Three logging strings were run in Hole 1118A: one triple combo string that logged from a total depth of 891 mbsf to above mudline; one FMS-sonic string that logged a first pass from total depth (891 mbsf) to 92 m above total depth and a second pass from total depth to the bottom of the pipe at 98 mbsf; and a VSP run (Table T16). The VSP data and operations are described in "Vertical Seismic Profile and Depth Conversion" and this section concentrates on the other runs.
For the first run, the pipe was raised to 99 mbsf, and the triple combo with the dual induction tool (DIT) (see Table T7, Fig. F15, both in the "Explanatory Notes" chapter) was lowered downhole. Natural gamma ray (HNGS) was monitored during the descent in the pipe to locate the mudline where the string was stopped for 3 min to provide a depth reference for the temperature-logging tool (TLT) temperature log. Upon reaching open hole, HNGS and DIT data were logged during the descent to total depth (Table T16). An upward log was then recorded at 300 m/hr. The tool string was stopped at the mudline for a few minutes both to run calibrations and to provide a depth reference for the TLT data, and logging continued up to 67 m above seafloor. After this pass the tool was lowered again to record the evolution of temperature at three stations (415, 625, and 835 mbsf) where the tool was stopped for 10 min at each station. The tool string was then pulled out of the hole.
For the second run, the bottom of the pipe remained at 99 mbsf and the FMS-sonic string (see Table T7, Fig. F15, both in the "Explanatory Notes" chapter) was lowered downhole. The string reached near total depth (Table T16), and a first pass was recorded up at 300 m/hr for 92 m to better cover the bottom part of the hole. The tool was lowered again to total depth for a second pass. The imaging log ended at the bottom of the pipe at 98 mbsf (Table T16) where the pads were closed, but natural gamma ray continued to be recorded up to 67 mbsf.
The mudline wireline depth (Table T16), which defines the depth shift from meters below rig floor to meters below seafloor applied to the logs (Fig. F71A, F71B), was located by its associated NGR decay for the triple combo run (Fig. F72). The depth shift for the second FMS-sonic pass was derived by correlating the NGR with that of the triple combo run within the 283-303 and 373-387 mbsf intervals that were chosen for their large characteristic variations (Figs. F73, F74). The depth shift for the first FMS-sonic pass was derived by correlating the NGR with that of the second FMS pass within the 855-865 mbsf interval. Note that differential cable stretching results in a depth mismatch up to 0.75 m between the FMS and the triple combo passes at the bottom of the log as shown in that interval (Fig. F75).
Ten logging units are defined (Fig. F76), based mainly on natural gamma ray, the relationship between the neutron (APLC) and density porosity (DPHI), the photoelectric effect (PEFL) (see "Downhole Measurements" in the "Explanatory Notes" chapter), and the results of core analysis (see "Lithostratigraphy").
The neutron porosity is larger than the density porosity, which indicates the dominance of clay. Coarse-grained beds are rare, although several thin silty streaks occur. A thin layer at 186 mbsf may be calcite rich as indicated by a spike in PEFL.
This unit is predominantly silty at the bottom, but grades upward into a formation that is primarily clay, again as indicated by the relationship between the density and neutron porosities. The unit is distinguished from log Unit L1 by its lower porosity and higher gamma-ray values. Near the top, several short (2-5 m) fining upward sequences can be seen, each with a silty base marked by a low in porosity.
This unit is similar to Unit L1, showing a high clay content and some thin silty beds, but no clean sand beds. However, it displays lower natural gamma ray.
There is generally more convergence between the density and neutron porosities here along with a 20 GAPI gamma ray increase, suggesting that the background sedimentation in this unit is silty. Thin intervals of greater convergence of the porosity curves are inferred to be sandy.
The porosity curves diverge again, showing that this unit is primarily clay with some thin sandy beds and some silty intervals a few meters thick. One interval from 374 to 389 mbsf shows elevated natural gamma-ray magnitudes associated with Th and K increases. The slight convergence of porosities over the interval suggests that it contains a significant amount of fine-grained volcaniclastic material.
