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doi:10.2204/iodp.proc.322.103.2010

Logging and core-log-seismic integration

Hole C0011A logging data quality

Near the end of Expedition 319, LWD and MWD data were collected in Hole C0011A. The MWD tool provided drilling parameters, and LWD data were obtained using Schlumberger's geoVISION tool (see "Operations" in Expedition 319 Scientists, 2010). Real-time data from MWD and real-time and memory data from geoVISION were collected. The geoVISION memory data were environmentally corrected. The mudline was identified at 4078 m LWD depth below rig floor (LRF) from LWD gamma ray and resistivity data. All LWD data were converted to LWD depth below seafloor (LSF) based on the mudline depth. Figure F69 shows the overview of the data from MWD and geoVISION. Generally, the data quality, except the resistivity images, is good and no vertical tool shocks were detected with sensors in the tools. The amount of stick-slip, caused by unstable bit rotation, increases with depth in the borehole. Between 596 and 629 m LSF, the stable ROP following a wiper trip resulted in high-quality resistivity images. Elsewhere in the borehole, variations in rotations per minute (rpm) and high angular acceleration index (AAI) indicate that the bit rotation was not stable. Sections of missing images are the result of the sudden rotation caused by stick-slip, consistent with AAI spikes (Fig. F70). The ship's heave also produced some discontinuities in the resistivity images, resulting in an overlap of images at the same depth (Fig. F70). Overall, the resistivity images are of poor quality. The close correspondence of deep- and shallow-button resistivity values indicates little or no significant drilling mud invasion into the formation (Fig. F69).

Log characterization and interpretation

Site C0011 logging units were characterized from visual inspection of gamma ray and ring resistivity log responses (Fig. F47). Five primary logging units were defined based on variations in trend lines and log character, with each logging unit further divided into subunits, defined by more subtle variations (Table T27). Statistical variations were calculated on gamma ray and resistivity values in logging units and subunits (Fig. F71). Lithologic interpretations were made based on log character and relative values, with some guidance from previous coring and logging results at Site 1173 in the Shikoku Basin (Moore, Taira, Klaus, et al., 2001; Mikada, Becker, Moore, Klaus, et al., 2002). We classified the background lithology as hemipelagic mud, with relatively constant gamma ray and ring resistivity values. A differentiation between volcanic ash and sand was based primarily on peaks in gamma ray and ring resistivity values: low gamma ray with high resistivity was interpreted to be ash, and low gamma ray with low resistivity was interpreted to be sand. Previous ODP studies (Legs 131, 190, and 196) characterized the upper Shikoku Basin sediments as rich in ash layers, whereas the lower Shikoku Basin sediments are dominated at Site 1177 by sand turbidites (Moore, Taira, Klaus, et al., 2001; Mikada, Becker, Moore, Klaus, et al., 2002; Taira, Hill, Firth, et al., 1991).

Logging Unit 1 (0–251.5 m LSF)

Logging Unit 1 (0–251.5 m LSF) exhibits an initial increase in gamma ray and resistivity over the upper 5 m, which is potentially caused by a transition from unconsolidated mud-rich sediments near the seafloor to more consolidated mud-rich sediments with depth. Then the gamma ray log maintains minor fluctuations around a constant trend (~72 gAPI), while the ring resistivity log exhibits a very gradual decrease (~0.70 to 0.50 Ωm) (Fig. F47). We interpret the response of low gamma ray and ring resistivity log values to be consistent with a lithology of sandy mud (Fig. F47); however, another factor could be the influence of high porosity. Both the gamma ray and ring resistivity logs exhibit only small variations through this interval, but we can still define three subunits. A stepped increase in the gamma ray log trend (~77 gAPI) corresponds to a gradual decrease in the ring resistivity trend at 100.0 m LSF and marks the base of logging Subunit 1A (0–100.0 m LSF). Logging Subunit 1B (100.0–212.0 m LSF) exhibits a gradual decrease in ring resistivity log values and slightly increasing gamma ray log values, potentially associated with simple increased compaction with depth. Because there is only a minor variance between logging Subunits 1A and 1B, they are both categorized as sandy mud intervals. Logging Subunit 1C (212.0–251.5 m LSF) exhibits a lower gamma ray trend line (~70 gAPI) with a sharp contact; a similar sharp decrease in ring resistivity log values marks the top of the subunit boundary. Ring resistivity values are low in comparison with the other subunits (~0.50 Ωm) and remain constant to the base of the subunit. The lower gamma ray signature suggests a coarser texture to this subunit, and therefore, it is categorized as muddy sand. Within this subunit, between 216 and 220 m LSF, a series of low-value gamma ray peaks correspond to high-value ring resistivity peaks, and these features are interpreted as thin ash layers. A sharp increase in both logs at 251.5 m LSF marks the logging Unit 1/2 boundary (Fig. F47).

