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doi:10.2204/iodp.proc.314315316.116.2009

Log characterization and lithologic interpretation

Log characterization and identification of logging units

Hole C0004B logging units were characterized through visual inspection of the gamma ray, resistivity, sonic velocity, and caliper log responses (Fig. F1). The resistivity images were used to define finer scale characteristics within the units. Three primary logging units were defined based on the variability of the log responses (Table T5).

Logging Unit I (0–67.9 m LSF) is characterized by low-frequency and high-amplitude variation of the gamma ray log (50–75 gAPI). This logging unit is also characterized by constant low ring resistivity values (0.8–1.1 Ωm) and highly variable deep resistivity responses ranging from 0.6 to 1.5 Ωm. Resistivity log values in the lower part of this logging unit (54–68 m LSF) decrease from 1.1 to 0.8 Ωm, ring resistivity values increase from 0.8 to 1.0 Ωm, and the deep resistivity response increases from 0.8 to 1.2 Ωm with depth, with low-amplitude fluctuations (Fig. F1). The lowermost part of this logging unit is characterized by small borehole diameter values to as little as 8.9 inches. Logging Unit I correlates to slope sediments recognized in the seismic section (see “Log-seismic correlation”).

Logging Unit II (67.9–323.8 m LSF) is divided into four subunits (Table T5). Logging Subunit IIA (67.9–96.2 m LSF) is characterized by high-frequency fluctuations in gamma ray value (55–75 gAPI) with an overall increasing trend. Ring resistivity values range from 1.0 to 1.3 Ωm, and deep resistivity values range from 1.3 to 1.8 Ωm. The sonic log shows a slight positive shift from 1570 to 1620 m/s at the top of this subunit. Caliper values increase from 9 to 10.5 inches through this subunit (Fig. F1).

Logging Subunit IIB (96.2–160.3 m LSF) is characterized by continuous high-frequency fluctuation in gamma ray values between 62 and 78 gAPI and constant ring resistivity values (1.1 and 1.5 Ωm). The deep, medium, and shallow resistivity logs significantly deviate from each other and exhibit strong fluctuations (Figs. F1, F9). The shallow resistivity log commonly exhibits strong conductive excursions to 0.7 Ωm throughout the subunit. The sonic log response shows a positive shift from 1660 to 1780 m/s at the top of the subunit and increases to 1890 m/s with depth.

Logging Subunit IIC (160.3–236.4 m LSF) is characterized by low-frequency and high-amplitude variations in gamma ray values (59–84 gAPI). The ring resistivity also exhibits high-amplitude variations (1.0 to 1.5 Ωm). The deep, medium, and shallow resistivity logs show similar trends to those in logging Subunit IIB, significantly deviating from each other and showing strong fluctuations. The shallow resistivity log commonly exhibits strong conductive excursions to 0.6 Ωm in this subunit. Sonic velocity values change from 1760 to 2005 m/s within logging Subunit IIC and increase slightly with depth.

Logging Subunit IID (236.4–323.8 m LSF) is characterized by a decreasing trend in gamma ray values (from 83 to 63 gAPI), with a minor increasing trend over a particular short interval (from 250 to 266 m LSF). The ring resistivity log exhibits large variations between 1.0 and 1.5 Ωm, with repeating intervals of decreasing values. The shallow, medium, and deep resistivity logs show similar trends to the ring resistivity log. The sonic velocity log exhibits a series of decreasing trends, punctuated by intervals with increasing values. There is a particularly sharp increase in sonic velocity values across the interval 283–298 m LSF, above an interval with gradually decreasing values (back to ~1985 m/s) (Fig. F1). This trend in sonic velocity log responses is not observed in any other subunit of logging Unit II or other logging units.

The boundary between logging Units II and III is based on changes in the resistivity and sonic velocity logs. A repeated decreasing trend in the ring resistivity in logging Subunit IID changes to a moderately variable trend in Unit III, and highly variable sonic velocity values in logging Subunit IID change to a normal increasing trend in Unit III (Fig. F1). The caliper baseline is shifted at this boundary from ~10.5 to 9.5 inches (Fig. F1). This boundary coincides with the lower limit of fractured Zone 8 (see “Structural geology and geomechanics”).