The top 9 m of this unit are a massive radioactive sand like those observed in Holes 1109D and 1115C. Density and neutron porosities converge well, and the Th/K value is ~6. Below this sand, this unit displays higher PEFL and resistivity and lower gamma ray than Unit L5, suggesting higher carbonate content. This lower interval is also clay rich, and no thin clean beds are visible.
The background sedimentation appears to be silty with several thin (<1 m) sandy beds. An interesting change occurs at ~604 mbsf: above this, all sandy beds are radioactive, whereas below this depth, sandy beds are associated with a decrease in natural gamma-ray magnitude. There also appears to be a change in the average Th/K value from ~5 above to ~3 below, suggesting less volcanic input.
Just 8 m below the top of this unit is a massive sand almost 40 m thick; it is characterized by low PEFL, convergence of porosities, and relatively low natural gamma-ray magnitude. Below and in the 8 m above this, several similar but thinner intervals are visible alternating with sequences of clay and silt where the porosities diverge. The bottom 7 m of the unit shows a decrease in porosity and an increase in gamma-ray magnitude, which is associated with an increase in Th and K content; this is likely another volcaniclastic sand. This sand is probably calcareous as well, particularly where the PEFL reaches 4 barns/e-.
This unit is dominated by calcium carbonate, as shown by the PEFL of 5 barns/e- and low natural gamma-ray magnitude. Two interesting features stand out: a radioactive sand and a calcareous conglomerate. From 860 to 861.5 mbsf, there is a highly radioactive sandy bed with good density-neutron convergence and a decrease in PEFL. The Th/K ratio shoots up to 10, and a mild increase in uranium is seen. This suggests the presence of mafic volcanic minerals. From 868.5 to 870 mbsf, there is an increase in bulk density and a subsequent large separation of the porosity curves. PEFL remains relatively high at 4.25. From the FMS image, we infer that this bed is a conglomerate. It corresponds to a calcareous, basaltic conglomerate recovered in the core (see "Lithostratigraphic Unit VIII").
The neutron and natural gamma-ray sensors did not reach this unit; however, the top 5 m is visible on the density and PEFL curves. Bulk density is high, close to 2.7 g·cm-3, and PEFL ranges between 4 and 5 barns/e-. The FMS log shows that the entire unit is a conglomerate; the large, boulder-sized clasts are likely to be the dolerite that was recovered in core at this depth (see "Lithostratigraphic Unit VIII"). A calcareous matrix may be boosting the PEFL.
The accelerometer data of the FMS show that the hole deviation first increases with depth up to 3.5º at 810 mbsf and then decreases (Fig. F77). The hole azimuth varies from 150° at the top to 188° at 810 mbsf (Fig. F78). Tool acceleration magnitudes (Fig. F79) deviate by 0.2 m·s-2 from the average, which is about twice the amount of a good quality run (such as the lower run in Hole 1109D, see "Downhole Measurements" in the "Site 1109" chapter). This indicates irregular tool movements and lower quality raw images, as was observed during logging.
The FMS caliper data reveal a significant hole ellipticity except in the 550-680 mbsf interval, which is totally included in Unit L7 (Fig. F80).
This data also revealed
two intervals where the smallest diameter is below the drill-bit size (25 cm [9
in]). The 204-255.5 mbsf interval corresponds exactly to Unit L2, and the
682-875 interval corresponds to Units L8 and L9. In these intervals, it can be
noted that the largest diameter recorded by the FMS caliper is smaller than
those recorded by the hostile environment lithodensity sonde (HLDS) 7 hr before.
This suggests that the hole was slowly closing during logging operations.
Washouts spaced out at ~10-m intervals are probably caused by the circulation that is maintained when the drill bit progression halts during core retrieval.
The FMS tool rotated counterclockwise as shown by the pad 1 azimuth, except in the 330-420 mbsf interval where the largest diameter azimuth remains steadily north-south (Fig. F80). This interval also corresponds to a large ellipticity. This is consistent with observations in Hole 1109D and 1115C (see "Site 1109" and "Site 1115" chapters) and could be related to a north-south minimum horizontal principal stress direction or to structural orientations.
