Hole 1085A was logged with a full suite of sensors to continuously characterize the sedimentary changes, to correlate the lithostratigraphy to other sites, and to provide data for core-log integration.
Hole 1085A was logged with four different tool strings. The first tool string (seismostratigraphy) included the NGT, LSS, DIT, and TLT sondes. The second tool string (lithoporosity) included the NGT, neutron porosity, gamma density, and TLT sondes. The third tool string (FMS, 1 pass) included the NGT, inclinometry, and FMS sondes. The fourth tool string (GHMT) included the NGT, magnetic susceptibility, and vertical component magnetometer sondes. The logs were run uphole from 606 mbsf (total depth) to pipe at 60 mbsf; the two first runs were logged to the seafloor. The natural gamma-ray intensity is the only parameter measurable through the pipe, but it should be interpreted only qualitatively in this interval. The pipe was set at 90 mbsf and pulled up to ~60 mbsf during logging for the first three runs and before logging for the fourth run (GHMT). The wireline logging heave compensator was started immediately upon entering the hole.
Hole 1085A is characterized by an irregular hole diameter size of ~9 to 14 in with numerous large enlargements from the bottom to 150 mbsf (see caliper measurements; Fig. 36). Above this interval, the hole conditions are very degraded and show critical washout zones at the top of the logged interval. The downhole measurements are affected by the poor hole conditions, and, thus, only the measurements that are less sensitive to hole conditions yielded reliable data (resistivity, sonic velocity, gamma-ray intensity, and magnetic susceptibility).
The lithologic succession recovered from Hole 1085A is controlled mainly by changes in the nature and intensity of biogenic production vs. the type and amount of detrital input. It is characterized by small changes in sediment composition and compaction, which should be reflected in the log physical properties measurements. Despite the uniform lithology defined from core observation and smear-slide studies (see "Lithostratigraphy" section, this chapter), the high resolution and sensitivity of downhole measurements allows us to identify numerous sedimentary changes in the logged formation.
Lithostratigraphic Unit II at the very bottom of the hole is characterized by decreasing magnetic susceptibility and increasing resistiv-ity; the other parameters were not measured at this depth. The lower part of lithostratigraphic Subunit IB is marked by a large change in magnetic susceptibility at 535 mbsf, which is also weakly reflected in the gamma-ray and the resistivity logs. This pronounced magnetic susceptibility feature suggests a possible change in the content of detrital magnetite. An overall regular decrease in gamma-ray intensity, resistivity, and magnetic susceptibility occurs between 430 and 400 mbsf and corresponds to a general increase in carbonate content (see "Organic Geochemistry" section, this chapter). This trend is particularly clear in the 400–360 mbsf interval where the carbonate content reaches a maximum. Exactly the same pattern is observed in the gamma-ray log at 75 mbsf, which approximately corresponds to the boundary between lithostratigraphic Subunits IB and IA.
Besides this general trend, the 480–460, 455–430, and 150–120 mbsf intervals are characterized by high values of gamma-ray intensity, magnetic susceptibility, uranium (U) content, and, occasionally, resistivity. The interval between 455 and 430 mbsf shows an unusual velocity decrease. In contrast, the intervals at 530–510, ~480, ~455, and 280–260 mbsf show a low gamma-ray intensity, magnetic susceptibility, and, occasionally, U content. Uranium correlates with the other parameters when the organic carbon content is high (e. g., between 480 and 420 mbsf; see "Organic Geochemistry" section, this chapter). These intervals probably reflect high-frequency changes in the ratio between clastic and biogenic components.
The temperature tool measures borehole fluid temperature. The results suggest a downhole thermal gradient of 25°C/km, an estimate that is low because of the cooling effect of circulation during drilling.
The core MST and log measurements of natural gamma-ray intensity are very similar. Core data are recorded in counts per second (cps), whereas log data are presented in API (Oil Industry Standard) units. Detailed correlations between the core and log data sets (Fig. 37) appear reliable, with a higher amplitude of the signal in the core. The interval with no recovery between 280 and 265 mbsf corresponds to an interval of low gamma-ray intensity values in the log. Thus, the missing sequence most likely contains carbonate-rich sediments. Log depth is close to core depth at Hole 1085A.