m.
At Site 1245, three holes were used for physical property analyses: Holes 1245B (0-473 mbsf), 1245C (0-221 mbsf), and 1245E (473-540 mbsf). Routine physical property measurements were performed on the recovered core sections from these holes, except for Hole 1245E where the RCB coring technique was used. RCB cores were not suitable for high-quality MST measurements; however, discrete samples were taken for moisture and density (MAD) analyses. The data quality for these RCB core samples is generally comparable to those obtained from APC and XCB cores, but there is a larger uncertainty in the coring depth of samples from these RCB cores because of the poor core recovery.
Standard IR imaging was performed at Site 1245 on cores from Holes 1245B and 1245C. RCB cores from Hole 1245E were not imaged. The first cold-spot anomalies associated with the presence of gas hydrate were detected in cores from Holes 1245B and 1245C, just below 50 mbsf. The deepest occurrence of cold anomalies in Hole 1245B (129.4 mbsf) matches well with the depth of the BSR at 134 mbsf, as estimated from seismic data.
IR imaging of cores drilled at Site 1245 provided on-catwalk identification of hydrate zones in each core, as described in the Site 1244 chapter and in the Explanatory Notes chapter (see "Physical Properties" in the "Site 1244" chapter and "Physical Properties" in the "Explanatory Notes" chapter). This information was used to facilitate hydrate sampling and preservation for all cores. The IR thermal anomalies for Holes 1245B and 1245C are cataloged in Table T15 and include an interpretation of the overall hydrate texture for each anomaly. The majority of the hydrates detectable by IR imaging for Hole 1245B (75%) are present as apparently disseminated layers. Nodular textures account for the remainder of hydrate occurrences (25%). For Hole 1245C, 66% of the IR images indicate disseminated hydrate; nodular and vein hydrate each account for 17%. The preponderance of both disseminated and stratigraphically conformable layers of hydrate in Holes 1245B and 1245C suggests that differences in permeability and porosity related to bedding may control the presence of hydrate at Site 1245.
Cores from both holes show strong cold anomalies between ~5 and 120 mbsf (Figs. F35, F36). Hole 1245B also has a single cold anomaly at 129 mbsf corresponding to the BSR depth (134 mbsf). Figure F36A shows the magnitude of the temperature anomalies as function of depth for both Holes 1245B and 1245C plotted back to back. Anomalies in the two holes are similar, with a sharp onset at 49.9-51.5 mbsf and cessation of anomalies at 119.5 to 121.3 mbsf, except for the single anomaly near the BSR in Hole 1245B. Although individual anomalies cannot be correlated between the holes, the depths at which clusters are observed are consistent, suggesting that hydrate occurrence is stratigraphically controlled.
The IR thermal anomalies associated with hydrate are also consistent with pore water saturation (Sw) estimated from LWD data (Fig. F36B) (see "Downhole Logging") except near the BSR, where LWD logging indicates greater presence of hydrate than observed using IR imaging. This difference may be explained by the possible inability of IR imaging to detect low concentrations of hydrate disseminated over a relatively large zone and by heterogeneity in the actual concentrations of hydrate between the two holes.
The sediment densities derived from MAD, GRA, and LWD data all generally increase downhole, which is the trend caused by normal compaction of the marine sediments (Fig. F37). Density values at the seafloor are ~1.6 g/cm3, but these increase downhole to ~2.0 g/cm3 at 540 mbsf, resulting from the effect of sediment compaction (Table T16). Porosities decrease from values of ~65%-70% at the seafloor to <50% at 540 mbsf. Grain densities do not show any apparent trend with depth and have an average value of 2.69 g/cm3.
Lithostratigraphic Units I and II have lower boundaries at 31 and 76 mbsf, respectively, and coincide with seismic Reflectors X and Y (Figs. F37, F38). These boundaries are also visible in the physical properties, especially in the LWD and MAD bulk density data. However, physical property anomalies cannot be correlated to the boundary between lithostratigraphic Units III and IV, which was defined at a depth of 212 mbsf (see "Lithostratigraphy").
