LITHOSTRATIGRAPHY

Introduction

A 1113-m thick sedimentary section of Holocene to middle Miocene age was cored in Holes 1151A, 1151C, and 1151D (Fig. F5). The common major lithology of the recovered sediments is olive green diatomaceous silty clay and diatomaceous silty claystone, similar to that at Site 1150. The similarity in lithogies is such that the upper 950 m of section at Site 1151 correlates well with the Site 1150 section, although the two sections differ slightly in degree of lithification and other physical properties.

The major lithology is composed mainly of siliciclastic, volcaniclastic, and biogenic components, whose relative proportions vary with depth. The siliciclastic component ranges from 30% to 60% of the total composition based on smear-slide observations but is more often the dominant component, comprising 50%-60% of the composition. The siliciclastic component is composed predominantly of clay minerals with lesser amounts of quartz, feldspar, hornblende, and mica. Small grains of clinopyroxene are more common at this site than at Site 1150, although they are still rare in smear-slide observation. The biogenic component is the next most abundant, comprising on average 26%-30% of the composition and ranging between 15% and 45%. The dominant fossils are fragments of diatom tests and siliceous sponge spicules. Nannofossils, radiolarians, foraminifers, and silicoflagellates, though rare, are also observed. In general, calcareous tests are better preserved at this site than at Site 1150. The volcaniclastic component consists of volcanic glass shards and rare palagonite, accounting for 5%-30% of the composition and averaging 12%. Other components include carbonaceous grains, iron oxides, iron sulfides, and opaque minerals.

Lithification increases with depth from soft through firm to hard. Brittle deformational structures dominate below 400 mbsf. Bioturbation can be seen in most cores below 300 mbsf and causes patchy color variations of the usual olive green.

Lithologic Units

The division of the cored sedimentary column into lithologic units (Fig. F5) is based mainly on the composition of the major lithology and the degree of lithification. Quantitative color of sediments and sedimentary rock, frequency of brittle deformational structures, occurrence of minor lithologies, and physical properties analyses are also used. Occurrences of minor lithologies such as ash layers, fractures, and silty layers are referred to as sparse or low (<0.4 occurrences/m), common or intermediate (<1.0 occurrences/m), and abundant or high (>1 occurrence/m). The degree of lithification is determined by visual core description (VCD) (see "Lithostratigraphy" in the "Explanatory Notes" chapter) and by resistivity logs.

Lithologic Unit I

Interval: Cores 186-1151A-2R through 13R; 186-1151C-1H through 11H; and 186-1151D-1H through 10H
Depth: 0-189.5 mbsf
Age: Holocene to late Pliocene
Description: Hemipelagic diatom-bearing silty clay, glass- and diatom-bearing silty clay, and diatom- and siliceous sponge spicule-bearing silty clay or diatomaceous silty clay. Minor lithologies (sand, silt, ash, and pumice) are abundant to sparse.
Subunit IA
Interval: Cores 186-1151A-2R through 5R; 186-1151C-1H through 11H; and 186-1151D-1H through 10H
Depth: 0-106.0 mbsf
Description: Soft hemipelagic diatom-bearing silty clay, diatom- and glass-bearing silty clay, and clayey silt, with sparse to abundant occurrence of minor lithologies.
Subunit IB
Interval: Cores 186-1151A-6R through 13R
Depth: 106.0-189.5 mbsf
Description: Soft hemipelagic glass- and diatom-bearing silty clay and diatom-bearing silty clay, with sponge-bearing silty clay in the lower part. Minor lithologies are commonly intercalated.

Unit I consists predominantly of homogeneous diatom-bearing silty clay and diatomaceous silty clay (Fig. F5). Volcanic glass and siliceous sponge spicules comprise between 10% and 24% of the total composition; therefore, we use the suffix "-bearing" for these components. Unit I is olive green in color, with intercalated variations between light and dark olive green at a 1- or 2-m scale. The color is more green compared to Units II and III (Fig. F6).

Primary tephra layers, reworked tephra layers, and bioturbated ash patches are present and may be intercalated with diatomaceous silty clay (Fig. F7). More than 50 of these ash intervals are correlatable between Holes 1151C and 1151D.

