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doi:10.2204/iodp.proc.323.109.2011

Physical properties

Site U1345 was spudded at a water depth of ~1008 m along the uppermost sector of the Beringian margin near the large, broad head of the Navarin submarine channel (Normark and Carlson, 2003). The underlying stratigraphic section, which exhibits sediment-filled canyon axes, is ~1 km thick and overlies a deeply subsided wave-cut platform of early Tertiary age. The platform is underlain by basement units of Cretaceous and Jurassic age (Cooper et al., 1987).

Five holes were drilled at Site U1345. Four of these holes (U1345A, U1345C, U1345D, and U1345E) were drilled to similar depths ranging from ~147 to 150 mbsf; the microbiology-dedicated hole (U1345B) was drilled to 36.7 mbsf. Cores from Holes U1345A, U1345C, U1345D, and U1345E were placed sequentially on the "fast track" Special Task Multisensor Logger (STMSL) and the Whole-Round Multisensor Logger (WRMSL) systems to measure bulk density and magnetic susceptibility; cores from Hole U1345B were scanned only on the STMSL before being stored in the cold room for microbiological sampling. These same cores were scanned for NGR, and at least one section, commonly the undisturbed Section 2, of each core was selected for thermal conductivity measurement. Determination of moisture and density (MAD) physical properties was conducted only on cores retrieved from Hole U1345A. Core continuity was typically broken and disturbed by gas-expansion cracking and gapping. As a consequence, WRMSL P-wave measurements were not taken for cores collected from Site U1345 holes.

Only one lithologic unit, Unit I, was defined for the sediment penetrated at Site U1345 (see "Lithostratigraphy"). Unit I is composed of silt and clay with subordinate but important volumes of sand. Lesser components are provided by diatom frustules and lesser amounts of other microfossils. The section recovered at Site U1345 is the most coarse grained and siliciclastic of all stratigraphic sequences drilled during Expedition 323.

GRA and MAD (discrete-sample) wet bulk density

Wet bulk density recorded by the WRMSL GRA densitometer is indicated by the continuous line in Figure F24A. Also shown on this figure are discrete sample measurements of bulk density by the MAD methodology (open circles). Figure F24B is a line plot of only MAD bulk density readings. The two profiles of downhole bulk density are generally similar in the uppermost ~70 mbsf but are dissimilar at greater depths, where MAD density readings are significantly higher than GRA measurements. Both profiles display a small downhole increase in density. GRA measurements increase from a near-seafloor value of ~1.65 to 1.75 g/cm³ at the bottom of the hole, and MAD determinations increase from a near-seafloor value of 1.64 g/cm³ to a hole-bottom average of ~1.85 g/cm³ (Fig. F24). The reason for the difference between the two profiles in the lower part of the hole is not understood. Calculation or calibration errors may have affected the accuracy of MAD density determination for the lower half of Hole U1345A. However, wet bulk densities in Hole U1345A appear to be higher by 0.1–0.2 g/cm³ compared to density measurements from Hole U1344A in the uppermost 150 m of the sedimentary section; the higher densities in Hole U1345A probably reflect the sediment's higher sand content.

Similar to stratigraphic sections drilled at Beringian margin sites (U1343 and U1344), GRA density values in Hole U1345A also document rhythmic fluctuations, with readings as high as ~2.05 g/cm³ and as low as ~1.40 g/cm³. The longer wavelength between oscillations is ~25–50 m. The change in wavelength presumably reflects time changes in the deposition rate and texture of siliciclastic material.

Magnetic susceptibility

The downhole distribution of magnetic susceptibility is displayed in Figure F25. The profile recorded by the WRMSL scanner outlines sharply defined highs separated by broad, low-relief lows. The profile is similar to the blade contour of a logger's high-toothed felling saw. Peak values rise to ~170 SI units, above lows averaging ~20 SI units. Although core sections that passed by the magnetic susceptibility detection coils were disrupted by gas-expansion cracks, the measurements seem to have realistically recorded downhole variations in magnetic susceptibility, which are functions of many factors (see "Paleomagnetism").

Natural gamma radiation

The downhole profile of NGR readings displayed in Figure F26 is broadly similar to that of GRA bulk density (Fig. F24A). Recorded NGR values are lowest (~25 counts/s) in poorly consolidated and relatively porous near-seafloor sediment. Below ~15 mbsf, average NGR readings increase rhythmically from ~33 to 38 counts/s at the bottom of Hole U1345A (~145 mbsf). The slight increase may reflect a downsection increase in clay mineral content, but it most likely also reflects sediment compaction and densification tracked by a downsection increase in GRA bulk density and a matching decrease in porosity and water content (Fig. F27). The estimated hole-average NGR reading of ~35 counts/s is slightly higher than the ~30 counts/s of the uppermost ~150 m of section cored at other Beringian margin sites (U1343 and U1344). The higher NGR in Hole U1345A may result from the coarser and higher content of siliciclastic mineral debris.

MAD porosity and water content

Downsection profiles of MAD-measured porosity and water content record a progressive decrease in average values (Fig. F27; Table T16). Near-surface porosity is ~68%, lower than that estimated (75%) for the finer grained sediment of nearby Hole U1344A. Porosity in the lower 15 m of Hole U1345A decreases to ~53%, the lowest average recorded at Beringian margin drilling sites. The downhole distribution of water content and porosity is rhythmic. Within the lithologically similar deposits of Unit I, the trend of downhole-decreasing porosity and water content is surmised to reflect compaction and dewatering. Oscillations about these general gradients are likely caused by textural variations in the dominantly siliciclastic clay, silt, and sand beds of Unit I.

Grain density

Figure F28 documents little change in average grain density with depth. The estimated hole average is 2.75 g/cm³, which is higher than that observed (2.69 g/cm³) in the uppermost 150 m of Hole U1344A. The higher average grain density of Unit I in Hole U1345A is interpreted to be a consequence of its greater abundance of coarse siliciclastic grains.

Thermal conductivity

The downhole distribution of thermal conductivity readings (Fig. F29) displays an overall trend of increasing conductivity, from a near-surface value near ~1.00 W/(m·K) to ~1.23 W/(m·K) at the deepest measurement just above ~140 mbsf. Oscillations about the average are high, ranging from 1.32 to ~0.810 W/(m·K). The estimated thermal conductivity hole average is ~1.07 W/(m·K), which is higher than the average thermal conductivity value (0.942 W/[m·K]) for the uppermost ~150 m cored in Hole U1344A. With respect to Unit I of Hole U1343A, the higher thermal conductivity of Unit I of Hole U1345A can be attributed to its higher sand and silt content and lower porosity.

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