Logging Unit 2 (251.5–478.5 m LSF)

Below the sharp increase in gamma ray and resistivity, which may correspond to a change from coarser grained sediment to a background hemipelagic mud, logging Unit 2 (251.5–478.5 m LSF) exhibits a gradual increase in the ring resistivity log until a constant trend line is reached (~0.85–0.90 Ωm) and dominates the logging unit. The gamma ray log has a stepped increase marking the top of the unit to a constant trend line (~95 gAPI); minor fluctuations around this trend line suggest a silt–sand alternation with the background hemipelagic mud. The most striking features within logging Unit 2 are a series of prominent, high-value resistivity spikes, corresponding to low-value gamma ray peaks. These features vary in thickness from a few decimeters to tens of meters and are interpreted to be ash and volcaniclastic sand layers (Fig. F72). The first occurrence at ~337.0 m LSF marks the upper boundary of logging Subunit 2B (337.0–478.5 m LSF). In this subunit, gamma ray log values exhibit a blocky character, with sharp contacts both at the top and bottom of each feature. At corresponding depths, ring resistivity values are high, with gradual increases from the baseline value and sharp basal transitions. The thickest layers are concentrated ~345.0–410.0 m LSF, although minor occurrences continue to the base of the logging unit at 478.5 m LSF. Logging Subunit 2A (251.5–337.0 m LSF) exhibits the more constant and uniform character of the unit and is interpreted to be dominated by silt-rich hemipelagic mudstone (Fig. F47). The base of logging Unit 2 (and Subunit 2B) is placed at the base of the lowermost low-value gamma ray peak (478.5 m LSF), below which there is a sharp increase in gamma ray and ring resistivity log values.

In Hole C0011A, based on the last occurrence of ash/volcaniclastic layers in the log-based lithology, we place a tentative boundary between the middle Shikoku Basin sediments and the lower Shikoku Basin sediments at the base of logging Unit 2 (478.5 m LSF). We place the upper boundary of the middle Shikoku Basin at the top of logging Subunit 2B (337.0 m LSF).

Logging Unit 3 (478.5–736.0 m LSF)

Logging Unit 3 (478.5–736.0 m LSF) exhibits an increased gamma ray log trend (~107 gAPI), with little variation. Ring resistivity log values also increase in this unit and exhibit little variation, with the exception of minor spikes. There is a small step decrease in the trend, potentially caused by a decrease in the clay content, which marks the base of logging Subunit 3A (478.5–542.5 m LSF); where this lower value section occurs is logging Subunit 3B (542.5–649.0 m LSF). An apparent change in gamma ray and resistivity baseline values at ~596 m LSF and a return to the previous baseline at ~629 m LSF is likely due to a reamed-down interval during the drilling process rather than any change in the formation. Around 649.0 m LSF, a change occurs in the log character, with the introduction of low-value gamma ray peaks and corresponding low ring resistivity spikes, which are interpreted as sand or sandstone beds. Through logging Subunit 3C (649.0–736.0 m LSF), these spikes increase in concentration, thickness, and amplitude. The initial spikes are only minor deviations from the trend line in both gamma ray and resistivity logs, changing to very low gamma ray values (~35–25 gAPI) that define features of meter-scale thickness with increasing depth. At ~700.00 m LSF, two thick sandstones are interpreted, below which the character of the spikes decreases until the top of logging Unit 4 (Fig. F73).

Logging Unit 4 (736.0–867.0 m LSF)

Logging Unit 4 (736.0–867.0 m LSF) exhibits a very different character from the units above and appears to correspond to a series of strong reflections in seismic data (Fig. F74). As Figure F74 illustrates, the variability in gamma ray and ring resistivity log values is greater than that in logging Units 1–3. Logging Unit 4 can be divided into four "packages" of sediment, which exhibit coarsening- and fining-upward trends (Fig. F74). These are interpreted to be more consolidated sandy turbidite packages. The base of logging Unit 4 (867.0 m LSF) is characterized by a change to more uniform gamma ray log values and a sharp increase in ring resistivity log values (Fig. F47).

