fig. F25) is based on grain thermal conductivities summarized in Table T6 in the "Explanatory Notes" chapter (Keen and Beaumont, 1990). Except for a small number of outliers, the majority of the measured thermal conductivities lie between the shale and sandstone power law relationship, giving confidence in the viability of the observed thermal conductivities. Given the predominance of carbonate through the various sections, the observed thermal conductivity is consistent with the mixing of siliciclastic/carbonate and clay/carbonate sediment facies.
High values of MS are a function of the existence and concentration of ferro- and ferrimagnetic minerals such as magnetite, hematite, goethite and titanomagnetite within a sediment. The source of this material may be associated with the coarse sediment fraction of, for example, proximal turbidites and/or single-domained magnetic material contained within the clay fraction. In the absence of ferro- and ferrimagnetic minerals, the MS often displays low values induced by paramagnetic and diamagnetic minerals such as clays and evaporites. NGR values are also a function of the terrigenous clay content within sediment. Clay minerals, being charged particles, tend to attract and bond with K, U, and Th atoms so that an increasing NGR count typically correlates with increasing clay/shale content. Both MS and NGR contain independent information concerning source provenance and magnetic mineral derivation. For example, a noncorrelation between NGR and MS may imply the existence of biogenically produced ferrimagnetic minerals or the mixing of distinct terrigenous sources.
As with the grain density (Fig. F21), the MS measured at Site 1192 can be divided into three zones with distinct patterns (Fig. F26):
In Zone 1, the magnetic susceptibility is characterized by high-frequency variability that ranges in amplitude from 0 to 25 x 10-6 SI units, which is in contrast with the subdued signal of Zone 2 where susceptibility values, apart from a few outliers, consistently range from 0 to 2 x 10-6 SI units. Zone 3 shows increased values ranging from 0 to 15 x 10-6 SI. It is inferred that Zones 1 and 3 represent times of enhanced terrigenous input and hemipelagic deposition in the basin. Coeval with the change in MS, there is an abrupt but relatively small decrease in the carbonate content of the section and a corresponding decrease in reflectance, again a proxy for clay content in a clay-carbonate two-component system (Fig. F26). If the MS values in Zone 2 were solely due to the dilution effects of carbonates and clays, then the ~10% decrease in carbonate in Zone 1 would result in a minimal change in susceptibility values, inconsistent with the observed tenfold increase. It is concluded that the MS is not the result of changing the mixing ratio between clays and carbonates but, rather, of an increase in continental clastic flux.
The NGR count was recorded on the MST for core from Holes 1192A and 1192B. However, a series of problems related to possible electrical "cross-talk" within the data acquisition circuitry produced significant amplitude and high-frequency noise. The form of this noise was a recursive NGR count whose periodicity was linked to section length and whose count amplitude rapidly increased toward the end of the section (Fig. F27A). This problem persisted for all of the NGR measurements for Hole 1192A. Only after shutting down the acquisition system and rebooting the controlling computers prior to MST measurements of Hole 1192B core did the problem disappear. Neither the exact reason for the source of the problem nor its solution is known. Remeasuring Cores 194-1192A-1H through 12H confirmed that the NGR count was now acceptable (Fig. F27B). However, in comparing the original and remeasured NGR, it is clear that the high-frequency and high-amplitude spikes are superimposed on the same "baseline" signal observed in the remeasured sections (Fig. F27B). It is concluded that the NGR count can be used, with care, for geological interpretation.
For Site 1192, the interpretational use of NGR data is limited because of poor core recovery from 100 to 180 mbsf and from 260 to 290 mbsf (Fig. F26). However, even with the limited NGR data, it is clear that the variations with depth are compatible with the sediment facies interpretations based on the MS, carbonate concentration and sediment reflectance (Fig. F26). In particular, the NGR count associated with the carbonate-dominated section between 180 and 245 mbsf is very low, ranging from 0 to 10 cps. In contrast, the NGR count ranges from 10 to 40 cps between 0 and 90 mbsf and correlates with the magnetic susceptibility and grain density of Zone 1, considered to be the result of increased terrigenous sediment flux (Figs. F22, F26).
GRA density was used to estimate the depth offset between Holes 1192A and 1192B (Fig. F28). This estimate is based on matching characteristic features in the data sets. For example, Figure F28 shows a small but general decrease and subsequent increase in GRA density between 204.5 and 205.1 mbsf in Cores 194-1192A-24H to 25H. The interpreted counterpart density variation exists within Cores 194-1192B-5H to 6H at 203.2-203.7 mbsf, suggesting an ~1.27 m offset between these two cores. This offset will not apply to other cores or even other intervals in these cores. However, it represents the typical margin of uncertainty in depth measurements with the drill string.
