Downhole logging was performed in Hole 1239A after it had been drilled to a depth of 517 mbsf with an 11.438-in APC/XCB drill bit (see "Operations") and displaced with sepiolite mud and the pipe was set at 80 mbsf. Two tool string configurations were run, the triple combo-MGT and the FMS-sonic (see "Downhole Measurements" in the "Explanatory Notes" chapter). No problems were encountered while logging, and all passes reached the base of the hole. Details of the intervals logged with each configuration, together with the position of the drill bit, are shown in Figure F30. The Dipole Sonic Imager (DSI) on the FMS-sonic was run in P&S (middle frequency), lower dipole (low frequency), and first motion detection (FMD) modes. Weather was excellent and the sea state was calm with peak heave <1.5 m. The wireline heave compensator was used throughout the logging operation.
The caliper log (Fig. F31) shows that the borehole was relatively smooth and that the diameter varied between 11.8 and 15.1 in (mean = 12.9 in; standard deviation = 0.9 in), resulting in excellent data from the density, porosity, and FMS tools that require good borehole contact. Although the hole deviation increased with depth, reaching 7.5° at the base, FMS pad contact was not affected and the images were good from the bottom of the hole to 110 mbsf, where the calipers were closed. Downhole log-derived densities mirror the downhole porosities and closely match with core measurements (Figs. F32, F33). NGR measurements are highly reproducible between tools and passes and are similar to the core-derived natural gamma record from Hole 1239A (Figs. F31, F34). Sonic velocities were low but reliable, reproduced well between passes, and are comparable to core measurements (Fig. F32). During the first pass with the MGT, a power failure occurred while logging between 489 and 479.5 mbsf and no data were acquired in this interval (Fig. F31).
Overall, the physical properties at Site 1239 are relatively homogeneous, suggesting a fairly uniform lithology throughout the sequence. Sonic velocities and densities increase gradually with depth, whereas mean resistivity and porosity both decrease downhole. These large-scale changes in the sediment physical properties are most likely related to sediment compaction and lithification with depth. At higher frequencies, covariations in density and resistivity, similar to those observed at Site 1238, occur throughout the sequence. As at Site 1238, these meter-scale density and resistivity fluctuations are most likely related to the biogenic silica to carbonate oscillations observed in the cores (see "Lithostratigraphy"). Color banding on the FMS images occurs on the same scale as the density changes associated with the nannofossil to diatom oscillations (Fig. F35).
A pronounced spike in resistivity, sonic velocity, and density (porosity minima) occurs at ~493 mbsf, similar in magnitude to the spikes in physical properties found near the base of Hole 1238A related to well-lithified chalk beds. The spikes in physical properties occur much deeper (>100 mbsf) in the section at Site 1239 relative to Site 1238, and the well-cemented chalks and chert layers found at Site 1238 were not recovered at Site 1239. A deeper diagenetic front at Site 1239 is consistent with the offset in pore water silicate concentration profiles between the sites, suggesting that the lower thermal gradient at Site 1239 may be important in increasing the depth of chert formation at this site (see "Geochemistry").
The NGR activity in Hole 1239A shows significant meter- and dekameter-scale variability superimposed upon a general decrease with depth (Figs. F31, F36). The spectral gamma results from the Hostile Environment Gamma Ray Sonde (HNGS) tool (Fig. F36) show low, regularly varying Th and K activity throughout the sequence. In contrast, the U activity and variability is much greater, reaching its highest values between 95 and 111 mbsf. The high overall U activity dominates the total gamma ray activity of the sediments. As at Site 1238, the U log shows a similar overall trend to the TOC measured in the cores (Fig. F36), indicating that changes in organic matter rather than terrigenous input controls sediment gamma ray activity at Site 1239 since the late Miocene. Although both the U and total counts are slightly higher at Site 1239 relative to Site 1238, the magnitude of the variability and overall trends are the similar at both sites.
Log-derived density and NGR records show close agreement with core measurements from the Hole 1239A down to the meter scale (Figs. F33 and F34, respectively). On average, downhole densities are often slightly higher (~0.5 g/cm3) than core measured densities and do not show the pronounced minima (<1.2 g/cm3) evident in the raw GRA density record. The difference in mean values may be due to core expansion, whereas the frequent minima evident in GRA density may be due to the higher resolution of core measurements or erroneous values resulting from coring disturbance. However, on the meter scale, variability in core measurements closely matches downhole changes in density. Using the downhole log density and NGR records as a depth reference, the core measurements were mapped to equivalent log depths (eld) using the software program Sagan in order to more precisely identify the size and position of core breaks within the XCB section (see "Composite Section"). Despite the high recovery, after mapping to the logs the resulting gaps between XCB cores (1-3 m) are similar in scale to the period of many of the density and natural gamma ray fluctuations. Hence, we are able to identify a number of missed cycles in the XCB cored interval.