| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.346.105.2015 LithostratigraphyDrilling at IODP Site U1424 penetrated to a maximum subbottom depth of 158.8 m in Hole U1424A, recovering a total of 161 m of sediment for a recovery rate of 101%. The shipboard lithostratigraphic program involved detailed visual assessment of sediment composition, color, sedimentary structures, and bioturbation intensity, supplemented by petrographic analysis of smear slides (64 from Hole U1424A, 48 from Hole U1424B, and 7 from Hole U1424C) and bulk mineralogic analysis by X-ray diffraction (XRD) (22 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 U1424 extends from the Pliocene to Holocene and is dominated by clay and diatom ooze. Volcaniclastic material represents a minor component throughout the sediment succession, except where concentrated in tephra (i.e., volcanic ash) layers. The recovered sediment is divided into two major lithologic units (I and II, following Tada [1994]) based on sediment composition, especially the biosiliceous fraction content. Units I and II are each further divided into two subunits. The character of the sedimentary physical properties, including natural gamma radiation (NGR), magnetic susceptibility, color reflectance parameters, and density, reflects the distribution of the various sediment components and lithologies (see “Physical properties”). The major characteristics of the sedimentary sequence at Site U1424, together with some of these additional properties, are summarized in Figures F2, F3, and F4, whereas the between-hole correlation of sedimentary units is shown in Figure F5. Note that for Hole U1424C, the top three cores were immediately sealed upon recovery for OSL dating and hence not described. Unit I
Lithologies and structuresUnit I consists of Holocene to early Pleistocene clay with small amounts of diatom-bearing, diatom-rich, foraminifer-bearing clay and rare (nonbiogenic) calcareous layers (Figs. F2, F3, F4). Pyrite and volcaniclastic materials represent a minor component throughout the sediment succession. Numerous discrete millimeter- to centimeter-thick tephra layers (vitric and scoria) are described throughout Unit I (total of 132 tephra beds, including 72 tephra beds >0.5 cm thick; Fig. F6). The most distinguishing sedimentary feature of Unit I sediment is the alternating decimeter-scale color-banded bedding, which 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 as well as the intensity of bioturbation are used as criteria to divide Unit I into Subunits IA and IB. Bulk mineralogyThe XRD analysis results are listed in Table T2. In general, Unit I sediment at Site U1424 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 was not detected in the measured samples but is sparsely present (foraminifers and nannofossils) in some layers within the upper 28 m of Unit I, as observed in smear slides. Figure F7 shows the downcore variations in peak intensity of the identified minerals at Site U1424. In general, quartz, plagioclase, smectite, illite, kaolinite and/or chlorite, and K-feldspar contents show a long-term trend toward increasing counts. Peak heights of these minerals tend to be higher in Unit I and lower in Unit II, which is similar to observations at the other two Japan Basin sites (Sites U1422 and U1423). Peak heights of opal-A are generally much lower in Unit I and higher in Unit II. The peak intensity of pyrite at 20.56 m CSF-A in Hole U1424A is very high. Halite is always present in samples with a higher intensity in Unit II and lower intensity in Unit I. Subunit IA
Lithology and structuresSubunit IA consists dominantly of clay with subordinate amounts of diatomaceous (i.e., diatom bearing and diatom rich) and foraminifer-bearing clay. Tephra layers intercalated in the clay sequence are a minor but common component of Subunit IA. Subunit IA is characterized by decimeter-scale alternations of light and dark colored sediment intervals (Fig. F8), which show up clearly in the L*, a*, and b* records (see “Physical properties”). The light colored intervals are mainly composed of light greenish gray clay with some diatoms. Within these intervals, millimeter- to centimeter-scale layers of gray, dark greenish gray, and very dark gray clays are observed. The light 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 described throughout the light intervals. Detailed examination of these layers reveals that they are composed of clay minerals. Some of them show abundant amounts of pyrite (sometimes visible to the naked eye). The contrasting dark layers that dominate Subunit IA correspond to dark grayish brown organic-rich clay intervals with some foraminifers and pyrite throughout. Although some of these intervals show evidence of slight disturbance from burrowing, the dark layers are mostly finely laminated with no apparent bioturbation. Foraminifers (mostly planktonic) are generally restricted to millimeter-scale (foraminifer-bearing) yellowish layers (Fig. F9). These yellowish layers are not necessarily present in all dark intervals. The lower contacts of the dark layers within Subunit IA are mostly sharp (i.e., not bioturbated), whereas the upper contact with the light greenish gray intervals usually appears gradual because of bioturbation (at the time of deposition of the overlying light greenish gray clay). Most of the dark brown intervals are underlain by centimeter-scale gray clay (e.g., Sections 346-U1424B-2H-3, 2H-6, and 3H-3; Fig. F10). 