PHYSICAL PROPERTIES

Only one hole was cored at Site 1247 in addition to the first hole drilled for the LWD program. Hole 1247B generally had good core recovery, but one core was not fully recovered around the BSR (at 124 mbsf). Standard procedures were used to measure the physical properties on recovered cores (see "Physical Properties" in the "Explanatory Notes" chapter). Hole 1247A, where the LWD data were acquired, is offset from Hole 1247B by 87 m. A complete set of IR images was acquired with the track-mounted camera prior to and after sectioning the core liner on the catwalk. In addition, the hand-held IR camera was used for quick identification of cold anomalies associated with the presence of hydrate. At this site, only two hydrate samples were recovered.

IR imaging provided rapid identification of hydrate on the catwalk. The track-mounted IR camera was used twice, both prior to and after sectioning the core liner. The second scan could then be directly correlated to the visual core descriptions and especially to the presence of the mousselike texture that is indicative of the presence of hydrate.

Data from the first IR scan were used to generate a downhole temperature profile at Hole 1247B (Fig. F22). The temperatures in the upper 20 mbsf are ~2°-3°C colder than those in the deeper part of the hole, but no discrete temperature anomalies were detected over this depth (Fig. F23). There was no indication of mousselike texture within the first three cores (see "Lithostratigraphy"), and the LWD resistivity data, converted to pore water saturation (S_w) using Archie's Relation, did not predict the presence of any hydrate in the upper 20 mbsf either (see "Downhole Logging"). The low temperatures observed may, therefore, be the result of a shorter core-handling time on the rig floor before the IR scan and/or seawater contact.

Significant T anomalies start to occur at a depth of 45 mbsf (Fig. F23). The anomalies cluster in two intervals, from 45 to 65 mbsf and from 80 to 120 mbsf. The BSR depth at Site 1247 is between 121 and 124 mbsf, which matches well with the last thermal anomaly at 118 mbsf (Table T10). There is an apparent mismatch between the S_w data and the specific depths of T anomalies, suggesting lateral heterogeneitiy in the presence of hydrate at this site. Note that the distance between Hole 1247A, where LWD data were obtained, and Hole 1247B, where cores were collected, is 87 m. Overall, 1 - S_w is relatively small with values below 0.2, indicating only small concentrations of hydrate present in the sediments, which is consistent with relatively small Ts from the IR imaging.

Hydrate samples were taken at 93 and 113 mbsf at Site 1247 after their identification by IR imaging. The temperature anomaly associated with interval 204-1247B-14H-5, 39-62 cm, was -2.8°C. However, close inspection of the sample after it had been preserved in liquid nitrogen did not show any visible hydrate crystals. Instead, the sediment texture suggested that disseminated gas hydrate had been present in this sample but was largely dissociated prior to storage in liquid nitrogen. Alternatively, the thermal anomaly could result from gas expansion or gas exsolution. Pore water chlorinities (Fig. F23) (see "Interstitial Water Geochemistry") are consistent with the presence of a few volume percent of hydrate at this depth, favoring the hypothesis that hydrate was present at 93-113 mbsf in Hole 1247B.

Sediment density increases with depth from values of ~1.6 g/cm³ at the seafloor to 1.8 g/cm³ at 220 mbsf (the bottom of Hole 1247B) (Fig. F24; Table T11). Generally, bulk density remains almost constant in the lower 50-60 m of Holes 1247A and 1247B at ~1.8 g/cm³.

There are several events in the density profile that can be correlated to lithostratigraphic units and seismic horizons (Figs. F24, F25). The bottom of lithostratigraphic Unit II at 60 mbsf is correlated with seismic Horizon Y. The results from moisture and density (MAD) samples indicate a zone of high variability within ~25 m above this unconformity. Individual bulk density values are higher by ~0.2 g/cm³ within this interval from 40 to 65 mbsf. The porosity of those samples is <50% compared to a background average of 55%-60%, and grain density is relatively higher. The scattering of bulk density values is the result of discrete sampling of individual sandy layers and the background clay-rich sediments. High sediment densities are not measured in the gamma ray attenuation (GRA) density data, probably as a result of the averaging effect of the GRA measurement.

