m)
between ~294 and 308 mbsf. These data have been edited out.Downhole logging was conducted in Hole 1171D after it had been drilled to a depth of 959 mbsf with a 9.785-in drill bit. The basal 711.4 m was cored, whereas the upper 247.6 m of the hole was drilled ahead with a center bit (see "Operations"). After a wiper trip (see "Downhole Measurements" in the "Site 1168" chapter) and displacing the hole with sepiolite mud, the RCB bit was released and the pipe was set at 151 mbsf in preparation for logging. Three tool-string runs were planned; the triple combo, the GHMT-sonic, and the FMS-sonic. However, operational difficulties only allowed triple-combo measurements to be obtained (see below).
There was substantial and increasing heave throughout the logging operations. The wireline heave compensator (WHC) stroked out on three occasions while logging with the triple combo near the base of the hole. A repeat pass of the triple combo, with no failures of the WHC, was conducted between 761 and 959 mbsf (Fig. F46), and the results from both passes have been spliced together to produce a composite depth-corrected data set.
While running in with the GHMT-sonic, heaves of ~9 m were recorded at the rig floor. During the transit of this tool down through the open formation, the WHC frequently stroked out and, possibly as a result of the extreme heave, the head-tension gauge malfunctioned. Shortly afterward, downhole telemetry with the tool string was also lost, so we decided to pull out of the hole. While pulling the tool back toward the rig floor, it became apparent that the tool had sat, bridged out, at ~280 mbsf. The tool was returned to the rig floor. To try and clear the bridge in the formation, the drillers attempted to push the open pipe down to ~294 mbsf. Unfortunately, after adding one stand of drill pipe, the end of the pipe deviated from the hole and became clogged with soft sediment. No further logging was possible.
The principal results are
shown in Figures F47
and F48. The
borehole was generally smooth, with diameters of ~12 in below 272 mbsf and ~19
in above 250 m (Fig. F47).
Despite the washed out conditions in the upper section of the hole, the pad
contact tools (HLDS and APS) appear to have produced good-quality data. The raw
spherically focused resistivity (SFLU) results contain anomalous spikes (9700 m)
between ~294 and 308 mbsf. These data have been edited out.
There is generally good agreement between log density and core density values, with both data showing a stepwise variation downhole (Fig. F49). However, although the log densities are almost the same as the core densities near the top of the logged section (e.g., 170-270 mbsf; log densities = 1.72 ± 0.04 g/cm3; core densities = 1.67 ± 0.04 g/cm3), toward the base of the hole log densities are slightly higher and less variable than the core densities (e.g., 845-845 mbsf; log densities = 2.3 ± 0.03 g/cm3; core densities = 2.17 ± 0.07 g/cm3). This disparity most likely results from postcoring sediment expansion affecting the core densities.
Log porosity and core porosity values also show good agreement (Fig. F50). Neutron porosity log values agree closely with core porosity measurements throughout. Density porosities are similar to neutron and core porosities between 150 and 750 mbsf but are ~8% lower than both below 750 mbsf. Spikes in downhole density and porosity are generally correlative with equivalent core measurements, demonstrating that fluctuations in the logs represent sedimentological variability and that there is relatively little depth mismatch between the two data sets. The failure of the core measurements to record every porosity minimum and density maximum between 273 and 465 mbsf reflects the poor core recovery over this interval.
Because there was no sonic velocity log from this site, the P-wave velocity data measured vertically on half cores (see "Physical Properties") have been used to produce an integrated traveltime data set. These data can be used to show a graph of increasing two-way traveltime vs. depth and also to plot the P-wave velocities against time for comparison with the seismic section (Fig. F51). The sharp increases in P-wave velocity between ~274 and 427 mbsf (~0.33-0.49 s) correspond to major reflectors in the seismic section. The reflector at ~0.59 s (~519 mbsf) correlates with an increase in P-wave velocities and also to a sharp increase in both core and log density values (see "Log Unit 4: 520-890 mbsf" and "Discussion").
All of the log data show marked and often correlative downhole fluctuations (Figs. F47, F48) that are used to divide the logged interval into five units. The two log parameters that show the greatest downhole variation are density and natural gamma; a crossplot of these data (Fig. F52) gives a good visual representation of the five different logging units, which are described in more detail below.
The natural gamma values are low (~5 API) and show only small variations (~3-8 API) throughout this unit, suggesting there is little terrestrial material in these sediments. Photoelectric log values remain near 4 barn/e-, suggesting a high carbonate content, which is consistent with core measurements of ~90 wt% CaCO3 over this interval (see "Organic Geochemistry"). Resistivity values are also low and fairly stable throughout this unit, consistent with the homogeneity observed in other log parameters.
Density, resistivity, and
photoelectric log values are generally low (~1.4 gm/cm3, ~0.6 m,
and ~1.1 barn/e-, respectively), whereas porosity values increase to
>80% and natural gamma logs also increase over much of the interval. The
increase in natural gamma and decrease in photoelectric values suggest greater
terrestrial material and lower carbonate content throughout this unit. However,
the high porosities (>80%) and low densities are typical of diatomaceous
sediments (Gersonde, Hodell, Blum, et al., 1999). This interpretation is
consistent with the clay-and diatom-rich sediments recovered in the core (see "Lithostratigraphy").
