DOWNHOLE MEASUREMENTS

Downhole logging was performed in Hole 1168A after it had been drilled to a depth of 883 mbsf with a 9.875-in APC/XCB drill bit (see "Operations"). Two tool-string configurations were run, the triple combo and the GHMT-sonic (see "Downhole Measurements" in the "Explanatory Notes" chapter). The dipole shear sonic imager (DSI) on the GHMT-sonic was run in P, S, and upper dipole mode (see Schlumberger, 1995). The weather was good, with a maximum recorded heave of <3 m. The wireline heave compensator was used throughout the logging operations.

Three logging runs were attempted, although the last one was unsuccessful. Details of the intervals logged, together with the position of the drill bit and the depth of the seafloor (calculated from results of the gamma-ray log) are shown in Figure F44. During the first logging run with the triple combo, the passage of the tool string up and down the hole was frequently hindered by bridges in the formation. For this reason a wiper trip was conducted after the first run, with the aim of clearing any constrictions. A wiper trip involves pushing the pipe all the way back to the base of the hole and then pulling the bit back up to the required depth. Despite this action, it was still only possible to reach a depth of 728.5 mbsf with the second tool string, the GHMT-sonic (see Fig. F44).

Because of very poor hole conditions (see "Results/Data Quality," below) it was decided that a second attempt to log the base of the hole with the GHMT-sonic was a higher priority than running the FMS. There were two principal reasons for this: (1) the hole conditions were so poor (see "Results/Data Quality," below) that any FMS data would likely be of limited or no use and (2) the scientifically important Eocene/Oligocene boundary was at ~750 mbsf (see "Biostratigraphy"), close to the position where logging had stopped during the second run (Fig. F44). A second wiper trip was conducted and the bit was placed at 664.5 mbsf. Unfortunately, it was still not possible to get the tool string past the constriction and no further data were obtained. The principal results are summarized in Figures F45, F46, and F47.

The caliper data show that the borehole was uneven and highly variable in width (from 4 to >19 in), with the greatest rugosity occurring below 250 mbsf (Figs. F45). The poor hole conditions affected the readings from the density and neutron porosity tools the most because the sensors used to make these measurements require good contact against the borehole wall. The frequent association of high caliper readings (>18 in) with density minima and porosity maxima (Fig. F48) provides qualitative evidence of the effect that poor hole conditions had on these log values.

Comparing bulk density values from the high-temperature lithodensity sonde (HLDS) with results from the core (Fig. F49) shows that where hole conditions were poor, densities have been underestimated. This is particularly apparent between 300 and 500 mbsf. The density spike at 712 mbsf is evident in both the core and logging results (Fig. F49), suggesting that there is very little depth mismatch between the two data sets at this point. This density spike is correlative with an 18-cm-thick indurated sandstone bed recovered in the core.

A comparison of density porosity, neutron porosity, and core porosity (Fig. F50) shows that where hole conditions were poor, porosities have been overestimated. Above 300 mbsf, where hole conditions were reasonable, density porosities and core porosities are similar. However, the neutron porosities above 300 mbsf are significantly higher than both the density and core values. Because the formation above 300 mbsf has a very similar composition to limestone (grain densities = 2.7 g/cm³; see "Physical Properties"), the offset between the neutron and density porosities are most likely a result of the effect of bound waters in clays or mica. The 18-cm-thick indurated sandstone bed, evident in the density log, produces minima in density and core porosities at 712 mbsf (Fig. F50).

The resistivity, gamma, and magnetic susceptibility logs are least affected by poor hole conditions and will be most useful for core/log integration and for providing information on fluctuating physical, chemical, and lithologic properties downhole. The sharp decreases in gamma and spectral gamma-ray ratios between 420-430 mbsf and 718-741 mbsf result from the fact that the caliper had to be closed to get past a bridge in the formation; this tool requires the caliper to be open to enable a correction to be made for borehole diameter.

The P-wave sonic data are also relatively unaffected by the hole conditions. The same is not true, however, of the shear wave (S-wave) data. The S-wave velocities between ~197.7 and 527.5 mbsf may be inaccurate because the amplitude of the S-wave arrival at the upper dipole was low throughout the logging of this interval. Down to ~240 mbsf, P-wave velocities from the DSI compare well with the velocities measured in the PSW-3 direction from core samples (see "Physical Properties") (Fig. F51). However below ~240 mbsf, there is an increasing downhole discrepancy between the core and log results (Fig. F51). P-wave velocities were used to construct integrated traveltimes for this hole, which enable comparisons with the seismic section (see "Principal Results").