A distinctive feature at this site is the Horizon A density anomaly at a depth of 180 mbsf. This horizon is characterized by very low sediment densities of <1.4 g/cm3 as determined from GRA and LWD measurements. However, the discrete MAD samples only show a change of 0.1 g/cm3 across this interval, possibly due to the spacing of discrete samples. Porosity is slightly reduced by ~5%, and grain density is slightly lower at 2.65 g/cm3. The decrease in GRA and LWD density is mainly caused by the presence of abundant ash layers (see "Lithostratigraphy"). Horizon A shows two distinctive intervals of reduced density (Fig. F39). This double low-density feature is also seen at Site 1248, where Horizon A was cored at a depth of 128 mbsf (see "Physical Properties" in the "Site 1248" chapter).
The MS record in the upper 200 mbsf is characterized by small variations with the exception of three major events at 86, 108, and 150 mbsf, respectively (Fig. F37). All three spikes are associated with turbidite events, as determined from core descriptions (see "Lithostratigraphy"). Below 200 mbsf, the overall magnitude of the MS increases and the record is characterized by many individual spikes over relatively short intervals. The most prominent spike at 372 mbsf does not clearly correlate to a seismic event or major lithostratigraphic boundary. Observation of the split core section at this depth, there is no indication of a turbidite layer or a greater abundance of sulfides.
The prominent Horizon A, clearly defined in the sediment density record, is not associated with large MS variations. Small variations in MS, however, can be correlated to the observed individual ash layers (see "Lithostratigraphy"). The ash layers are associated with low MS, whereas the sandy/silty layers show higher MS values.
Resistivity was determined using the Non Contact Resistivity (NCR) system on cores recovered from Holes 1245B and 1245C (Fig. F40). However, the measurements are dominated by the gas expansion cracks, which act as electrical insulators. It is interesting to note that the degree of cracking as determined by the noncontact resistivity (NCR) decreases dramatically below the BSR at 134 mbsf. Where gas cracks are less abundant, the measured sediment resistivity values vary around 1 m.
At Site 1245, compressional (P)-wave velocity (VP) was measured on shallow cores recovered from all holes. In Hole 1245B, velocities were measured using the MST on Core 204-1245B-1H from seafloor to a maximum depth of 8.5 mbsf. On the split half sections, measurements were carried out using all three VP sensors mounted on the Hamilton Frame up to a maximum depth of 10 mbsf (Fig. F41).
At shallow depths from the seafloor to 10 mbsf, there is a good correlation among the MST, VP , NCR, and the GRA density (Fig. F41). Gas expansion cracks started at ~8.5 mbsf and made further measurements impossible. From cores recovered from Hole 1245E, individual small samples were selected and trimmed with a saw for VP measuring using the PWS3 sensor only (Table T17). The VP values at depths greater than 470 mbsf vary between 1700 and 2090 m/s, and have an average value of 1994 m/s. Because of the higher noise level in the velocity measurements, the automated picker of the Hamilton Frame was not able to determine reliable velocity picks, and all VP values were determined based on handpicks of the waveforms.
Thermal conductivity was measured in Hole 1245B only (Table T18). Values range from 0.9 to 1.12 W/(m·K) (average = 0.978 W/[m·K]). No distinct downhole trend is evident, and no major correlation with bulk density variations was observed.
Shear strength was measured in Holes 1245B and 1245C using the automated shear vane and the handheld Torvane (Fig. F37; Table T19). Shear strength shows a linear increase with depth in the upper 10 mbsf (Fig. F41) but is dominated by large scatter at depth. This is partially the result of gas expansion cracks, which caused relatively unreliable measurements, in spite of the care taken to select the intervals with minimum disturbance.
At Site 1245, cores were recovered up to a total depth of 540 mbsf, which is the deepest hole penetration of this leg. Physical properties have been correlated to major seismic events, such as Horizons X, Y, and A. Horizons X and Y mark boundaries between the lithostratigraphic Units I, II, and III, respectively. Horizon A is the most prominent seismic reflector at this site (besides the BSR) and is characterized by low sediment densities. This horizon is characterized by sand and silt layers including large amounts of volcanic ash components. There is no MS anomaly associated with Horizon A, which is in contrast to other turbidite sequences observed during this leg.
IR imaging provided a robust and fast method of detecting gas hydrate at this site. In Hole 1245B, the deepest thermal anomaly at 129.4 mbsf is in good agreement with the seismically defined depth of the BSR at 134 mbsf.
