m).
The base of log Subunit 1b (55-122 mbsf) is defined by an abrupt decrease in
gamma ray (from 60 to 45 API), density (from 1.75 to 1.65 g/cm3), and
resistivity (from 0.54 to 0.46 m)
values.For the following descriptions of LWD data, Hole 1173C was used for the upper 150 m and Hole 1173B for below 150 mbsf.
An overview of log data with log units is shown in Figure F8. Five first-order log units and two second-order units were defined through a combination of visual interpretation and multivariate statistical analysis (see "Identification of Log Units through Visual Interpretation and Multivariate Statistical Analysis" in the "Explanatory Notes" chapter). For the multivariate statistical analysis, factor logs were calculated from gamma ray, photoelectric effect, bulk density, ring resistivity, differential caliper, and neutron porosity logs. The P-wave velocity LWD log was not used because of the delay in processing it. The first two factor logs account for 76% of the total variance in the selected data. Cluster analysis of these two factor logs identified four clusters, each with a specific and distinct set of log properties. The distribution of log units is shown in Figure F9, together with the log units and LWD gamma ray, density, and resistivity curves. Mean and standard deviation values of the log properties for each log unit are summarized in Table T5 and graphically shown in Figure F10. Additionally, a good overall differentiation of the five log units according to their physical properties can be observed in the crossplots (Fig. F11).
Log Unit 1 (0-122 mbsf)
shows a high mean value of neutron porosity (0.63 porosity units [pu]), a low
mean value of density (1.55 g/cm3), and a relatively high standard
deviation of density (0.176 g/cm3) (Fig. F10;
Table T5). A
high variability in the differential caliper log (standard deviation of 0.55 in)
and a large number of values >1 in, which reflect bad borehole conditions,
characterize log Unit 1. Log Unit 1 is subdivided into Subunits 1a and 1b. Log
Subunit 1a is characterized by high variability in resistivity (Fig. F8).
The base of Subunit 1a (0-55 mbsf) has a positive shift in gamma ray (from 35 to
65 API), density (from 1.5 to 1.7 g/cm3), and resistivity (from 0.5
to 0.7 m).
The base of log Subunit 1b (55-122 mbsf) is defined by an abrupt decrease in
gamma ray (from 60 to 45 API), density (from 1.75 to 1.65 g/cm3), and
resistivity (from 0.54 to 0.46 m)
values.
Log Unit 2 (122-340 mbsf)
is identified by a decrease in resistivity of 0.3 m,
in density of 0.1 g/cm3, and in gamma ray of 10 API with depth. The
decrease in resistivity and density reflects an abnormal compaction trend. The
base of log Unit 2 is clearly marked by a positive shift in the mean values of
resistivity (from 0.37 to 0.53 m),
density (from 1.65 to 2.06 g/cm3), neutron porosity (from 0.62 to
0.51 pu), and gamma ray (from 51.27 to 74.32 API) (Table T5).
Log Unit 3 (340-698 mbsf)
is characterized by high mean values of gamma ray (74.32 API), density (2.06
g/cm3), and photoelectric effect (3.38 b/e-). Density and
gamma ray values increase continuously with depth (from 1.6 to 2.2 g/cm3
and from 60 to 90 API). Log Unit 3 also has a significant cyclicity in
resistivity. The base of this log unit is defined by an abrupt decrease in gamma
ray (by 30 API), density (by 0.15 g/cm3), resistivity (by 0.2 m),
and photoelectric effect (by 0.6 b/e-).
Log Unit 4 (698-731 mbsf) is characterized by a high variation of values that is reflected in its high standard deviation values (Table T5). The mean values of neutron porosity (0.20 pu), density (1.03 g/cm3), and photoelectric effect (1.62 b/e-) are very low.
The top of log Unit 5
(731-735 mbsf) is characterized by an abrupt increase in resistivity (from 2.5
to 11 m)
and a decrease in gamma ray (from 100 to 10 API), the only logs that are
available for this depth.
