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doi:10.2204/iodp.proc.317.105.2011

Physical properties

Gamma ray attenuation (GRA) densitometer bulk density, magnetic susceptibility (loop sensor; MSL), natural gamma radiation (NGR), and P-wave velocity measured with the P-wave logger (PWL) were measured on all whole-round sections from Holes U1353A and U1353B. Magnetic susceptibility (point sensor; MSP), discrete P-wave velocity measured using the P-wave caliper (PWC), and spectrophotometry and colorimetry were measured on all section halves from the same holes. Discrete P-wave velocity measured with P-wave bayonets (PWB), moisture and density (MAD), and sediment strength were measured only on section halves from Hole U1353B. Unless otherwise noted, all depths in this section are reported in m CSF-A.

Gamma ray attenuation bulk density

GRA bulk density was measured with the densitometer at 2.5 cm intervals (measurement time = 3 s). The raw data range from –0.26 to 2.42 g/cm³ (Fig. F23). Cyclic variations in GRA bulk density may reflect varying concentrations of sands, which may not always be in their stratigraphic location within the section.

A comparison of GRA densitometer data with MAD data from Holes U1353A and U1353B yields an unexpected relationship (Fig. F24). Typically, GRA densities are equal to or lower than MAD densities because of coring disturbance, but at this site GRA density estimates are consistently higher than MAD estimates by ~0.08 g/cm³. A calibration problem with the GRA instrument was suspected, and measurement of the water standard after core measurements had been completed seemed to support this conclusion, giving a density of 1.1 g/cm³. However, density measurements of distilled water standards during each run of the actual measurements were only 1.03 ± 0.05 g/cm³ at the time Site U1353 cores were measured. This bias was observed during the entire expedition and is insufficient to account for the total differences between GRA and MAD bulk densities at Site U1353.

An alternative explanation is that MAD densities are too low. However, we could not imagine any mechanism that could cause this. One possibility is that systematic addition of water to the samples after whole-round analysis could have somehow affected MAD results. However, MAD samples were collected both before and after whole-round measurements, and both data sets are consistently lower than GRA densitometer measurements (Fig. F24). Systematically low measurements of MAD sample mass or overestimates of volume in the pycnometer seem unlikely because of regular calibration. To date, a satisfactory explanation has not been put forth.

Magnetic susceptibility

Magnetic susceptibility (MSL) was measured at 2.5 cm intervals (measurement time = 2 s), and magnetic susceptibility (MSP) was measured at 5 cm intervals. MSP measurements were made on all sections unless drilling disruption or surface disruption precluded collection of meaningful results (Fig. F23).

Raw MSL data range from 5.5 to 227 instrument units in Hole U1353A and from 0 to 199 instrument units in Hole U1353B (Fig. F23). The signal was filtered using a Gaussian low-pass filter (30 passes; Fig. F23).

In the uppermost ~260 m, where core recovery was greatest, MSL and MSP data are highly variable. In contrast, below ~260 m, where core recovery was poor, magnetic susceptibility is much less variable. Overall, magnetic susceptibility decreases downhole, but within this trend several shifts were observed. Major shifts between 75 and 79, 160 and 174, 205 and 217, and 258 and 270 m may reflect the presence of unconformities. Below ~575 m, magnetic susceptibility appears to increase slightly. However, the corresponding interval is too short for reliable interpretations.

Two abrupt increases in magnetic susceptibility occur at 13–17 and 27–36 m. These intervals are associated with thick sandier units (see "Lithostratigraphy;" Fig. F25). Coeval changes were also observed in NGR (low values likely reflect low clay content), GRA bulk density (lower densities), and P-wave velocities (higher velocities) (Figs. F23, F26). Several prominent peaks in magnetic susceptibility occur (e.g., at 26, 36, 43, 49, 57, 62, and 68 m) (Fig. F25). These peaks may be linked to caved shell-hash material at the top of each core liner, as observed previously at Site U1351. These findings suggest that the noisy magnetic susceptibility signal below ~100 m may be mainly the result of caved material at the top of cores or sandy material sucked into the bottom parts of core liners (flow-in). High magnetic susceptibility related to shell hash might be caused by fine-grained magnetic minerals, which were found to be associated with these sediments in both smear slides and in association with foraminifer samples, or might be caused by rust or other magnetic minerals used in shipboard operations (Richter et al., 2007).

Natural gamma radiation

NGR was measured on all core sections at 10 cm intervals down to 587 m. However, poor core recovery means that only the record from the uppermost 260 m is usable, with mainly "spot" values available below this depth. Even the upper record contains a coring gap between 80 and 105 m.