Logging Unit III (323.8 m LSF to TD) is characterized by a decreasing trend in gamma ray values (74 to 60 gAPI) with a minor increasing trend over a short interval (337–350 m LSF). The resistivity logs exhibit moderately variable values (1.0 to 1.4 Ωm) for the ring resistivity and 1.1 to 1.6 Ωm for the deep resistivity. The sonic log shows a gradually increasing trend from 1980 to 2150 m/s with moderate fluctuations. The average borehole diameter in this logging unit is smaller than in logging Subunits IIB, IIC, and IID. Two distinct negative spikes in the resistivity log at 349–350 m LSF and 366–368 m LSF are interpreted as localized conductive fractures based on the borehole images (Fig. F1). Another negative spike in the resistivity, at 389–390 m LSF, corresponds to a negative spike in the gamma ray log, which is interpreted as a sand layer based on the borehole images. Logging Unit III is correlated to the underthrust sediments estimated from the seismic data (see “Log-seismic correlation”).

Figure F10 illustrates the ring and deep resistivity, gamma ray, and sonic transit time (slowness) distributions for the logging units and subunits. The gamma ray and ring resistivity logs exhibit a similar trend of gradually increasing from logging Unit I to Subunit IIC then gradually decreasing to logging Unit III. The deep resistivity shows an increasing trend from logging Unit I to Subunit IIA and then gradually decreases with depth to logging Unit III. Figure F11 shows cross-plots of sonic velocity versus gamma ray and sonic velocity versus ring resistivity. Logging Subunit IIA is isolated from both logging Unit I and other subunits in logging Unit II in both plots. Logging Subunit IID is widely distributed and overlaps with logging Subunit IIC and Unit III in both plots.

Log-based lithologic interpretation

RAB image data from Hole C0004B show distinctive textural, sedimentary, and structural features (Fig. F12). Deformation zones as defined in “Structural geology and geomechanics” at given depth intervals are easily recognizable in Figure F12 within logging Unit II as conductive bands. Internal trends of the gamma ray log show that each of the logging units, and to some degree the subunits, exhibit different trends. The most distinctive characteristics are observed in logging Unit I (0–67.9 m LSF) and Subunit IIC (160.3–236.4 m LSF).

Logging Unit I (slope sediments)

Logging Unit I exhibits two different trends. The upper ~30 m LSF exhibits low-frequency, decameter-scale fining-upward profiles, whereas the lower section exhibits higher frequency, meter-scale fining-upward profiles (Fig. F13). Across the logging Unit I/II boundary, the gamma ray log exhibits a near-constant baseline with only minor fluctuations in the ~6 m thick zone. From borehole images, southward-dipping well-stratified sediments with layering of centimeter to decameter scale are observed in logging Unit I. A conductive layer at the boundary between Unit I and Subunit IIA is presumed to be an unconformity (Fig. F14). The most likely lithology of logging Unit I is hemipelagic mud.

Logging Subunit IIA (mass transport deposits)

Although logging Subunit IIA can be correlated to the uppermost part of the wedge-shaped body with the major seismic reflector at ~70 m seismic depth below seafloor (SSF) forming its upper boundary, this subunit exhibits unique characters in facies and structure within logging Unit II. Logging Subunit IIA is obviously isolated from other subunits and units based on the cross-plot examination of sonic velocity versus gamma ray and sonic velocity versus ring resistivity (Fig. F11). No bedding surfaces or internal structures were identified from borehole images (Fig. F14). The structureless character with patchy texture suggests a chaotic, mixed, or deformed deposit, probably gravitational in origin.

Logging Subunit IIB (thrust sheet)

The uppermost interval of Subunit IIB (96.2–112.5 m LSF) corresponds to a fractured zone (Fig. F12) (see “Structural geology and geomechanics”). Image facies are characterized by several clear fractures and shear deformation with no clear bedding or sedimentary features throughout this interval. Below 112.5 m LSF, a few north–northeast dipping bedding planes can be identified and measured (Fig. F12). The most likely lithology of logging Subunit IIB is hemipelagic mudstone.

