Two logging runs were made in Hole 1230A with the triple combo and the FMS-sonic tool strings (see "Downhole Logging" in the "Explanatory Notes" chapter). The FMS-sonic run was added to the program at this site in order to better characterize the gas hydrate deposit that had been detected at Site 685 during Leg 112 (Shipboard Scientific Party, 1988) (see "Gas Hydrate"). After recovery of Core 201-1230A-39P at 2300 hr on 15 March, the hole was conditioned for logging. The wiper trip indicated that there was no fill at the bottom of the hole. The hole was then displaced with 150 bbl of sepiolite, and the bottom of the drill string was positioned at 80 mbsf. Logging rig-up started at 0300 hr on 16 March. The 35-m-long triple combo tool string started downhole at 0530 hr, and two passes were made without difficulty. Both passes reached the bottom of the hole at the wireline depth of 282 mbsf, and the triple combo run was completed at 1400 hr. The 33-m-long FMS-sonic tool string started downhole at 1515 hr, and two passes were made without problem over the entire open hole. Because of some concern over a tight spot felt at every exit of the bit, the drill string was raised by 10 m only during the last pass, allowing data recording in the open hole to 70 mbsf. Logging operations and rig-down were completed by 2200 hr on March 16 (see Table T13 for a detailed summary of the operations).
The caliper log measured by the two arms of the FMS (Fig. F29A) shows that the borehole wall was irregular but generally smooth and that the caliper arms maintained a good contact with the formation over most of the interval logged. This suggests that the data should be of excellent quality. There is a good correlation between the density log and the density measurements made on core samples (Fig. F29F). However, the good borehole conditions make it difficult to explain the discrepancy between the porosity log and the core measurements in Figure F29G. The logging values are consistently ~20% higher than the core measurements. The presence of bound water with clays generally increases the porosity measured with the Accelerator Porosity Sonde (see "Downhole Logging" in the "Explanatory Notes" chapter), but there was no similar discrepancy in the previous Peru margin sites (Sites 1228 and 1229), where the clay content was generally higher than at this site. Because the dominant component of these sediments is diatom ooze (see "Lithostratigraphy"), the core measurements might be underestimating the voids in the diatom skeletons, which are sensed by the porosity log. However, this should not account for the extent of the difference between the core and log measurements. We used the grain density measured on core samples (see "Density and Porosity" in "Physical Properties") to derive a porosity curve from the density log, by interpolating the grain density measurements at the logging sampling interval (0.1524 m) and assuming a uniform water density of 1.05 g/cm3. The resulting profile (red line in Fig. F29G) is in much better agreement with core measurements. This agreement is a direct consequence of the good match between the log and core density data because the same grain density is used to derive the core sample porosity from the core density measurements.
The sonic compressional and shear velocities in Figure F29E were calculated during the acquisition by an automatic slowness-time coherence algorithm applied to the monopole and dipole waveforms (see "Downhole Logging" in the "Explanatory Notes" chapter). The gaps and some dubious spikes in this figure show that the waveforms will need to be reprocessed, particularly in the upper part of the hole, in order to determine more reliable values.
The general trend of the logs is controlled by the nature of the sediments and influenced by the location of the site on the lower slope of the Peru Trench, at the transition between the continental crust and the accretionary complex. The sediment composition, as shown by the average gamma radiation values, is intermediate between the equatorial Pacific sites (Sites 1225 and 1226), which were dominated by biogenic sediments with extremely low gamma radiation counts, and the shallow margin sites (Sites 1228 and 1229), which contained mostly terrigenous sediments. Thorium concentrations consistently higher than uranium suggest that the clastic sediments are of continental origin (Rider, 1996). However, the low variability in most logs is similar to the data recorded in the open-ocean sites. Because of this low variability in the logs, we distinguish only two logging units, which correspond to the two lithostratigraphic units identified from the cores (see "Description of Lithostratigraphic Units" in "Lithostratigraphy").
Logging Unit 1 (80-216 mbsf) is characterized by a general downhole increase in natural gamma radiation, thorium, resistivity, and density. This general trend corresponds to sediment compaction. This logging unit can be divided into five subunits with slightly different characteristics.
