m)
and neutron porosity (0.61 pu). The base of log Unit 1 is defined by positive
shifts in the gamma ray and density logs of ~50 API and ~0.4 g/cm3,
respectively.The uppermost 156 m of LWD data was not used because of the 20-in casing that influenced logs considerably. Below 156 mbsf the following logs are available for a composite log interpretation and a determination of logs units: gamma ray, resistivity, density, neutron porosity, photoelectric effect, and differential caliper.
Because the velocity log was processed after the cruise at Anadrill-Schlumberger in Houston, Texas, it was not available on the ship and thus is not used in the following.
The logs and log units are shown in Figure F6. Four log units and six subunits 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). The cluster analysis shows four prominent clusters. The calculated cluster log, log units, and log curves are shown in Figure F7. The mean values and standard deviations of log properties for each log unit or subunit are summarized in Table T2 and Figure F8.
Log Unit 1 (156-268 mbsf)
is characterized by the overall lowest mean values of gamma ray (46.5 API),
density (1.68 g/cm3), and photoelectric effect (2.93 b/e-)
and overall highest mean values of resistivity (0.84 m)
and neutron porosity (0.61 pu). The base of log Unit 1 is defined by positive
shifts in the gamma ray and density logs of ~50 API and ~0.4 g/cm3,
respectively.
Log Unit 2 (268-530 mbsf)
is divided in three subunits. Log Unit 2 is characterized by constant values of
gamma ray (50-70 API) and neutron porosity (0.4-0.7 pu) and a downhole
decreasing resistivity log (1 to 0.5 m).
This interpretation should be used cautiously because many intervals have
differential caliper values >1 in, which suggests that the log data and
especially density data may be unreliable. Log Subunit 2a (268-389 mbsf) has
relatively high mean values of density (1.88 g/cm3) and neutron
porosity (0.56 pu). The base of log Subunit 2a is placed at the top of an
increasing resistivity log trend (from 0.5 to 1.05 m).
Log Subunit 2b (389-415 mbsf) may be significantly influenced by a highly
variable differential caliper (0.4-2.2 in). Here, all logs have high mean values
and high standard deviations: 0.91 ± 0.12 m
for resistivity, 3.00 ± 0.28 b/e- for photoelectric factor, 1.87 ±
0.25 g/cm3 for density, and 57.6 ± 14.6 API for gamma ray. The base
of Subunit 2b is defined by an abrupt decrease in resistivity from 0.85 to 0.65 m.
Log Subunit 2c (415-530 mbsf) has a rather low mean value of resistivity (0.63 m).
All other logs have similar mean values as those of log Subunit 2a. The base of
log Subunit 2c has a significant increase in gamma ray from 65 to 75 API.
Log Unit 3 (530-620 mbsf)
is defined by a significant increase of the mean values of gamma ray (68.6 API),
density (1.98 g/cm3), and photoelectric effect (3.15 b/e-).
The base of log Unit 3 is defined by an increase in resistivity (from 0.5 to
0.65 m)
and gamma ray (from 55 to 70 API).
Log Unit 4 (620-1035 mbsf)
is generally characterized by the overall very high values of gamma ray (60-90
API) and density (1.6-2.4 g/cm3) and the very low values of
resistivity (0.4-0.9 m).
Log Unit 4 is divided into three subunits: Subunit 4a (620-776 mbsf) is
characterized by the highest mean value of photoelectric effect (3.20 b/e-)
and a relatively low mean value of resistivity (0.62 m).
This log unit shows a slight downhole decrease in gamma ray (from 90 to 60 API),
resistivity (from 0.85 to 0.45 m),
and neutron porosity (from 0.6 to 0.4 pu). Close to the base of log Subunit 4a
(~730 mbsf) the differential caliper increases to >1 in and obviously
influences the density log. The base of log Subunit 4a shows a positive shift in
the resistivity log from 0.5 to 0.85 m.
Subunit 4b (776-965 mbsf) is characterized by high mean values of density (2.17
g/cm3) and gamma ray (76.5 API) and low mean values of neutron
porosity (0.48 pu) and resistivity (0.65 m).
The base of log Subunit 4b is defined by a decrease in density (from 2.3 to 1.3
g/cm3) and resistivity (from 0.65 to 0.55 m).
Subunit 4c (965-1035 mbsf) is characterized by very low mean values of
resistivity (0.58 m)
and density (1.82 g/cm3) and very high mean values of gamma ray (76.8
API) and neutron porosity (0.53 pu).
Leg 196 Hole 808I is located ~107 m northeast at an azimuth of 48° from Leg 131 Hole 808C. The core descriptions from Leg 131 Site 808 (Shipboard Scientific Party, 1991) are presented in Figure F9. Each lithologic description that follows is partially reproduced hereafter from the Leg 131 Initial Reports volume (Taira, Hill, Firth, et al., 1991). Here only lithologic Units II to IV, which correlate with the defined log units, are described.