Logging Unit 5 (867.0–950.5 m LSF)

Logging Unit 5 (867.0–950.5 m LSF) is characterized by a sharp increase in ring resistivity log values, which then remain constant with minor fluctuations around the trend line. In contrast, no corresponding sharp change occurs in gamma ray, but rather gamma ray values decrease slightly with depth and exhibit less variation than logging Unit 4. Logging Subunit 5A (867.0–938.5 m LSF) is characterized by minor fluctuations in gamma ray values, with corresponding fluctuations in ring resistivity log values, and encompasses most of logging Unit 5. The gamma ray log values suggest a hemipelagic mudstone, but higher ring resistivity log values indicate a more consolidated or compacted nature (Fig. F71). In the lowermost section, a sharp decrease in gamma ray log values marks the Subunit 5A/5B boundary. Ring resistivity values in Subunit 5B (938.5–950.5 m LSF) remain very similar to Subunit 5A (Fig. F71). The contrast between the change in gamma ray log and continuation of the resistivity log suggests a compositional change between the two subunits. Lower gamma ray log values imply an increase in sand/silt, leading to a category of consolidated silty/sandy mudstone.

Structural image analysis

Resistivity images were generated from the deep-, medium-, and shallow-button resistivities, and both static and dynamic processing were applied. Large-scale variations are most clearly observed in static images, while smaller-scale features are highlighted in dynamic images. Bedding, resistive and conductive fractures, faults, and breakouts were recorded.

Bedding and fractures

Bedding dip and orientation were fairly constant throughout the whole drilled interval, dipping consistently toward to the north, at a shallow 5°–20° angle (Figs. F22, F47). This dip direction is consistent with the regional dip of reflections, observed in seismic data (Fig. F1) as well as core observations (see "Structural geology"). Abundant bedding-parallel conductive and resistive fractures were also observed, although it is possible that some of the features interpreted as resistive fractures are actually ash layers (Fig. F75A), whereas some of the conductive fractures may be silty layers. However, the resolution of the logging data does not allow us to make this distinction.

Some lithologic variations can be distinguished in the resistivity images. For example, the volcaniclastic sandstones found in cores within lithologic Unit II (see "Lithology") can be identified in the dynamic resistivity images as having a more "mottled" appearance (Fig. F75B). Local high-angle faults (>50°) are observed cutting across fractures and bedding and, in some cases, offsetting borehole breakouts (Fig. F75C). These faults exhibit a general southward dip (Fig. F22) and increase in concentration through logging Unit 3. Only one fault is recorded in logging Units 4 and 5. However, the lack of faults in the lower units may be due to the degradation of image quality over this interval. The chaotic deposit recognized in the cores from 400.86 to 411.15 m CSF (see "Lithology") can be identified in resistivity images at 399.5–409.5 m LSF (Fig. F75D).

Borehole breakouts

Breakout analysis was performed to assess the orientation of the maximum horizontal stress direction within the borehole. Breakouts appear as two vertical conductive zones, with 180° of separation between them (e.g., Fig. F75C). They typically form perpendicular to the direction of maximum horizontal stress in the borehole. Assuming that they are Coulomb shear failures because of circumferential stress of the borehole wall, the breakout width is related to the horizontal differential stress. The parameters needed to calculate the differential horizontal stress (S_Hmax – S_hmin) from breakout width were not available, however, and the calculation was not done on board.

At Site C0011, breakouts are most prevalent in the hemipelagic sediments of logging Units 3 and 4, particularly in an interval between ~600 and 650 m LSF (Fig. F47). Analyses indicate a mean azimuth of 115° and, therefore, an azimuth of 205° for the maximum horizontal compressive stress (S_Hmax) (Fig. F76). This north-northeast–south-southwest trend is roughly perpendicular to the convergent direction of the Philippine Sea plate (Fig. F77) and to the dominant maximum horizontal compressive stress determined at IODP sites within the accretionary prism during NanTroSEIZE Stage 1 (Kinoshita et al., 2008) and Site 808 (Ienaga et al., 2006; McNeill et al., 2004). The shallow sediments of the Kumano Basin at IODP Site C0002 exhibit a similar perpendicular-to-convergence azimuth (Kinoshita et al., 2008).

