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doi:10.2204/iodp.proc.346.103.2015 LithostratigraphyDrilling at Site U1422 penetrated to a maximum subbottom depth of 205 m in Hole U1422C, recovering a total of 215.78 m of sediment for a recovery rate of 105%. The shipboard lithostratigraphic program involved 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 the facies and facies associations from each hole. A total of 212 smear slides were examined from Hole U1422C to help determine lithologic names, whereas fewer were taken from Holes U1422D (49) and U1422E (27). From Hole U1422C, 36 samples were selected for XRD analysis. The sedimentary succession recovered at Site U1422 extends from the Pliocene to Holocene and closely follows the lithologic sequence previously identified at Site 795 (~40 km to the northeast), which is dominated by clay, silty clay, and diatomaceous clay with minor volcaniclastic material. The section is divided into two major lithologic units (I and II), distinguished on the basis of sediment composition. Unit I is further divided into two subunits based on the frequency of alternating dark and light color variations. The character of the sediment physical properties, including natural gamma radiation (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 U1422, together with some of these additional properties, are summarized in Figures F2, F3, and F4. Hole-to-hole stratigraphic correlation is shown in Figure F5. Unit I
Lithology and structuresUnit I consists of Holocene to early Pleistocene silty clay and clay with lesser amounts of diatom-bearing and diatom-rich silty clay. Minor calcareous layers containing foraminifers and carbonate nodules are rare but present. Pebble-sized clasts are also found at various depths, whereas discrete tephra (volcanic ash) layers ranging in thickness from a few millimeters to >10 cm are numerous (Table T2). Pyrite is 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. A few black spherules, likely microtektites (Fig. F6), were found in Sample 346-U1422C-4H-CC near the horizon of the Brunhes/Matuyama paleomagnetic boundary (~0.78 Ma: Sample 346-U1422C-5H-1, 50 cm; 37.5 m CSF-A) during foraminiferal analysis. One of the most striking features of Unit I sediment is the alternating, decimeter-scale color-banded bedding that characterizes much of the sequence. Previous work (e.g., Föllmi et al., 1992; Tada et al., 1992) suggested that the color variations are primarily due to the organic carbon content of the dominant silty clay and clay, with dark, organic-rich intervals (dark gray to dark olive-gray) interspersed between lighter colored, organic-poor intervals (light green to light greenish gray). In addition, dark green centimeter-scale banding is common throughout the unit. The relative frequency of these color alternations is the basis 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 was not detected in XRD analyses from Unit I. There may be other minor minerals that were not identified by the XRD analysis of bulk samples. Figure F7 shows the downcore variations in peak intensity of the identified minerals at Site U1422. In general, quartz, plagioclase, illite, kaolinite and/or chlorite, and pyrite contents show a long-term trend toward increasing counts uphole. In general, the peak heights of these minerals are higher in Unit I and typically lower in Unit II. In contrast, K-feldspar drops sharply downhole across the Unit I/II boundary and is largely absent in the upper part of Unit II but increases again below ~130 m CSF-A before largely disappearing near the base of the site. Peaks of opal-A are generally lower in Unit I and higher in Unit II. The peak intensity of pyrite at 6.98 m CSF-A in Hole U1422C is very strong. Halite is always present in samples, reaching a maximum content at 9.98 m CSF-A in Hole U1422C. Subunit IA
Lithology and structuresSubunit IA is composed mainly of silty clay and clay with subordinate amounts of diatomaceous clay. Tephra layers are a minor but common component in Subunit IA and intercalate in the silty clay sequence. Subunit IA is primarily recognized by having a much higher frequency of dark layer occurrence than Subunit IB (Fig. F8). In general, low L*, a*, and b* values in color and high NGR values with significant fluctuations are characteristic of Subunit IA. Bioturbation is slight to absent throughout this subunit and is another criterion for separating Subunit IA from IB. Where evident, bioturbation is more common in lighter colored intervals, whereas dark colored layers often show evidence of fine parallel lamination. Rare beds contain fining-upward size grading. Other features observed in Subunit IA include scattered pebble- to cobble-sized clasts and rare small normal microfaults. CompositionThe principal components of lithologies in Subunit IA are terrigenous, volcanic, and biogenic. The biogenic fraction is dominated by diatoms and sponge spicules and includes rare foraminifers. Terrigenous components are dominated by clay and fine silty clay fractions. Clay minerals are the dominant siliciclastic component of the clay and silt, usually in contents of between 10% and 50%. Lithic and quartz grain contents are also relatively high, ranging from abundant to dominant (up to 75%). Volcanic glass shards and pumice combine to nearly 100% in tephra layers but are sometimes mixed with minor siliciclastic and biogenic materials as well as opaque minerals. Pyrite, or more likely a precursor black iron sulfide, is present as submillimeter-scale disseminated framboids (or irregular masses) coexisting with other opaque minerals. Diagenetic calcite in small size needle-like crystals is observed at multiple depths and is typically associated with layers of more indurated sediment (see “Lithology and structures” and “Bulk mineralogy”). In Section 346-U1422C-5H-1A, 63.5 cm (33.735 m CSF-A), a thin (<0.5 cm thick) volcaniclastic layer containing large biotite flakes is found interbedded in greenish silty clay. This particular composition is quite unique at Site U1422 (monospecific and in phenocrystal texture), as the tephra layers