NGR values measured over the uppermost 260 m range from ~3 to >80 cps, with higher values associated with muddy lithologies and lower values associated with sands (Fig. F23). No particular trend occurs in the record, which is, however, divided into two portions. The interval between 0 and 80 m exhibits a striking "bimodal" cyclicity between high values that correspond to mud and low values that correspond to sand (see "Magnetic susceptibility"). Assuming, tentatively, that the first two cycles downward correspond to marine isotope Stages (MIS) 1–5 and 6–7, these last two glacial–interglacial cycles have a combined thickness at Site U1353 of ~30 m (36 m to the base of the inferred MIS 7 sand, minus 6 m of Holocene muddy sand at the top of the section). An alternative interpretation is that the cyclicity in the uppermost 36 m at Site U1353 represents only MIS 1–5, which would imply that MIS 2 and 3 are, unusually, fully manifested in the sand/mud packages present.

The section between 105 and 260 m has a more closely spaced, peaked cyclicity than the cycles above, which is consistent with either lower sedimentation rates, sediment compaction, or both. This part of the section is not entirely continuous, but, allowing for the minor gaps, the cyclicity appears to represent at least an additional eight glacial–interglacial cycles. Because of the coring gap between 80 and 105 m, which also corresponds to a demonstrated hiatus, these cycles cannot yet be correlated with the MIS scale and therefore remain as a "floating" package of cycles of broadly mid-Pleistocene age.

The striking cyclic bimodality of the upper two cycles at Site U1353, noting the associated lithologic variations recorded during core description, are similar to extremely shallow water, near-shoreline sedimentary successions described in early Pleistocene cycles in the Nukumaru coastal succession, Wanganui Basin (Birdgrove and Maxwell motifs of Abbott et al., 2005). These successions are distinguished, in part, by the position of the shoreface sand within each type of cycle—toward the top of the cycle ("regressive") for the Birdgrove motif and at the base ("transgressive") for the Maxwell motif.

P-wave velocities

P-wave velocities were recorded continuously in Holes U1353A and U1353B at 2.5 cm intervals using the PWL. Velocities were also recorded with the PWC in both holes and with the PWB in Hole U1353B (Fig. F26). P-wave measurements yielded unexpectedly good results in the muddy portions of the sediment to below 585 m. This is remarkable because P-wave velocity records from Sites U1351 and U1352 were obtained only in the first ~22.5 and ~20 m of soft sediments, respectively. The long record at Site U1353 is a result of the absence of sediment cracking caused by high gas content that was observed at the previous sites. With this data set, an excellent positive correlation was found between PWL estimates from both holes, and a good correlation was found between PWL, PWC, and PWB estimates in Hole U1353B. Nevertheless, PWB P-wave velocities (Fig. F26B) are generally lower than velocities recorded with the PWL and PWC.

Overall, P-wave velocities increase downhole (from averages of ~1500 m/s to ~1800 m/s) in three major steps at 6, 76–106, and 224–229 m. The steps between 76 and 106 m and between 224 and 229 m might reflect the presence of unconformities.

Two abrupt changes in P-wave velocity to higher values between 14 and 18 m and between 26 and 28 m are associated with thick sand layers (see "Lithostratigraphy"). A similar pattern was observed in magnetic susceptibility, NGR, and GRA bulk density (Fig. F23).

Spectrophotometry and colorimetry

Spectrophotometric measurements and associated colorimetric calculations were made on section halves at 5 cm intervals at the same positions as MSP measurements. Color data were recorded as L*, a*, and b* variations. Changes in color are more abrupt and pronounced at Site U1353 than at Sites U1351 and U1352 (Fig. F27). This is most conspicuously evident in the uppermost ~250 m of Hole U1353B, where core recovery was greatest (Fig. F28). In particular, two clear, abrupt excursions to lower L* and b* values and higher a* values are evident in the uppermost 36 m of Hole U1353B. These shifts coincide with sandy intervals (see "Lithostratigraphy"). At the stratigraphic level of these excursions, pronounced changes in magnetic susceptibility and NGR also occur (see "Magnetic susceptibility"), with the shift to lower L* values being coincident with high magnetic susceptibility and low NGR (Fig. F28). Downhole, between ~183 and ~189 m, a similar shift to low L* and b* and high a* values is present, coeval with excursions to minimum values in both magnetic susceptibility and NGR and reflecting the occurrence of gravel (Fig. F28; see also "Lithostratigraphy"). Based on these observations, it is clear that the lithologic factors that determine sediment color, magnetic susceptibility, and NGR do not always produce the same signatures at Site U1353.

