),
which were corrected for the reduced sample volume using Equation 1:All whole-round core
samples from Holes 1201A, 1201B, 1201C, and 1201D longer than 40 cm were run
through the MST. The XCB cores from Hole 1201B and the RCB cores from Hole 1201D
have a diameter of ~5.75 cm, smaller than the nominal 6.6-cm core diameter
recovered by the APC at Site 1201. The values measured by the MST are corrected
for the reduced core diameter as at Site 1200 (see "Physical
Properties" in the "Site
1200" chapter), with the exception of the magnetic
susceptibility logger (MSL) measurements (),
which were corrected for the reduced sample volume using Equation 1:
corr
= (meas/1.03)
x 10-5.
(1)
The values in the database do not reflect these corrections.
The P-wave velocity logger was able to obtain P-wave velocity measurements only through the APC cores. The degree of consolidation in the sediment at depths >46 mbsf prevented both discrete P-wave velocity measurements using the insertion transducers (PWS1 and PWS2) and formation factor measurements on split core samples. At depths >46 mbsf, P-wave velocity was measured along the x-, z-, and, occasionally, the y-axis of the core using discrete minicore samples and the contact transducers (PWS3). The minicore samples were subsequently used for index properties measurements.
RCB cores from Hole 1201D were recovered in pieces ranging in length from a few centimeters to in excess of 1 m; very little of the core was affected by biscuiting. The gaps, or fractures, between each piece reduce the magnetic susceptibility values by a small amount at a distance of up to ~10 cm on either side. Gamma ray attenuation (GRA) density values are reduced almost to zero by the presence of a gap. As at Site 1200, the effect of the gaps was not removed from the MSL data set but anomalously low density values (<1.0 g/cm3 or values obviously outside the local trend) were eliminated from the GRA data set by hand (see "Physical Properties" in the "Site 1200" chapter).
Results from the corrected MST runs are presented in Figure F66, and results with the depth scale expanded in 100-m intervals are presented in Figures F67, F68, F69, F70, F71, and F72. Holes 1201B and 1201C cored a similar depth interval, and results from the MST are very similar (Fig. F73). Index and physical properties measurements are presented in Tables T11 and T12.
The values of volume magnetic susceptibility measured in the pelagic muds are relatively low and constant at ~200 x 10-5 (SI units). The values increase at the boundary between the pelagic muds and the turbidites. The mean value for the turbidites is ~500 x 10-5. The variability of the susceptibility values is higher in the turbidites compared to the muds, with values ranging from near zero to >2000 x 10-5. High values occur only over very short intervals, giving the data a "spiky" appearance. The spikes likely correspond to greater concentrations of magnetite, which may be present at the base of turbidite sequences because of magnetite's higher than average grain density or because of better preservation of magnetite in the coarser clasts. The sharp decreases in magnetic susceptibility at depths of 341.0 and 347.0 mbsf correlate with boundaries between larger-scale turbidite sequences. The values of magnetic susceptibility increase below the boundary between the sedimentary succession and basaltic basement. Although the highest values of magnetic susceptibility in the basalts are less than high values in the turbidites, the values in the basalts have a higher mean value of ~800 x 10-5 and are more evenly distributed.
It is possible to identify several scales of turbidite sequences in the magnetic susceptibility data measured by the MST. For example, the interval between 340 and 347 mbsf contains one large-scale turbidite sequence (~6 m thick) (Fig. F74). Evident within this sequence are smaller-scale sequences on the order of 1-2 m, and within these sequences are smaller sequences on the order of 0.2 m.
Whole-core density measurements obtained on the MST (Figs. F67, F68, F69, F70, F71, F72) are in close agreement with the bulk density measured on discrete samples. Grain density was also measured on the discrete samples.