Logging Subunit IIC (thrust sheet)

Logging Subunit IIC is characterized by the difference observed in the gamma ray trends compared to logging Unit II above and below (Fig. F15). The baseline gamma ray value is slightly higher than the sections above and below and exhibits decameter-scale cycles dominated by fining-upward profiles, some of which have sharp bases, whereas others exhibit gradational coarsening-upward profiles. The subunits above and below logging Subunit IIC exhibit meter-scale variations of mostly fining-upward profiles, which have sharp bases (Fig. F15).

Based on interpretation of borehole images the upper part of Subunit IIC (160–190 m LSF) is strongly deformed and bedding features are not easily identified (Fig. F12). Over the interval 190–220 m LSF the formation is characterized by slightly higher resistivity values and weak deformation. Northwest-dipping bedding planes were measured consistently within this interval. The orientation of individual beds of relatively high resistivity can be identified and measured (e.g., at 204 m LSF), matching the general dip trend for this section. Below 220 m LSF bedding measurements indicate a slight change in orientation to northward dipping (Fig. F12). The log responses suggest that the lithology of logging Subunit IIC is hemipelagic mudstone.

Logging Subunit IID

The upper part of logging Subunit IID from 237–292 m LSF is a highly deformed zone. Although northward-dipping alternation of conductive–resistive layers are recognized locally in this interval, most of the original sedimentary features were disrupted at fractured Zones 6 (247–269 m LSF) and 7 (284–292 m LSF) (see “Structural geology and geomechanics”). The lower part of this subunit (292–324 m LSF) is weakly deformed. The angle of bedding dip changes from 20°–50° to <25° at ~298 m LSF (Fig. F12). In the interval 308–324 m LSF corresponding to fractured Zone 8 (see “Structural geology and geomechanics”), bedding-parallel shear zones are commonly observed as conductive bands. These sharp conductive bands can be easily distinguished from the surfaces of subtle alternating conductive–resistive beds. The most likely lithology of logging Subunit IID is hemipelagic mudstone.

Figure F16 shows the boundary between logging Units II and III. There is a distinct sandy interval of negative excursions in gamma ray, resistivity, and sonic velocity values at 311.7–312.6 m LSF. No significant washout is observed from the caliper log over this interval. The ring resistivity values exhibit oscillation over the intervals 301–311 and 312–324 m LSF above and below the sand layer at 311–312 m LSF. This fluctuation zone is surrounded by stable intervals (294–301 and 324–337 m LSF). The caliper log exhibits high values to 12 inches over the intervals 306–311 and 312–324 m LSF above and below the sand layer. These intervals coincide with a zone of parallel low-angle conductive fractures (fractured Zone 8) and are surrounded by intervals of small borehole diameter (293–306 and 324–336 m LSF). These log responses and images look symmetrical around a central sand layer, which implies a localized deformation zone.

Logging Unit III (underthrust sediments)

Logging Unit III is characterized by reduced deformation. Alternating conductive–resistive bedding is common throughout the unit. Bedding dip in this unit is <25°, except for the base of the unit (~390 m LSF) where it reaches >30°. The top of this unit corresponds to the top of underthrusting sediments at 316 m SSF (see “Log-seismic correlation”). Taken together, all log responses suggest that the lithology of logging Unit III is hemipelagic mud with thin sandy layers.

Gamma ray value trends

The gamma ray values exhibited in logging Units I and II in Hole C0004B are very similar to those exhibited by logging Units I and II in Hole C0001D (Fig. F17). Considering the proximity of the two sites (Fig. F2) it is feasible that the slope sediments are similar in composition (logging Unit I from each site) and that the thrust sheet sediments are similar in composition (logging Unit II from each site). We suggest that the slope sediments at Site C0004 are muddy deposits with thin sand layers, whereas logging Unit II is composed of hemipelagic mud and silt.

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