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doi:10.2204/iodp.proc.346.106.2015 LithostratigraphyDrilling at Site U1425 penetrated to a subbottom depth of 427 m in Hole U1425D, recovering a total of 417.5 m of sediment for a recovery rate of 98%. The shipboard lithostratigraphic program involved detailed visual assessment of sediment composition, color, sedimentary structures, and bioturbation intensity, supplemented by petrographic analysis of smear slides (141 from Hole U1425B and 157 from Hole U1425D), bulk mineralogic analysis by X-ray diffraction (XRD) (73 samples), and thin section analysis (3 samples). 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. The sedimentary succession recovered at Site U1425 extends from the Miocene to Holocene and is dominated by clay, silty clay, diatomaceous ooze, and siliceous claystone. Numerous discrete tephra (i.e., volcanic ash) layers occur throughout the sedimentary sequence and volcaniclastic material represents a minor component of the sediment succession. The section is divided into three major lithologic units (I, II, and III, similar to Tada [1994]), distinguished on the basis of sediment composition, referring particularly to the abundance of biosiliceous and clay fractions. These units are each further divided into two subunits. The character of the sediment physical properties, including NGR, magnetic susceptibility, color reflectance parameters, and density, records the distribution of the various sediment components and lithologies (see “Physical properties”). The major characteristics of the sedimentary sequence at Site U1425, together with some of these additional properties, are summarized in Figures F3, F4, and F5. A lithostratigraphic correlation between the three holes is shown in Figure F6. Unit I
Lithologies and structuresUnit I consists of clay and silty clay with small amounts of diatom-bearing, diatom-rich, and foraminifer-bearing clay and rare (nonbiogenic) calcareous layers. Pyrite and volcaniclastic materials represent a minor component throughout the sediment succession. Numerous discrete millimeter- to centimeter-thick tephra layers (vitric and scoria) and pumice clasts are described throughout Unit I (total of 51 tephra beds that are >0.5 cm thick; Fig. F7). The most distinguishing sedimentary feature of Unit I is the alternating decimeter-scale color-banded bedding that characterizes much of the sequence, with dark, organic-rich clay intervals interspersed with lighter colored, organic-poor intervals. The relative frequency of these color alternations and the intensity of bioturbation are used as criteria to divide Unit I into the Subunits IA and IB (Figs. F3, F4, F5). Bulk mineralogyThe XRD analysis results are listed in Table T2. In general, 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 foraminifers and nannofossils). Other minor minerals are observed in smear slides, but it was not possible to detect these through XRD analysis of bulk samples. Figure F8 shows downcore variations of the major identified minerals in Hole U1425B. 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 (II and III), suggesting higher terrigenous input relative to biogenic material. In contrast, the peak height of opal-A is generally much lower in Unit I compared to other units, especially Unit II. Dolomite is only detected in two selected samples of carbonate-rich concreted yellowish sediment (Samples 346-U1425B-13X-1W, 7–9 cm, and 56X-CC, 0–2 cm), although it occasionally occurs in sediment as a minor component in smear slides. Subunit IA
Lithologies and structuresSubunit IA is dominated by clay with subordinate amounts of diatomaceous (i.e., diatom bearing) and foraminifer-bearing clay. Tephra layers are intercalated in the clay sequence and form a minor but common component of Subunit IA. The subunit is characterized by pronounced decimeter-scale alternations of light and dark sediment intervals (Fig. F9). These alternations are also evident in the L*, a*, and b* records (see “Physical properties”). In detail, the light intervals are mainly composed of light greenish gray clay with some diatoms. Within these lighter intervals, millimeter- to centimeter-scale layers of gray, dark greenish gray, and very dark gray clay are observed forming color banding within the sediment. The lighter intervals are slightly bioturbated, although 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 clay minerals with abundant amounts of pyrite (sometimes visible to the naked eye). The contrasting dark layers, which dominate Subunit IA, correspond to dark grayish brown organic-rich silty clay intervals with some foraminifers and pyrite. Some