Sandy and silty layers and accumulations are more common in this unit than in the other units at this site, though they are still a minor component. These layers and accumulations are common to abundant in Subunit IA, which contains as many as 22 occurrences per core (Fig. F8). The high abundance of these minor lithologies produces a relative high in magnetic susceptibility (Fig. F7).

Subunit IA is composed of hemipelagic diatom-bearing silty clay and diatom- and glass-bearing silty clay. Siliciclastic components occupied ~50% in smear slides (Fig. F9A) in this subunit, corresponding to high peaks of clay minerals by X-ray diffraction (XRD) analysis (Fig. F10). The biogenic component, mainly consisting of diatoms, is ~30% of the total composition in Subunit IA. Volcanic glass and siliceous sponge spicules comprise between 10% and 24% of the total composition.

Subunit IB consists of hemipelagic diatom-bearing silty clay and diatom- and glass-bearing silty clay. Siliciclastic components in this subunit are high (as much as 60%) and show a gradual decrease downhole. In contrast, the biogenic component decreases relative to Subunit IA to ~20% of the total composition. The volcaniclastic component is relatively high at ~20%. The siliciclastic and volcaniclastic components are rich in Subunit IB compared to Subunit IA. In Subunit IB, occurrences of sandy and silty layers as well as volcanic ash and its reworked layers and patches are less common than at Subunit IA, although more common than in other units.

The Subunit IA/IB boundary is determined to be where the occurrence of minor lithologies such as ash layers and sandy/silty layers significantly decreases downhole, with a corresponding lack of abrupt peaks of magnetic susceptibility in Subunit IB. The base of Unit I is placed at the point where magnetic susceptibility decreases below about 1.5 × 10-5 SI units (~25 raw meter units) (Fig. F7) and where opal-A increases (Fig. F10).

Lithologic Unit II

Interval: Cores 186-1151A-14R through 38R
Depth: 189.5-430.3 mbsf
Age: early Pliocene to late Miocene
Description: Soft and firm hemipelagic siliceous sponge spicule-bearing diatomaceous silty clay, diatom-bearing silty clay, and diatomaceous silty clay. The major lithology is locally glass and nannofossil bearing. Common intercalation of minor lithologies occurs at the top of the subunit, but minor lithologies otherwise have sparse occurrence.
Subunit IIA
Interval: Cores 186-1151A-14R through 24R
Depth: 189.5-295.0 mbsf
Description: Soft hemipelagic diatom- and siliceous sponge spicule-bearing silty clay that becomes diatomaceous with depth. Locally glass bearing and nannofossil bearing. Common intercalation of minor lithologies occurs at the top of the subunit.
Subunit IIB
Interval: Core 186-1151A-25R through Section 30R-2, 114 cm
Depth: 295.0-346.72 mbsf
Description: Soft and firm hemipelagic siliceous sponge spicule-bearing diatomaceous silty clay and diatom-bearing silty clay, locally diatom bearing, with sparse occurrence of minor lithologies.
Subunit IIC
Interval: Section 186-1151A-30R-2, 114 cm, through Core 38R
Depth: 346.72-430.3 mbsf
Description: Firm hemipelagic diatomaceous siliceous sponge spicule-bearing silty clay with local diatom-bearing silty clay and sparse occurrence of minor lithologies.

Unit II is composed of hemipelagic siliceous sponge spicule-bearing diatomaceous silty clay, diatom-bearing silty clay, and diatomaceous silty clay. Relative to other units, this unit has the highest amount of the biogenic component, mainly in the form of opal-A. On the other hand, siliciclastic components, such as quartz and clay minerals, decrease in this unit (Fig. F10). The 14-Å minerals decrease at the lowest level between ~300 and 350 mbsf. The major lithology is locally glass and nannofossil bearing. Lithification of sediments is soft in Subunit IIA, becoming firmer downhole in Subunits IIB and IIC (see shallow resistivity logs in Fig. F5). Sandy and silty layers and patches are common at the top of the subunit, but otherwise are sparse. The unit has an olive-green color with some brownish and dark olive regions in Subunit IIA, with the color becoming more patchy in Subunits IIB and IIC. Bioturbation is moderate to common in this unit.