Seismic analysis

Seismic stratigraphy

A high-resolution, three dimensional (3-D) multichannel seismic (MCS) reflection survey was carried out in the Nankai Trough off the Kii Peninsula using R/V Kairei of the Japan Agency for Marine-Earth Science and Technology in March 2006 (Park et al., 2008). A ~5 km, 204 channel streamer and ~100 m separated dual source (two G-guns plus one GI-gun) were used for the 3-D MCS survey. Flip-flop shooting with a 30 m interval yields ~50 m separated two common midpoint lines, resulting in a 3.5 km × 52 km 3-D seismic volume. Recording length with a 1 ms sampling interval is 10 s. Depths of source and streamer cable are 5 m and 8 m, respectively. After applying a bandpass filter, amplitude recovery, deconvolution, multiple suppression, 3-D geometry (bin size 25 m × 50 m), flexible binning, and normal moveout velocity analysis for the 3-D data, an interval velocity volume model for 3-D prestack depth migration (PSDM) was constructed and updated. A velocity uncertainty test demonstrated that the final 3-D PSDM velocity has an ~5% maximum velocity uncertainty at ~6 km depth.

We show a small part of In-line 93 and Cross-line 816 of the 3-D PSDM across Site C0011 and their interpretations in Figures F78 and F79, respectively. Based on reflection characteristics, we identify seven seismic reflection units, which are denoted as A–G. According to Ike et al. (2008b), seismic Units A–C and D–F correspond to upper and lower Shikoku Basin strata, respectively, whereas Unit G is igneous oceanic crust of the Philippine Sea plate. Unit A shows a very flat seafloor reflection and northwest tilting with a slope of ~4°. Several continuous and subparallel bedding planes are observed within this unit. The upper boundary of Unit B at 4325 mbsl shows relatively weak and discontinuous reflection, while its lower boundary at 4435 mbsl shows very strong reflection. We observe several normal faults cutting the upper boundary. Unit B represents chaotic reflection character. This unit shows northeast tilting with a gentle slope of ~2° on Cross-line 816. On In-line 93, the upper boundary of Unit C at 4435 mbsl shows very strong reflection except for several regions of faulting, while its lower boundary at 4574 mbsl shows discontinuous reflection. In contrast, the lower boundary exhibits almost continuous reflection on Cross-line 816. We did not observe the normal faults on Cross-line 816, which seem to cut through this unit as shown on In-line 93. Unit D exhibits transparent reflection character. We observe three continuous reflectors within this unit on Cross-line 816. The upper boundary of Unit E at 4835 mbsl shows continuous reflection with low frequency and is displaced by normal faults. We observe successive strong and continuous reflections within this unit. Unit F is characterized by transparent reflection character. The top of basement Unit G is at 5204 mbsl.

Synthetic seismograms

Synthetic seismograms calculated from an acoustic impedance model, or more simply from a velocity model based on a geological model or well logs, are simulated traces comparable to the seismic sections obtained after processing of seismic data (e.g., surface seismic reflection after common depth point stacking). They are used to identify the causes of seismic markers and to calibrate surface seismic events in time and amplitude (Boyer and Mari, 1997). Displaying the synthetic seismogram beside the seismic data in the vicinity of the borehole provides information about specific boundaries of interest and a quality check on velocity models and density logs.

Most conventional synthetic seismograms are calculated from well logs (sonic and density). In order to create a synthetic seismogram, a source wavelet is convolved with a reflection coefficient series that is generated by using the check shot curve, sonic log, and density log. Wireline logging operations, unfortunately, were not conducted at Site C0011, so a check shot curve, sonic log, and density log were not available for conventional synthetic seismogram calculation. As an alternative, we attempted to create a synthetic seismogram using P-wave velocity (V_P) and bulk density data obtained from measurements on discrete core samples (see "Physical properties"). We used Paradigm Ltd.'s software package Epos3 for the synthetic seismogram calculation.

Depth intervals of measured V_P and density data from Hole C0011B are 340.4–866.9 m CSF and 340.4–876.0 m CSF, respectively. We edited some erroneous spikes in the V_P and density data. V_P was measured on each core sample in three components (x-, y-, and z-axis). We adopted z-axis V_P data, whose direction is similar to that of a seismic wave during a check shot survey. For a check shot curve, we used the z-axis V_P data for a range of 340–876 mbsf. The 3-D PSDM velocities of Park et al. (2008) guided the check shot curve for the depth ranges of 0–340 mbsf and 876–1200 mbsf. We adopted the Ricker wavelet as a source wavelet. Figure F80 shows the synthetic seismograms beside the seismic data along PSDM Cross-line 816.