Interspersed within this unit are thin layers marked by high density, resistivity, photoelectric effect, and natural gamma values but relatively low porosities, most pronounced at 273-275 mbsf and 290-296 mbsf. The spectral gamma results (Fig. F48) show that most of the natural gamma increase is caused by greater K concentrations over these intervals. In conjunction with the high photoelectric values, such high K concentrations are characteristic of glauconite-bearing sediments (Rider, 1996). Glauconite was recorded in the core over this interval (see "Lithostratigraphy"), although core recovery was very low. Indeed, the inferred glauconite-rich interval between 290 and 296 mbsf was entirely missed because of poor core recovery. Density, resistivity, and photoelectric log values increase at the base of Unit 2 (334-345 mbsf), marking the bottom of the high-porosity diatomaceous interval.
Density, resistivity, neutron porosity, and photoelectric log values generally covary in this unit, although no overall downhole trend is discernible. This is particularly apparent in the density log (Fig. F49), which shows no apparent compaction trend from top to bottom. The total gamma and spectral gamma-ray logs show increased values at 423-436, 467-476, and 490 mbsf, and the resistivity, density, porosity, and photoelectric effect (PEFL) show spikes at 418 and 465 mbsf. The strongly fluctuating log values suggest that this unit contains variations in lithology and/or mineralogy (see "Discussion").
Densities show a sharp downhole increase at the top of log Unit 4 (from ~1.8 to ~2 g/cm3) and then continue to increase toward the base of the hole (Fig F49). Natural gamma values also show a generally linear increase down through this unit. Figure F52 shows that Units 3 and 4 have very different distributions on a crossplot of density against natural gamma. The increase in natural gamma down through log Unit 4 is mainly a result of increasing Th and K content rather than U (Fig. F48), suggesting terrestrial components increase downhole. This hypothesis is consistent with the magnetic susceptibility data from the core, which show an increase in values below 520 mbsf (see "Physical Properties").
Log Unit 5 has been defined entirely on the presence of increased U levels, which reach their maximum value of 8.3 ppm at 915 m (Fig. F48). This peak in U is at a depth where Th values are high but relatively stable and where K values show a slight increase above already high levels. This depth also correlates with high magnetic susceptibilities in the core (see "Physical Properties") and the highest TOC content of the sediment (see "Organic Geochemistry"). All these data imply an increase in organic matter into a sedimentary system already receiving a significant input of terrigenous material.
The sharp increase in densities at the top of log Unit 4 divides the underlying sediments, which show a normal compaction trend, from the overlying sequence (log Unit 3), which shows no clear density trend. At this depth (520 mbsf), Th, K, and core magnetic susceptibility also show a downward trend to increasing values, and P-wave velocities measured in the core show a sharp rise from ~1878 to 2056 m/s (see "Physical Properties"). There is clearly a change in sediment properties at this depth even though the other log curves and the lithostratigraphy are relatively constant across this horizon. The reason for the sudden shift in the character and value of the density log at 520 mbsf is not clear, but it may possibly relate to a hiatus in sedimentation. Magnetostratigraphic interpretations (see "Paleomagnetism") were unable to produce a conformable reversal stratigraphy across this horizon; geochemical analyses show the presence of low TOC and high HI values at ~520 mbsf, possibly indicating a condensed section (see "Organic Geochemistry"); and dinocyst biostratigraphy suggests the presence of a ~2 to 3-m.y. hiatus between Cores 189-1171A-29R and 35R (~510-570 mbsf; see "Biostratigraphy").
The strongly varying log parameters in log Unit 3 suggest that changes in the sedimentary environment occurred throughout this interval. Preliminary interpretation of the logs suggests that spikes in resistivity, density, and PEFL are often correlative and that they tend to be directly above increased natural gamma values (e.g., 400-500 mbsf; Fig. F53). This pattern of log variations may well be a response to alternating marine vs. terrestrial influences. Figure F53 shows the zones where Th and K values are highest, possibly indicating an increased input of terrigenous clays. Interestingly, in the area of high Th values between ~425 and 439 mbsf, the density and porosity logs have a noticeable mismatch (Fig. F53), implying that some of the neutron porosity values may be derived from water bound in clays. The spikes in resistivity, PEFL, and density probably indicate areas of indurated carbonate. An exception to this is present at 422 mbsf, where increased PEFL values and densities, with no concomitant increase in resistivity, correlate with an increase in K. This indicates the presence of glauconite. The presence of glauconite may indicate a slightly anoxic, sediment-starved environment, whereas the indurated carbonate may be indicative of more open marine conditions.
The fluctuating sedimentary environments indicated by log Unit 3 could be caused by variations in relative sea level or climate. However, any changes in paleowater depths must be relatively small because middle neritic (~50-100 m) benthic foraminifer faunas are found throughout this interval (see "Biostratigraphy"). Confirmation that the variations in log data from log Unit 3 are a result of sea-level change will require postcruise core/log analyses and detailed comparison between the logs and seismic stratigraphy, as well as the results of further microfossil and sedimentological analysis.
On a finer scale, downhole logs appear to be strongly cyclic in many intervals. For example, the Th spectrum of the natural gamma-ray log, which showed strong cyclicity in the middle to late Eocene at Site 1170, also shows strong cyclicity in the middle Eocene at this site (Fig. F54). A prominent peak in the middle Eocene Th power spectrum is present at ~2 m as opposed to the ~4-m period found at Site 1170. A shift in the apparent periodicity between sites could be explained by a lower sedimentation rate (~50%) at Site 1171 relative to Site 1170. Preliminary magnetic and biostratigraphic results from this site (see "Paleomagnetism" and "Biostratigraphy") are consistent with lower sedimentation rates in general, but confirmation that the shift in periodicity is the result of sedimentation-rate differences will require further lithostratigraphic, magnetic, and biostratigraphic integration.