Holes 1173B and 1173C are located ~52 and ~98 m, respectively, from Hole 1173A, which is sufficiently close to correlate the logged sections with recovered sediments. Lithologies based on core descriptions from Hole 1173A are summarized below, along with bulk mineralogy from shipboard X-ray diffraction (XRD) analyses. A detailed description of the sedimentary units, depositional environments, and core photographs are given in the Leg 190 Initial Reports volume (Moore, Taira, Klaus, et al., 2001).
Sediments from Hole 1173A were divided into five lithologic units. Unit I is characterized by sandy and silty turbidites. Units II and III are composed of hemipelagic silty clay/claystone with and without volcanic ash layers, respectively. Units IV and V consist of volcaniclastics and basalt. Sedimentation rates based on biostratigraphic age assignments indicate high accumulation rates for Units I and II and low accumulation rates for Units III and IV.
Lithologic Unit I comprises Quaternary sandy to muddy turbidites of the outer Nankai trench-wedge facies. Unit I is divided into two subunits. Subunit IA (outer trench-wedge facies) consists of 83.37 m of silty clay interbedded with silt, sandy silt, silty sand, and rare beds of volcanic ash. The dominant lithology is greenish gray silty clay to clayey silt. Hemipelagic settling and fine-grained (muddy) turbidity currents probably deposited the silty clay to clayey silt of Subunit IA.
The base of Subunit IB (trench-basin transition facies) is defined by the deepest occurrence of medium-bedded silty sand. The top of this subunit is defined by the uppermost interval containing multiple ash layers. Subunit IB is 18.77 m thick and contains silty clay with scattered interbeds of silt- to clay-sized volcanic ash and rare volcanic lapilli. Subunit IB is stratigraphically transitional between upper Shikoku Basin and outer trench-wedge sediments.
The transition to lithologic Unit II is marked by the deepest occurrence of turbidites. The silty claystone of Unit II appears to be finer grained than equivalent lithologies of Unit I and contains fewer siliceous microfossils.
Unit II is Pliocene to Quaternary in age and consists of hemipelagic mud with abundant interbeds of volcanic ash probably derived from the Kyushu and/or Honshu volcanic arcs (upper Shikoku Basin facies). The most common lithology ranges in texture from silty clay to clayey silt and changes with increasing compaction to silty claystone and clayey siltstone. Interbeds of volcanic ash and tuff are common. The deepest unequivocal ash bed defines the base of Unit II (343.77 mbsf). Unit II represents the upper part of the Shikoku Basin succession.
The Unit II/III boundary is controlled by the last ash bed and, in part, by silica diagenesis. There is an abrupt loss of unequivocal ash beds and replacement by siliceous claystone, most of which may be altered tephra beds.
Bioturbated silty claystone and a minor amount of calcareous and siliceous claystone characterize lithologic Unit III. Unit III is of Pliocene to middle Miocene age (lower Shikoku Basin facies). The dominant lithology of silty claystone in Unit III is gray to greenish gray with local faint laminae.
The sediments consist of variegated siliceous claystone and silty claystone (volcaniclastic facies). The probable sediment age is middle Miocene. The silty claystone of Unit IV is probably a hemipelagic deposit, whereas the light gray siliceous intervals appear to be altered volcaniclastic deposits.
Lithologic Unit V consists of a single piece of middle Miocene (13-15 Ma) basalt. It is unclear whether or not this fragment was part of a basaltic breccia or lava flow.
The trends observed in the bulk mineralogy column of Figure F12 can generally be related to the changing influence of hemipelagic sedimentation vs. lateral influxes of terrigenous and calcareous sediment, with intermittent deposition of volcanic ash layers. The percentage of total clay minerals varies between 40% and 50% in the upper 330 m. Below 330 mbsf, the percentage of total clay increases to 45%-60%. The increase in clay content corresponds to the log Unit 2/3 transition.
The calcite content varies significantly. The intervals 0-125 and 360-615 mbsf are characterized by low calcite content (0%-5%). The intervals 125-360 and 515-680 mbsf have higher calcite contents of up to 25% and 70%, respectively. Even the carbonate-rich intervals are interbedded with noncalcareous claystone layers. The plagioclase content decreases continuously with depth from 15%-20% at the top to 5%-10% at the bottom.