Logging Subunit 1A (80-97 mbsf) is characterized by very high porosity and a strong downhole increase in gamma radiation and density. Spikes in the resistivity log and, less reliably, in the velocity log could indicate the presence of gas hydrate that was recovered in this unit (Core 201-1230B-12H) (see "Lithostratigraphy"). A highly resistive feature that steeply dips to the north in the FMS images at 85 mbsf (Fig. F30A) could represent a hydrate-rich layer.
The top of logging Subunit 1B (97-118 mbsf) is marked by a sharp drop in density, resistivity, gamma radiation, and thorium followed by a steady downhole increase in these same measurements. The overall lower thorium content of this unit indicates a more marine origin of the sediments.
Logging Subunit 1C (118-160 mbsf) shows almost uniform values in gamma radiation, radioactive element concentration, and density, as well as some variability in resistivity that could be attributed to the presence of gas hydrate. Gas hydrate was identified in three cores from this subunit during Leg 201 (Cores 201-1230A-15H, 18H, and 19H) (see "Lithostratigraphy") and during Leg 112 (Shipboard Scientific Party, 1988). The FMS images in this subunit (Fig. F30B, F30C) show a series of resistive features that generally dip to the north, northeast, or to the southwest. These features indicate tilted layers, faulting, and conjugate planes, particularly between 135 and 140 mbsf and between 145 and 155 mbsf. Gas hydrate might be filling some of these fault joints.
Logging Subunit 1D (160-201 mbsf) is characterized by a steady increase in density and gamma radiation, mostly due to an increase in thorium. The top of logging Subunit 1E (201-216 mbsf) is marked by a sharp drop in gamma radiation, and this subunit is characterized by a decrease in gamma radiation and density, opposite to the compacting trend of the overlying subunits.
Logging Unit 2 (216-280 mbsf) corresponds to lithostratigraphic Unit II (see "Description of Lithostratigraphic Units" in "Lithostratigraphy") and is composed of sediments under intense deformation belonging to the accreted complex. The top of this unit is marked by a sharp increase in resistivity and density. It is also clearly defined in the FMS images (Fig. F30D) by a bright massive feature that dips ~69° to the northeast at 216 mbsf. All static images from below this depth (Fig. F30D, F30E) are much brighter than those from above this boundary. This relative brightness indicates the higher resistivity of the accreted sediments. Some of the most striking features in the images (at 223, 225, and 241 mbsf) are steeply dipping to the northeast (by 55°-70°). This inclination indicates the general deformation of the accreted sediments. Whereas logging Subunit 2A (216-246 mbsf) shows a very high variability in resistivity and density, the underlying logging Subunit 2B (246-280 mbsf) is characterized by almost uniform, slightly lower, resistivity and density.
Temperatures were recorded with the Lamont-Doherty Earth Observatory Temperature/Acceleration/Pressure (TAP) memory tool attached at the bottom of the triple combo tool string. Because only a few hours had passed since the end of drilling operations and hole conditioning, the borehole temperature is not representative of the actual equilibrium temperature distribution of the formation. In the case of Hole 1230A, the surface seawater cooled though the 5-km water column generated borehole fluid temperatures lower than the equilibrium formation temperatures. Discrete measurements made with the DVTP indicate a maximum measured temperature of 10.34°C at 256.6 mbsf (see "In Situ Temperature Measurements" in "Downhole Tools"), whereas the maximum temperature recorded by the TAP tool at 284 mbsf is 8.9°C (see Fig. F31). The generally higher temperatures during the second pass indicate a progressive return to equilibrium. The variations measured at 80 mbsf while logging downhole correspond to the tool exiting the drill string. The temperature fluctuations within the pipe recorded during the two passes logging downhole are difficult to explain. Overall, this profile suggests that besides mechanical drilling disturbances, the operations in Hole 1230A did not raise temperatures and cause decomposition of gas hydrate, which remains stable to temperatures of up to 30°C at this depth (Kvenvolden and McMenamin, 1980). Therefore, the present data should allow an accurate estimation of the hydrate content of these sediments.