Reasonable total field measurements were also recorded by the GHMT tool during the second run and may allow at least a partial magnetic reversal stratigraphy to be generated after shore-based processing. The difficulties encountered in the upper carbonate-rich section (logging Unit 1) by the shipboard paleomagnetists will likely also apply to downhole polarity stratigraphy in this interval.

Log units have been defined using mainly resistivity, magnetic susceptibility, and sonic velocity data. These parameters are least affected by poor borehole conditions and tend to show the most variability downhole.

Log Unit 1: Base of the Pipe (97 mbsf) to 247.5 mbsf

Throughout this unit, the mean values of resistivity, magnetic susceptibility, sonic velocity, and natural gamma remain relatively low (0.958 ± 0.076 m, 340 ± 11.8 SI, 1846 ± 43 m/s, and 12.7 ± 3.4 gAPI, respectively) (Figs. F45, F46, F47). Density values are also quite constant (mean = 1.77 ± 0.11 g/cm³), whereas porosities gradually decrease from 84% at the base of the pipe to ~50% at the bottom of this unit. This downhole porosity decrease is most likely caused by sediment compaction. The photoelectric effect (PEFL) values are consistently high between 145 and 247.5 mbsf (average = 4), indicating a high carbonate content, consistent with the calcareous biogenic ooze and chalks that dominate the core material down to 260 mbsf (lithostratigraphic Unit I; see "Lithostratigraphy"). In addition, the low magnetic susceptibility and natural gamma values suggest the terrigenous component of the sediments is low. The lack of any distinct variability in the other log parameters suggests a relatively uniform lithology throughout the unit.

Log Unit 2: 247.5-540.0 mbsf

Magnetic susceptibility values increase sharply from ~350 ppm at 247.5 mbsf to ~550 ppm at 305 mbsf, implying an increase in the terrigenous component of the formation. Magnetic susceptibilities then remain relatively high through the remainder of the logged section. Similarly, gamma-ray, resistivity, and sonic velocity values all show a stepwise increase near the top of Unit 2 (247.5-305.0 mbsf) consistent with an increased terrigenous fraction and general consolidation of the sediments. The variability in all log parameters is greater throughout this interval and most likely reflects a more lithologically and compositionally varied section relative to the upper portion of the hole.

Log Unit 3: 540.0-580.0 mbsf

At 540 mbsf P- and S-wave velocities show a marked increase; P-wave velocities increase from ~1960 to ~2190 m/s, and S-wave velocities increase from ~500 to ~840 m/s. Densities and resistivities also increase within this unit, whereas porosities and magnetic susceptibilities decrease. The decrease in magnetic susceptibility and an associated increase in PEFL values suggest carbonate contents increase relative to the siliciclastic component of the sediment.

Log Unit 4: 580.0-848.0 mbsf

There is a decrease in resistivity (from 1.72 to 1.17 m), P-wave velocity (from 2350 to 2050 m/s) and density at the top of this unit. Magnetic susceptibilities are generally higher in Unit 4 than in Unit 3, suggesting a greater terrigenous component.

Log Unit 5: 848 mbsf-Base of the Hole

Resistivities increase very suddenly at 848 mbsf, to maximum values of 2.5 m in this unit. Densities also increase slightly at this depth. Unfortunately, this region is too deep in the section to be captured by most of the other logs.

The transition between log Units 1 and 2 corresponds with a lithologic transition from calcareous biogenic oozes (75-97 wt% CaCO₃) in the upper 260 m of the sequence (lithostratigraphic Unit I) to increasingly siliciclastic sediments below, consistent with the compositional shift inferred from log parameters. The highest values of core magnetic susceptibility below 260 mbsf are associated with clay-rich zones (see "Lithostratigraphy" and "Physical Properties") implying that magnetic susceptibility is strongly influenced by changes in the siliciclastic fraction of the sediment. However, the presence of pyrite in many intervals must also influence the magnetic susceptibility and may explain some of the differences between the magnetic susceptibility and natural gamma logs. Accurate assessment of the periodicity and ultimate origin of the observed cyclicity in core magnetic susceptibility (see "Lithostratigraphy") require that a continuous sequence with accurate depth control be used in the analysis. The strong correlation between continuous log and discontinuous core magnetic susceptibility records should allow the recovered XCB sections to be mapped back to their true stratigraphic depths, providing an estimate of the size and location of core gaps and the number of cycles missed. A good example of this is shown in Figure F52.