Subunit IIA consists of soft hemipelagic diatom- and siliceous sponge spicule-bearing silty clay that becomes diatomaceous with depth. Primary and reworked ash layers, bioturbated ash, and pumice accumulation are common. The Subunit IIA/IIB boundary is determined to be where the color of the sediments becomes slightly dark and more yellowish, based on L* and b* value, respectively (Fig. F6), and where 14-Å minerals abruptly increase (Fig. F10).

The sediments of lithologic Subunits IIB and IIC consist mainly of diatomaceous siliceous sponge spicule-bearing silty clay. Clay minerals are richer than in Subunit IIA and increase slowly downhole. Minor lithologies such as ash and sandy layers are rare. The Subunit IIB/IIC boundary is determined to be where lithification starts to increase, as shown by resistivity logs (Fig. F5; see "Physical Properties").

Lithologic Unit III

Interval: Core 186-1151A-39R through Section 87R-4, 35 cm
Depth: 430.3-896.75 mbsf
Age: late Miocene
Description: Firm hemipelagic diatom-bearing silty clay, with volcanic glass and siliceous sponge spicule bearing, and common intercalations of minor lithologies. The unit is marked by the first downhole occurrence of brittle deformational structures, which reach their peak occurrence within the unit.
Subunit IIIA
Interval: Cores 186-1151A-39R through 57R
Depth: 430.3-613.2 mbsf
Description: Firm hemipelagic diatom- and siliceous sponge spicule-bearing silty clay with common glass-bearing minor components and sparse intercalations of other minor lithologies. The first downhole occurrence of brittle deformational structures is within this subunit.
Subunit IIIB
Interval: Cores 186-1151A-58R through 68R
Depth: 613.2-718.8 mbsf
Description: Firm and hard hemipelagic diatom-, glass-, and siliceous sponge spicule-bearing silty clay, with common occurrences of minor lithologies. The number of brittle deformational structures increases gradually downhole within this subunit.
Subunit IIIC
Interval: Core 186-1151A-69R through Section 87R-4, 35 cm
Depth: 718.8-896.75 mbsf
Description: Hard diatom-, glass-, and siliceous sponge spicule-bearing silty claystone, with sparse occurrences of minor lithologies. Brittle deformational structures reach their peak in this subunit and then decrease downhole to the base of this subunit.

Lithologic Unit III consists mainly of firm diatom-bearing silty clay. Volcanic glass and siliceous sponge spicule typically comprise 10%-24% of the composition. The top interval of the unit is determined by the first downhole occurrence of brittle deformational structures (Fig. F5). The frequency of structures increases downhole to a high peak value followed by a rapid decrease.

The sediments have a predominantly olive-green color with patchy variations due to moderate to common bioturbation. Subunit IIIB is a slightly lighter olive green relative to the other subunits (Fig. F6). In Subunits IIIB and IIIC, sediments are more greenish in some intervals and become yellowish to bluish with depth. The siliciclastic components generally increase downhole in this unit (Fig. F10), with the exception of 10-Å minerals, which are low in Subunit IIIA, increase at the top of Subunit IIIB, and then decrease with depth in Subunits IIIB and IIIC. Opal-A, in contrast, decreases with depth and has peaks at ~550, 790, and 859 mbsf. These peaks correspond to lows in siliciclastic minerals and natural gamma radiation (NGR).

Minor lithologies have common or abundant occurrences in this unit. The occurrence of reworked ash layer and pumice accumulation is relatively low overall, although slightly more common in Subunits IIIA and IIIC (Fig. F7). Sand and silt accumulations are common to abundant. Silty layers are rich at ~550 mbsf in Subunit IIIA, and sandy layers are rich at ~800 mbsf in Subunit IIIC (Fig. F8).