Subunit IIA (20.55-120.60 mbsf) is characterized by silty to very coarse grained sands in beds/units showing considerable variation in bed thickness and internal structure. A few very thin (<1 cm thick) ash layers also occur. The sand-rich nature of Subunit IIA, together with its seismostratigraphic position, suggests that it represents axial trench, sandy channel and nonchannel (overbank and sheet) deposits. Hemipelagic settling and fine-grained turbidity currents probably caused sedimentation of muddy interbeds in Subunit IIA.
Subunit IIB (120.60-264.90 and 365.90-409.54 mbsf) comprises very thin to thin-bedded, very fine grained sandstones, the mudstone-pebble conglomerate used as a marker bed, very thin to thin-bedded siltstones, and mud (clayey siltstone/silty claystone). Ash constitutes a minor lithology. The conglomerate provides an important correlative deposit that is duplicated across the frontal thrust zone. The stratigraphic position of Subunit IIB, the relative fine bulk grain size, and thin beds, when compared to overlying Subunit IIA, suggest that it represents axial trench deposits, probably deposited in the outer part of the trench floor.
Subunit IIC (264.90-355.50 and 409.54-556.80 mbsf) is defined from immediately below the matrix-supported, mudstone-pebble conglomerate to the first occurrence of a thick tuff in Hole 808C. Subunit IIC differs from overlying Subunit IIB in that it contains little sand. There are very few ash/tuff layers relative to underlying Unit III.
Unit III is defined from the first occurrence of a thick tuff unit to the last ripple-laminated siltstone turbidite in Hole 808C. Unit III is dominated by thoroughly bioturbated clayey siltstone/silty claystone with ash/tuff layers up to 25 cm thick. The stratigraphic position of Unit III, between trench-fill deposits and the predominantly hemipelagic Shikoku Basin deposits, suggests that this unit represents trench-basin transition deposits, perhaps including sediments that were deposited on the outer trench.
The characteristic feature of Subunit IVA (618.47-823.74 mbsf) is the abundance of thin layers of tuff and vitric sandstone, intercalated within a thoroughly bioturbated (mottled) mud succession rich in foraminifers. Volcaniclastic beds reach thicknesses up to 20-50 cm. The seismostratigraphic position of Subunit IVA, together with the absence of terrigenous sandy or silty turbidites, suggests that this subunit represents the upper Shikoku Basin deposits dominated by hemipelagic sedimentation and the accumulation of volcanic layers.
The top of Subunit IVB (823.74-1243.0 mbsf) corresponds to the disappearance of the abundant, intact ash/tuff layers and the base corresponds to the appearance of predominantly acidic volcaniclastic deposits. Subunit IVB comprises an essentially monotonous succession of thoroughly bioturbated clayey siltstones and silty claystones with traces of disseminated volcanic glass. The seismostratigraphic position of Subunit IVB, between upper Shikoku Basin deposits and the acidic volcaniclastic unit that rests on basaltic basement, suggests that Subunit IVB represents hemipelagites of the lower Shikoku Basin deposits.
The following descriptions of bulk mineralogy are taken from the Site 808 core descriptions (Shipboard Scientific Party, 1991).
Calcite percentages are overall low (0-5 wt%) but there are some intervals (600-880, 950-1000, and 1120-1210 mbsf) with values >10 wt%. The calcite content increase coincides with the lower part of lithologic Unit III and all of Unit IV, which are composed of siltstone turbidite, clayey siltstone/silty claystone with ash/tuff layers (Unit III), and tuff and vitric sandstone, a mud succession, clayey siltstones, and silty claystones (Unit IV).
The overall scatter in quartz content decreases downhole in response to a decrease in the amount and frequency of sediment influx via turbidity currents. The quartz content remains constant between 156 and 850 mbsf in the range of 40 to 55 wt%. At 850 mbsf quartz content increases abruptly to 55-60 wt%, which is caused by silt and ash-rich units in the finer grained hemipelagic facies within lithologic Subunit IVB. Below 850 mbsf the section is characterized by a fluctuation in quartz content.
Plagioclase content decreases continuously downhole from 30 to 20 wt% in response to the decrease in amount and frequency of turbidity currents. There are intervals with higher variability that may reflect sand layers that are typically enriched in plagioclase relative to the muddy interturbidite deposits.
Total clay content is constant from 156 to 860 mbsf in the range between 10 and 25 wt%. Below 860 mbsf clay mineral content varies.
The depth boundaries differ between the log and lithologic units because casing influenced the logs significantly; thus the interval between 0 and 156 mbsf is not used to determine log units.