of these intervals show evidence of slight bioturbation, but the dark layers are predominantly finely laminated with no apparent bioturbation (interval 346-U1425B-4H-1, 121–143 cm; Fig. F10). Foraminifers (mostly planktonic) are generally restricted to millimeter-scale (foraminifer-bearing) yellowish layers. These yellowish layers, however, are not present in all the dark intervals. The lower contacts of the darker layers within Subunit IA are normally sharp, suggesting a lack of bioturbation, whereas upper contacts with the light greenish gray intervals appear more gradational because of bioturbation (at the time of deposition of the overlying light greenish gray clay). Centimeter-scale gray clay typically underlies these dark brown sediment intervals (Fig. F11). This gray clay is slightly bioturbated but only with the underlying light greenish gray clay (i.e., not with the overlying dark brown interval). As a result, these layers do not form from mixing of the dark brown and greenish gray intervals and probably reflect transitional depositional or early diagenetic conditions. Despite this, heavy bioturbation can occur with the formation of burrows in Subunit IA (Fig. F12). Tephra layers in Subunit IA are mostly thicker than 1 cm and are interbedded within both the light greenish gray and dark brown/grayish brown clay intervals. The number of tephra per core with thickness >0.5 cm is highest in Subunit IA (Fig. F7; Table T3). Tephra are mostly white and light gray in color (i.e., vitric), but there are some very dark to black tephras (i.e., scoriaceous), with occasional gray to white pumiceous tephra layers. A notably prominent tephra layer in Hole U1425B is 26 cm thick and is made of gray vitric tephra (interval 346-U1425B-4H-7, 8.5–34.5 cm), with its equivalent found in Hole U1425D at interval 346-U1425D-5H-2, 37–67 cm. CompositionSubunit IA sediment is composed of a mixture of terrigenous, volcanic, and biogenic grains (see Site U1425 smear slides in “Core descriptions”). Terrigenous components in this subunit are dominated by clay and fine silty clay fractions. In Subunit IA, the light greenish gray intervals are mostly composed of siliciclastic fine-grained material (up to 80%) dominated by clay minerals. Small pyrite framboids are distributed mainly in the dark layers. Discrete accumulations of well-developed pyrite crystals are also observed on the surface of the split section half, as well as in smear slides (Fig. F13). Volcanic glass accounts for nearly 100% of the tephra 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 Unit II. It is identified by a decrease in the frequency of dark and light color alternation and increasing dominance of light greenish gray and light gray clay (Fig. F14). Some millimeter- to centimeter-scale layers of gray, dark greenish gray, very dark gray, and olive-gray clay (previously described in Subunit IA) are observed. Distinct diatom-rich laminae are evident at the top of Subunit IB (intervals 346-U1425B-6H-5, 22–54 cm, and 6H-6, 7–39 cm; Fig. F10), with some discrete calcite layers occurring within these structures. Bioturbation increases gradually with depth, and sediment mottling and disruption of laminae and color banding is more prevalent. Tephra layers are intercalated in the clay sequence and form a minor but common component of Subunit IB. As with Subunit IA, tephra layers are mostly centimeter-scale light (vitric) deposits. Some of the (light) tephra layers described within the light greenish gray intervals show distinct millimeter-scale gray and grayish green laminations or cross laminations. A notable example of a thick (~18 cm) tephra made up of gray vitric tephra (interval 346-U1425B-7H-2, 9–27 cm) shows clear internal layering with normal grading of the volcaniclastic material. CompositionThe principal lithologic components of Subunit IB are terrigenous, volcanic, and biogenic in origin (see Site U1425 smear slides in “Core descriptions”). The major difference between the lithologies of Subunits IA and IB is the reduced occurrence of calcareous microfossils and the slightly higher contents of the biosiliceous fraction. Terrigenous materials compose the bulk (>80%) of Unit I sediment, which is dominated by clay. Volcanic glass usually occurs as a minor dispersed component (~5%) throughout the sediment. The biogenic fraction is generally low (<10%) in Subunit IB and is dominated by diatoms and sponge spicules with few calcareous microfossils. Unit II
