Wireline logging operations were conducted in Hole 1166A, and LWD/MWD was done in Hole 1166B (Fig. F40). For wireline logging, the drill string was placed at 41.2 mbsf to prevent borehole collapse and was raised to 32.0 mbsf as the tool string approached on its way up the hole. The depth to the seafloor for Hole 1166A was determined to be 480.0 mbrf, based on a drop in total gamma-ray emission to zero at that depth.
Three tool strings were run during wireline operations: the triple combo (resistivity, density, porosity, and natural gamma), the GHMT-Sonic combination (magnetic field, susceptibility, sonic wave velocities, and natural gamma), and the FMS (resistivity images and natural gamma) (Table T10).
The combination of the dipole shear imager sonic (DSI) and GHMT into a single tool string is new and was run for the first time in Hole 1166A. Advantages of this combination over the usual arrangement, in which the FMS and Sonic tools are combined, are (1) the Sonic tool is able to record ~10 m lower in the hole because the GHMT is much shorter than the FMS tool; (2) both the DSI sonic tool and the FMS acquire data at high resolution, which limits the logging speed because there is a limit to the data transmission rate up the logging cable; and (3) the GHMT alone is relatively light, which makes it less likely to be able to pass through obstructions in the borehole.
Hole 1166B was logged using Anadrill LWD/MWD logging tools located a few meters above the drill bit. Only low weight on bit could be applied to drill the upper part of the hole because the hard sediments at shallow depth greatly increased the risk of snapping the LWD tools off the end of the heavier drill collars above. Consequently, drilling was slow and time constraints permitted only 40 m of LWD logs to be obtained.
The CDR LWD tool measured spectral gamma-ray and resistivity, which were recorded and stored within the tool's memory. The Power Pulse MWD tool measured weight on bit and downhole torque, and it transmitted these data, along with some of the CDR data, up to the ship in real time via pressure pulses in the borehole fluid (12 Hz; 3 bits/s).
The caliper log on the triple combo shows that the borehole diameter varies from ~13 to >19 in. For the most part, the hole is smooth and <16 in wide, but there are two intervals where washouts (widenings) of the borehole are found: 163-193 and 103-142 mbsf (Fig. F41). In the wider washouts, the density and porosity tools loose contact with the borehole wall, causing poor data quality. The location of the washouts is typically controlled by the lithology. The MS and resistivity logs are relatively insensitive to borehole size.
The DSI sonic tool was run in P and S and dipole shear acquisition mode for the first pass and in the P and S mode (with more waveforms recorded) in the second pass. P-wave velocities were generally good for both passes, but both contain occasional erroneous spikes. The shear-wave velocity log from the dipole shear acquisition mode was good, but shear-wave velocities were not resolved from the P and S mode.
Although the logging units were chosen independent of lithology, they closely match the lithostratigraphic units (see "Lithostratigraphy"; Figs. F41, F42).
Unit 1 covers most of the
LWD section and is characterized by a steady increase in resistivity from near
zero at the surface to 5 
m
in the lowermost 3 m of the unit. High resistivities, such as 5 m,
close to the surface are typical of diamict. Apart from low gamma-ray values in
the uppermost 2 m, the natural gamma log is fairly steady at ~130 gAPI. The base
of the unit is defined by a sharp decrease in natural gamma and resistivity.
Logging Unit 2 corresponds to lithostratigraphic Unit I. Overall, Unit 2 is characterized by high bulk-density, low neutron porosity, and high gamma-ray values. Unit 2 has higher potassium values than the underlying Units 3 and 4, whereas the thorium values are only slightly higher than in the underlying two units. This likely reflects the relative abundance of potassium-rich K-feldspars and micas throughout this unit, as noted by XRD analysis (see "Lithostratigraphy"). If the drop in potassium values was caused by lower clay content, there would be a corresponding drop in the thorium log. Unit 2 is also characterized by high magnetic susceptibility, reflecting the presence of metamorphic and igneous clasts in the diamict.
