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doi:10.2204/iodp.proc.346.104.2015 LithostratigraphyDrilling at Site U1423 penetrated to a maximum subbottom depth of 249.1 m in Hole U1423B, recovering a total of 250 m of sediment for a recovery rate of 100%. The shipboard lithostratigraphic program involved detailed visual assessment of sediment composition, color, sedimentary structures, and bioturbation intensity, supplemented by petrographic analysis of smear slides and bulk mineralogic analysis by X-ray diffraction (XRD). These were used to describe and define facies and facies associations in each hole. A total of 61 smear slides from Hole U1423A, 66 smear slides from Hole U1423B, and 12 smear slides from Hole U1423C were made and examined to help determine lithologic names. A total of 25 samples were selected for XRD analysis. The major characteristics of the sedimentary sequence at Site U1423 are summarized in Figures F2, F3, and F4, and the stratigraphic correlation between the three holes is shown in Figure F5. The sedimentary succession recovered at Site U1423 extends from the Pliocene to Holocene and is dominated by clay, silty clay, and diatom ooze with discrete foraminifer-bearing clay levels. Volcaniclastic material represents a minor component throughout the sediment succession, except in tephra layers where it is the dominant component. The section is divided into two major lithologic units (I and II), distinguished on the basis of sediment composition, referring particularly to the biosiliceous fraction content. Unit I is further divided into two subunits based on the occurrence of alternating dark and light color variations and the intensity of bioturbation. 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”). Unit I
Lithologies and structuresUnit I consists of Holocene to early Pleistocene silty clay and clay with lesser amounts of diatom-bearing and diatom-rich silty clay, as well as rare calcareous layers containing abundant foraminifers. Discrete tephra (volcanic ash) layers ranging in thickness from a few millimeters to >10 cm are numerous. The total thickness of tephra layers in each core (Fig. F6; Table T2) reaches a broad maximum in lower Subunit IA but with high thickness totals also recorded in discrete intervals of Subunits IIA and IIB. Pyrite can be found as a minor component in most lithologies, whereas fine-grained tephra occurs as a dispersed component throughout much of the section based on smear slide analysis. Unit I is characterized primarily as representing fine-grained material derived from terrigenous sources. Color banding suggested to be related to variable content of organic matter and pyrite are the most diagnostic features of Unit I, with dark, organic-rich intervals (dark gray to dark olive-gray) interspersed between lighter colored, organic-poor intervals (light green to light greenish gray). The relative frequency of these color alternations as well as the intensity of bioturbation are the criteria used for recognizing and distinguishing between Subunits IA and IB. Bulk mineralogyThe results of XRD analysis are listed in Table T3. In general, Pliocene–Pleistocene 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 (mostly foraminifers and nannofossils) is present in the upper 46 m of Unit I, reaching a maximum at 36.71 m CSF-A in Hole U1423A. Figure F7 shows the downhole variations in peak intensity of the identified minerals at Site U1423. In general, the contents of quartz, plagioclase, smectite, illite, and kaolinite and/or chlorite show a long-term trend toward increasing counts toward the top of the sequence. In general, the peak heights of these minerals are higher within Unit I and typically lower in Unit II. In contrast, K-feldspar generally increases uphole throughout Unit II and Subunit IB but then fluctuates markedly in Subunit IA sediment, with several pronounced minima where K-feldspar contents drop to near zero. Peaks of opal-A are generally much lower in Unit I and higher in Unit II. The peak intensity of pyrite at 2.5 m CSF-A in Hole U1423A is very high. Halite, presumably precipitated from pore water, is present in all samples with higher intensities in Unit II and lower intensities in Unit I. Subunit IA