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 gray clay layers do not result from the mixing of the dark brown and greenish gray intervals but constitute an additional facies. These deposits probably reflect transitional depositional conditions between those prevailing during deposition of the organic-poor, light greenish gray and the organic-rich, dark brown intervals. Tephra layers in Subunit IA are mostly centimeter-thick deposits interbedded within the light greenish gray and dark brown/grayish brown clay intervals. The number of tephra layers per core with a thickness >0.5 cm is highest in Subunit IA (Fig. F6; Table T3). Tephra are mostly white and light gray in color (i.e., vitric) but some very dark to black tephra (i.e., scoriaceous) are also described. We tested the potential of using the L* data from the color spectrophotometer as an indicator for tephra occurrence in Hole U1424B. When only L* values ≥44 are considered (i.e., the brightest sediment layers), a good match is found between the number of tephra layers and the L* data (Fig. F6). The highest peaks in L* occur at ~40 m CSF-A, consistent with a high cumulative tephra thickness and the largest number of individual thick tephra layers. Between ~90 and 130 m CSF-A, only a few high L* values ≥44 occur. This is also consistent with the relatively small number of tephra layers in this interval. The results of this simple test suggest that high L* values can be used as a guide to identifying tephra layers at this site and potentially others. This technique works well for Site U1424 because the major lithologies are carbonate poor and greenish to brownish colors dominate. Thus, highly reflective layers like whitish tephra layers stand out from the darker background color and can be identified by measured L* profiles. Core 346-U1424B-7H is an exception because it contains an interval with pale yellow color that is rich in carbonate content. Black or dark colored scoriacious tephra layers with low L* values would not be detected by this method. CompositionThe principal lithologic components of lithologies in Subunit IA are terrigenous, volcanic, and biogenic (see Site U1424 smear slides in “Core descriptions”). The 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 observed at the sediment surface as well as in the smear slides (Sample 346-U1424C-4H-3, 78 cm; Fig. F11). Volcanic glass and pumice account for nearly 100% of the tephra layers, even though sometimes the volcanic material appears to be mixed with a siliciclastic component. Diatoms and siliceous sponge spicules dominate the biogenic fraction with some radiolarian-enriched levels. Calcareous nannofossils and foraminifers (mostly planktonic) can be abundant (up to 30%) in the dark brown layers (and in the yellowish laminae in particular). A significant biogenic component is found at levels in the light greenish sediment, made of planktonic foraminifers mixed to high abundance (~60%) of microcrystalline calcite (micrite). Subunit IB
Lithology and structuresSubunit IB is transitional downward from Subunit IA and is identified by a decrease in the frequency of dark and light color alternation and the dominance of light greenish gray and light gray clay (Fig. F12). Subunit IB consists dominantly of clay with subordinate amounts of diatomaceous clay. 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. Olive-gray clay intervals are particularly well expressed in Core 346-U1424A-7H. Contrary to Subunit IA, some distinct centimeter-scale layers of pale yellow carbonate-rich (calcite) sediment are described in Subunit IB (Sections 346-U1424A-6H-1, 6H-3, and 6H-4). Bioturbation increases gradually with depth, and sediment mottling and disruption of laminae and color banding is more prevalent. Tephra layers intercalated in the clay sequence are a minor but common component of Subunit IB. As described in Subunit IA, tephra layers are mostly centimeter-scale light (vitric) deposits. A 4 cm thick dark tephra (scoria) is observed in Subunit IB (e.g., interval 346-U1424B-7H-6, 118–122 cm). 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. The latter are interpreted as tephra layers that have been remobilized and emplaced as turbidite deposits (e.g., Section 346-U1424A-6H-5; Fig. F13). CompositionThe principal components of the lithologies in Subunit IB are terrigenous, volcanic, and biogenic in origin (see Site U1424 smear slides in “Core descriptions”). The major difference between the lithologies of Subunits IA and IB is the reduced occurrence of calcareous microfossils and slightly higher contents of the biosiliceous fraction. Terrigenous materials compose the bulk (>80%) of Unit I sediment, which is dominated by clay. Clay minerals are found to be abundant, whereas the lithics are generally rare. Very light gray clay beds are frequent in Subunit IB and made of microcrystalline calcite (from 50% to 100%). Volcanic glass usually occurs as a minor dispersed component (~5%) throughout the sections. A very fine grained tephra layer (clay size) is observed in Sample 346-U1424A-6H-6, 102 cm. The biogenic fraction is generally low (<10%) in Subunit IB and is dominated by diatoms and sponge spicules, with few calcareous microfossils. Unit II