This Subunit is marked by
a relatively constant resistivity of ~4 m
and total gamma-ray values of ~125 gAPI. The low porosity of this subunit (~25%)
results in high bulk-density, resistivity, and sonic velocity values. An
increase in photoelectric effect (PEF) from 55 to 90 mbsf possibly reflects an
increase in heavy mineral concentration, although this interpretation is not
supported by the natural gamma logs. The increase could possibly reflect an
increase in carbonate clasts, although the PEF rises higher than the carbonate
value of 5 barn/e-. XRD analysis from Core 188-1166A-9R (76 mbsf)
suggests the presence of heavy minerals at concentrations of 1%-5%. The FMS
image of this subunit shows the presence of numerous relatively small resistive
clasts supported by a mixed sand/clay matrix, again in agreement with the
observation of diamict in the cores (Fig. F43).
The prominent features of
Subunit 2b are a lowering of resistivity to ~2 m,
a sharp decrease in magnetic susceptibility, and a decrease in P-wave
velocity. A slight decrease in bulk density and a minor increase in total
gamma-ray counts occur at the same time. These changes point to a clay-rich
interval, as observed in the cores. Clays are generally of higher porosity than
coarser grained lithologies, with clay beds supported by a "cardhouse"
alignment of platy grains, whereas coarser grains are more densely packed. In
addition, a sediment consisting of similar-sized grains has a higher porosity
than a poorly sorted sediment (e.g., diamict), whose smaller grains fill the
spaces left between the larger grains. The increase in gamma-ray values suggests
that the lithology is clay rather than sand (in the absence of biogenic
dilution). The sharp decrease in magnetic susceptibility relative to the
subunits above and below indicates the lack of the reworked clasts bearing
magnetic minerals in this subunit. The FMS image of Subunit 2b reveals a layered
lithology with relatively low resistivity.
The resistivity values in Subunit 2c are the highest of the entire log; porosity and bulk-density values exceed those of Subunit 2a. The FMS image clearly shows the presence of larger and more frequent clasts than in the diamict of Subunit 2A (Figure F43). The high resistivity values probably reflect the larger and more frequent clasts.
The upper part of Unit 3 has low resistivity, density, and P-wave velocity values. However, the S-wave velocity remains quite high and the caliper log indicates a smooth borehole wall, which is not usually associated with high porosities. Thus, it appears that the sediments are highly cemented yet retain a high porosity. The core recovered from this unit (lithostratigraphic Unit II) is a hard claystone. The FMS images reveal a layered unit, lacking clasts.
There is a step increase in the gamma-ray values in the lower third of the unit. This step in gamma-ray counts corresponds to a similar increase in density, resistivity, and P-wave velocity, a decrease in neutron porosity, and a slight increase in susceptibility. Diatoms, radiolarians, and sponge spicules are present only in the upper half of Hole 1166A from 0 to 151.6 mbsf. The increase in bulk density and velocity with a corresponding increase in gamma-ray counts and magnetic susceptibility in the lower part of Unit 3 is likely related to a significant reduction in the amount of siliceous microfossils relative to the upper part.
This unit is divided into two distinct subunits, the upper subunit showing alternations between high and low density and porosity and the lower subunit showing much less variation. The lower subunit corresponds to the massive sandstone of lithostratigraphic Unit III. The sandy texture of this subunit is apparent in the FMS images (Fig. F43). From the logs, we infer interbedded sandstone and claystones for the upper subunit; the negligible core recovery in this interval means the lithologies cannot be determined directly from core.
Subunit 4a contains a number of alternations between high and low density, porosity, resistivity, and velocity, in contrast to the uniform Subunit 4b. The low density and high porosity values are partly the result of bad contact because of borehole washouts, which may themselves be partly lithologically controlled. The alternating log character is consistent with an interbedded sand/clay lithology. In the velocity log, fluctuations appear to vary between a clay "baseline" velocity of Unit 3 and a sand "baseline" value for Subunit 4b, further suggesting interbedding of the sands and clays (Fig. F44). Poor core recovery in this part of the section prevented direct observation, however. The FMS image for Subunit 4a is also affected by washouts but indicates a generally sandier lithology than the base of Unit 3.