The zone of increased consolidation inferred from log responses (log Unit 3; 540-580 mbsf) is consistent with the increased lithification observed at 540 mbsf (the top of lithostratigraphic Subunit IIC, see "Lithostratigraphy"). The relative stabilization of log caliper readings below 540 mbsf is also consistent with an in-formation consolidation at this point.

The top of logging Unit 5 (848 mbsf), defined by the sudden increase in resistivity, corresponds to an observed change in clay mineralogy in the core. Between Cores 189-1168A-90X and 88X, there is an upward transition from predominantly kaolinite to smectite (see "Lithostratigraphy"). This mineralogic change may be related to an increase in tectonism leading to a sedimentary environment more typical of passively subsiding regions (see "Lithostratigraphy"). The short interval (<2 m) over which resistivity increases in the log section suggests that this environmental change is confined to a relatively brief depositional period.

One valuable application of the sonic velocity logging is the creation of a curve of depth (in meters below seafloor) vs. two-way traveltime (in seconds). For Hole 1168A, the velocity information from the sonic log was collated with the P-wave data from the MST from the top of the hole, where the drill pipe prevented the recording of good log data. The one-way velocities increased steadily down the hole from ~1600 m/s at the top to ~2100 m/s at the base of the log at ~720 m. The cumulative velocities (in two-way traveltime) have been plotted against depth in Figure F53 and are assumed to approximate to seismic velocities. By using the time/depth curve, we can correlate the seismic reflection profiles with well horizons, thus putting Site 1168 into the regional context. Figure F5 in the "Leg 189 Summary" chapter, indicates that some major reflectors correspond to various biostratigraphic/lithostratigraphic boundaries; "Me" to the top of the lower Miocene and to the Unit I/II boundary; "El" to the top of the upper Eocene and the Unit III/IV boundary.

Gas Hydrates

Geochemical and lithologic results from the core indicate the possibility of gas hydrates at this site. Headspace measurements show that methane concentrations reach a maximum of ~50,000 ppm at ~400 mbsf (see "Organic Geochemistry"), and observations of soft-sediment deformation observed in the core at ~325 mbsf may be indicative of the dissociation of methane clathrates (see "Lithostratigraphy"). At this stage, log measurements are unable to confirm or deny the presence of gas hydrates at Site 1168.

Simple thermodynamic models give a general estimate for the base of the gas hydrate stability zone at ~300 mbsf (see "Inorganic Geochemistry"). If hydrates were present at this site, then sonic data should show a sharp decrease in P-wave velocities at the depth of the inferred change from gas hydrates to free gas (see Guerin et al., 1999; Paull et al., 1996). There is indeed a sharp decrease in P- wave velocities at ~295 mbsf, from ~1940 to 1830 m/s (Fig. F46). Nevertheless, there is also a concomitant decrease in S-wave velocities and densities at this point, which should not happen if this zone marks a change from hydrate- to gas-bearing sediments (see Anstey, 1991; Guerin et al., 1999). However, the quality of the S-wave and density data are not particularly good in this interval (see "Results/Data Quality"). Gas hydrates are generally associated with increased resistivity values (e.g., Matthews and von Huene, 1985). There is a slightly anomalous spike in resistivities in Hole 1168A at 257 mbsf (Fig. F45), which could possibly indicate the presence of gas hydrates, although this depth does not correlate with the decrease in sonic velocities. Further postcruise work is required.

Organic-Rich Shales

The spectral gamma-ray values downhole (Fig. F47) show that a good correlation exists between Th and K concentrations. This is commonly observed in ODP holes, as both Th and K tend to be contained within the terrigenous clay component of the sediment (Hassan et al., 1976). At this site, U concentrations vary semi-independently of Th and K and, to a certain extent, of total gamma (HSGR). Increasing U concentrations are generally associated with increasing organic matter within the sediment, and particularly high U concentrations (>~5 ppm) and low Th/U ratios (<~2) are often associated with black shale deposits (Adams and Weaver, 1958; Doveton 1991). The logging results (Fig. F47) show that U concentrations increase steadily downhole from <0.1 ppm in log Unit 1 to >4.5 ppm at the base of the hole. The increasing contribution that U makes toward total gamma-ray radioactivity downhole is illustrated by the increasing offset between the HSGR (Th, K, and U radiation) and HCGR (Th and K radiation) curves (Fig. F47). This increase in U concentrations downhole is consistent with the lithostratigraphic results, which show increasing organic matter with depth. However, although Th/U ratios generally decrease downhole, it seems unlikely that the extremely acidic, reducing conditions necessary for the formation of black shale were ever achieved (Fig. F54).