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doi:10.2204/iodp.proc.346.110.2015 LithostratigraphyDrilling at Site U1430 penetrated to a subbottom depth of 274.4 m CSF-A in Hole U1430A, recovering a total of 258.2 m of sediment for a recovery rate of 94%. The shipboard lithostratigraphic program involved detailed visual assessment of sediment composition, color, sedimentary structures, and bioturbation intensity, supplemented by petrographic analysis of smear slides (98 from Hole U1430A and 22 from Hole U1430B), bulk mineralogic analysis by X-ray diffraction (XRD) (39 samples), and thin section analysis (1 sample). These objective criteria were used to describe the sediment succession, to define facies and facies associations, and to divide the stratigraphic section into major lithologic units (Figs. F2, F3, F4). The sedimentary succession recovered at Site U1430 extends from the middle Miocene to the Holocene and is dominated by clayey silt, silty clay, nannofossil ooze, diatom ooze, claystone, and sandstone. Numerous discrete tephra (i.e., volcanic ash) layers occur throughout the sedimentary sequence, especially within the upper 50 m (pumice more frequently) (Fig. F5), and volcaniclastic material represents a minor component of the entire sediment succession. The section is divided into four lithologic units (I, II, III, and IV, similar to Site U1425 and Tada [1994]), distinguished on the basis of sediment composition, referring particularly to the relative abundance of the biosiliceous and siliciclastic fractions. Units I–III are further divided into two subunits. The character of the sediment physical properties, including natural gamma radiation (NGR), magnetic susceptibility, and color reflectance parameters, also record downhole variations of the sediment components and lithologies (see “Physical properties”). The major characteristics of the sedimentary sequence at Site U1430, together with some of these additional properties, are summarized in Figures F2, F3, and F4. Between-hole correlation of lithologic units for Site U1430 is shown in Figure F6. Unit I
Lithologies and structuresUnit I consists of alternating interbedded clayey silt (greenish gray to dark olive-gray), silty clay (dark olive-gray to dark gray), foraminifer-rich diatom-bearing clayey silt, nannofossil ooze (light greenish gray), and tephra (white, gray, and black). Pyrite and volcaniclastic materials represent minor components throughout the sediment sequence. Numerous discrete millimeter- to centimeter-thick pumice clasts (~1–5.0 mm diameter) and tephra layers (vitric and scoria) occur throughout Subunit IA (total of 71 tephra beds) (Fig. F5; Table T2). The most distinguishing sedimentary feature of Unit I sediment is the alternating, decimeter-scale color-banded bedding that characterizes much of the sequence, with dark organic-rich silty clay intervals interspersed with lighter colored organic-poor intervals. The relative frequency of these color alternations as well as the intensity of bioturbation are used as criteria to divide Unit I into Subunits IA and IB (Figs. F2, F3, F4). In comparison with Site U1425, the siliciclastic grains of Unit I at this site are much coarser (greater silt fraction) and have significantly higher quantities of quartz as observed in smear slides. Bulk mineralogyThe results of XRD analyses are listed in Table T3. In general, the sediment at this site is composed mainly of quartz, plagioclase, and clay minerals (including smectite, illite, and kaolinite and/or chlorite), as well as biogenic opal-A and minor amounts of halite and pyrite. Calcite is sparse throughout the upper 50 m of Unit I (mostly in the form of nannofossils and foraminifers). Other minor minerals are observed in smear slides, but it was not possible to detect these through XRD analysis of bulk samples. Figure F7 shows the downcore variations in peak intensity of the identified minerals in Hole U1430A. In general, the peak intensities of quartz, plagioclase, feldspar, and clay minerals (smectite, illite, and kaolinite and/or chlorite) in Unit I are higher than in other units except Unit IV, suggesting higher terrigenous input relative to biogenic material. The peak intensity of calcite is also higher than in other units but disappears abruptly downhole at 50 m CSF-A. In contrast, the peak height of opal-A is generally much lower in Unit I compared to other units. Subunit IA