The logs of Subunit 4b show little variation, very low porosities (~20%), and a slight compaction trend with depth. Only three thin intervals with higher porosities and lower density and resistivity are displayed. The FMS images have a gritty texture and are interpreted as massive sands with occasional large clasts (Fig. F43). Toward the bottom of the unit, the images contain variably inclined bedding (Fig. F43), which confirms a similar observation of deformed beds in the core.
Unit 5 shows a baseline shift to higher porosity and lower density, resistivity, and velocity values. Gamma-ray values start to increase rapidly, exceeding 300 gAPI in parts of the unit. Analysis of the gamma-ray log shows increases in all three components, particularly in thorium and uranium; the increase in potassium is proportionately less. Thorium is as high as 70 ppm in places, uranium peaks at around 7 ppm, and potassium peaks at 4%. Potassium values of Unit 5 are only slightly higher than those observed in the diamictites of Unit 2, which were K-feldspar rich.
The only possible sources of such large thorium and uranium values are heavy minerals such as zircon or monazite, derived from erosion of a continental igneous or metamorphic body and concentrated by sedimentary processes. Zircon was observed in low concentrations in the cores (see "Lithostratigraphy"). The thorium content of zircon ranges from 100 to 2500 ppm, and its uranium content ranges from 300 to 3000 ppm; the thorium content of monazite ranges from 4% to 12% (Rider, 1996). Taking the midpoints of these ranges, it would require ~0.2% zircon and ~0.03% monazite to generate the observed uranium and thorium increases above the background values (3 and 30 ppm, respectively).
The magnetic susceptibility mimics the gamma-ray log through this unit, suggesting that the concentration of magnetic minerals (likely to be magnetite) covaries with the concentration of heavy minerals.
Unit 5 corresponds to lithostratigraphic Unit IV, which is identified as sandy silts and organic-rich, laminated dark silty sands. The FMS shows the distinction between the two lithologies (Figs. F43). The porosity in this unit is low, which is consistent with a mudstone and suggests that the lithology remains the same to the bottom of the unit.
Subunit 5a (273-308 mbsf) differs from Subunit 5b (308-373 mbsf) primarily in the resistivity signal. Subunit 5a shows a gradual increase of resistivity followed by a rapid return to lower values at the top of Subunit 5b. Throughout Subunit 5b, the resistivity stays fairly constant but with an occasional spike.
This short interval was only reached by the GHMT, resistivity, and FMS tool strings. It is defined based on the resistivity and FMS values. The resistivity log shows a baseline increase, and the FMS log shows indications of a layered claystone with more clasts than the overlying unit. Without more data it is difficult to say whether this interval is distinct from Unit 5 or represents just a brief change.
A synthetic seismogram was created using the IESX seismic software, part of Schlumberger Geoquest's Geoframe package. The P-wave slowness log from the second pass of the Sonic-GHMT tool string was converted to a velocity log. Bulk density was not used with the velocity to calculate an impedance log because the density log contained spikes caused by hole washouts. A subjective correction of these spikes could have led to more artifacts in the synthetic than would be generated by just using the velocity log alone. Thus, a reflection coefficient series was created from the velocity log (Fig. F45) and converted to two-way traveltime using a depth-two-way traveltime relation also derived from the velocity log.
A minimum phase source wavelet was extracted from the 20 traces centered on the projection of the site location on the seismic line BMR-33-23B. This wavelet was convolved with the reflection coefficient series to produce the synthetic seismogram displayed in Figures F45 and F46.
Site 1166 lies ~1 km southeast of the seismic line 33-23B. Thus, the match between synthetic seismogram and seismic survey data was unlikely to be exact; however, the overlay (Fig. F46) gives an indication of what is causing the reflection pattern in the seismic.
