One logging run was made at Site 1226 in Hole 1226B with the triple combo tool string (see "Downhole Logging" in the "Explanatory Notes" chapter). After the last core was recovered at 1900 hr on 22 February, the hole was conditioned for logging. A wiper trip determined that there was no fill at the bottom of the hole. The hole was then displaced with 220 bbl of sepiolite, and the bottom of the drill string was positioned at 80 mbsf. Logging rig-up started at 2300 hr on 22 February. The 35-m-long logging string started downhole 0130 hr on 23 February, and two passes were made without difficulty. Both passes reached the bottom of the hole at the wireline depth of 423 mbsf (3732 mbrf). Logging operations and rig-down were completed by 1030 hr on 23 February (see Table T13 for a complete summary of the operations).
The caliper log (Fig. F31A) shows that the borehole wall was generally smooth and that the caliper arm maintained good contact with the formation over most of the interval, a requirement for good quality data recording. Between 200 and 250 mbsf, the caliper appeared to reach its maximum extent, but the very good agreement between the density and porosity logs with core measurements (shaded circles in Fig. F31E, F31F) show that these measurements, which are the most sensitive to hole quality, were not affected. As at the previous site, the far porosity readings are generally lower than the near porosity because of their greater depth of investigation and they agree well with the MAD data. The excellent agreement between logs and the MAD properties extends to the entire interval logged and allows fine-scale core-log correlation of some distinctive features. A local density and resistivity minimum in the logs at 138.15 mbsf corresponds to a similar density low in Section 201-1226B-16H-1, from a sample that was taken at the bottom of a 30-cm-thick diatom-rich dark green band. Similarly, a density and resistivity peak at 262.64 mbsf can be associated with a density maximum measured at the top of Section 201-1226B-34X-2, underlying a dark green diatom- and nannofossil-bearing carbonate ooze layer. In such favorable hole conditions, similar core-log correlations can be extended further to correlate core description and downhole measurements.
Figure F32 compares the logs from Hole 1226B (solid red lines) with the data recorded in Hole 846B during Leg 138 (dashed green line). The comparison between the two calipers in Figure F32A shows that Hole 846B was slightly narrower, but both holes are of comparable quality, which is confirmed by the excellent match between the two sets of logs. Both density and resistivity logs are almost identical, and the only differences are in the thorium and potassium yields (Fig. F32C, F32E). As at Site 1225, this difference is due to the low natural radioactivity of the nannofossil ooze that constitutes the bulk of the formation and the higher sensitivity of the gamma ray tool used during Leg 201 even though gamma readings were higher in Hole 1226B than Hole 1225A. However, a comparison of gamma ray measurements shows that yields are higher from Hole 1226B (average = >10 gAPI) than those recorded from Hole 1225A (average = <10 gAPI). The higher values measured in Hole 1226B are within the sensitivity range of the two generations of gamma ray sondes used (Hostile Environment Natural Gamma Ray Sonde and Natural Gamma Ray Tool) (see "Downhole Logging" in the "Explanatory Notes" chapter), allowing a good match between the total counts of the logs. As in Hole 846B, the almost perfect parallelism between total gamma ray counts and the uranium content (Fig. F31B, F31C) shows that the local radioactivity is dominated by uranium and that uranium content increases steadily uphole. This observation, made during Leg 138, had suggested that this was the indication of high organic carbon in sufficiently reducing sediments, which would accumulate insoluble uranium(IV) from the seawater (Shipboard Scientific Party, 1992).
As at Site 1225, the sediments at Site 1226 can be generally described by low natural radioactivity and physical attributes (density and resistivity) typical of high porosity. Both porosity logs and MAD measurements show that the porosity is consistently >60% (average = ~70%), explaining the generally low density and resistivity. The low NGR values are typical of the nannofossil-rich sediments dominant at Site 1226, and the main compositional change indicated by the logs is the general increase of uranium uphole. Our characterization of the formation from the logs is based on the physical measurements that show the clearest distinctions, specifically the resistivity and the density. Because of the limited variations in all data, we consider that the sediments penetrated by Hole 1226B correspond to only one logging unit. Subtle covariation differences allow definition of five subunits (Subunits 1A-1E) (see Fig. F31).
Logging Subunit1A (80-200 mbsf) is characterized by generally higher gamma radiation and uranium readings, decreasing steadily with depth. Resistivity and density both increase slightly downhole, and porosity decreases accordingly. The bottom of this subunit is marked by a sharp drop in gamma radiation and uranium, which is marked also in the density and resistivity logs.
Logging Subunit 1B (200-263 mbsf) displays almost uniform values for all logs, except for the decrease in gamma radiation and uranium that follows the general downward trend of the natural radioactivity in the entire hole. This subunit coincides with the only interval in the hole where the caliper indicates a consistently larger hole. Although this does not seem to have affected the data, which compare well with the core measurements and the data from Site 846, it indicates an interval with lower mechanical cohesion than the formations above and below.
In logging Subunit 1C (263-312 mbsf), gamma radiation counts, resistivity, and density decrease steadily with depth while porosity increases. This subunit corresponds to the upper part of lithostratigraphic Subunit ID, described as alternating clay- and radiolarian-bearing diatom nannofossil ooze with clay- and radiolarian-bearing diatom-rich nannofossil ooze, with a generally higher diatom-ooze proportion than the surrounding sediments (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). The sequence of variously clay-bearing intervals is apparent in the generally larger variations in the NGR log. The higher porosity and lower density are typical of diatom-rich sediments.
Logging Subunit 1D (312-363 mbsf) is defined by a steady increase in resistivity and density with depth, whereas porosity decreases and gamma radiation counts are almost uniform. This subunit generally coincides with the middle part of lithostratigraphic Subunit ID, composed of diatom-bearing nannofossil chalk (see "Description of Lithostratigraphic Units" in "Lithostratigraphy").
The top of logging Subunit 1E at 272 mbsf is marked by a sharp increase in density and resistivity and an opposite decrease in porosity. This logging subunit coincides with the deeper section of lithostratigraphic Subunit ID, described as diatom-bearing nannofossil chalk with chert layers (see "Description of Lithostratigraphic Units" in "Lithostratigraphy"). The spikes in the resistivity and density logs throughout this subunit could be associated with these layers.
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 1226B, the drilling fluids circulating through the water column generated borehole fluid temperatures lower than the formation temperatures. Discrete measurements made with the DVTP-P indicate a maximum formation temperature of 23.7°C at 400 mbsf (see "In Situ Temperature Measurements" in "Downhole Tools"), whereas the maximum temperature recorded by the TAP tool is 14.0°C at 421 mbsf (see Fig. F33). The absence of significant anomalies in this profile not attributable to operations shows that any hydrologic activity is absent or of low intensity.