Low recovery below ~250 m precludes a determination of long-term trends in color data in the lower part of Hole U1353B, although the variation in values below ~250 m demonstrates that high variability in sediment color is a pervasive feature of the entire cored interval (Fig. F27).

Moisture and density

MAD measurements were expanded somewhat at Hole U1353 to perform a sampling experiment. MAD samples were taken on the catwalk from one end of whole-round core sections (MAD catwalk samples) immediately after the sections were cut. One MAD catwalk sample was taken from each core in the uppermost 54 m of Hole U1353A and from cores in the uppermost 178 m and in the interval of 547–617 m (base of the hole) of Hole U1353B in order to extend the test to lower porosity samples. In Hole U1353A, two subsequent samples were taken on the sampling table, one next to the MAD catwalk sample and one near the location of the thermal conductivity measurement. One sample table sample was taken adjacent to the MAD catwalk sample for Hole U1353B cores. Generally, one or two additional samples were taken per core so that the total number of MAD samples was not much greater than the normal 3–4 samples per complete core or 1 sample per section for poor-recovery sections. Unfortunately, poor-recovery sections characterize the bulk of Hole U1353B below ~80 m.

A comparison of MAD catwalk sample porosity results with those of samples taken from the sampling table reveals that, on average, the results are essentially the same (Fig. F29). In fact, they are correlated with a regression coefficient (R) of 0.96. Although MAD catwalk samples have higher porosities in a few cases (notably those taken in the uppermost 20 m), at least as many samples have higher porosities after arriving on the sample table. It would be good practice to take MAD samples prior to discrete P-wave measurements. In any case, analytical variability in MAD data is not unexpected, and some of the variations may be due to real variations in porosity and lithology over very short distances. In some cases, the differences between results from the two sampling methods are as high as 4%–5% and, as such, are higher than the expected analysis error of at least 1%–2%.

Porosities decrease much more rapidly at Site U1353 than they do at either Site U1351 in a more distal shelf setting or at Site U1352 in an upper slope environment (Fig. F30). With the exception of grain density, curve fitting revealed an excellent relation between these physical properties and depth of burial (Fig. F30). The higher rate of compaction in the uppermost 20 m reflects faster compaction, where the grains move from loose packing toward closest packing, which takes relatively little overburden to achieve (e.g., Bond and Kominz, 1984). Bulk density increases linearly with depth, whereas grain density does not show any statistically significant change with depth. Correlations of porosity and void ratio with depth are stronger as exponential curves:

and

where

• ρ_b = bulk density in g/cm³,
• ρ_g = grain density,
• ϕ = porosity in percent,
• v = void ratio, and
• z = depth in kilometers.

Porosity and density trends shift to higher and lower values, respectively, across an interval of poor recovery at ~90 m. This corresponds quite well to a hiatus indicated by biostratigraphic datums at 80 m (Fig. F30). A second hiatus at ~120 m is less well represented in the porosity and bulk density trends. A slight increase in bulk density and a corresponding decrease in porosity occur at the lithologic Unit I/II boundary at ~151 m (see also Fig. F24; see "Lithostratigraphy").

Sediment strength

Sediment strength measurements were conducted on working section halves from Hole U1353B using automated vane shear (AVS) and fall cone penetrometer (FCP) testing systems (Fig. F31). A comparison of both measurement methods is shown in the cross-plot in Figure F31C.

Shear strength indicates that sediments range from very soft (0–20 kN/m²) to very stiff (150–300 kN/m²). Vane shear and fall cone shear strength correlate well in very soft and soft sediments, but AVS values are about three times lower in firm to very stiff sediments (standard deviation = 28.3 kPa) than FCP values (standard deviation = 83.9 kN/m²). A similar pattern was observed at Sites U1351 and U1352. These findings suggest that the applicability of vane shear in firm to very stiff sediments is limited and that vane shear tests underestimate the strength of stiffer sediments. Overall, vane shear and fall cone strength data from Hole U1353B are positively correlated (Fig. F31A–F31B). Between 0 and ~250 m, shear strength increases generally, indicating a change from very soft to firm sediments. The generally lower sediment strength below ~250 m in cores from Hole U1353B coincides with the change from APC to XCB drilling.

A change in sediment strength to lower values between 14 and 18 m is consistent with sharp changes in magnetic susceptibility, NGR, GRA bulk density, and P-wave velocity and may suggest the presence of an unconformity (Figs. F23, F26). The pronounced cyclicity observed in the upper portion of Site U1351 was not observed at Site U1353.

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