Downhole logging was performed in Hole 1238A after it had been drilled to a depth of 430.6 mbsf with a 11.438-in APC/XCB drill bit (see "Operations" in the "Explanatory Notes" chapter) and displaced with sepiolite mud, and the pipe initially was set at 99.4 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 tool configuration, together with the position of the drill bit, are shown in Figure F34. During each pass, the pipe was pulled to 84 mbsf to allow the upper unit to be logged. 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 <2 m. The wireline heave compensator was used throughout the logging operations.
The caliper data show that the borehole was relatively smooth and varied between 11.5 and 15 in (Fig. F35), resulting in excellent data from the density, porosity, and FMS tools that require good borehole contact. Although the hole deviation increased with depth, reaching 6.5° at the base, FMS pad contact was not affected and the images were good from the base of the hole to 117 mbsf, where the calipers were closed. Downhole log-derived densities mirror the downhole porosities (Fig. F36). Natural gamma radiation measurements are highly reproducible between tools and passes and also closely match the core-derived natural gamma radiation record from Hole 1238A (Figs. F35, F37, F38). Sonic velocities were also generally good and reproduced well between passes, apart from the interval between 387 and 410 mbsf, where the main pass should be used (Fig. F36).
Three logging units have been defined using porosity, density, resistivity, and sonic velocity, which show coherent downhole shifts associated with changes in formation lithology, competence, and lithification.
Throughout this unit, the mean values of velocity, resistivity, and density are relatively low, whereas porosity remains high. Sonic velocities and densities increase slightly with depth through this interval, most likely because of sediment compaction. Formation NGR is relatively high and variable within this unit. Density and porosity values show subtle but regular fluctuations throughout this interval, which are most likely associated with the nannofossil to diatom ooze oscillations observed in the cores (see "Lithostratigraphy").
At ~209 mbsf, mean velocities and densities increase stepwise while mean porosities decrease, marking the boundary between logging Units 1 and 2. This shift suggests an increase in sediment consolidation/lithification at this depth and is within the interval covered by the first XCB core. Sonic velocities within this unit continue to increase gradually with depth, whereas mean resistivity, porosity, and density values show no trends with depth and have variance similar to that in logging Unit 1. Color banding on the FMS images occurs on a scale similar to the density changes attributed to nannofossil/diatom oscillations (Fig. F39).
The increase in mean and variance of sonic velocity, resistivity, and density (and the attendant decrease in porosity) at 341 mbsf marks the top of logging Unit 3. In addition, a number of strong spikes in resistivity, sonic velocity, and density (porosity minima) punctuate logging Unit 3, occurring first at ~345 mbsf and with increased frequency below the most pronounced event at ~386 mbsf (Fig. F40). This mean rise in resistivity and velocity implies a general increase in formation lithification below ~341 mbsf, near the lithologic Subunit IA/IB boundary, which marks the ooze to chalk transition (see "Lithostratigraphy"). The most pronounced spikes in physical properties are likely caused by particularly well cemented or lithified layers. Indeed, the most prominent spike in physical properties occurs within the depth range of Core 202-1239A-42X (384.5-394 mbsf), where coring was difficult and recovery was only 34 cm of well-lithified chalk (see "Operations" and "Lithostratigraphy"). The statically normalized FMS image from the same interval shows a series of highly lithified (resistive) beds separated by centimeter- to decimeter-thick layers of less lithified (resistive) material (Fig. F41). The attendant decrease in pore water silicate concentrations (see "Geochemistry") with increased lithification suggests that sediment diagenesis drives the physical property changes in Logging Unit 3.
The NGR activity in Hole 1238A shows significant meter-scale variability superimposed upon a general decrease with depth (Figs. F35, F37, F38). The spectral gamma results from the Hostile Environment Gamma Ray Sonde (HNGS) tool (Fig. F37) show low, regularly varying Th and K activity throughout the sequence. In contrast, the U activity and variability is much greater, increasing significantly between 120 and 160 mbsf and again near the top of the open sequence (~95 mbsf) to the bit (84 mbsf), where the tool enters the pipe and gamma rays are attenuated. The high U activity dominates the total gamma ray activity of the sediments. The strong correlation between U and TOC measured in the cores (Fig. F42) indicates that changes in organic matter rather than terrigenous input has controlled sediment gamma ray activity at Site 1238 since the late Miocene (see "Geochemistry").
Log-derived natural gamma radiation and density records show close agreement with core measurements from the Hole 1238A down to the meter scale (Figs. F38 and F41, respectively). Using the downhole log records as a depth reference, the core measurements were mapped to equivalent log depths 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 are often similar in scale (~1-3 m) to the dominant cycle length in the density and NGR records. Hence, we are able to identify a number of missed cycles in core records (Figs. F38, F41).