Downhole logging was conducted in Hole 1170D after it had been drilled to a depth of 780 mbsf with a 9.785-in drill bit. The basal 355 m was cored, whereas the upper 425 m of the hole was drilled ahead with a center bit (see "Operations"). The presence of indurated limestone horizons above and below an unconsolidated glauconitic sand unit between 474 and 529 mbsf led to the decision to log the hole in two parts. The pipe was initially set at 529.2 mbsf to insure that TD was achieved by each of the tool strings. After a wiper trip (see "Downhole Measurements" in the "Site 1168" chapter) and displacing the hole with sepiolite mud, the RCB bit was released in preparation for logging.
Three tool-string configurations were run, the triple combo, the GHMT-sonic, and the FMS-sonic (see "Downhole Measurements" in the "Explanatory Notes" chapter). The dipole shear sonic imager (DSI) was run in combination with both the GHMT and the FMS in order to compare the quality of sonic data from different tool-string configurations. In combination with the GHMT, the DSI was run in P and S, upper and lower dipole mode (see Schlumberger, 1995). With the FMS-sonic tool string, the DSI was initially run in P and S, upper dipole mode. However, because of problems with data acquisition during this tool run, the DSI was discontinued after two aborted attempts to log the formation (see "Results/Data Quality"). The weather was moderate and heave was typically between 3 and 4 m. Occasionally heave increased sufficiently to cause the wireline heave compensator to stroke out, which occurred once during each pass of the triple combo and GHMT-sonic tool strings.
Upon completion of logging operations in the bottom portion of the hole, the wireline tools were rigged down and the drillers attempted to raise the pipe to ~200 mbsf for the second stage of logging. However, after removing two stands of pipe, the drillers were unable to raise the drill string above 468 mbsf. After trying unsuccessfully to free the drill string, the pipe was severed at 317 mbsf, preventing any logging in the upper section of the hole (see "Operations"). Details of the intervals logged, together with the position of the pipe and the depth of the seafloor, are shown in Figure F38.
The principal results are shown in Figures F39, F40, F41. Borehole conditions were excellent. No ledges or obstructions were encountered and, with one exception, caliper readings from both the triple-combo and FMS tool strings show a smooth borehole with a diameter typically between 10.5 and 12 in (Fig. F39). The one exception can be seen at ~645-650 mbsf, where the borehole width rapidly increased to a maximum value of 16.6 in. This washout corresponds to a decrease in natural gamma, resistivity, sonic velocity, and density values and to an increase in porosity values (Figs. F39, F40). The excellent hole conditions over the rest of the interval resulted in particularly good measurements in the contact tools such as density, porosity, and FMS.
Density and porosity values tend to increase and decrease respectively downhole, implying increased lithification with depth. A comparison between core and log density (Fig. F42) shows that log values are consistently higher than the core results and that this discrepancy increases with depth, perhaps because of core expansion when it is unloaded. Density porosity, neutron porosity, and core porosity values show generally good agreement downhole (Fig. F43). However, the slight offset between density porosity and neutron porosity increases below ~683 mbsf, at the log Unit 2/3 boundary (see "Log Units"). This may be caused by the increased influence of organic material and terrestrial clays (see "Discussion") on the neutron porosity values because the neutron tool fails to differentiate between hydrogen in pore spaces and hydrogen bound in clays and organic matter.
Although heave was significant enough to shut down the heave compensator during each pass of the first two tool strings, subsequent passes of each string covered the intervals lacking active heave compensation during the original passes. Heave also affected the FMS raw data, resulting in apparently constant conductivity over depth intervals as the tool stalled its ascent; however, routine processing removed most of the problems.
The GHMT total field measurements were within the acceptable range, and the susceptibility log shows all of the same fine-scale structure as the core measurements from the MST track, which should allow a core-log correlation capable of determining more precisely the core gaps and postcoring expansion (see "Downhole Measurements" in the "Site 1168" chapter).
The only major lithologic variation within the logged interval was a distinct limestone horizon at 589 mbsf, which is recognizable as a strong minimum in porosity and natural gamma radiation and a maximum in resistivity corresponding to a white stripe in the FMS image (Fig. F44). This limestone horizon can be clearly identified at 589 mbsf in both the neutron and core porosities (Fig. F43), indicating that there is very little depth mismatch between the two data sets at this point. The limestone is also evident in the core densities (Fig. F42) and core sonic velocities (Fig. F45), although it can not be identified in the log densities (Fig. F42) and log sonic velocities (Fig. F45). Log sonic velocities are typically 50-100 m/s below those measured in the core. The higher discreet sample values may be the result of a natural sampling bias toward consolidated (higher velocity) vs. unconsolidated (lower velocity) layers because they are less likely to biscuit or crack during coring and splitting, resulting in an overestimate of the mean core velocities.