We recognize a reasonably good match between the PSDM and synthetic seismogram waveforms at the upper boundary of seismic Unit C, even though there is a mismatch of depth. In contrast, we identify mismatches of both waveform and depth at the lower boundary of Unit C as well as the upper boundary of Unit E. Consequently, a highly correlated synthetic seismogram could not be achieved. The mismatch is probably because of (1) sparse intervals of core recovery and subsequent measurements, (2) errors in physical properties data due to widespread coring disturbance, and (3) uncertainty of PSDM velocities. Such a poor recovery of core samples may affect the waveform mismatch between the PSDM and synthetic seismogram. The depth mismatch is attributed to the uncertainty of PSDM velocities. Figure F80A shows that the PSDM velocities are overestimated for the discrete core sample velocities, particularly at depths between 4450 and 4650 mbsl.

Core-Log-Seismic integration

Log units, seismic, and lithology

Once logging units had been defined, we compared them to the available core data in an attempt to correlate features from the core with features in the logs. We used the MSCL-W data as the intermediate step between discrete measurements from core and the log data. An average consistent offset of ~4 m was observed between the core and log data, with features in the core data regularly appearing deeper than the same features in the logs (e.g., Fig. F72). This offset may be due to uncertain seafloor identification in drilling/coring depths or local variations in lithology between Holes C0011A and C0011B.

Logging Unit 1 (0–251.5 m LSF)

Logging Unit 1 comprises the uncored upper section of Hole C0011B; therefore, no comparison with core can be made at this site. However, a lithologic interpretation can be made based on the core and log data obtained at Sites 1173 and 1177 during previous Legs 190 and 196 (Moore, Taira, Klaus, et al., 2001; Mikada, Becker, Moore, Klaus, et al., 2002). The log character of logging Unit 1 closely resembles that of log Unit 2 defined at Site 1173, which Leg 190 showed is composed of hemipelagic mud (silty clay) with interbedded volcanic ash (Shipboard Scientific Party, 2001b, 2002). Cores show that the sand content of the hemipelagic mud is consistently low. The character of resistivity images and identification of high-resistivity, bedding-parallel features that may be ash layers at Site C0011 (e.g., Fig. F75A) are also comparable to Site 1173 images, where cores were available to confirm the presence of ash. Logging Unit 1 correlates with seismic Unit A (Fig. F81).

Logging Unit 2 (251.5–478.5 m LSF)

Coring began at the upper boundary of logging Subunit 2B (337.0 m LSF), which corresponds to the top of lithologic Unit II (340 m CSF). There is a good stratigraphic correlation between LWD data from Hole C0011A and physical properties data from Hole C0011B, particularly between LWD gamma ray and MSCL-W natural gamma ray (NGR) data.

The correlation between tuffaceous and volcaniclastic sandstones found in cores (lithologic Unit II) and log character (logging Subunit 2B) is shown in Figure F72. In the logs, these sandstones of the middle Shikoku Basin facies are characterized by low gamma ray with sharp tops and bases, gradually increasing resistivity intervals with sharp bases (both characteristic of channels), and high resistivity intervals in images (Fig. F75B). In physical property data, these channel-like features are characterized by low NGR (Fig. F72), low to background electrical resistivity values, and high magnetic susceptibility. This interval of sandstones correlates with a package of strong and discontinuous seismic reflections at the upper boundary of seismic Unit C (Fig. F78).

The hemipelagic sediments observed between Unit II sandstones in the core also correspond well to the areas of high gamma ray and moderate resistivity, and the silt turbidites are characterized by low gamma ray and low resistivity, which were initially interpreted as sandstones from the logging data (Fig. F72; e.g., 388 m LSF).

The base of lithologic Unit II (~479 m CSF) is below a thin cemented sandstone bed, which is characterized in the logging data by a low gamma ray peak and a very high resistivity spike (Fig. F72), allowing a good tie between the logging units and lithologic units throughout this section. The base of logging Unit 2 also corresponds to the lower boundary of seismic Unit C (Fig. F81).