A similar observation can be made at a depth of 181 mbsf (Sample 204-1247B-23X-1, 108-110 cm) (Fig. F26A). The sample was taken in the sand layers, resulting in a high density of 2.019 g/cm³ and a low porosity of 43%. It is interesting to note that the GRA density over this interval is also increased. A sample taken a few meters above (Sample 204-1247B-22X-6, 27-29 cm [171 mbsf]) was taken from a highly disturbed interval with apparent high porosity and low bulk density of 1.67 g/cm³ (Fig F26B). These samples are not representative of the general background trend and should be used with caution.

The GRA density data at a depth of 112 mbsf show elevated values that are not evident in the MAD or LWD density data. This interval corresponds to Section 204-1247B-14H-5 (Fig. F26C). Sample 204-1247B-14H-5, 30-32 cm, has a lower bulk density (1.81 g/cm³) than the two samples taken above. This core section shows higher sulfide content and a structure that probably reflects bioturbation.

Horizon A was cored at Site 1247 at a depth of 163 mbsf. There is a prominent decrease in the LWD density data, but this decrease is not so evident in the GRA and bulk density data. At Site 1247, Horizon A is not glass rich and does not contain ash layers, in contrast to its nature at other Leg 204 sites.

Below Horizon A, at a depth of 164 mbsf, the grain density shows a decrease to values of ~2.6 g/cm³. This decrease is consistent with the onset of lithostratigraphic Subunit IIIB, which is characterized by a high percentage of calcareous nannofossils (see "Lithostratigraphic Subunit IIIB" in "Lithostratigraphic Unit III" in "Lithostratigraphic Units" in "Lithostratigraphy").

The MS record is characterized by a series of individual spikes that can be correlated to the presence of sulfides and turbidites (Fig. F24). However, it is difficult to relate individual features in the MS data to the exact location of the turbidites in the subdecimeter range of the core description. The uncertainties that remain are related to relative shifts of sediments and voids in the core liner during the core-splitting procedure and at section boundaries where whole-round samples were taken on the catwalk.

The boundary between lithostratigraphic Unit II and Subunit IIIA is marked by a MS peak at a depth of 60 mbsf. MS is generally increased in the last 2 m of Core 204-1247B-8H and shows detailed variations that are related to turbidite sequences.

Another remarkable feature of the MS record is the presence of a sulfide vein at 55 mbsf that contained abundant concretions (Fig. F27). The concretions are rich in pyrrhotite as inferred from XRD analyses.

At this site, no compressional (P)-wave velocities (V_P) could be measured with the MST as a result of intensive gas expansion cracks. An equipment failure on the Hamilton Frame, also precluded V_P measurements on the split cores.

Thermal conductivity was measured following standard procedures (see "Physical Properties" in the "Explanatory Notes" chapter). Values are scattered and vary between a minimum of 0.83 W/(m·K) and a maximum of 1.112 W/(m·K) (Table T12). The average thermal conductivity is 0.99 W/(m·K). There is no obvious correlation between thermal conductivity and other physical properties, especially bulk density.

As a result of pervasive gas-expansion cracks, no shear strength measurements were carried out at this site.

Physical properties measured in Hole 1247B match well with the defined lithostratigraphic units. The boundaries are associated with peaks in MS (e.g., the boundary between Units II and III) or correlate to changes in sediment and grain density. Horizon A was cored at this site at a depth of 163 mbsf but did not contain high amounts of ash and does not show any significant change in bulk density. This is in contrast to the LWD data, which show a prominent drop in LWD density across this horizon in Hole 1247B.

IR thermal imaging provided the on-catwalk identification of gas hydrates, and at this site, two hydrate samples were taken based on negative temperature anomalies.