Downhole logging was performed in Hole 1124C. The drill string was placed at 95 mbsf as the logging tools were lowered to the bottom of the hole. During logging, the drill string was raised to 78.5 mbsf. The drill string had to be maintained at 78.5 mbsf to keep the upper hole wall from collapsing.
Three tool-string configurations were run in Hole 1123B in the following order: the triple combination, the FMS-sonic, and the GHMT (see "Downhole Measurements" in the "Explanatory Notes" chapter). A repeat interval was measured with all three tool strings. Logging operations are summarized in Figure F32. Logging operations began at 2100 hr on 29 September 1998 and finished at 1600 hr on 30 September 1998. There was less than 2 m of heave throughout operations, and the wireline heave compensator was used during all measurements. The hole conditions were good, with a relatively uniform borehole diameter (~13 in) through the lower part of the hole and a more variable diameter in the upper portion (Fig. F33A). The NMRS (total field) sonde on the GHMT was again not working.
The Lamont temperature tool was left off the bottom of the triple combination to maximize the chances of recording the K/T boundary with the resistivity tool (DIT). The temperature tool is 1.3 m long. The sensors on the DIT are 1.6 m above the base of the tool. The location of the K/T boundary was estimated to be between Cores 181-1124C-48X and 49X (see "Biostratigraphy"), ~6-8 m above the base of the hole.
Observations of the core lithology detected horizons containing altered tephra. Altered tephra is frequently high in smectite, which is susceptible to swelling, and may cause constrictions downhole. With this in mind, clay-swelling tests were performed on two samples: a greenish clay, thought to contain very little smectite; and an altered tephra, thought to be almost entirely smectite. The results are shown in Figure F34. The greater the time shown on the y-axis the more susceptible the sample is to swelling. Figure F34 shows that the altered tephra sample was highly susceptible to swelling, but that the addition of KCl (an anti-swelling agent) considerably reduced the propensity of the sample to swell. Before logging operations, the borehole was flushed with a seawater-based drilling mud containing 276 kg/m3 of sepiolite (an anti-swelling agent). This helped to reduce swelling of the altered tephra horizons in the borehole.
Because of good weather and good hole conditions, the data quality is excellent. The uneven caliper in the upper portion of the hole may be caused by tephra bed washouts, as tephras were abundant in this part of the core (see "Lithostratigraphy"). Nevertheless, caliper deviation was generally no more than 1 to 2 in from the average (13 in). In some places, however, the caliper did exceed 15 in, and in these regions the FMS data are unreliable (see "Downhole Measurements" in the "Explanatory Notes" chapter). For example, a washout can be seen on the FMS image at 300 mbsf. This point corresponds to a gap in core recovery.
The log magnetic susceptibility data were correlated with the core magnetic susceptibility results (Fig. F35). Although core recovery was good for the majority of the hole, there was virtually no core recovered between 280-300 mbsf, and, consequently, the core-based magnetic susceptibility record is incomplete here. The log/core magnetic susceptibility correlation diagram shows a log susceptibility spike at 280-300 mbsf. The log/core susceptibility correlation also confirms that coring did not recover the K/T boundary. There is a small gap in the core-based magnetic susceptibility at 467 mbsf, which is the predicted location of the K/T boundary (see "Stratigraphic Correlation: the K/T Boundary and the Brown Mudstone").
Sonic traveltime data contained 5-10 cycle skips, which were edited from the log. The traveltime data were converted to velocities and used to create a depth-integrated travel time profile to compare with a pre-cruise seismic survey of the area. The profile demonstrates that the major lower seismic reflector correlates with the top of the brown mudstone at 419.5 mbsf (see "Lithostratigraphy").
The predicted location of the K/T boundary, toward the base of the hole, was monitored by the lowest tool on each of the three tool strings. The tools that recorded the boundary were the resistivity tool on the triple combination, the FMS tool on the FMS-sonic, and the magnetic susceptibility tool on the GHMT. The sensor on the resistivity tool is 1.6 m above the base of the triple combination, the FMS is at the base of the FMS-sonic, and the susceptibility sensor is 3.96 m above the base of the GHMT.
The log data from Hole
1124C show noticeable variations downhole (Fig.
F33A, F33B), in response
to fluctuating depositional environments. Distinct logging units can be
identified on the basis of average log values and trends in the data. The range
of values observed within the data from Hole 1124C is greater than that recorded
from the previous two logged holes, possibly suggesting a more marked change in
sedimentary environments through time. Magnetic susceptibility values vary from
around 390 × 10-6 (uncalibrated log units) in log Subunits 3A and
3B, to more than 1600 × 10-6 at the base of the brown mudstone.
Resistivity values are generally around 0.5 
m
toward the top of the logged section, but exceed 3 m
near the base of the hole. Natural gamma-ray values reach a maximum of 88 API
(American Petroleum Institute units) at the base of the brown mudstone, but drop
to only slightly more than 10 API above and below this unit. As observed at Site
1123, downhole variations in thorium and potassium appear to correlate (Fig. F36).