Lithologies and structuresUnit II consists of clay, diatomaceous clay, and diatom ooze (Figs. F3, F4, F5). Pyrite and volcaniclastic materials represent a minor component throughout the succession. Discrete millimeter- to centimeter-scale tephra layers (vitric) are found throughout Unit II (a total of 18 tephra beds each with a thickness >0.5 cm). Although the majority of these layers are at most a few centimeters thick, some of the peaks in tephra thickness in Unit II (Fig. F7) are driven by rare but much thicker tephra deposits. This can be illustrated by a thick (37 cm) deposit of gray vitric tephra in interval 346-U1425B-28H-6, 61–98 cm. 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 that are related to the relative increase of the diatomaceous to terrigenous fraction downhole (see “Physical properties”). Furthermore, XRD analyses show a large increase in opal-A content (see “Bulk mineralogy”; Fig. F8). Unit II sediment is moderately to heavily bioturbated and is often mottled (Figs. F15, F16). The degree of bioturbation changes vertically. The diatom content and intensity of bioturbation are criteria used to further divide Unit II into Subunits IIA and IIB. Bulk mineralogyThe results of XRD analyses conducted on Hole U1425B sediment are listed in Table T2. In general, the bulk mineral composition of Unit II sediment is very similar to that of Unit I. The major difference is the disappearance of foraminifers and nannofossils in Unit II and the large increase in opal-A content (Fig. F8). The XRD results show mineral peak intensities of Subunit IIB sediment differ from Subunit IIA sediment and are more similar to those of Unit III. For example, the terrigenous minerals show much lower peak intensities in Subunit IIB and Unit III than Unit I and Subunit IIA (Fig. F8). This likely reflects the diluting influence of diatoms in units where diatom ooze dominates (Subunit IIB and Unit III). Subunit IIA
Lithologies and structuresSubunit IIA consists dominantly of brownish and greenish diatom-bearing and diatom-rich clay, as well as clay (Fig. F15). 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, sediments of this unit are heavily bioturbated (Fig. F17), leading to poor preservation of original sedimentary structures, which inhibits their recognition (e.g., color banding, laminae, etc.). Tephra layers intercalated in the diatom-bearing clay and clay sequence are a minor but common component of Subunit IIA. CompositionSubunit IIA is predominantly composed of a mixture of fine-grained material, mostly clay minerals and biosiliceous components (see Site U1425 smear slides in “Core descriptions”). The abundance of these components varies throughout this transitional unit, alternating with diatom ooze, clayey diatom ooze, diatom-rich clay, and clay. Subunit IIB
Lithologies and structuresSubunit IIB sediment is dominated by brownish diatom ooze with a few limited clay intervals. The abundance of diatoms and other siliceous components is key to the recognition of Subunit IIB (Fig. F16), and they typically comprise >70% (and up to 95%) 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 and distinctive mottling are also displayed in some sections (Fig. F17). Tephra layers (vitric and scoriaceous) and occasional individual pumice stones are a minor but common component of Subunit IIB. The thickest tephra layer, with a maximum thickness of 37 cm, occurs in the lower part of Subunit IIB (see discussion above). Most of the layers, however, have thicknesses <1 cm. Carbonate-rich zones in Hole U1425B are present as well-lithified, pale gray dolomite beds and concretions, with a 10 cm bed in the top of Section 346-U1425B-13X-1 (104.6–104.7 m CSF-A; Fig. F18A). Dolomitic concretions occur throughout Unit II, with 11 dolomite concretions identified in Hole U1425B and 5 dolomite concretions in Hole U1425D. Thin sections of the dolomite bed and concretions show the prevalence of pyrite within their structure along with dolomitized biosiliceous remains (Fig. F18). Carbonates also frequently occur as poorly lithified, yellowish brown to olive chalky patches or cements rather than as nodules or concretions. There are 27 and 16 such cements documented in Holes U1425B (~132–172 m CSF-A) and U1425D (130–210 m CSF-A), respectively. A number of slump folds and discordant bedding are seen in Sections 346-U1425D-9H-1 to 9H-2 (71.46–72.92 m CSF-A) (Fig. F19) and Section 10H-6 (88.8–89 m CSF-A). These sedimentary structures are only observed in Hole U1425D. The thickness of slumped material ranges from tens of centimeters to >1.5 m. The genesis of these slumped deposits remains unknown, but it is likely to be instability or slope-related phenomena, although other types of sediment flow, such as turbidites, are not common in Hole U1425D. CompositionThe major lithologies in Subunit IIB are dominated by biosiliceous components (>70% and up to 95%) from Sections 346-U1425B-11H-1 and 346-U1425D-11H-1 downward (i.e., the Subunit IIA/IIB boundary transition) (see Site U1425 smear slides in “Core descriptions”). Diatoms and siliceous sponge spicules are dominant in the biosiliceous fraction (Fig. F20), whereas radiolarians and silicoflagellates are found only in rare or trace amounts (1%–5%). These siliceous fossil assemblages characterize both the brownish and greenish sediment in the “diatom ooze” category. Scattered glauconite grains are occasionally observed in the diatom ooze between Samples 346-U1425B-11H-5, 75 cm, and 23H-A, 75 cm. Unit III