Lithologies and structuresSubunit IA is dominated by silty clay with subordinant amounts of foraminifers and/or nannofossil-rich ooze and diatom-bearing clayey silt. The subunit is characterized by decimeter-scale alternations of light and dark sediment intervals (Fig. F8). These alternations are also expressed in the L*, a*, and b* records (see “Physical properties”). Light colored sediment intervals are dominantly composed of light greenish gray clayey silt or nannofossil ooze, occasionally with minor amounts of diatoms. Within these intervals, millimeter- to centimeter-scale layers of gray, dark greenish gray, and very dark olive-gray silty clay are observed that form distinct color banding within the sediment. The lighter intervals tend to be slightly bioturbated, though not enough to disrupt preservation of the thin, darker banding. Some prominent millimeter- to centimeter-scale olive-gray layers are also observed throughout these light intervals. Detailed examination of these layers shows they are composed of quartz and clay minerals with abundant amounts of pyrite and organic matter. The contrasting dark layers, which dominate and help define Subunit IA, correspond to dark grayish brown silty clay intervals with more organic matter and pyrite. Tephra layers are intercalated in the silty clay sequence and form a minor but common component (~5%–20%) of Subunit IA. Some of these intervals show evidence of moderate to heavy bioturbation, but the dark layers are predominantly finely laminated with no apparent bioturbation (interval 346-U1430A-1H-2, 150–290 cm) (Fig. F8). Foraminifers (mostly planktonic) are generally restricted to millimeter-scale, foraminifer-bearing yellowish layers (e.g., interval 3H-6, 206–221 cm). These yellowish layers, however, are not present in all dark intervals. The lower and upper contacts of the dark layers with the lighter greenish gray intervals within Subunit IA are normally subtle and gradual, suggesting moderate to heavy bioturbation. This character is different from the weak bioturbation observed in dark layers at Site U1425. A rather remarkable series of three very thin (0.5–1.0 cm) yellowish calcareous ooze–rich laminated intervals (relatively more foraminifers, pyrite, and organic matter) are observed interbedded with three light green nannofossil ooze layers (relatively fewer foraminifers, less pyrite, and less organic matter) (intervals 345-U1430A-3H-5, 90 cm, to 3H-6, 55 cm; Fig. F9). Similar carbonate-rich/ooze sequences that appear to be correlative are also observed in Sites U1423 (Section 346-U1423B-4H-6), U1424 (346-U1424A-2H-7), U1425 (346-U1425B-3H-1), and U1426 (346-U1426C-6H-3), suggesting a basin-wide event. Tephra layers in Subunit IA are typically thicker than 1 cm and are interbedded within the light greenish gray and dark brown/grayish brown silty clay intervals. The number of tephras per core with a thickness >0.5 cm is highest in Subunit IA (Fig. F5; Table T2). Pyrite and volcaniclastic materials represent a minor sediment component (~5%–10%) throughout the succession. Discrete millimeter- to centimeter-scale tephra layers (mostly pumice) are found throughout Subunit IA (a total of 71 tephra beds each with a thickness >1.0 cm), but only five pumice layers are found in Subunit IB. In contrast with the occasionally pumiceous tephra found at other sites in the marginal sea (IODP Sites U1422–U1427), the prominent feature of tephra layers at this site is the much higher frequency (47% probability) of thicker (2–30 cm) and coarser sized (~1–5.0 mm diameter) pumice. The pumice is usually white to light gray in color, although the gray color likely comes from mixing with gray-colored fine ash. Tephra layers are mostly white and light gray in color (i.e., vitric), but there are some very dark to black ash layers (i.e., scoriaceous). A notably thick (29 