Figure F75A and F75B show the grain and bulk densities as a function of depth. The bulk density of the pelagic mud shows little variation with depth and has a mean value of 1.28 g/cm3. The mud has a mean grain density of 2.66 g/cm3 and an average porosity of ~85%. The bulk density increases rapidly as the lithology changes from pelagic mud to turbidites. The mean bulk density for the turbidites is 1.86 g/cm3, and the mean grain density is 2.54 g/cm3. There is a general trend of decreasing grain density with increasing depth in the turbidites, which can be attributed to increasing diagenesis (see "Lithostratigraphy"). At a smaller scale, there are several intervals where the grain density increases almost linearly with depth (e.g., 340-350 mbsf). These trends likely correspond to sorting of more dense minerals and clasts toward the bottom of larger-scale turbidite sequences. However, from 100 to 140 mbsf within the turbidites, there is a clear decrease in grain density with increasing depth.
At the interface between the turbidites and the basalt, bulk density increases significantly from ~2.0 to ~2.5 g/cm3. At a depth of ~20 m below the top of the basalt, the bulk density increases to a mean value of 2.70 g/cm3 for the remainder of the section. The grain density of the basalt shows similar variability with depth and has a mean value of 2.79 g/cm3 at depths >20 m below the top of the basalt. The relatively low density in the top 20 m of the basalt may be due to alteration. The mean porosity of the basalt is 5.1%.
Porosity values (Fig. F75C) decrease from a mean of 84% in the pelagic mud to 44% in the turbidites and 9% in the basalt. On the scale of the whole core, porosity appears to be inversely related to the bulk density. This relationship follows from the generally low variability in the grain density with depth, as the large variations observed in the bulk density must be related to the water content.
Natural gamma ray (NGR) emissions (Figs. F66, F67, F68, F69, F70, F71, F72) show significant variations in the pelagic mud. Count rates are highest at the seafloor, with nearly 50 counts per second (cps), compared to an average count rate of ~5 cps at this site. Three large increases in the count rate occur in the muds, at 16.5, 30-35, and 45-50 mbsf. In the turbidites, NGR emission variations appear to follow the same general trends as the density log, except that where the density decreases, the NGR count rate increases.
Toward the bottom of the turbidites between 505 and 510 mbsf, an interval with relatively high count rates can be identified. The high count rates coincide with intervals of reddish brown silty claystone.
Count rates in the basalt are slightly higher in the top 15-20 m, with an average of ~6 cps. At depths greater than ~530 mbsf, the NGR emissions reduce to ~4 cps then appear to increase slightly toward the bottom of the section. Overall, in comparison to the sediments, NGR emissions in the basalt are relatively low and uniform throughout the interval.
P-wave velocity increases in discrete steps from ~1.6 km/s at the seafloor, to ~2.5 km/s in the turbidite sequences, and ~5.5 km/s in the basalts (Fig. F75D). The velocities measured in the pelagic muds are relatively uniform, varying ±0.1 about an average of 1.63 km/s. In contrast, the velocities of the turbidites vary much more, ±0.8 about an average of 2.46 km/s. This variation is due to the changes in porosity, mineralogy, and cementation within individual turbidite sequences and within the entire turbidite section. Within a single turbidite sequence, for example the more massive sequence between 90 and 130 mbsf, velocities decrease with increasing depth. In general though, the velocities are aliased, that is, the sampling interval for the velocity measurements was too large to show the individual turbidite sequences. Also, because the velocity depends on both mineralogy and porosity, a decrease in velocity toward the top of an individual turbidite sequence due to the presence of clay minerals might be offset by a concomitant decrease in porosity. The velocities of the basalts increase from just over 4.0 km/s at the top of the basalts to >5.0 km/s ~10 m into the basement. This increase in velocity is likely due to decreased alteration of the basalts with depth.
Acoustic impedance (Z),
defined as the product of density (
)
and velocity (V) (Equation 2), is calculated for each index properties
sample (Fig. F75E):
x V.