Lithologies and structuresUnit III consists of clay, diatom-rich clay, diatom ooze, and siliceous claystone (Figs. F3, F4). Unit III is distinguished from Unit II on the basis of sediment color and an increase in the terrigenous content of the sediment, though there is still significant numbers of diatoms. Unit III sediment is further divided into two subunits (IIIA and IIIB) based primarily on the degree of lithification. The upper sediment of Unit III is moderately to heavily bioturbated and often mottled, making changes between diatom ooze, diatom-rich clay, and clay difficult to distinguish. The diatom content (i.e., biogenic silica component) decreases at the deeper end of Subunit IIIA, and the indurated siliceous claystone of Subunit IIIB develops. Bulk mineralogyThere is very little variation in peak intensities of minerals in sediment from Units II to III (Fig. F8), except for the general decrease in halite intensity and gradually increasing pyrite content between ~90 and 340 m CSF-A. The peak intensity of a number of minerals, including clay minerals, quartz, feldspar, halite, and amorphous opal (opal-A), decreases abruptly at ~341 m CSF-A, coincident with the first downhole occurrence of siliceous claystone in the sediment sequence and indicating the diagenetically enhanced compaction of soft sediment to well indurated claystone (Figs. F3, F4). At the Subunit IIIA/IIIB boundary transition, smear slides show a sharp decrease in the amount of biosiliceous material within the sediment (from 70% to 0%) over a depth range of only a few centimeters (Fig. F21). An important change in the Subunit IIIB sediment is the distinct appearance of opal-CT at 378 m CSF-A. Opal-CT shows up as a large peak in intensity centered at 22° (Δ2θ) in the XRD diagram (Fig. F22), suggesting opal-A is recrystallizing into opal-CT by dissolution/reprecipitation reactions. However, opal-CT likely forms a distinct peak here because the siliceous content of the claystone at 378 m CSF-A depth is high. There are small but detectable amounts of opal-CT in XRD analyses as shallow as ~342 m CSF-A, with its peak intensifying downhole as the siliceous content of the claystone increases. Subunit IIIA
Lithologies and structuresSubunit IIIA sediment is composed of alternating layers of heavily bioturbated diatomaceous ooze, clayey diatomaceous ooze, and diatom-rich clay (Fig. F23). These alternating layers show decimeter- to meter-scale cycles of dark gray (diatom ooze, relatively clay poor) and gray (diatom-rich 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 observed in Unit III (Fig. F24). There are very few sedimentary structures within Subunit IIIA, although there is an excellent example of finely laminated diatom ooze that extends ~1.5 m in thickness and is observed in both Holes U1425B and U1425D (Fig. F25; 275 m CSF-A). These laminations (of approximately middle Miocene age) occur within a dark organic-rich layer in Subunit IIIA and are not seen elsewhere in the sediment sequence during the Miocene. In addition, there is evidence of slump folds from Core 346-U1425D-38H (Fig. F19; 294 m CSF-A). As before, their genesis is not known. CompositionThe principal lithology of Subunit IIIA is characterized by abundant diatoms (>75%) associated with the common occurrence of siliceous sponge spicules (5%–25%) and an increasing clay mineral and quartz content (5%–25%) compared to Unit II. Other siliceous components, such as radiolarians, remain rare (<5%), but organic matter, based on smear slides, increases to become more common. Toward the bottom of Subunit IIIA, the 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. Subunit IIIB