Lithification increases with depth (Fig. F5), with the sediments becoming firm in Subunit IIIA and transitioning from firm to hard in Cores 186-1151A-62R, 63R, and 64R in Subunit IIIB. In Subunit IIIC, the core becomes hard enough to be called "stone."

The Subunit IIIA/IIIB boundary is determined to be at the interval where color of the sediments become less yellowish (Fig. F6), siliciclastic components increase (Fig. F10), and lithification starts to increase as constrained by resistivity logs (Fig. F6). The Subunit IIIB/IIIC boundary is determined to be where resistivity logs reached almost constant level, which occurs at the top of the interval classified as "stone." The base of Unit III is determined to be at the interval where 10-Å minerals decrease (Fig. F10) and the lightness of sedimentary rocks increases (Fig. F6) and at the boundary between where the sediments are considered consolidated and overconsolidated (see "Physical Properties").

Lithologic Unit IV

Interval: Section 186-1151A-87R-4, 35 cm, through Core 98R
Depth: 896.75-1007.4 mbsf
Age: late to middle Miocene
Description: Hard hemipelagic diatom- and siliceous sponge spicule-bearing silty claystone. Minor lithologies have sparse to common occurrence. Brittle deformational structures increase downhole through this unit.

Lithologic Unit IV consists of hard diatom- and siliceous sponge spicule-bearing silty claystone, which is commonly glass bearing. The sedimentary rocks have an olive-green color with grayish and brownish patches caused by moderate bioturbation. In this unit the color is lighter and more blue greenish than in Units II and III. The biogenic component, represented mainly by opal-A, is low in the top of the section and increases from ~970 mbsf to the bottom.

Minor lithologies have sparse to common occurrences, with ash occurrence being very low. Pumice grains and layers are common (Fig. F7). Sandy and silty layers and accumulations are common, with silt-sized grains being dominant. The frequency of brittle deformational structures increases with depth.

The base of Unit IV is determined to be at the interval where quartz, 14-Å, and 7-Å mineral abundances decrease (Fig. F10), where the color becomes lighter and more greenish and bluish (Fig. F6), and where magnetic susceptibility and NGR begin to increase.

Lithologic Unit V

Interval: Cores 186-1151A-99R through 109R
Depth: 1007.4-1113.6 mbsf
Age: middle Miocene
Description: Hard hemipelagic glassy or glass-bearing silty claystone that is locally siliceous sponge spicule bearing. Bioturbation is common. Brittle deformational structures increase with depth.

Lithologic Unit V consists of glassy or glass-bearing silty claystone that is locally siliceous sponge spicule bearing and diatom bearing, although less so than in other units. Opal-A is relatively high in this unit (Fig. F10). XRD intensities of quartz and clay minerals are low and decrease with depth (Fig. F10).

The color of the sedimentary rocks of this unit is quite characteristic relative to the other units, with a more bluish green with gray color rather than olive green (Fig. F6). Brownish patches caused by common bioturbation are also present.

Pumice grains and layers occur together, especially in the bottom part of the unit (Fig. F7). Sandy and silty accumulations are also high in the bottom of the unit. Feldspars, hornblende, and clinopyroxene, coarser grains of which are included in silty or sandy layers and accumulations, increase with depth. These high occurrences may explain the high values of magnetic susceptibility. The frequency of brittle deformational structures increases with depth (see "Structural Geology").

Major Lithology

The major lithologies are described following the scheme of three main constituting components of siliciclastic, biogenic, and volcanogenic origin. The percentages of their occurrence were determined by smear-slide analysis, and quasiquantitative variations with depth were derived from XRD analysis (Table T2, also available in ASCII format).

Siliciclastic Components

The siliciclastic components are the main constituent of the major sediments and sedimentary rocks of Sites 1151 and 1150. Clay minerals are the dominant siliciclastic constituent, with feldspars, quartz, pyroxenes, hornblende, and mica (brown biotite) present in much lower percentages. Clay mineral content varies between 25% and 50%. Between 200 and 400 mbsf, the values are slightly lower; below 700 mbsf, the values are slightly higher. Mica, hornblende, and pyroxene are present only in trace amounts in major lithologies and range from accessory (<1%) to maxima of 2% in minor lithologies such as sand or silt layers. Quartz and feldspar grains are present in almost all cores with values between 1% and 7%. The siliciclastic component decreases with depth from the top of the section down to ~300 mbsf and then increases down to ~800 mbsf. A correlation between the increase of siliciclastic components and grain density is observed below 400 mbsf (see "Physical Properties").