Uranium values often fluctuate independently of thorium and potassium (Fig. F36).
There is generally a good correlation between the log units identified in Hole 1124C and the main lithostratigraphic units (Figs. F33A, F33B). Log Subunits 1A and 1B correlate well with lithostratigraphic Subunit IB, and log Unit 4 (the brown mudstone) correlates well with lithostratigraphic Unit IV. There may be some correlation between log Subunits 2A and 2B and lithostratigraphic Subunit IC, and between log Subunits 3A and 3B and lithostratigraphic Unit II. The only major seismic reflector identified at this site occurs at the top of the brown mudstone (419.5 mbsf). The log units identified in Hole 1124C are outlined below and shown in Figures F33A, F33B.
This subunit has a
relatively low resistivity (shallow resistivity = 0.64 ± 0.04 m)
and high natural gamma values (54 ± 5 API). Sonic travel times are consistently
high (197 ± 1.5 µs/ft). Porosity, density, and photoelectric effect show
relatively little long-term variation. Magnetic susceptibilities fluctuate
regularly, by ~500 × 10-6. This is probably caused by the widespread
occurrence of tephra horizons.
Within this subunit, resistivities gradually increase with depth, and sonic traveltimes begin to decrease. However, the other log data are very similar in character to Subunit 1A, implying there is only a very subtle change from the subunit above.
At the top of this subunit, both density and magnetic susceptibility show marked increases. Photoelectric effect values begin to show greater fluctuations, indicating that the composition of the sediment is more variable. Toward the base of Subunit 2A, natural gamma-ray values begin to decrease.
Resistivity and magnetic susceptibility values begin to decrease from the top to the bottom of this subunit. In addition, magnetic susceptibility values show far less variability than those seen in the subunits above. This may be a result of the lack of tephra in this section of the hole (see "Lithostratigraphy").
Within both of these subunits, gamma-ray values are generally lower (43 ± 5 API) than above. Magnetic susceptibility values are also low (mean = 393 × 10-6) and have a very small variation (standard deviation = 24 × 10-6). However, Subunits 3A and 3B can be subdivided using (1) resistivity, density, and photoelectric effect, which show two distinct cycles of increasing value downhole; and (2) sonic traveltime and porosity, which show two cycles of decreasing value downhole. These log responses suggest that compaction and lithification increase in two steps, which overlap at the boundary of Subunits 3A and 3B.
This log unit is noticeable by a sharp increase in magnetic susceptibility, sonic traveltimes, and gamma-ray values. Porosity increases within this unit and density decreases. The photoelectric effect also decreases, suggesting a change in the mineralogy within this unit. This unit correlates with the brown mudstone (see "Lithostratigraphy").
In Unit 5, all of the log data except for resistivity remain relatively constant. Gamma-ray values are at their lowest in this unit (14.5 ± 3 API). At 466.8 mbsf, however, there is a sharp increase in resistivity and an associated decrease in magnetic susceptibility, which reflect a sudden change in environmental conditions. These results are discussed in more detail below.
The drilling of Hole 1124C was of particular scientific interest, as it penetrated the K/T boundary (see "Biostratigraphy"). However, the exact contact between Cretaceous and Tertiary rocks is missing from the core. It lies between the base of the recovered section of Core 181-1124C-48X, which is Tertiary, and the top of Core 181-1124C-49X, which is Cretaceous. Three tools were able to take readings from the base of the hole (see "Data Quality and Log/Core Correlation"), where the first appearance of Cretaceous rocks was observed (see "Biostratigraphy"). The K/T boundary appears to occur at slightly more than 7 m above the base of the hole. This interpretation is based on a large spike in resistivity and a concomitant decrease in magnetic susceptibility at this point (Fig. F37). The FMS data show a 30-cm-thick resistive bed, with a sharp lower contact at 466.8 mbsf. Although the lithologic conditions responsible for these log responses are somewhat ambiguous (see below), they clearly represent a sudden and short-lived change in the environment at this point.
The sediment above and below the K/T boundary is highly lithified (see "Lithostratigraphy"). Although the low resistivity recorded in these lithified sediments appears incongruous, it may be explained by the presence of conductive clays. Terrigenous clays occur in varying amounts throughout the sediment column. High resistivities seen at the K/T boundary may represent the absence of these clays. Low magnetic susceptibility seen at the K/T boundary appears to support this interpretation, as it may be recording the absence of the terrestrially derived, clay-sized material that occurs above and below.
A distinct stratigraphic marker that was observed in the core was the brown mudstone (see "Lithostratigraphy"). The brown mudstone can also be clearly seen in the log data, as a horizon of increased natural gamma intensity and magnetic susceptibility between 420 and 430 mbsf (Fig. F37). Interestingly, the spectral gamma-ray values from the brown mudstone show no increase in uranium concentrations (Fig. F37), suggesting that the concentration of organic matter is low. Furthermore, the high magnetic susceptibilities and thorium concentrations at this point suggest an increased input of terrigenous clay.