The magnetometer measurements yield a stable inclination around -30°, except at the bottom of the hole in log Units L9 and L10, where it becomes variable and reaches up to -27° (Fig. F81). The magnetic intensity shows the same stability above log Units L9 and L10 and similar variation within these units (Fig. F81).
One main FMS pass and one short pass were recorded at Site 1118 (Table T16). FMS processing steps for this site included speed correction, depth shifting to meters below seafloor using an estimated mudline of 2313.7 m for each FMS log, and static and dynamic normalization using a 1-m window. The image quality is good between 138 and 190 mbsf for the main pass and 825 and 890 mbsf for the short pass. The high contrast in resistivity between the low-resistivity clays and silts in the top portion of the hole and the high-resistivity limestone and dolerite in the bottom portion resulted in poor resolution of the smaller scale resistivity variations in the statically normalized images. Comparison of the gamma-ray logs indicates a depth offset between the triple combo and the lower portion FMS images (Fig. F75); that is, the FMS depths within log Units L9 and L10 are actually ~75 cm deeper than the triple combo logs. Postcruise processed data available in the LDEO CD-ROM minimize the depth offset between the triple combo and FMS-sonic logs by applying a differential depth shift.
Very thin (2-3 cm), moderately resistive, flat-lying beds characterize log Unit L1. These beds are interpreted to be dominantly clays based on both the FMS images and the conventional logs. Thin, more resistive beds are commonly interbedded within this unit. Approximately 1-m scale coarsening upward sequences are recognized as units with relatively sharp, low-resistivity bases and gradational increases in resistivity toward the tops as shown in Figure F82. The thickness of the high-resistivity layers increases up to ~10 cm toward the base of log Unit L1.
FMS-image quality is poor within log Unit L2 because of poor pad contact with the borehole wall. The large-scale resistivity is low in the statically normalized FMS, indicating high clay content. Sedimentary layering is not well defined within this unit, but this may be an imaging artifact. A few meter-scale fining upward sequences with resistive bases are more clearly observed in this unit.
The top of log Unit L3 is clearly imaged in the FMS data with the reappearance of thinly layered clayey beds similar to those of log Unit L1. Bed thicknesses are slightly greater, including thicker (~15 cm) resistive interbeds. At 268 mbsf, beds show a sudden increase in dip of ~15° to the west that decreases to nearly horizontal with depth and changes to a northwest dip direction at 282 mbsf. This structure appears to be a depositional feature as no fractures are observed. The base of log Unit L3 is comprised of wavy, steeply dipping foresets with variable dip directions as shown in Figure F83.
The top of log Unit L4 displays similar bedding structures as log Unit L3; however, the bed boundaries are more diffuse. Bed dips throughout this unit generally range between 15° and 20° to the west, changing to a more northwestward dip direction with depth. Beds lie nearly horizontal at the base of this unit. A few 50° to 60° southward-dipping resistive fractures occur between 316 and 323 mbsf.
A decrease in large-scale resistivity indicating increased clay content between 347 and 389 mbsf marks the top portion of log Unit L5. Beds are flat lying, but occasionally dip up to 10° to the southeast and southwest. Resistivity is greater in the lower portion of this unit below 389 mbsf, and sandy, resistive interbedded layers are more common; however, bedding structures remain thin and flat lying.
Thin (<5 cm), flat-lying, conductive, clayey beds with generally thicker (10 cm) resistive interbeds characterize log Units L6 and L7. Large-scale resistivity varies throughout this section, whereas bed forms are generally uniform. Log Unit L6 is composed of more conductive, clayey material, whereas log Unit L7 contains both conductive and resistive intervals. Resistivity generally increases with depth; however, below ~568 mbsf, highly conductive, interbedded clay layers appear in the form of irregular wavy beds between the more uniform, flat-lying resistive layers as shown in Figure F84.