(2)
Major changes in impedance coincide with changes in lithology from pelagic mud to turbidite to basalt. The reflection coefficient (R), defined as the ratio between the amplitude of the incident and reflected seismic waves (Equation 3), was determined at the interface between each lithologic unit (Fig. F75E):
Although the seismic record shows strong reflections at these interfaces, it also shows reflections with as great or greater strength throughout the entire top half of the turbidite succession. Because the impedances of the turbidites vary widely, these strong reflections are likely caused by the constructive interference of seismic waves reflected within the turbidites. Figure F76 compares the seismic record to a synthetic trace that convolves the impedances shown in Figure F75E with a 15-Hz sine wavelet. The synthetic trace does not include the effects from attenuation, multiples, or gain control imposed during analysis of the seismic data. It does, however, show that many of the reflections shown in the seismic record are caused by the rapidly varying impedance contrasts.
Seismic anisotropy,
measured as the ratio of the horizontal P-wave velocity (Vh)
to the vertical P-wave velocity (Vz), is shown in
Figure F77. The
pelagic muds show a slight anisotropy (Vh/Vz
1.03) with little variation. The anisotropy in the turbidites is
initially very close to unity, with values ranging from 0.95 to 1.1. At ~300
mbsf, the variability and magnitude of the anisotropy in the turbidites
increases. These increases correspond to smaller-scale and finer-grained
turbidite sequences, as shown in the sedimentary record (Fig. F66D).
The anisotropy of the basalts varies from 0.9 to 1.04. The values of anisotropy
are likely caused by microcracks created either by thermal stresses as the
basalt cooled or possibly by stress relief and fracturing during recovery of the
core.
The shear strength of the pelagic muds was measured once per section on the working half of the core from Hole 1201B within 30 min of splitting the core. The measured peak shear strengths are relatively constant, with values ranging between 3.9 and 24.0 kPa at depths of <40 mbsf. From 40 to 46 mbsf, the peak shear strength rapidly increases to a maximum of nearly 100 kPa as the interface between the pelagic muds and the turbidites is crossed (Fig. F78).
The formation factor was determined from resistivity measurements made using a four-probe array with a 13.3-mm probe spacing. The probes were inserted to their full 15-mm length in the split core surface to provide consistent apparent resistivity measurements but not absolute values. Pore water was assumed to have the same electrolytic properties as surface seawater.
Formation factors range from 1.92 to 3.67, with a mean value of 2.62 (Fig. F79). The formation factor does not vary linearly with increasing depth. There appears to be a region between 20 and 35 mbsf where the formation factor is reduced by ~25%.
Thermal conductivity measurements fall into three distinct ranges that are defined by major changes in lithology. Measurements vary between 0.73 to 0.86 W/(m·K) in the pelagic muds, between 0.94 and 1.21 W/(m·K) in the turbidites, and between 1.41 and 1.74 W/(m·K) in the basement (Fig. F80). The thermal conductivity values vary little with depth in each lithologic layer, with the possible exception of the basement, which shows a slight increase in thermal conductivity within the first 50 m.
In situ temperature measurements were made in Hole 1201C using the Adara temperature probe. The temperature record is shown in Figure F81. Also shown on the figure are the seafloor or mudline temperature (Tml) and the estimated sediment temperature at depth (Tsed).
The sediment temperatures at depth were determined using the curve fitting program TFIT. The estimates of sediment temperatures can vary several degrees depending on the value of the thermal conductivity of the sediment and the region of the decay curve chosen for the estimation. The estimated sediment temperature shown in Figure F80 was found using only the latter portion of the middle temperature series, the temperature decay curves.
The mudline temperature was determined to be 1.51°C. The sediment temperature at a depth of 44.6 mbsf was measured to be 7.08°C, yielding a thermal gradient of 0.12°C/m. A mean thermal conductivity for the pelagic mud of 0.786 W/(m·K) was determined from the measured values in Table T12. Because there was negligible variation of the thermal conductivity with depth in the pelagic muds (Fig. F79), the integrated thermal resistivity was not calculated. Instead, the heat flow was calculated directly using Equation 4:
T/z),
(4)
where,
T/z
= the thermal gradient.
It should be noted that this value of heat flow assumes that all heat flow is due to conduction. The heat flow in Hole 1201C of 98 mW/m2 is higher than the global average of ~50 mW/m2 (Garland, 1979).