The siliciclastic components are divided into quartz, feldspars, hornblende, clinopyroxene, and clay minerals based on XRD identification (Table T2; Fig. F10). Variations of XRD intensities with depth of quartz, feldspars, and 14- and 7-Å minerals show similar patterns downhole, decreasing from the top of the section down to ~300 mbsf, then increasing from ~300 to ~600 mbsf, and finally decreasing downhole with ~100-m scale variation. Quartz and 14- and 7-Å minerals decrease in Units IV and V. The 10-Å minerals, which include glauconite and the illite group, behave differently from other minerals, decreasing with depth from the top of the section down to ~500 mbsf, abruptly increasing at ~610 mbsf (Fig. F10), and then decreasing with depth down to the bottom of the hole.

Biogenic Components

The biogenic components represent the second most common constituent of the sediments. Biogenic components are mainly represented by diatoms, siliceous sponge spicules, and their fragments with small amount of radiolarians and siliceous silicoflagellate. The sum of these biogenic components is constant at 35% from 0 to 80 mbsf and then increases, albeit with large scatter, from ~20% at 80 mbsf to 40% at 350 mbsf. Below that, the sum decreases to values of ~15% at the bottom of the hole (Fig. F9). The percentages lie between 5% and 35% for diatoms and between 5% and 19% for siliceous sponge spicules in smear-slide observation. Minor calcareous components of nannofossils and foraminifers are more abundant than at Site 1150. Foraminifers show a peak abundance between 0 and 100 mbsf, where they reach up to 15% but then decrease to below 2% of the total composition. Siliceous sponge spicule aggregates (Fig. F11), which are often found in the core during visual core description, are abundant to common above Core 186-1151A-87R and very rare or absent below Core 88R.

Diatoms have an average value of 20% from 0 to 80 mbsf (Cores 186-1151C-1H to 9H) and then change, with large scatter, to an average of 15% below 80 mbsf, increasing subsequently to a maximum of 25%-30% at 300 mbsf (Cores 186-1151A-25R and 26R) (Fig. F9). From 500 to 650 mbsf (Cores 186-1151A-46R to 61R), biogenic content stays at an average of 15% and then decreases to <10% in the bottom of the sedimentary section.

Opal-A, as measured by a hump in the XRD intensities, increases from the top of the section down to 350 mbsf (Fig. F10) with a small-scale variation. Below 350 mbsf, opal-A decreases with depth and may have a 100-m scale variation. At the bottom of the section, the opal-A hump increases. This is probably related to an increase in volcanic glass associated with the common accumulation of pumiceous grains and layers (Fig. F7).

Volcaniclastic Components

The volcaniclastic component has the lowest abundance of the three main components. The volcaniclastic component is composed mainly of volcanic glass shards of clay to sand grain size, with a predominance of silt grain size. The majority of the ash shards in the major lithology are colorless, but rare dark altered shards are also present. The grains are highly angular, typically with sharp broken edges and clear surfaces. The dark shards are of the same habit as the translucent shards but show different amounts of brown and black fine inclusions. The dark glass is far more common in minor lithologies (see "Minor Lithologies"). Very minor amounts of palagonite grains can be found as alteration products of the volcanic glass. The overall proportion of volcaniclastic components lies between 5% and 30%. Between 0 and 90 mbsf, as well as between 200 and 380 and between 420 and 700 mbsf, the values stay at intermediate levels slightly below or above 10%.

Other Components

Minor minerals in order of decreasing abundance are glauconite, pyrite, calcite, hematite, and dolomite. As mentioned above, for simplicity, the iron minerals and calcite are included in the sum of siliciclastic components for the triangle diagram. The very minor and sporadic amounts of minerals in the major lithology or related to minor lithologies normally range below 3%.