The first large positive reflection at 0.785 s two-way traveltime is caused by the increase in velocity from logging Unit 2b to 2c (clays to clast-rich diamict). The reflection at 0.855 s is probably caused by the contact between logging Units 4a and 4b, although the offset between borehole and seismic line make this correlation tentative. The reflection at 0.95 s appears to be a negative polarity reflection, which might correlate with the drop in sonic velocity between logging Units 4 and 5 (although the synthetic seismogram would locate this reflection 0.03 s shallower in the section).
An interesting detail in the synthetic seismogram is that the unconformity reflection in the section (0.855 s) seems to be caused by the contrast between Units 4a and 4b, rather than the contrast between Units 3 and 4 (lithostratigraphic Units II and III). The unconformity would then separate what appears to be a proglacial outwash sand (below) from a lowstand and transgressive unit (above), speculatively indicating melting and recession of the initial glacial advance to the Prydz Bay margin and concomitant sea level rise.
There is a close similarity in the resistivity and sonic logs for Sites 742 and 1166, which are separated by ~40 km (Fig. F47). Both the shape and the amplitude of the logs are very similar in logging Unit 2, indicating that the upper diamict (logging Subunit 2a), the clay-rich interval (logging Subunit 2b), and the lower diamict (Subunit 2c) are continuous across a wide area of Prydz Bay. Subunit 2a is about the same thickness at both sites, whereas Subunits 2b and 2c are about twice as thick at Site 742. The unconformity between Units 2 and 3 is well represented at both sites by an abrupt change in the resistivity log values; below this, the log-based correlation between the two sites becomes speculative.
Total magnetic field and magnetic susceptibility logs were recorded by the GHMT tool string. Under favorable conditions, a magnetic polarity stratigraphy can be derived from such logs. A correlation analysis of the logs (see "GHMT Tool String" in "Downhole Measurements" in the "Explanatory Notes" chapter) was performed postcruise by the ODP wireline logging services GHMT processing center and interpreted by the logging staff scientist (Fig. F48).
The remanent component of the total field log was determined by removing the background field at the site (~53,380 nT), the field caused by the pipe, and the field caused by the induced magnetization of the sediment (calculated from the susceptibility) from the total field log. The correlation analysis of the remanent and induced components appears to have been successful in determining the polarity stratigraphy at the site. The GHMT polarity is consistent with the available core polarity (Fig. F48).
Dating based on the GHMT polarity stratigraphy is made difficult by the discontinuous sedimentation at the site and by the inherent uncertainties in the GHMT polarity determination (which should be used with caution). However, the lowermost part of the log (below 361 mbsf) seems to be clearly reversed polarity, and hence, not within the long Cretaceous normal superchron (83-118 Ma) (Cande and Kent, 1995).
The Lamont-Doherty Earth Observatory temperature-acceleration-pressure tool recorded the temperature of the fluid in Hole 1066A as part of the triple combo tool string (Fig. F49). These measurements underestimate the formation temperature, as the fluid temperature does not have time to equilibrate to the formation temperature. A temperature of 6°C was recorded at the bottom of the hole (383 mbsf). The downgoing and upgoing curves have an offset of ~1°C, owing to the continuing borehole reequilibration during acquisition.
An excellent correlation exists between the downhole logs and the lithostratigraphy at Site 1166. The logs fill the gaps in the section where core recovery was poor. Most of the logs displayed high amplitude changes coincident with changes in lithology. For example, diamicts and sands have lower porosity and higher density, resistivity, and velocity than the clay-rich lithologies. The FMS was able to image individual clasts in the diamict and massive sands, clay layering, sand beds, and lithologic boundaries throughout the drilled section. The susceptibility drops by an order of magnitude as the lithology changes from diamict to clays and sands. Potassium values are high in the diamictite where K-feldspars and micas are abundant, but they drop in the clays and sands with the decrease in potassium-bearing minerals. Thorium and uranium values increase to very high values in the lower mudstone unit, indicating the presence of heavy minerals such as zircon and monazite.