cm) pumice layer was identified in Hole U1430A (interval 346-U1430A-5H-1, 0–29 cm), with its equivalent found in interval 346-U1430B-4H-2, 108–138 cm. This particular layer provides an especially useful tie point between holes. In Hole U1430C, the interval between Sections 346-U1430C-4H-3, 83 cm (26.4 m CSF-A), and 5H-5, 40 cm (39.7 m CSF-A), showed inconsistent stratification that could not be correlated with the corresponding intervals in Holes U1430A and U1430B. Two slump structures found in intervals 346-U1430C-4H-3, 83–109 cm, and 4H-3, 122–133 cm, did not occur in either Holes U1430A or U1430B. A chaotic mud underlain by an ash layer found in interval 4H-5, 5–35 cm, and a light colored homogeneous mud layer found between Sections 4H-6, 66 cm, and 4H-7, 19 cm, also occur only in Hole U1430C. At Section 5H-3, 81 cm, there is no apparent sequence corresponding to the 60 cm thick interval observed at 346-U1430A-5H-1, 40–100 cm. At Section 346-U1430C-5H-5, 40 cm, there is also no apparent sequence corresponding to the 170 cm thick interval found between Sections 346-U1430A-5H-3, 140 cm, and 5H-5, 10 cm. CompositionThe principal lithology of Subunit IA consists of terrigenous and biogenic grains and pumice or volcanic glass (see Site U1430 smear slides in “Core descriptions”). Terrigenous components in this subunit are dominated by silt and clay fractions. In Subunit IA, the light greenish gray intervals are mostly composed of siliciclastic fine-grained material (up to 80%) dominated by quartz and clay minerals. However, sometimes the light greenish layers are dominated by nannofossil ooze with only a few siliciclastic grains. This subtle difference in color but distinct change in lithology is difficult to identify by the naked eye. Small pyrite framboids are distributed mainly in the dark layers. Volcanic glass accounts for nearly 100% of the ash layers, even though sometimes the volcanic material appears to be mixed with a siliciclastic component. Subunit IB
Lithologies and structuresSubunit IB is a transitional sediment unit between Subunit IA and Subunit IIA. It is identified by a decrease in the frequency of the dark and light color alternations and by the increasing dominance of light greenish gray and light gray clayey silt (Fig. F10). In contrast with the ~48 m stratigraphic thickness of Subunit IB at Site U1425, Subunit IB at Site U1430 is very condensed (12.7 m), suggesting a much slower sedimentation rate during deposition of this subunit at this site, or even a possible hiatus. Heavy bioturbation and large burrows (1–5 cm diameter) were observed frequently in Core 346-U1430A-6H. Tephra layers are intercalated in the silty clay sequence and form a minor but common (~5%–10%) component of Subunit IB. As compared to Subunit IA, tephra layers in Subunit IB are less frequently observed. Some of the (light) tephra layers within the dark gray intervals (346-U1430A-6H-2, 60–80 cm) show distinct millimeter-scale gray and dark gray laminations. In addition, green sediment with abundant silt-size glauconite mixed with scattered pumice was observed in interval 346-U1430A-6H-7A, 23–59 cm. Glauconite occurs in Cores 346-U1430A-6H and 9H (Fig. F11), corresponding to the interval of very low sedimentation rate between 50 and 80 m CSF-A (see “Biostratigraphy” and “Stratigraphic correlation and sedimentation rates”). CompositionThe principal lithologic components in Subunit IB are terrigenous, volcanic, and biogenic in origin (see Site U1430 smear slides in “Core descriptions”). The major difference between the lithologies of Subunits IA and IB is the reduced occurrence of calcareous microfossils downhole and the generally higher contents of the biosiliceous fraction (mostly diatoms). Terrigenous materials compose the bulk (>80%) of Unit I sediment, which is dominated by quartz and clay minerals. Volcanic glass usually occurs as a minor dispersed component (~5%–10%) throughout the sediment. The biogenic fraction is generally low (<30%) in Subunit IB and is dominated by diatoms and sponge spicules, with a few calcareous microfossils. Unit II