Log units have been differentiated mainly by using natural gamma-ray and magnetic susceptibility data, with sonic velocity, density, photoelectric effect (PEFL), and resistivity data showing similar but more subtle changes over the unit boundaries. Three log units have been identified, even though the core results categorize all but the top 9 m of this interval as lithostratigraphic Subunit VB (see "Lithostratigraphy").
Magnetic susceptibility and natural gamma-ray values remain relatively low and show the smallest amplitude of variability within this unit (436 ± 9 ppm and 48 ± 2 API, respectively). In addition, resistivity and sonic velocities are generally higher than in Unit 2 below.
Magnetic susceptibility and natural gamma values increase between 583 and 595 mbsf, where they level off and vary about a mean of 492 ± 14 ppm and 58 ± 4 API, respectively, until the base of logging Unit 2. Porosity shows a strong minimum and spherically focused resistivity (SFLU) shows a distinct maximum at ~589 mbsf, near the top of Unit 2, correlative with the indurated limestone horizon recovered in the core (see "Results/Data Quality").
The most marked change in log parameters is below 684 mbsf, where the mean and variance of magnetic susceptibility and gamma-ray values increase to 527 ± 21 ppm and 80 ± 8 API, respectively. Furthermore, mean density and PEFL values increase toward the base of this unit, whereas porosity shows a minor decrease downhole.
The three log units described above appear to correlate with a general downhole trend toward a higher terrigenous component in the sediments. This trend is illustrated well by the magnetic susceptibility and Th log data (Fig. F46), both of which usually vary as a function of the detrital component. Fluctuations in the magnetic susceptibility and Th data also appear to correlate with downhole variations in the TOC of the sediment (Fig. F47), suggesting that the increases in TOC are associated with the input of terrigenous material. Increased magnetic susceptibility, Th, and TOC values also tend to correlate with Rock-Eval pyrolysis index values, indicative of mainly terrestrially derived organic carbon (see "Organic Geochemistry") implying that the periodic increases in sediment TOC are caused by higher fluxes of terrestrial rather than marine organic matter. Interestingly, U values do not covary with the TOC above ~675 mbsf (Fig. F47). Uranium tends to be adsorbed by organic matter (Adams and Weaver, 1958) and, therefore, could be expected to show a correlation with TOC.
The spectral gamma-ray data from this site are also of considerable interest because they show a pronounced cyclicity, particularly between ~660 and 715 mbsf (Fig. F46). The Th data, which contain the clearest cyclicity, have a dominant peak with a wavelength centered on 4.1 m. Although it is tempting to relate such regular periodicity to an orbital frequency, preliminary biostratigraphic analyses from the base of Hole 1170D (see "Biostratigraphy") indicate that sedimentation rates may have been as low as ~7.1 cm/k.y. (dinoflagellate dates) or as high as ~23 cm/k.y. (nannofossil dates). Such a high range in possible sedimentation rates makes it impossible to assign a particular Milankovitch periodicity to the spectral gamma-ray cycles at this stage. For example, a 4.1-m cycle in the sediments could be related to obliquity if sedimentation rates were ~10 cm/k.y., or precession if sedimentation rates were closer to 20 cm/k.y.
The P-wave velocity data recorded by the DSI in Hole 1170D have been combined with P-wave velocities measured in the cores from Holes 1170A and 1170D to produce an integrated traveltime data set for this site. 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. F48). The two major spikes in P-wave velocity at ~474 and ~529 mbsf are caused by the presence of indurated limestone horizons (see "Physical Properties"). These spikes probably correspond to the two main reflectors in the seismic section (Fig. F48). The fact that calculated traveltimes to these two spikes in P-wave velocity differ considerably from the traveltimes to the two reflectors in the seismic section (Fig. F48) suggests that the core measurements underestimate true sonic velocities over at least part of the interval. One explanation for this could be that the unconsolidated carbonate sediments from the uppermost ~350 m of the core have undergone considerable expansion or have been adversely affected by rotary coring and cutting with the saw during section splitting. The noticeable drop in core sonic velocities at the switch over to XCB drilling and saw splitting supports this hypothesis (see "Physical Properties").