Lithology and structuresSubunit IA is primarily recognized by having a much higher frequency of dark layers (Fig. F8). It is also characterized by low L*, a*, and b* values in color reflectance and high NGR values with significant fluctuations. Subunit IA is composed mainly of silty clay and clay with subordinate amounts of diatomaceous clay and foraminifer-bearing silty clay. Bioturbation is moderate to intense throughout this subunit and is most noticeably observed at the interface between lighter colored intervals and darker colored beds. Two types of dark intervals can be recognized based on the color shade. Somewhat lighter brown layers are affected by slight bioturbation at various levels, which possibly hides any original lamination. Dark brown layers, on the other hand, typically show fine parallel lamination, frequently highlighted by the presence of biogenic carbonate (mostly planktonic foraminifers), which is light in color and visible to the naked eye. The lower contact of the darker brown layers is typically sharp, whereas the upper contact often appears to be gradual at the transition to the light gray-greenish bed. In some cases, the opposite pattern is observed where the top contact of a dark brown layer is sharp and the dark color fades gradually downhole (see Fig. F8, Section 346-U1423B-5H-3 for examples). Among the dark colored layers, a very dark greenish brown bed is observed in interval 346-U1423B-5H-6, 30–123 cm. It appears to be different compared to other dark colored beds because of its considerable thickness (~100 cm) and very dark color. This interval is finely laminated with marked color banding (very dark gray and lighter gray alternation) visible in its upper portion. A peculiar characteristic of the very dark greenish interval at 346-U1423B-5H-6, 30–123 cm, is that it contains abundant small pyrite framboids (see “Composition”) distributed throughout the layer, which is otherwise composed of terrigenous components and clay minerals. Diatoms and sponge spicules are abundant in this layer, but no biogenic carbonate is found. Tephra layers are a minor but common component in Subunit IA. An especially prominent tephra layer is found in interval 346-U1423B-8H-4, 50–60 cm. This 10 cm thick tephra is fine grained and white in color and consists of silt-sized volcanic glass and crystals. The volcanic glass contains semiregular shapes with sharp borders on the fragments mixed with rectangular, subparallel pipe vesicles. The volcanic material does not show any trace of postdepositional alteration. CompositionThe principal lithologic components in Subunit IA are terrigenous, volcanic, and biogenic in origin (see Site U1423 smear slides in “Core descriptions”). Diatoms dominate the biogenic fraction, with a minor component of calcareous microfossils, mostly foraminifers and nannofossils (Fig. F9). Terrigenous components in this subunit are dominated by clay and fine silty clay fractions. Volcanic glass and pumice account for nearly 100% of the tephra layers. In the upper part of Subunit IA, the light greenish gray intervals are mostly composed of siliciclastic fine-grained material (up to 80%) dominated by clay minerals. A very low biogenic component is observed. Conversely, the dark brown laminated beds are relatively rich in biosiliceous microfossils and biogenic carbonates. Planktonic foraminifers are observed, but the biogenic carbonate fraction is estimated to be no greater than 20%. Subunit IB
Lithology and structuresSubunit IB sediment is dominated by silty clay that contains variable amounts of biosiliceous material (mostly diatoms and siliceous sponge debris). The frequency of the dark and light color alternation is considerably less pronounced than in Subunit IA (Fig. F10). Bioturbation increases gradually with depth, and sediment mottling and disruption of laminae and color banding is more prevalent. A subtle decrease in NGR values in Subunit IB sediment suggests decreasing amounts of organic matter and/or clay mineral content relative to Subunit IA. CompositionThe principal lithologic components in Subunit IB are terrigenous, volcanic, and biogenic materials (see Site U1423 smear slides in “Core descriptions”). The major difference between lithologies of Subunits IA and IB is the less frequent occurrence of calcareous microfossils and slightly greater contents of the biosiliceous component (mostly diatoms and sponge spicules) in Subunit IB than in Subunit IA. In Subunit IB, terrigenous materials are the major component (>80%) of the sediment, and they are dominated by clay and fine silty clay fractions. The biogenic fraction is generally low (<10%) in Subunit IB and is dominated by diatoms and sponge spicules with fewer calcareous microfossils (mostly rare nannofossils). Volcanic glass usually occurs as a minor dispersed component (~5%) throughout the sections. Unit II