Lithologies and structuresUnit II consists of grayish green silty clay, olive-gray diatom-rich silty clay, and dark olive-gray diatom ooze (Figs. F2, F3, F4). Unit II is distinguished from Unit I on the basis of sediment color and an increase in overall diatom content relative to terrigenous sediment. This lithologic change is supported by NGR measurements, which show lower values in Unit II than in Unit I and are likely to be related to variations in the relative contents of the diatomaceous and terrigenous fractions downhole (see “Physical properties”). Furthermore, XRD analyses show a large increase in opal-A content (see “Bulk mineralogy”; Fig. F7). Unit II sediment is heavily bioturbated and is often mottled. The degree of bioturbation changes vertically. The diatom content in bulk sediment is the primary criterion used to further divide Unit II into Subunits IIA and IIB. Bulk mineralogyThe results of XRD analyses conducted on Hole U1430A sediment are listed in Table T3. In general, the character of the bulk mineral composition and peak intensity of Unit II sediment is intermediate between Units I and III. Figure F7 shows the downcore variations in peak intensity of the identified minerals in Hole U1430A. In general, the peak intensities of quartz, plagioclase, feldspar, and clay minerals (smectite, illite, and kaolinite and/or chlorite) decrease through Unit II. In contrast, the peak height of opal-A increases through Unit II. These characteristics are consistent with the observed lithologic changes. Glauconite in this unit is observed in smear slides at Samples 346-U1430A-6H-CC, 2 cm, and 9H-4, 100 cm (Fig. F11). However, it is difficult to detect glauconite by XRD because the glauconite peak overlaps with the illite peak. Subunit IIA
Lithologies and structuresSubunit IIA consists dominantly of meter-scale alternating dark olive-gray, grayish green diatom-bearing and diatom-rich silty clay and very dark gray diatom ooze (Fig. F12). This subunit is considered transitional from Subunit IB to the underlying Subunit IIB, which is defined by the consistent appearance of diatom ooze. In general, the sediments of this unit are heavily bioturbated, leading to poor preservation of original sedimentary structures, which inhibits their recognition (e.g., color banding, laminae, etc.). CompositionSubunit IIA is predominantly composed of a mixture of fine-grained material, mostly clay minerals, quartz, and biosiliceous components (see Site U1430 smear slides in “Core descriptions”). The abundance of these components varies throughout this transitional unit, with alternations of silty clay, diatom-bearing/rich silty clay, and diatom ooze. Subunit IIB
Lithologies and structuresSubunit IIB sediment is dominated by dark olive-gray to very dark gray diatom ooze with a few dark gray silty clay intervals. The color changes within the unit are subtle. The abundance of diatoms and other siliceous components (sponge spicules and radiolarians) is key to the recognition of Subunit IIB (Fig. F13) and typically composes >50% (mostly 90% or more) of the sediment based on smear slides. A significant decrease in NGR values from Subunit IIA to Subunit IIB coincides with the increasing diatom content of the sediment (see “Physical properties”). Moderate to heavy bioturbation is also displayed in some sections but is weaker in general than in Subunit IIA. Only three pumiceous tephra layers were observed in Subunit IIB (Table T2). The thickest layer, with a maximum thickness of 30 cm (not continuous and mixed with normal sediment), occurs in the lower part of Subunit IIB (interval 346-U1430A-9H-5, 30–60 cm). The other two tephra layers are very thin, ~1 cm. Although very low sedimentation rates or the possible presence of a hiatus has been inferred for the upper part of Subunit IIB (see “Biostratigraphy” and “Stratigraphic correlation and sedimentation rates”), there is no clear visual evidence of an erosional surface or lithologic change in the upper Subunit IIB sediment except for a slightly coarser bed containing glauconite grains (interval 346-U1430A-9H-4A, 68–150 cm). Well-lithified, pale gray dolomite beds and concretions frequently occur in Subunit