Logging Unit 3 (478.5–736.0 m LSF)

Given the homogeneous nature of the lithology and log character through much of logging Unit 3, it is not possible to make a detailed correlation between the logs and the core NGR data. Correlation between logs and core data is also more limited in intervals of poor core recovery and core quality. However, the lower Shikoku Basin lithofacies of hemipelagic mudstone confirms the log-based lithologic interpretation of a background hemipelagic mud. The shift in both gamma ray and resistivity baseline values ~550 m LSF that marks the logging Subunit 3A/3B boundary may correspond to a change in sedimentation rate observed in the core data between ~550 and 570 m CSF (see "Paleomagnetism" and "Lithology").

In logging Subunit 3B, we interpret the logs to represent the increasing appearance of silt and sandstone interbeds within the hemipelagic mudstone (Fig. F73); however, no sand layers were recovered in cores throughout this interval. Two significant features at the base of logging Unit 3, interpreted as thick sand layers from the logs, may actually correspond to cemented siltstones at the base of a fairly homogeneous interval of silty claystone in cores. These features have very low gamma ray values, which would be expected from such a lithology, but the high resistivity of these features is unexpected. The discrepancy between cored lithology and log interpretation may also be related to poor core recovery through some of the section. The seismic character within this unit is mainly transparent, with discontinuous, faint reflections, so it is also possible that the sandstones interpreted from logs at Hole C0011A are not laterally extensive and, therefore, are not present in Hole C0011B.

Below this point there is no longer a straightforward correlation between logging units and lithologic units. Logging Subunit 3C continues to ~736 m LSF, maintaining the fluctuating nature of the gamma ray and resistivity log responses; however, a lithologic change from homogeneous silty claystone to thin interbeds of clayey siltstone at ~674 m CSF marks the base of lithologic Unit III (see "Lithology"). The logging Unit 3 boundary also corresponds to the lower boundary of seismic Unit D (Fig. F81).

Logging Unit 4 (736.0–867.0 m LSF)

Logging Unit 4 is characterized by a repeating pattern of coarsening-up and fining-up intervals in gamma ray, as well as a corresponding pattern of increasing and decreasing resistivity. In these patterns, we recognize the appearance of four packages, all with similar log character (Fig. F74). Within this logging unit, the observed mid-frequency variability in both gamma ray and resistivity is greater than that in any other logging unit.

The upper and lower parts of logging Unit 4 were cored (~730–783 and ~844–870 m LSF); the intermediate interval was washed down. NGR data from the MSCL-W provided a good means of correlating the cores and logs for both cored sections of this logging unit (Fig. F74). Lithology from cores indicates that the shallowest package is composed mainly of silty claystone (i.e., mudstone) with alternations of mixed silty claystone and siltstone. The upper portion of the second package was also recovered in cores and contains a transition from alternating silty claystone and siltstone to sandstone. The third package was not cored, but the fourth package was cored and found to be composed of a transition from siltstone and silty claystone to interbeds of silicic tuff, tuffaceous siltstone, and silty claystone, corresponding to lithologic Unit IV (see "Lithology"). Although we cannot confirm the lithology of the intermediate packages without cores, the log character suggests that the second package may be composed of mixed hemipelagic mudstone and siltstone, while the third package may be tuff and tuffaceous siltstone and silty claystone.

In seismic character, logging Unit 4 corresponds to an interval (4835–5204 mbsl) with a series of strong, continuous reflections (seismic Unit E; Figs. F78, F81), similar in seismic character to the lower Shikoku Basin–turbidites class (LSB-T) defined by Ike et al. (2008b), which correspond to the Miocene turbidites drilled at Site 1177 (Shipboard Scientific Party, 2001b).

Logging Unit 5 (867.0–950.5 m LSF)

Very little core was recovered in this logging unit, so a stratigraphic correlation between core and logs is not possible. Both cores from this unit (Cores 322-C0011B-60R and 61R) had minimal core recovery because of the degradation of the drill bit as well as an increase in the hardness of the formation. This change in material properties agrees with the character of the log data, which show a distinct increase in resistivity across the boundary between logging Units 4 and 5 (Fig. F47). Gamma ray decreases in this unit, which we interpreted as a change in lithology, from a consolidated mudstone to a more coarse grained, sandy mudstone. This logging unit may correspond to seismic Unit F, which is characterized by transparent reflections.

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