Lithology and structuresUnit II is dominantly composed of moderate to heavily bioturbated diatomaceous silty clay and clay and diatom ooze. Unit II is distinguished from Unit I on the basis of a significant increase in diatom content relative to terrigenous sediment from top to bottom. Color banding is less common in Unit II than in Unit I and nearly disappears in the lowest part of the unit. Sediment of Unit II is moderately to heavily bioturbated and often shows a mottled facies (Figs. F11, F12). In contrast to Site U1422, turbidites are rarely observed in Unit II sediment at this site. Tephra beds are frequently found in Subunit IIB (Table T2), but their numbers decrease significantly downhole even if single beds are generally much thicker than in Unit I. Rare but thick tephra deposits strongly contribute to the total thickness peaks of tephra in Unit II (Table T2; Fig. F6). CompositionThe major lithologies in Unit II are dominated by fine-grained material (see Site U1423 smear slides in “Core descriptions”). In Core 346-U1423A-10H and 11H smear slides of Subunit IB, the dominant fine-grained component is clay sized, in which abundant clay minerals and quartz are commonly found. Biosiliceous components are present in these cores but occur in low abundances (5%–10%). Biosilica rapidly becomes more abundant (>70%) in smear slides from Core 12H downhole, and this increase is adopted as the boundary between Unit I and II sediments. Diatoms and siliceous sponge spicules dominate the biosilica fraction in Unit II, whereas radiolarians and silicoflagellates are found only in rare or trace amounts (1%–5%). The above-mentioned siliceous fossil assemblages are observed both in the brownish and greenish colored sediment in the “diatom ooze” category. Dispersed pyrite is found mostly in brownish intervals, but the total amount of pyrite (framboids and/or irregular masses) and other opaque minerals is not particularly abundant (<5%) except in one dark layer found at Section 346-U1423A-12H-4, 2 cm. Here, pyrite is abundant (~10% of the bulk sediment). Tephra layers that are several centimeters thick were found in Unit II. The volcaniclastic layer found in interval 346-U1423A-8H-7, 46–48.5 cm, is unusual. It shows normal grading, and the bottom layer (4 mm) yields coarse sand-sized pyroclastic material (interpreted to be ballistic debris from eruptions), which is opaque under transmitted light microscope analyses. Another noteworthy tephra is observed in interval 20H-6, 74–93 cm. This volcanic deposit is coarse grained (sand size) and contains abundant biotite flakes and bubble-wall shards with cuspate and lunate forms (Fig. F13). In general, tephra beds consist of pristine volcanic glass shards. Bulk mineralogyThe results of XRD analyses conducted on Hole U1423C sediment are listed in Table T3. In general, the bulk mineral assemblage of Unit II sediment is similar to that of Unit I. The major difference is the higher opal-A peak intensities and the disappearance of the calcite peak in samples of Unit II. Figure F7 indicates a slight downhole decrease in quartz, plagioclase, illite, kaolinite and/or chlorite, and pyrite contents, countered by the significant increase in opal-A and halite. Subunit IIA
Unit II is divided into two subunits (IIA and IIB) with Subunit IIA considered to be somewhat transitional between the silty clay of Unit I and the ubiquitous diatom ooze of Subunit IIB. Subunit IIA consists of diatom-rich clay (Fig. F11), but compared to Subunit IIB, the siliciclastic fraction still remains dominant. NGR values in Subunit IIA gradually decrease with respect to Unit I as the biosiliceous component rises but the values remain highly variable (Fig. F14). Subunit IIB
Subunit IIB is composed of diatom ooze and clayey diatom ooze (Fig. F12). The abundance of diatoms and other siliceous components is uniformly high throughout Subunit IIB (90% and above) and is recorded by reduced variability and a significant decrease in NGR values. The siliciclastic fraction is a minor component and usually includes abundant clay minerals and rare quartz fragments. It may be mixed with vitric tephra and sparse pyrite. Bioturbation is moderate to intense, and distinctive mottling is displayed in some sections. DiscussionOverall, the sedimentary succession at Site U1423 records a history of terrigenous and biosiliceous deposition since the Pliocene. Unit II sediment, which