IIB sediment of Site U1425. However, very few were observed at Site U1430, which is consistent with only trace amounts of dolomite being observed in the normal sediment by microscope and possibly indicating much weaker diagenesis at this site. In addition, some intervals of faint dark gray laminations (~60 cm thick, diatom ooze) are found in Sections 346-U1430A-11H-3 and 11H-6, which can be compared with the very fine laminations in Sections 346-U1425B-38H-2 and 38H-3 of the same age (~8 Ma). CompositionThe major lithologies in Subunit IIB are dominated by biosiliceous (mostly diatom) components (>50% and up to 95%) from Section 346-U1430A-9H-3 (i.e., the Subunit IIA/IIB transition) downhole (see Site U1430 smear slides in “Core descriptions”). Diatoms dominate in the biosiliceous fraction, whereas siliceous sponge spicules are common to rare (20%–5%) and radiolarians and silicoflagellates occur only in rare or trace amounts (1%–5%). These siliceous fossil assemblages characterize both the dark olive-gray and very dark gray sediment in the “diatom ooze” category. Scattered glauconite crystals are occasionally observed in the diatom ooze in the interval 346-U1430A-9H-4, 68–150 cm. Observed in smear slide, some glauconite grains (~50–120 µm) show ovoid and lobate crystal forms and a very fresh green color (Fig. F11). In some cases, glauconite is observed growing within empty diatom frustules, replacing the biosiliceous composition and preserving the original texture of the diatoms. Unit III
Lithologies and structuresUnit III consists of silty clay, diatom-rich/bearing silty clay, and diatom ooze (Fig. F14), siliceous claystone (Fig. F15), and sandstone (Fig. F16). Unit III is distinguished from Unit II on the basis of a decrease in diatom contents and an increase in the terrigenous content of the sediment; nonetheless, the upper part of Unit III still has a significant amount (5%–90%) of diatoms. Unit III sediment is further divided into two subunits (IIIA and IIIB), and the upper sediment is moderately to heavily bioturbated and often mottled. Alternations between diatom ooze and diatom-rich/bearing silty clay are difficult to distinguish visually without smear slide observations. The diatom content (i.e., biogenic silica component) decreases near the base of Subunit IIIA, and siliceous claystones and sandstones develop within Subunit IIIB and Unit IV, respectively. Bulk mineralogyThe results of XRD analyses are listed in Table T3. In general, Unit III sediment is composed mainly of quartz, K-feldspar, plagioclase, clay minerals (including smectite, illite, and kaolinite and/or chlorite), biogenic opal-A, and minor amounts of halite and pyrite. Figure F7 shows the downcore variations in peak intensity of the identified minerals in Hole U1430A. In Subunit IIIA, the peaks of quartz, K-feldspar, plagioclase, and clay minerals (including smectite, illite, and kaolinite and/or chlorite) have lower intensities and variation than in Unit I. In contrast, biogenic opal-A has higher intensities and greater variability than in Unit I. This character is the same as Subunit IIIA sediment at Site U1425. As a minor lithology, a concretion layer (Sample 346-U1430A-21H-6W, 43.0–44.0 cm) and two rock samples (346-U1430A-22H-2W, 87.0–88.0 cm, and 346-U1430C-26H-CC, 11.0–12.0 cm) were analyzed by XRD. A hydroxyl-apatite peak was detected in the concretion layer and in one of the rocks analyzed (Sample 346-U1430A-22H-2W, 87.0–88.0 cm). In the other rock (Sample 346-U1430C-26H-CC, 11.0–12.0 cm), a strong dolomite peak was detected. In Subunit IIIB, only one sample (Sample 346-U1430A-30X-CC, 22.0–23.0 cm) was analyzed by XRD, so it is difficult to characterize the subunit lithology by this means. An important change is the appearance of opal-CT at 249.72 m CSF-A, which appears as a large peak in intensity centered at 22° (Δ2θ) in the XRD diagram (Fig. F17). The appearance of opal-CT is very close to the Subunit IIIB/Unit IV boundary (249.5 m CSF-A). Subunit IIIA
