When the bit of the PCS was lost while coring Core 201-1228A-23P (see "Hole 1228A" in "Operations"), it became impossible to drill any deeper with the APC in this hole and we decided to log Hole 1228A. One logging run was made with the triple combo tool string (see "Downhole Logging" in the "Explanatory Notes" chapter). After recovery of the damaged PCS at 1300 hr on 5 March, the hole was conditioned for logging. A wiper trip indicated that there was no fill at the bottom of the hole. The hole was then displaced with 100 bbl of sepiolite, and the bottom of the drill string was positioned at 80 mbsf. Logging rig-up started at 1715 hr. The 35-m-long logging string started downhole at 1930 hr, and two passes were made without difficulty. Both passes reached the bottom of the hole at the wireline depth 201 mbsf (477 meters below rig floor), and the bottom of the drill string was raised by 10 m during logging, allowing us to record data in the open hole to 70 mbsf. Logging operations and rig-down were completed by 0000 hr on 6 March (see Table T10 for a detailed summary of the operations).
The caliper log (Fig. F23A) shows that the borehole wall was generally smooth and that the caliper arm maintained good contact with the formation over the entire logged interval, a requirement for high-quality data recording. Because of the generally poor core recovery below 90 mbsf, physical property measurements were made on fewer samples than at the previous sites, but the MAD density and porosity data available generally agree with the logs (Fig. F23E, F23F). This agreement is particularly critical to the interpretation and relocation of the discrete measurements, as the average recovery below 70 mbsf was ~40%.
Because of the short length of the open hole interval (~130 m), a second pass was made over its entire length to ensure data quality. Figure F24 compares some of the logs of the two passes (pass 1 = dashed green line and pass 2 = solid red line). The density and resistivity logs from the two passes overlap almost perfectly. The gamma radiation values from the second pass are slightly higher than those for the first pass, indicating that the formation is still activated because of the minitron source of the Accelerator Porosity Sonde used during the first pass (see "Downhole Logging" in the "Explanatory Notes" chapter). However, despite this generally uniform offset, both gamma ray logs agree very well, except for a spike recorded during the first pass at 164 mbsf, which does not appear in the second pass. Figure F23C indicates that this spike is created by an increase in uranium. Because the two independent gamma ray sensors recorded this spike during the first pass (see Fig. F23B), it must correspond to some lithologic feature and could indicate the presence of a uranium-rich nodule near the borehole wall. Because the sensors follow different paths along the borehole wall during successive logging passes, a discrete uranium-rich module might be detected on one pass and missed on another.
The general trend of the logs is characteristic of a transgressive margin. A high concentration of terrigenous sediments, predominantly clays and silts (see "Lithostratigraphy") generates generally high gamma ray values. Most logs, particularly the resistivity, gamma ray, and density logs, show a succession of sedimentation cycles indicating fluctuations in sea level over the depositional history of the site. Most cycles are 10 to 20 m thick and are characterized by an upward increase in density, resistivity, and gamma radiation. This is the signature of fining-upward facies typical of transgressive margins, where the finer clay-rich sediments settle on top of the sequence as sea level rises or the basin sinks (Rider, 1996). Based on variations in the trends of these cycles, we have identified three logging units, divided into logging subunits corresponding predominantly to individual cycles.
Logging Unit 1 (70-91 mbsf) is characterized by very strong peaks in gamma radiation and in uranium content. These peaks are present at the top of the two sedimentation cycles corresponding to logging Subunit 1A (70-82 mbsf) and logging Subunit 1B (82-91 mbsf) and are similar to features observed in the logs recorded in Hole 679E, located seaward of the Salaverry Basin. Similar spikes were attributed to the presence of uranium-bearing phosphorite layers created by the reworking of phosphate nodules (Shipboard Scientific Party, 1988a). Logging Unit 1 corresponds to the lower part of lithostratigraphic Subunit IIB, described as an alternation of diatom- and quartz-bearing clay and silt with quartz-rich lithic sand (see "Description of Lithostratigraphic Units" in "Lithostratigraphy").
The top of logging Unit 2 (91-135 mbsf) is identified by a strong peak in gamma radiation, resistivity, and density and by a sharp decrease in porosity, from values averaging 70% to values of ~50% (see Fig. F23F). Like the underlying logging Unit 3, this portion of the section was characterized by a low core recovery and is included in lithostratigraphic Subunit IIC, composed of diatom- and quartz-rich silts and sands. This logging unit is made of three sedimentation cycles identified as logging Subunits 2A (91-101 mbsf), 2B (101-118 mbsf), and 2C (118-135 mbsf). Logging Subunits 2A and 2B display clear increases upsection in resistivity and density, with a more subtle increase in gamma radiation. By comparison, the upsection increase in gamma radiation in logging Subunit 2C is more pronounced than the resistivity and density trends.
Logging Unit 3 (135-200 mbsf) is characterized by almost uniform porosity and density values and by a slight but steady decrease in resistivity with depth, interrupted by several spikes. These spikes are possibly created by layers of hard sandstone cemented by calcitic dolomite that were recovered by the Shipboard Scientific Party (1988b). Logging Unit 3 can be divided into three subunits based on the occurrence of these spikes, which could also correspond to deposition cycles less well defined than those in logging Unit 2. Logging Subunit 3A (135-156 mbsf) shows almost uniform gamma ray counts, despite a subtle increase in thorium, and a slight decrease with depth in resistivity and density. The top of logging Subunit 3B (156-176 mbsf) is defined by a negative spike in resistivity and density and by an increase in uranium. Logging Subunit 3B (176-200 mbsf) was logged fully only by the resistivity sonde, located at the bottom of the tool string (see "Downhole Logging" in the "Explanatory Notes" chapter) but is defined by a clear decrease in resistivity and by a series of strong and closely spaced resistivity spikes, indicating cemented dolomitic 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 1228A, the surface seawater and the sepiolite mud pumped during and after drilling generated borehole fluid temperatures higher than the formation temperatures. Discrete measurements made with the DVTP indicate a maximum formation temperature of 19.1°C at 194.9 mbsf (see "In Situ Temperature Measurements" in "Downhole Tools"), whereas the maximum temperature recorded by the TAP tool at 201 mbsf is 24.75°C at 190 mbsf (see Fig. F25). The generally lower temperatures during pass 2 indicate a progressive return to equilibrium. The apparent anomalies measured at 70 and 80 mbsf while logging downhole correspond to the tool exiting the drill string. The changes in trends on the uphole passes correspond to increases of the tool velocity at the end of each pass. The absence of significant anomalies in this profile that could not be attributed to operations shows that any hydrologic activity is absent or of low intensity.