The interval from log Unit 1 through Subunit 4b shows overall increasing trends in gamma ray and density, which suggests an increase in clay content with depth.
Log Unit 1 (156-268 mbsf) coincides with lithologic Subunit IIB (120.6-264.9 mbsf), which consists of very fine grained sandstones, siltstones, and clayey siltstone/silty claystone. The very low gamma ray and density values confirm the coarse-grained lithology described in the cores. The mudstone-pebble conglomerate mentioned in the core description as a marker for the base of lithologic Subunit IIB is observed in the logs at the base of log Unit 1 at ~260 mbsf, where gamma ray, resistivity, and density are characterized by very low values and neutron porosity by very high values.
Log Unit 2 is divided into three subunits. Log Subunits 2a and 2c are both characterized by similar trends and value ranges of gamma ray, density, photoelectric effect, and neutron porosity. Additionally, log Subunit 2a (268-389 mbsf) shows positive shifts in the gamma ray and density logs and a negative shift in the neutron porosity log compared to log Unit 1. Log Subunit 2a is equivalent to the upper segment of lithologic Subunit IIC (264.9-365.9 mbsf), which is composed of a mudstone-pebble conglomerate and the first occurrence of a thick tuff. Lithologic Subunit IIC contrasts with overlying Subunit IIB in containing little sand, which is reflected in higher gamma ray values. Log Subunit 2b (389-415 mbsf) is defined by its high variability of LWD data, which may be caused by large borehole washouts indicated by the high differential caliper values. The density, resistivity, and photoelectric effect logs are characterized by high values. Log Subunit 2b coincides with the lower part of lithologic Subunit IIB (365.9-409.54 mbsf), which comprises very fine grained sandstones, mudstone-pebble conglomerate, siltstones, and mud (clayey siltstone/silty claystone). The pronounced drop in gamma ray values at the base of log Subunit 2b may mark the mudstone-pebble conglomerate noted at ~409 m in the cores and may correlate to a similar log signature at the base of Log Unit1. Log Subunit 2c (415-530 mbsf) coincides with lithologic Subunit IIC (409.54-556.80 mbsf), which consists of silt turbidites and hemipelagic mudstones.
Log Unit 3 (530-620 mbsf) coincides with lithologic Unit III (556.80-618.47 mbsf), which is composed of clayey siltstone/silty claystone with ash/tuff layers. Log Unit 3 is characterized by high gamma ray, density, and photoelectric effect and an overall high variation of values.
Log Subunit 4a (620-776 mbsf) coincides with lithologic Subunit IVA (618.47-823.74 mbsf), which consists of ash/tuff, sandstone, and hemipelagic mud. Log Subunit 4a is characterized by high photoelectric effect and low resistivity values. High photoelectric effect is also probably related to a relatively high carbonate content.
Log Subunit 4b (776-965 mbsf) is characterized by high values of density and gamma ray and low values of neutron porosity and resistivity. The transition between log Subunits 4b and 4c is defined by a decrease in density and photoelectric effect and an increase in neutron porosity and resistivity. Log Subunit 4b is equivalent to the lower part of lithologic Subunit IVA (618.47-823.74 mbsf) and the upper part of lithologic Subunit IVB (823.74-1243 mbsf). The lower part of log Subunit 4b represents the décollement zone identified at Leg 131 Site 808.
Log Subunit 4c (965-1035 mbsf) is characterized by low values in resistivity and density and high values in neutron porosity and gamma ray. This log subunit coincides with the lower part of lithologic Subunit IVB.
The crossplot of gamma ray vs. photoelectric effect reflects the change in lithology according to log units (Fig. F10A). These two logs were used because of a lower sensitivity to sediment compaction than density and resistivity logs. Overall, a positive correlation between gamma ray and photoelectric effect is observed. A continuous increase in gamma ray and photoelectric effect is observed from log Unit 1 which reflects the increase in clay and calcareous content. The data of log Units 2, 3, and 4 overlap significantly, which shows the similarity of their sediment composition. High gamma ray values reflect a high clay content and a low content of material such as sand and silt, which fits quite well with lithologic descriptions. In contrast, log Unit 1 is characterized by much lower gamma ray values, which coincides with a higher sand content.
The crossplot of resistivity vs. density shows the increasing compaction with depth (Fig. F10B). The entire data set shows no apparent correlation between resistivity and density (correlation coefficient = -0.03). However, focusing in detail log Units 3 and 4 are characterized by a positive correlation (+0.52 and +0.54, respectively), whereas log Units 1 and 2 show no clear correlation (+0.19 and +0.18, respectively). Overall, the four log units differentiate well. This is based on a continuous increase in density and a slight decrease in resistivity with depth.