Lithologies and structuresSubunit IIIA sediment is composed of alternating layers of heavily bioturbated diatomaceous ooze and diatom-rich/bearing silty clay (Fig. F14). These alternating layers show decimeter- to meter-scale cycles of dark gray diatom ooze (relatively clay poor) and very dark gray diatom-rich silty clay (relatively fewer diatoms and more clay), although these color changes can be subtle. Moderate to heavy bioturbation is generally restricted to the light layers in Unit III, but faint burrows (possibly Chondrites-type) are evident in the dark layers, with a large vertical burrow (~2 cm diameter) observed near the top of Unit III. An important feature of Subunit IIIA is that some (yet not all) diatom ooze sequences display very fine laminations that range from ~5 to ~240 cm in thickness and are observed in both Holes U1430A (Fig. F18) and U1430B. These laminated intervals are not observed in the diatom-rich silty clay sequences (Fig. F14). The laminations (~225–232 m CSF-A; ~11.8 Ma) appear within a dark organic-rich layer in Subunit IIIA that was not found at Site U1425, possibly a result of poor recovery and greater diagenesis. An unusual interval of yellowish concretion/clay-size sediment occurs at 191.4 (Sample 346-U1430A-21H-6A, 41–46 cm) and 195.35 m CSF-A (Sample 22H-2A, 86–92 cm) (Fig. F19). The sediment did not react with 10% hydrochloric acid and goes nearly extinct under cross-polarized light in the microscope, suggesting no calcite is present. The composition resembles hydroxyl-apatite based on XRD results, but the exact classification and origin of these unusual yellowish layers needs further onshore study. CompositionThe principal lithology of Subunit IIIA is characterized by varying diatom content (5%–75%, average = 40%) associated with the common occurrence of siliceous sponge spicules (5%–25%) and increasing clay mineral and quartz contents (5%–25%) compared to Unit II. Other siliceous components such as radiolarians remain rare (<5%), but organic matter based on smear slides becomes more common. Toward the bottom of Subunit IIIA, pyrite content also increases to become a more important component of the lithology. Overall, the biosiliceous and clay-size composition changes with the alternating color changes described above. In order to further understand the color and lithology change responsible for the fine laminations in Subunit IIIA, four samples were picked from the different color laminations (Samples 346-U1430A-24H-5, 25 cm, 24H-5, 26.5 cm, 24H-5, 33 cm, and 24H-5, 35 cm) to make smear slides (Fig. F20). The normal dark gray sediment (25 cm) consists of about 10% diatoms (centric), 90% siliciclastic grains, and abundant organic matter. The very dark gray layer sediment (26.5 cm) consists of 30% pyrite and 70% diatoms (pennate), whereas the white layer sediment (33 cm) has 50% centric and 50% pennate diatoms. In contrast, the light gray layer sediment (35 cm) is rich in organic matter and pennate diatoms. The fine laminations thus reflect rapid ecologic and lithologic changes of the sediment. Subunit IIIB
Lithologies and structuresSubunit IIIB sediment is characterized by dark gray siliceous silty clay and claystone with few sedimentary structures (Fig. F15). Subunit IIIB at Site U1430 covers a very short interval (7.9 m), in contrast to the much thicker (63.1 m) Subunit IIIB sequence at Site U1425. At this location it more likely represents the transition from a shallower, coarse-grained marine glauconitic facies (Unit IV) to the fine-grained and deeper hemipelagic diatomaceous sediment of Subunit IIIA. Furthermore, the estimated age of Subunit IIIB at Site U1430 appears to be somewhat older than that of Subunit IIIB at Site U1425. In addition, only two very thin (Samples 346-U1430A-28H-2, 144–152 cm, and 29H-1, 91–97 cm) claystones were found interbedded with siliceous silty clay in Hole U1430A. The transition from Subunit IIIA to Subunit IIIB is defined by the diagenetic loss of biosiliceous material (Fig. F21) and the formation of siliceous claystone. XRD results show that opal-CT first occurs between Subunit IIIB and the top of Unit IV (Fig. F17). However, it is difficult to exactly define the first downhole occurrence of opal-CT (top of Unit IV?), as only two samples in the interval were analyzed by XRD because of time limitations. CompositionThe principal lithology of Subunit IIIB is characterized by abundant clay minerals and quartz (i.e., falling within the category of 25%–75%, see the “Methods” chapter [Tada et al., 2015b]) and the common occurrence of pyrite and organic matter (5%–25%). All biosiliceous material is absent from smear slides in this unit. Unit IV