Lithology and structuresUnit II consists of early Pleistocene to Pliocene clay, diatomaceous clay, and diatom ooze (Figs. F2, F3, F4). Pyrite and volcaniclastic materials represent minor components throughout the succession. Discrete millimeter- to centimeter-scale tephra layers (vitric) are found throughout Unit II (89 tephra beds, including 30 tephra beds >0.5 cm thick). Some peaks in tephra thickness in Unit II indicate that rare but thick tephra deposition occurred during the deposition period of Unit II (Fig. F6). Unit II is distinguished from Unit I on the basis of the sediment color and a significant increase in diatom content relative to terrigenous sediment from top to bottom. This lithologic change is supported by NGR measurements, which show lower values in Unit II than in Unit I that are likely related to the increasing/decreasing content of the diatomaceous/terrigeneous fraction downhole in the sedimentary sequence. Sediment of Unit II is moderately to heavily bioturbated and is often mottled. The degree of bioturbation changes vertically. A few turbidite deposits are observed throughout Unit II. The diatom content and the intensity of bioturbation are the criteria used to further divide Unit II into Subunits IIA and IIB. Bulk mineralogyThe results of XRD analyses conducted on Hole U1424A sediment are listed in Table T2. In general, the bulk mineral composition of Unit II sediment is similar to that of Unit I. The major difference is the higher opal-A peak height, the higher peak intensity of halite, and the occurrence of dolomite in some concretions in Unit II. Figure F14 shows the image of one section (346-U1424A-13H-5A) with a representative concretion (dolomite). The sediment scratched from the surface of the concretion consists of very fine (~2–4 µm) dolomite grains (see Fig. F15 observed by smear slide), which was confirmed by XRD (Fig. F16). This dolomite may suggest that biogenic nannofossils and/or foraminifers have been affected by diagenesis (dolomization), at least in this horizon. Subunit IIA
Lithology and structuresSubunit IIA consists dominantly of brownish and greenish diatom-bearing and diatom-rich clay, as well as clay with a few turbidite beds (Figs. F13, F17). This subunit is considered transitional to the underlying Subunit IIB, which is defined by the consistent appearance of diatom ooze in the section. Subunit IIA sediment is heavily bioturbated. This bioturbation leads to poor preservation of the original sedimentary structures, which inhibits their recognition (e.g., color banding, laminae, etc.), with the exception of some fine-grained turbidite deposits within the diatom-bearing and diatom-rich clays (e.g., Sections 346-U1424B-8H-4 and 8H-7). Each turbidite bed is a few centimeters thick. The turbidite deposits show homogeneous centimeter-scale olive-gray clay intervals overlying millimeter- to centimeter-scale silty to sandy beds composed mainly of volcaniclastic particles. The latter intervals show sharp erosional basal contacts, and parallel and/or cross lamination are observed (Fig. F13). Tephra layers, intercalated in the diatom-bearing clay and clay sequence, are a minor but common component of Subunit IIA. CompositionSubunit IIA is dominated by fine-grained material, mostly clay minerals (see Site U1424 smear slides in “Core descriptions”). Biosiliceous components (diatoms, siliceous sponge debris, radiolarians, and silicoflagellates) are present but occur in low abundances (5%–10%). Subunit IIB
Lithology and structuresSubunit IIB consists dominantly of brownish and greenish diatom-bearing clay and diatom ooze, with a few clay intervals corresponding to fine-grained turbidite beds (in the upper part of the subunit in particular, see Section 346-U1424A-9H-5; Figs. F13, F18). The abundance of diatoms and other siliceous components is key in the recognition of Subunit IIB, which typically comprises >70% 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 Subunit IIB sediment. Bioturbation is moderate to heavy and distinctive mottling is displayed in some sections (Fig. F18). Tephra layers (vitric and scoriaceous) intercalated in the diatom-bearing clay and diatom ooze are a minor but common component of Subunit IIB. The thickest tephra layer, with a maximum thickness of 15 cm, occurs in the lower part of Subunit IIB (interval 346-U1424A-13H-6, 143–158 cm; 120.06–120.21 m CSF-A). Most of the layers, however, had a thickness of <1 cm. A characteristic tephra containing “bubble-junction” type glass shards was found at 138.37–138.46 m CSF-A (interval 346-U1424A-15H-6, 57–66 cm). This distinctive type of glass shard is known for the Znp-Ohta tephra in Pliocene sequences in central Japan. Thus, there is a possibility that this marine tephra can be correlated to the Znp-Ohta tephra. This tentative correlation will be tested by shore-based petrographic and geochemical analyses. A large and abrupt NGR offset is observed near the base of the stratigraphic succession, at 138.08 m CSF-A in Hole U1424A (Sample 346-U1424A-15H-6, ~28 cm) and at 133.24 m CSF-A in Hole U1424B (Sample 346-U1424B-15H-5, ~102 cm). The NGR offset corresponds to a centimeter-scale tephra layer that represents a