Glauconite is often found in smear slides from this site at <5%, reaching values up to 10% in the major lithology only in three intervals. Glauconite shows two different kinds of grain size depending on its detrital or authigenic origin. The detrital glauconite occurs either as sand grain size in the major lithology (Fig. F12) or as layers of minor lithology within the major lithology (Fig. F13). It is most abundant in interval 186-1151A-78R-4, 70 cm, to 79R-4, 120 cm, where a dense scattering of sand-sized glauconite grains occurs. The authigenic glauconite, found in both the major and minor lithologies, is of fine-silt grain size. The detrital glauconite is always found in association with the finer grained authigenic glauconite, but the latter may occur in the absence of the former. Alteration to limonite often occurs below 300 mbsf in brownish bioturbated patches.

Pyrite and hematite are ubiquitous, with pyrite accounting for 2%-4% and hematite usually <1%. Both are of medium- to fine-silt grain size. Pyrite is rarely shapeless and mainly framboidal and is common in diatom, radiolarian, and siliceous sponge spicule tests.

Inorganic calcite is present as sporadic accumulations of very fine grained aggregates and remains below an average of 3%. Within certain layers of the minor lithology, calcite and dolomite reach higher values.

Minor Lithologies

The minor lithologies are volcaniclastic ash layers and patches, commonly of gray to light gray color; sand and silt layers and patches, commonly dark gray and black; and distributed sand grains, commonly of black or white color (Tables T3, T4, both also available in ASCII format). Further, there are some rare carbonate layers consisting of dolomite or calcite (Table T5). The NGR data (see "Physical Properties") show these intercalated lithologies well with clear peaks.

Primary and Reworked Tephra, and Pumice Grains and Accumulations

Volcaniclastic material can be divided into primary and reworked tephra layers, their bioturbated patches, and accumulated grains and layers of granule- and pebble-sized pumice. Primary tephra layers (Fig. F14) and bioturbated patches (Fig. F15) are light gray and white, and reworked layers and patches are dark gray, sometimes lightening upward in the layers. The layer thickness varies from 0.5 to 20 cm, and grain size varies from silt to coarse sand. Layer boundaries are commonly sharp, sometimes bioturbated and occasionally gradational. Where an erosional boundary occurs, a reworked origin can be inferred for the layer. The upper boundary is often gradational but can be sharp. Grading rarely occurs within each layer. Patches are typically a few centimeters in diameter with diffuse boundaries. Pumice grains occur singly or in layers. The grains are white, gray, or brownish gray in color and of granule to pebble size (Fig. F16). Pumice content peaks at 150-230, 440-490, and 600-660 mbsf, and from 800 mbsf to the base of the section. Overall pumice content increases below 800 mbsf.

Sandy and Silty Layers and Accumulations

Sand layers display a range of colors from black through dark gray to light gray and white. Layer thickness is typically at centimeter scale (1-2 cm), and both upper and lower boundaries can be sharp or gradational. Erosional lower boundaries are also found. The sand layers show a range of grain size from coarse to fine, and a fining-upward texture is occasionally present. Sand patches are similar in color and grain size to sand layers. Thicknesses are typically at a centimeter scale, with average values of 1-2 cm.

Silty layers range in color from dark gray through dark olive gray to light olive green. Layers are typically 0.5-2 cm thick with a gradational upper boundary and sharp lower boundary. A fining-upward texture is occasionally present, and bioturbation of the unit is common. Silt patches have the same color as the layers and are typically 0.5-3 cm in diameter with diffuse or sharp boundaries. Patches can occur singly and together.

Sandy and silty layers are rich in feldspars, hornblende, and pyroxene by smear-slide observation and XRD analysis. Glauconite is often found with sand-sized particles.

Sand frequency shows a slight decrease over the interval of 220-680 mbsf relative to that at shallower depths and a sharp increase at 680 mbsf with relatively high values. These high sand abundances continue to the base of the hole (Fig. F8). Silt frequency (Fig. F8) shows two major peaks between 100 and 160 mbsf and between 530 and 590 mbsf and a minor peak from 900 to 1000 mbsf.