Lithologies and compositionBecause of poor recovery and heavy disturbance during drilling, a detailed description of lithologic changes in Unit IV is difficult (Fig. F16). In general, Unit IV is characterized by greenish black, hard glauconite-quartz-feldspar–rich sandstone (Fig. F22), and light gray glauconite-rich dolomite-dominated sandstone (Fig. F23). A thin section of a glauconite sandstone clearly shows that glauconite occurs in the form of pellets and infilling of siliceous microfossils and coexists with pyrite, quartz, and feldspar (Fig. F24), strongly suggesting its authigenic origin in the marine environment. Moreover, the transverse section of most glauconite minerals shows an internal yellow core and a green outer crust. The mineral compositions of Unit IV show high variability. Though based on only a few samples, XRD data (Fig. F7) are divided into three types. The first type is pure volcanic glass (Sample 346-U1430B-33X-1W, 39.0–40.0 cm). The second type shows very high peaks of quartz, plagioclase, and feldspar (Samples 346-U1430B-37H-1W, 28.0–29.0 cm, 346-U1430A-30X-CCW, 22.0–23.0 cm, and 346-U1430B-32X-1W, 34.0–35.0 cm). The last type shows high peaks of dolomite, quartz, plagioclase, and feldspar (Sample 346-U1430A-32X-CCW, 16.0–18.0 cm). In Unit IV, the intensities of plagioclase and feldspar peaks are much higher than in the other units. This character could be explained by grain size change or provenance change of the siliciclastic material. Thick tephra layers were observed in Hole U1430B. A 39 cm thick tephra layer (vitric) appears at the bottom of Hole U1430B (interval 33X-1A, 5–44 cm). Smear slide observations show that the volcanic glass has undergone significant alteration, especially the glass proximal to the adjoining sediment (Fig. F25). The alteration products resemble clay minerals but their mineralogy needs further confirmation. Summary and discussionThe sedimentary sequence recovered from Site U1430 records a nearly continuous history of terrigenous input and paleoceanographic evolution of the Ulleung/Tsushima Basin since the middle Miocene, as well as the gradual diagenetic development of claystones and sandstones with burial at deeper depth. The dominant lithology at Site U1430 has varied through time and is characterized by nannofossil ooze and calcareous-rich silty clay in the late Pliocene–Holocene and by diatom ooze and diatom-rich silty clay from the middle Miocene to early Pliocene. Sedimentation is dominated by pelagic and/or hemipelagic biogenic grains punctuated by periods of volcanic (tephra layers) and terrigenous input (silty clay or clayey silt layers), although both terrigenous and volcanic materials represent only minor components of the sediment succession since the Miocene. Over the last 15 m.y., the changes in major lithology at Site U1430 (and Site U1425) seem to have been largely controlled by subsidence and local sea level change in the marginal sea, which in turn was influenced by regional tectonic evolution and global sea level change. At present, the marginal sea is restricted and semi-enclosed, only connected with other seas by shallow, narrow straits, namely to the East China Sea to the south through the Tsushima Strait (130 m water depth), to the North Pacific to the east through the Tsugaru Strait (130 m water depth), and to the Okhotsk Sea to the north through the Soya and Mamiya Straits (20–50 m water depth) (Tada, 1994). The regional tectonic setting is thought to have switched from transtensional in the Miocene to compressional in the Pliocene and to convergence in the late Quaternary (Tada, 1994), which is well demonstrated by the high-frequency occurrence of tephra layers in Unit I at all sites in the marginal