is late Pliocene to early Pleistocene in age, is characterized by dominantly diatom rich strata, as compared to Unit I, possibly corresponding to a period of elevated biological productivity and good overall circulation in the basin. In contrast to the counterpart Unit II sediment at Site U1422, no turbidites were identified at this site. This lack of turbidite deposits at Site U1423 has allowed further division of Unit II sediment into Subunits IIA and IIB, which extends beyond the original lithostratigraphy described at nearby ODP Site 794 (Tamaki, Pisciotto, Allan, et al., 1990; Tada and Iijima, 1992; Tamaki et al., 1992; Tada, 1994). Tada and Iijima (1992) and Tada (1994) later refined and completed the lithostratigraphy of sedimentary units recognized at Site 794, but nonetheless retained only a single, undifferentiated Unit II. Visually, there is no clear characteristic allowing straightforward division of Unit II into Subunits IIA and IIB, but the decrease in NGR values at the transition between Units I and II and the further decline at the top of Subunit IIB appear to be significant. This decrease is mainly due to a drop in uranium content and thus probably reflects the overall decrease in organic matter content (Fig. F14) in Unit II (see “Geochemistry”). However, the decrease in organic matter is itself accompanied by a concomitant increase in diatom content, which suggests that the low NGR values in Unit II are driven by dilution from the high input and accumulation of biogenic opal. The siliceous biogenic component remains relatively low in the transitional Subunit IIA except for a very few horizons, whereas Subunit IIB mainly comprises of diatom ooze. The occurrence of authigenic pyrite is frequent throughout the sediment succession and is found most abundantly in the brownish and dark brown layers in both Units I and II. The size and distribution of this mineral is of great interest for determining the transition from oxic to suboxic/euxinic conditions in the basin, as well as the water column stratification. Pyrite is ubiquitous in modern anoxic sediment and preserved in many ancient sediment rocks. Small (<10 µm) scattered framboids can be considered a reliable marker of low oxygen conditions within the water column (Wilkin et al., 1996, 1997; Wilkin and Arthur, 2001), as previously observed in dark layers deposited during the last glacial in the marginal sea (Masuzawa and Kitano, 1984). On the other hand, the diagenetic pyrite is commonly found in discrete layers (few millimeters thick) and is composed of larger (~100 µm), sometimes euhedral, crystals. More than 100 visible tephra beds were observed in cores from Hole U1423A (Table T2). The thickest tephra layer (maximum thickness = 17 cm) occurs in Subunit IIB, although most of the layers were <1 cm thick. The total thickness of tephra layers in each core shows a peak in the transition from the lower part of Subunit IA to Subunit IB (Fig. F6). Tephra occurrence decreased remarkably in Unit II, although a few thick tephra deposits strongly contribute to the total thickness peaks in Unit II. Some especially distinctive tephra beds were observed. One is a thick white tephra found at 45.41–45.57 m CSF-A in Hole U1423A (interval 346-U1423A-6H-1, 11–27 cm). This tephra contains abundant thin bubble-wall type volcanic glass shards. The distinctive glass morphology suggests that this layer may be correlative to the Hakkoda-Daiichi (Hkd-1 or Hkd-Ku) tephra erupted from Hakkoda Volcano in northern Honshu, Japan. This tephra occurs just above the Brunhes/Matuyama paleomagnetic boundary (Machida and Arai, 2003). Another characteristic tephra was found at 186.37–186.54 m CSF-A in Hole U1423A (interval 20H-6, 77–94 cm). This tephra is dominated by “bubble-junction” type glass shards with biotite as the dominant heavy mineral. These features are the same as in the Znp-Ohta tephra that occurs in central Honshu, Japan (Satoguchi and Nagahashi, 2012). Another characteristic tephra was observed at 69.29–69.395 m CSF-A in Hole U1423A (interval 8H-4, 49–59.5 cm). This tephra consists of thick bubble-wall glass shards and biotite, as well as clinopyroxene and orthopyroxene as heavy minerals. One possible match for this tephra is the OM-SK110 tephra, which occurs in the Pliocene sequence above the Olduvai event in central Japan (Satoguchi and Nagahashi, 2012). These initial correlations will be further examined by shore-based research. |
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