The high quartz content reflects the influence of terrigenous silts and sands. Below 570 mbsf the quartz content varies between 15% and 40%. Most samples contain ~35% quartz.
Log Unit 1 (0-122 mbsf) does not exactly coincide with lithologic Unit I (0-102 mbsf), which comprises Quaternary sandy to muddy turbidites of the outer trench-wedge facies (Fig. F12). Thus, log Subunit 1a (0-55 mbsf) correlates with the upper part of lithologic Subunit IA (0-83 mbsf), which consists of silty clay with interbedded coarser sediments such as silt, sandy silt, and volcanic ash. The spiky variability in gamma ray, density, resistivity, and photoelectric effect logs in log Subunit 1a is attributed, at least in part, to borehole washouts. Given the presence of less cohesive lithologies in the vicinity of washouts and absence of change in drilling parameters, it seems likely that the enlargement might be lithologically influenced. The difference between log and lithologic unit boundaries might be explained by high values of differential caliper that influence the log quality (e.g., gamma ray and density) significantly. A marked increase in the photoelectric effect log at the bottom of log Subunit 1b (55-122 mbsf) correlates with an abrupt increase in carbonate content. But there is no decrease in the observed gamma ray as expected for calcareous-rich sediment. Additionally, there is no evidence in the logs (such as resistivity peaks) of high ash content in the cores that define lithologic Unit I.
Log Unit 2 (122-340 mbsf), reflecting the upper Shikoku Basin facies, is characterized by low values in the density, resistivity, and gamma ray logs. This log unit correlates with lithologic Unit II (102-344 mbsf), which consists of hemipelagic mud (silty clay to clayey silt) with abundant interbeds of volcanic ash. The high neutron porosity and low resistivity logs indicate a high and uniform porosity. The low density could be related to a cementation effect caused by the formation of cristobalite.
Log Unit 3 (340-698 mbsf) correlates with lithologic Unit III (344-688 mbsf), the lower Shikoku Basin facies, and includes the upper part of lithologic Unit IV (688-725 mbsf). Log Unit 3 is characterized by a normal compaction trend where density and resistivity logs increase continuously with depth. Also, resistivity and, less obviously, gamma ray logs show a characteristic cyclicity of values that may reflect changes in lithology, which may in turn reflect an interbedding of coarser and finer grained sediments (e.g., silty clay and clayey silt).
Log Unit 4 (698-735 mbsf) is characterized by broad variations in photoelectric effect, resistivity, neutron porosity, and gamma ray logs that correlate well with the presence of the volcaniclastic facies (see "Log Interpretation"). The difference in depth of the volcaniclastic facies might be caused by low core recovery (~12%).
Log Unit 5 (731-735 mbsf)
can be correlated with lithologic Unit V (725-735 mbsf), although the upper
boundary is significantly lower. Lithologic Unit V consists of middle Miocene
basalt. Low gamma ray (10 API) and high resistivity (10 m)
values differ significantly from the units above and characterize Log Unit 5.
The very low core recovery (~3%) and the poor definition of the top of the
lithologic unit (based on a single small piece of basalt) might explain the
depth difference of basement between log and lithologic units.
The crossplot of gamma ray vs. photoelectric effect reflects changes in lithology based on log units (Fig. F11). These two logs were used because of lesser sensitivity to sediment compaction than the density and resistivity logs. Overall a positive correlation between gamma ray and photoelectric effect is observed. From log Unit 1 to Unit 3 a continuous increase in gamma ray and photoelectric effect is observed. This may reflect an increase in clay and calcareous content. The data of log Units 2 and 3 overlap significantly, because of the similarity of the sediment composition (silty clay and clayey silt). This observation of higher clay content and lower content of sand and silt fits quite well with lithologic descriptions. Log Unit 4 (volcaniclastic facies) is an exception, because gamma ray and photoelectric effect values overlap with log Units 2 and 3. This might be caused by its lithologic composition, a mixture of siliceous claystone, silty claystone, and sand.