sea. Correspondingly, paleogeographic reconstructions suggest that the sill depths of the channels generally became shallower, from ~500–1000 m in the middle Miocene to ~130 m in the Pleistocene (Tada, 1994). During the middle Miocene to the early Pliocene (15–3.5 Ma), the deeper sill depths and the higher global sea level (around +100 m) relative to the Quaternary (between 0 and –130 m) suggests more open paleoceanographic conditions in the pre-Quaternary. The relatively unlimited inflow of the upper part of Pacific Deep Water would provide more nutrients such as silica that maintain the high productivity of diatoms, which are dominant from Units III to II. In contrast, the late Quaternary was significantly more restricted because of the very shallow inlet sills. During the glacial lowstands, the marginal sea was almost disconnected from the East China Sea and Pacific Ocean. The high-frequency climate and sea level changes of the Late Pleistocene induced the variable contributions of biogenic grains and terrigenous materials, corresponding to the alternating bands of light and dark layers observed in Unit I. In general, terrigenous materials are not the dominant component of sediment recovered in the marginal sea. There are no large/great rivers in the Japan islands or draining the Korean mainland. Considering that >70% of the freshwater discharged from the Yangtze River is carried into the marginal sea with the TWC (Isobe et al., 2002), some very fine clay minerals from the East China Sea are likely transported to the southern portion of the marginal sea. In any case, a more important terrigenous source may be dust transported by wind from the Asian inland. Previous studies have observed significant contributions of eolian dust to the bulk Quaternary hemipelagic sediment of the sea (Irino and Tada, 2000). Preliminary XRD results from analysis of bulk minerals at Site U1430 clearly show that quartz (mostly silt size) and clay mineral intensities have generally increased since the late Pliocene (Unit I), consistent with a strengthened East Asian winter monsoon and Asian inland aridity (Sun and An, 2005). Another important feature of the marginal sea’s sediment is the frequent occurrence of green layers and of scattered green silt- to sand-size glauconite grains within the sediment and dominant in the glauconite sandstone. Well-developed glauconite crystals usually occur in tephra layer/patches (Fig. F11) and sandstones (Fig. F24). Some glauconite seems to have grown in diatoms and to have replaced the biosiliceous composition, only to keep the original texture of the diatoms. In the green sediment that makes up much of the section, only a few (mostly <2%) silt-size light green glauconite grains were observed in smear slides; however, very light green aggregates (sometimes ~5%–10% abundance) were often seen that are similar in appearance to glauconite. Nonetheless, it was not possible to confirm their mineralogy. Many theories on the origin of green layers have been proposed, and the most common one is to attribute their genesis to the diagenetic alteration of volcanic material. Gardner et al. (1986) suggested that the pale green laminae found in sediment from the Lord Howe Rise east of Australia were the product of alteration of volcanic material, as their occurrence correlated with the distribution of volcanic material and their mineralogy corresponded to bentonitic material. Based on the distribution of volcanic glass/pumice and the typical glauconite light green aggregates in the sediment of the marginal sea, it seems that the formation of glauconite may be closely linked to tephra alteration here as well, with the light green aggregates in the green layer sediment representing the initial stage of glauconitization. However, further onshore study is needed to assess the authigenic origin and process of glauconite formation. In summary, the changes in sedimentation observed at Site U1430 since the Miocene largely reflect local and eustatic sea level changes, climate oscillations, and volcanic and diagenetic processes in the marginal sea. |
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