significant boundary between dark gray bioturbated sediment (above) and brown sediment (below). This unusually rapid change in color and the associated NGR offset could potentially reflect a discontinuity in the sediment record (genetically related to the tephra layer?). CompositionThe major lithologies in Subunit IIB are dominated by biosiliceous components (>70% from Section 346-U1424A-9H-4 downhole [i.e., the Subunit IIA–IIB transition]) (see Site U1424 smear slides in “Core descriptions”). Diatoms and siliceous sponge spicules are dominant in the biosiliceous fraction, whereas radiolarians and silicoflagellates are found only in rare or trace amounts (1%–5%). These siliceous fossil assemblages characterize both the brownish and greenish color sediment in the “diatom ooze” category. Scattered glauconite crystals are occasionally observed in the diatom ooze (Fig. F19). Summary and discussionThe sedimentary succession at Site U1424 records a history of terrigenous sedimentation since the Pliocene (~4.7 Ma), with the sequence showing an uphole decrease of the biosiliceous fraction (diatoms in particular) throughout the stratigraphic succession. The sedimentation at Site U1424 is largely dominated by hemipelagic and pelagic processes (Figs. F8, F12, F17, F18), although sedimentologic evidence also indicates occasional downslope (i.e., turbiditic) (Fig. F13) and volcanic processes. The deposition of Unit II spanned the Pliocene through early Pleistocene. Unit II sediment is mainly composed of moderate to heavily bioturbated clays and diatom ooze (Fig. F17, F18). This composition reveals the predominance of both pelagic and hemipelagic sedimentation at Site U1424 during the Pliocene–early Pleistocene period, whereas the high content of diatoms (in Subunit IIB in particular) suggests high biological productivity at that time. Active circulation and oxygenation of the bottom water likely prevailed, as indicated by the high intensity of bioturbation. Bioturbation at Site U1424 reaches a maximum in the upper part of Unit II (i.e., Subunit IIA; Fig. F17; late Pliocene–early Pleistocene), accompanying a significant decrease in diatom content relative to terrigenous sediment. The latter indicates a significant change in environmental conditions at Site U1424 during the late Pliocene–early Pleistocene. Bioturbation inhibits the recognition of former sedimentary structures (e.g., color banding, laminae, etc.) and associated sedimentary processes in Subunit IIA. However, some fine-grained turbidite deposits are described within the heavily bioturbated diatom-bearing and diatom-rich clay (Fig. F13), suggesting an increase in downslope processes at Site U1424 in the late Pliocene–early Pleistocene. Site U1424 is located west of the Sado Ridge, and as observed for Site U1422, the onset of turbidite deposition at that time is potentially related to the initiation of subduction in the region during the early Pleistocene (Tamaki et al., 1992). In such a case, the turbidite deposition may reflect sediment destabilization linked to tectonic movements (e.g., earthquakes) along the Sado Ridge. Nonetheless, these sediments represent a minor component in the sedimentary succession. Sedimentation at Site U1424 greatly changed in the early Pleistocene, with the deposition of slightly bioturbated light greenish gray and light gray clay (Subunit IB; Fig. F12) followed by the progressive appearance of the dark brown organic-rich (laminated) layers during the Middle and Late Pleistocene (Subunit IA; Fig. F8). This pattern parallels previous observations from Site 794, as well as from Sites U1422 (Figs. F7, F10 both in the “Site U1422” chapter [Tada et al., 2015c]) and U1423 (Figs. F8, F10 both in the “Site U1423” chapter [Tada et al., 2015d]). For the last glacial cycle, the centimeter- to decimeter-scale alternations in sediment color reflect millennial-scale variations associated with Dansgaard-Oeschger cycles, with each dark layer appearing to correspond to an interstadial (Tada et al., 1999). Detailed examination of the sediment immediately below the dark brown layers typically reveals a centimeter-scale gray clay interval that shows evidence of slight bioturbation and upward mixing of the light greenish gray clay that forms the light colored end member of the alternating light–dark color pairs (Fig. F10). This probably indicates transitional depositional conditions between deposition of the organic-poor, light greenish gray intervals (inferred oxic conditions; Tada et al., 1999) and the organic-rich, dark brown intervals deposited during suboxic to fully euxinic conditions (Tada et al., 1999). Further shore-based research will test this hypothesis. Overall, the changes in sedimentation observed at Site U1424 since the Pliocene reflect the combined effects of climate oscillations, eustatic sea level changes, and local tectonism in the sea (Tada, 1994). Site U1424 also records the volcanic history of the Japanese Islands and the East Asian continent as shown by the numerous tephra layers found throughout the sedimentary succession (Fig. F6). These environmental changes are recorded in Holes U1424A–U1424C (Fig. F5), and further shore-based research will help decipher the role of climate, sea level, and tectonic changes on sedimentation at Site U1424. |
|||