Resistivity variations between lithologic units are well illustrated by 360° RAB images, which can be displayed as "unwrapped" planar images with Schlumberger's Geoframe software. Figure F13 illustrates contrasts between the resistivity images of the thinly bedded, sandy to muddy turbidites of lithologic Unit I and log Unit 1 (trench-wedge facies) and the hemipelagic mud with interbedded volcanic ash layers of lithologic Unit II and log Unit 2 (upper Shikoku Basin facies). Figure F13A shows conductive (dark color in RAB image) probable coarser grained basal turbidite layers overlain by resistive (light color) probable finer grained sediments. Average turbidite bed thicknesses are 20-80 cm and are dominated by high-resistivity sediments (interpreted to be finer grained clays and silts). The resolution of the RAB images does not allow the identification of grading patterns or erosive vs. gradational boundaries. Average resistivity decreases in lithologic Unit II and log Unit 2 and contains highly resistive thin interbeds (Fig. F13C), which correlate with ash beds interpreted from Hole 1173A cores and Formation MicroScanner (FMS) images. This unit is characterized by more homogeneous RAB images compared with the well-stratified turbidites of lithologic Unit I and log Unit 1. Resistive (dark color) spots, corresponding with poorly stratified sections, are more common (Fig. F13B) and are inferred to represent bioturbation-related reduction spots (based on a correlation with Hole 1173A core data).
The trench-wedge zone (log Unit 1; 0-122 mbsf) consists of silty and sandy turbidites in the uppermost 100 m (Moore, Taira, Klaus, et al., 2001) (Fig. F14). The upper 75 m is characterized by >1 in caving of the borehole, which makes further log interpretation difficult. Below 75 m the caliper values stabilize, and so do the gamma ray, resistivity, and density logs, which show values that are typical for a clayey siltstone.
On the density log on Figure F14, several log trends have been marked with small arrows that show fining-upward trends, which are typical in turbidite zones (Rider, 1996). The same trends are seen in the resistivity log. On the other hand, the gamma ray log shows opposite trends in the intervals 20-35 and 55-90 mbsf. The explanation for this is bad borehole conditions, which are seen in the differential caliper values and the RAB image logs.
The transition zone
comprises the log boundary between log Units 2 and 3 (340 mbsf) and the
lithologic boundary between lithologic Units II (upper Shikoku Basin facies) and
III (lower Shikoku Basin facies) (Fig. F15).
Lithologic Unit II ranges in texture from silty clay/claystone to clayey
silt/siltstone. Interbeds of volcanic ash and tuff are common (Moore, Taira,
Klaus, et al., 2001). The section between 310 and 322 mbsf is interpreted as a
clayey siltstone on the basis of low gamma ray (~50 API) and resistivity (~0.3 m)
values. The transition zone is placed in the section between 322 and 340 mbsf on
the basis of two high density and resistivity peaks at 324-327 and 333-337 mbsf.
The log Unit 2/3 boundary is defined at the base of this transition zone where
gamma ray, resistivity, and density logs show low values.
The lithologic Unit II/III
boundary is located at the base of the deepest unequivocal ash bed (~344 mbsf).
This ash layer is clearly indicated by the low gamma ray (50 API) and density
(2.0 g/cm3) values and the high resistivity (0.6 m)
values. This is confirmed by the RAB image log as a thin band with very low
resistivity.
LWD measurements were made
in the depth interval between 680 and 735 mbsf, where the core recovery of Hole
1173A is very low (~12%) (Fig. F16).
The volcaniclastic facies (lithologic Unit IV; 688-725 mbsf) consists of
variegated siliceous claystone and silty claystone, calcareous claystone, and
sand. Between 698 and 702 mbsf the logs are characterized by significantly low
gamma ray (<70 API), photoelectric effect (<3 b/e-),
resistivity (<0.4 m),
and density (<2.1 g/cm3). This layer may be coarser grained
material such as sand and is recognized in the RAB images by its dark color that
reflects low resistivity. Between 702 and 708 mbsf gamma ray is still
characterized by low values (<70 API), but resistivity (0.4-0.6 m),
density (>2.2 g/cm3), and photoelectric effect (>3.5 b/e-)
values are much higher. This may be a carbonate-cemented, highly compacted sandy
material.