Lithologies and structuresSubunit IIIB sediment is characterized by well-lithified gray siliceous claystone (Fig. F26) with occasional parallel laminations (Fig. F25), burrows, and carbonate concretions that appear as layers and nodules. Recovery of Subunit IIIB sediment was limited, and therefore a detailed description of lithologic changes is difficult. However, the transition from Subunit IIIA to Subunit IIIB was captured and is characterized by a rapid decrease in the amount of biogenic silica (opal-A) in the sediment (Fig. F27). This diagenetic loss of biosiliceous material and the formation of siliceous claystone defines the Subunit IIIA/IIIB boundary at 341.3 m CSF-A in Hole U1425B and 343.91 m CSF-A in Hole U1425D. Biosiliceous-rich clay occurs in Section 346-U1425B-57X-1 (368.4–369.07 m CSF-A), which is immediately above a distinct increase in opal-CT as documented by XRD analysis (see “Summary and discussion”). CompositionThe principal lithology of Subunit IIIB is characterized by abundant clay minerals and quartz (i.e., falling between 25% and 75%) and the common occurrence of pyrite and organic matter (5%–25%). The amount of glauconite varies from rare (<5%) to common (5%–25%) throughout Subunit IIIB (Fig. F28). All biosiliceous material is absent from smear slides in this unit. Summary and discussionThe sedimentary sequence from Site U1425 records a history of terrigenous and pelagic sedimentation since the middle Miocene, as well as the diagenesis of silica-rich Miocene sediment and the lithification of clay at depth. The dominant lithology of Site U1425 is diatom ooze and diatom-rich clay, which extends from the late Miocene to early Pleistocene (Figs. F3, F4). Sedimentation is dominantly pelagic and/or hemipelagic, coupled with volcanic inputs. There is no evidence of significant turbidite inputs, but in Hole U1425D there are slump folds that likely represent downslope processes linked to either instabilities in the sediment plateau or tectonically triggered movement of sediment (Fig. F19). The Holocene to Middle Pleistocene sediment at Site U1425 shows alternating dark brown organic-rich (laminated) and greenish gray and light gray clay layers (Subunit IA), followed by the appearance of slightly bioturbated light greenish gray and light gray clay (Subunit IB) (Figs. F9, F11, F14). This pattern parallels observations from Sites U1422 (Figs. F8, F11 both in the “Site U1422” chapter [Tada et al., 2015c]), U1423 (Figs. F9, F11 both in the “Site U1423” chapter [Tada et al., 2015d]), and U1424 (Figs. F8, F12 both in the “Site U1424” chapter [Tada et al., 2015e]). However, it differs from Unit I at nearby Site 799 located in the Kita-Yamato Trough, which is characterized by coarse-grained and normally graded beds (fining upward) that have been interpreted as turbidite deposits (Shipboard Scientific Party, 1990). The centimeter- to decimeter-scale alternations in Subunit IA are thought to reflect millennial-scale variations associated with Dansgaard-Oeschger cycles, with organic-rich dark layers associated with interstadials over the last glacial cycle (Tada et al., 1999). Similar to Site U1424, the dark–light alternations show dark brown intervals that are often preceded by centimeter-scale gray clay that is slightly bioturbated with the underlying light greenish gray clay (Fig. F11). Tada et al. (1999) suggest that deposition of organic-poor gray clay occurs under oxic conditions, and the organic-rich dark brown layers are deposited under suboxic to euxinic conditions as a result of changes in ventilation of the marginal sea. The late Miocene to early Pleistocene sedimentation at Site U1425 corresponds to the deposition of Unit II, which is composed of moderately to heavily bioturbated diatom ooze and diatom-rich clay (Figs. F15, F16). This unit reflects significant pelagic and hemipelagic sedimentation at Site U1425 from the late Miocene until the early Pleistocene, whereas the high diatom abundance in the sediment indicates high biological productivity, especially in Subunit IIB. The dominance of diatoms, together with other biosiliceous components (i.e., sponge spicules and radiolarians), and the heavy bioturbation of the sediment suggests that bottom water of the marginal sea was well oxygenated and there was active circulation during at least the entire Pliocene (Tada, 1994). Bioturbation reaches a maximum in the upper part of Unit II (Subunit IIA; early Pleistocene), although the clay component increases, suggesting greater terrigenous sedimentation in the sea during this time. Middle to late Miocene sediment is characterized by diatom ooze, clayey diatom ooze, and bioturbated diatom-rich clay that alternate in layers (Subunit IIIA; Fig. F23). The color changes associated with these sedimentary variations, however, are subtle. The changing relative abundance of biosiliceous components to clay minerals defines this sedimentary unit; these oscillations in lithology are likely related to changes in bottom water oxygenation