Dolomitic and Calcareous Layers

Fourteen carbonaceous intervals were recovered (Table T5). The intervals are very hard and have a light gray, khaki, or white color. In smear slides from the major and minor lithologies, the carbonate grains occur both as aggregates of very fine grained (clay to silt size) and coarser grained detrital grains. The XRD data (Table T5) indicate that these intervals are predominantly composed of dolomite, though three intervals contain calcite as well. Only one thick limestone layer was recovered at interval 186-1151A-101R-2, 20-55 cm (Fig. F17). Another significant interval of carbonaceous concretions several centimeters in diameter and of oval shape was recovered in Core 186-1151A-109R (Fig. F18).

Color Variation

The color of the sediments in the homogeneous upper part of Site 1151 is dominantly olive green. In the lower bioturbated part of the section, the color is olive green and light greenish gray with light brownish gray or light gray patchy burrows that have green and dark gray rims. The color in the lowermost part of the section, lithologic Unit V, becomes bluish green with light brownish gray or light gray patchy burrows.

The quantitative color variation from spectrophotometric measurements shows a characteristic variation with depth (Fig. F6). L* values, which refer to the lightness of the sediment color, show three broad variations from 80 to 400 mbsf, from 400 to 870 mbsf, and from 870 mbsf to the base of the section. The lightness increases with depth down to ~280 mbsf and decreases rapidly at ~280-300 mbsf. The color from 280 to 520 mbsf is slightly darker. The lightness increases gradually from 480 to 720 mbsf, then the color becomes lighter in lithologic Units IV and V.

The a* values are dominantly negative, with positive and negative a* values corresponding to the degree of redness and greenness respectively. The a* values increase from the top of the section to ~200 mbsf and have an almost constant value of nearly zero between 200 and 600 mbsf with a slowly decreasing trend. From 700 down to 850 mbsf, a* increases with depth, corresponding to an increasing degree of bioturbation by light brownish gray and light gray patchy burrows. Below 850 mbsf, the color becomes greenish, as reflected by the decrease in a* values. A decrease of a* is also found between 1000 to 1080 mbsf. The color becomes bluish green in this interval.

The b* values are positive except for a few spikes, with positive and negative values corresponding to the degree of yellow and blue, respectively. Between 280 and 580 mbsf, b* is slightly higher, corresponding to the homogeneous olive-green sediments. Below 580 mbsf, yellow decreases with depth, but there are intervals with increasing values at ~800, 850, and 1000 mbsf. The lowest and only negative values occur in lithologic Unit V.

These color variations should be related to mineral and chemical compositional changes and physical properties variations within the recovered sediments and sedimentary rocks. The decreasing variation of a* values with depth shows a good correlation with grain density and/or porosity (see "Physical Properties"). The b* values also show a good correlation with void ratio. These similarities are not observed in Unit V.

Lithification

As is common, sediment lithification increases with depth with few exceptions (Fig. F5). Sediments from the top of the section to 350 mbsf in Units I and Subunits IIA and IIB are mainly unconsolidated. Below 285.9 mbsf, the recovered cores became firm enough in some intervals that we began splitting the cores with a saw rather than a wire. Between 350 and 430 mbsf, soft and firm sediments are intercalated, as noted by the small variations in the resistivity logs (Fig. F5). Between 613 and 720 mbsf, sediments are firm and much more lithified, as noted by the sediment surface, which becomes difficult to scratch by fingernail. Below 720 mbsf, the sediments are hard enough to warrant the use of the suffix "-stone" in the lithologic descriptions.

These lithification boundaries correspond well to the changes in resistivity logs and physical properties (see "Physical Properties"). In some cases these logs indicate downhole decreases in resistivity, which we might expect to be associated with downhole decreases in lithification. Though counter to the general increase of lithification with depth, these decreases in resistivity are still likely indicative of decreases in lithification. For example, resistivity logs show a decreasing trend with depth at Units IB, IIB, and IIIC (Fig. F5). Dolomitic layers, however, are found at the top those intervals and are generally more lithified than surrounding lithologies.

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