Log Unit L8 displays a grainy texture in the FMS images, which is characteristic of carbonate-rich material as observed in the nearby Sites 1109 and 1115 (see "Downhole Measurements" in the "Site 1109" chapter and "Downhole Measurements" in the "Site 1115" chapter). Beds are relatively thick (<20 cm or more) and flat lying throughout most of this unit, except where bedding is highly disturbed. The base of this unit is composed of a highly resistive sandy or carbonate-rich interval between 850.5 and 857 mbsf.
High resistivity and a grainy texture characterize log Unit L9, which is interpreted to be composed of carbonate-rich material based on both the FMS and the conventional logs. The dynamically normalized FMS image displays 5-10 cm thick, relatively conductive and resistive layers of limestone between 857 and 860 mbsf. Between 861.7 and 870 mbsf, beds display irregular, wavy bedding with more sharply defined boundaries, which overlie a 1.5-m-thick, more resistive interval containing angular clasts. Based on the analysis of logs and core samples, three lithologic units are recognized in this interval: a sandy unit between 860.1 and 861.7 mbsf; a limestone conglomerate unit between 861.7 and 868.5; and a conglomerate containing basalt and dolerite cobbles between 868.2 and 870 mbsf (see "Lithostratigraphy"). The base of this unit between 870 and 874 mbsf is comprised of a limestone unit with wavy and disturbed bedding structures.
Large (<40-cm diameter), resistive, rounded clasts within a more conductive, clayey matrix indicate that log Unit L10 is a conglomerate (Fig. F85). Clast sizes display a wide range in diameter, but show an overall decrease with depth to the base of the logged interval at 890 mbsf. This unit is capped by a ~45-cm-thick highly resistive layer that is interpreted to be a dolomite based on comparison with the conventional log data.
TLT borehole temperature recorded during the triple combo string's first downward and upward passes are shown in Fig. F86. Because no in situ temperature data could be obtained for Hole 1118A using the Adara or DVTP tools, a second pass was conducted that was dedicated to temperature measurements after the first triple combo log was completed. The tool string was lowered to 415, 625, and 835 mbsf, and held for ~10 min at each station (Fig. F87).
A mudline temperature of 2.2°C was recorded during the first downward pass of the triple combo (Fig. F86). On the first upward logging pass, a distinct increase in temperature is observed above 700 mbsf. This increase is also visible, but less pronounced, on the second upward pass (Fig. F87), during which the tool string was brought up much more quickly (~2700 m/hr). This increase in temperature strongly suggests an influx of warm fluids at ~700 mbsf. Fluid flow up the borehole had been noted during the prelogging wiper trip (see "Operations") and was treated by pumping mud into the hole. Interstitial water chemistry shows complex behavior in Hole 1118A, which suggests the possibility of elevated temperatures but does not definitively indicate fluid migration (see "Inorganic Geochemistry").
The probable occurrence of fluid migration complicates estimation of in situ temperatures from open hole temperatures, because the classical extrapolation accounts only for conductive heat flow. Upward flow of warm fluid may have warmed the hole, and subsequently pumping of mud would have cooled the borehole and slowed upward fluid migration. As a result, only data from the deepest station (835 mbsf) were used to estimate equilibrium temperature, because this depth is likely to be less affected by fluid advection.
Equilibrium temperature at 835 mbsf was approximated by plotting the temperature as a function of ln [t/(t-s)], where t is the total time elapsed since the drill bit penetrated that depth, and s is the total time elapsed between the initial penetration and the cessation of circulation (Fig. F88). The line was then extrapolated to infinite time (where ln [t/(t-s)] = 0). This method was introduced by Bullard (1947) and previously applied to open-hole temperature measurements from ODP Leg 123 (Castillo, 1992). Results suggest an approximate equilibrium temperature of 55°C at 835 mbsf. From this approximation and the mudline temperature, a gradient of 0.06°C·m-1 (60°C·km-1) can be calculated. However, this should only be considered a very rough estimate because of the strong evidence that the thermal profile is not linear above this depth.