of the sea (Tada, 1994). Fluctuations in biogenic silica content during the middle to late Miocene have been previously interpreted in terms of changes in the flux of terrigenous material to the marginal sea (Tada, 1991, 1994). Under this scenario, bottom water becomes oxic during high influx of terrigenous material and becomes anoxic during periods of reduced terrigenous input. Tada (1991, 1994) suggests that oscillations in eustatic sea level during this period changed the delivery of terrigenous material to the sea, which occurs over periodicities similar to orbital cycles. In Subunit IIIB (middle Miocene) the diatom ooze/diatom-rich clay is replaced by siliceous claystone (Figs. F3, F4), the appearance of which defines the top of Subunit IIIB (Fig. F26) and marks a diagenetic transition from high biogenic silica content to siliceous content (Fig. F21). This lithologic change is supported by the start of a significant decrease in silica concentration in interstitial water at this time (see “Geochemistry”) and a switch to higher sediment resistivity suggesting decreasing biogenic silica content in the downhole log of Hole U1425B (see “Downhole measurements”). Therefore, Subunit IIIB signifies the start of the diagenesis of diatoms and the formation of siliceous claystone. During burial, opal-A undergoes dissolution and reprecipitation as opal-CT (Murata and Nakata, 1974). Within Subunit IIIB, opal-CT starts to precipitate at ~342 m CSF-A and with increasing depth becomes the main phase of opal, reaching a distinct peak at ~378 m CSF-A in Hole U1425B (Fig. F22). Tada and Iijima (1992) suggest that dissolution of diatom frustules (opal-A) occurs within a few meters of the shallowest occurrence of opal-CT. Therefore, the dissolution of biogenic silica and the formation of siliceous claystone mark the transition from Subunit IIIA to Subunit IIIB. The last biosiliceous-bearing clay occurs in Section 346-U1425B-57X-1 (at the top of Subunit IIIB), followed by the detection of opal-CT in XRD analyses at ~342 m CSF-A. However, there is not a distinctive peak in opal-CT until Section 58X-CC at ~378 m CSF-A (Fig. F22), likely a result of the change in siliceous content of the lithology. Opal-A and opal-CT can coexist within a stratigraphic interval (Tada and Iijima, 1992), and therefore a transitional zone between the lower opal-A zone (i.e., disappearance of diatom frustules) and the main opal-CT phase is not unexpected. Figure F22 shows a downhole XRD profile for Hole U1425B and discrete samples from the top of Subunit IIIB. It shows there is no clearly defined opal-CT peak in Section 346-U1425B-57X-1 (which may be related to the low siliceous content), but there is a clear opal-CT peak in the sample from Section 58X-CC, suggesting the boundary between opal-A and opal-CT is likely to be between ~340 and 352 m CSF-A. Tada (1994) defined a Miocene Unit 4 in the marginal sea, the upper portion of which is characterized by alternation of dark gray to black chert and light gray siliceous claystone. The top of this unit was found to be deeper than the first recorded appearance of opal-CT in sites drilled during ODP Leg 127. At Site U1425, there is no clear indication from the recovered lithologies or from downhole logs that this Unit 4 has been reached; therefore, our Subunit IIIB is defined to the base of the hole. Interestingly, Tada (1994) noted that Unit 4 recovery was quite poor because of the presence of chert. In Hole U1425D, drilling deeper than ~403 m CSF-A yielded only a highly disturbed slurry of coarser materials described as drilling breccia. Despite there not being an obvious change in lithology, this decrease in recovery quality near the base of the hole suggests that penetration at this site may have fallen just short of reaching the Subunit 4A previously recognized by Tada (1994). In summary, the changes in sedimentation observed at Site U1425 since the Miocene likely reflect a combination of effects from climate oscillations, eustatic sea level changes, and volcanic processes in this marginal sea region (Tada, 1994). Site U1425 also records the volcanic history of the Japan and East Asian continent, as shown by the numerous tephra layers found throughout the sedimentary succession (Fig. F8). These environmental changes are recorded in Holes U1425B, U1425D, and U1425E (Fig. F6), although the attenuation of Subunit IIA in Hole U1425D is a consequence of the slump folds in the sediment succession. Further shore-based research will help to decipher the relative roles of these and other processes on the sedimentation history of Site U1425. |
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