commonly found in the section most typically consist of volcanic glass and crystals. The vitric components of the tephra layers have a wide range of forms, sizes, and shapes. Glass shards and fragments have sharp angles (so-called “bubble-wall” type), and vesicular glass fragments that include bubbles (so-called “pumice” type) are frequent. Some of the glass grains are rectangular fragments presenting a fibrous structure that clearly reflects the presence of subparallel pipe vesicles (Fig. F9). Alteration may occasionally affect the vitric grains, but most of the volcanic material appears pristine. In Section 346-U1422D-6H-6A, 23 cm, a thin level of authigenic zeolite, possibly resulting from the alteration of tephra, is observed yielding cruciform crystals of phillipsite (Fig. F10). The biogenic components found in Subunit IA deposits are rarely dominant. However, where found, biosiliceous fossils (diatoms, sponge spicules, radiolarians, and rare silicoflagellates) can reach up to 75% in contents in the bulk sediment. Calcareous nannofossils and foraminifers are nearly absent except in the uppermost part of the unit (Cores 346-U1422C-1H and 2H). Subunit IB
Lithology and structuresSubunit IB is transitional downhole from Subunit IA and is identified by a decrease in the frequency of the dark and light color alternation (Fig. F11). Silty clay and clay are again major lithologies in this subunit with subordinate laminated diatom-rich silty clay that is typically brown in color. Bioturbation increases gradually with depth, and sediment mottling and disruption of laminae and color banding is more prevalent. Discrete tephra layers with a thickness of 1–2 cm commonly occur in the subunit. Fining-upward turbidite beds with sharp and erosional basal contacts occur at the lowermost part of Subunit IB. Each turbidite bed is a few to several centimeters in thickness. Sediments comprising the light colored layers in Subunit IB are generally lighter (with higher L* value) than those in Subunit IA. A subtle decrease in NGR values of Subunit IB sediment suggests decreasing amounts of organic matter and/or clay mineral content relative to Subunit IA. CompositionThe results of smear slide analyses show that the most common lithology consists of terrigenous components dominated by clay and fine silty clay fractions. As in Subunit IA, tephra layers occur interspersed in the section, but these are more frequent in Subunit IB than in Subunit IA (see “Lithology and structures”) and are mainly composed of well-preserved volcanic glass and crystals (see “Subunit IA”). Clay and silty clay sediment contains abundant clay minerals, rock fragments, and other siliciclastic minerals (mainly quartz). Scattered sand layers are observed in Subunit IB, containing ~60% siliciclastic material (lithics) and ~35%–40% volcanic glass and crystals. Occasionally, small amounts of calcareous debris (fragmented foraminifers) are found in the sediment (usually <5%). In general, calcareous biogenic components are poorly preserved in Subunit IB (<5% or absent). Biogenic silica is common at various depths, and smear slide analyses reveal that diatom-rich sediment mainly corresponds to brown laminated intervals (see “Lithology and structures”). In these intervals, the siliceous biogenic component can reach up to 70% of the bulk composition (Fig. F12). Unit II
Lithology and structuresUnit II is dominantly composed of moderate to heavily bioturbated diatom ooze and diatomaceous silty clay and clay with numerous turbidite beds (Fig. F13). Minor hard micritized layers and calcareous nodules occasionally occur. Unit II is primarily distinguished from Unit I based on a significant increase in diatom content relative to terrigenous sediment downhole. A remarkable decrease in NGR values coincides with the increasing diatom content of Unit II sediment, whereas a* and b* color reflectance values show a slight overall increase in Unit II. Unit II sediment is also more heavily bioturbated than that of Unit I, with the degree of bioturbation changing vertically. Tephra beds are frequently found in the upper part of Unit II, but their number and bed thickness decrease significantly below ~130 m CSF-A. The thickest tephra layer (maximum thickness = 27 cm) occurs in the upper part of Unit II (107.36–107.72 m CSF-A in Hole U1422C). The lower 9 cm of the tephra is darker (dark gray) and coarser (very fine sand size) than the upper 18 cm (light gray and coarse silt size). Intercalation of numerous turbidite layers within the diatomaceous silty clay is another characteristic of Unit II. Each turbidite bed is a few centimeters thick. The frequency of turbidites changes remarkably from only a few to >10 in a section. Turbidite beds show fining-upward grading with sharp erosional basal contacts (Fig. F14) and can be divided into what are termed here turbidite mud and turbidite sand. Turbidite mud is relatively thick (several centimeters in thickness) and has a mixture of grain sizes compared with thinner, coarser turbidite sand (normally <1 cm in thickness). Parallel and/or cross lamination can be recognized in many turbidite sands above their base. Turbidite mud is characterized by brown color in the upper part and olive-gray in the lower part. The upper contact of turbidite mud is usually gradual and sometimes bioturbated. The grain composition of turbidite sand gradually changes from more volcanic origin (volcanic glass shards with quartz and rock fragments) in the upper part of Unit II to more siliciclastic grain dominated (volcanic glass shards, quartz, and rock fragments with volcanic glass shards) in the lower part. Both tephra layer occurrence and turbidite sand composition suggest a greater volcanic influence on sedimentation in the upper part of Unit II compared to the lower part. CompositionAs in Subunits IA and IB, the major lithologies in Unit II are dominated by fine-grained terrigenous material, except in turbidite layers where the grain size is generally sand or silty sand. On average, sediment in turbidite beds consists of ~70% siliclastic grains, <10% biogenic silica (mostly diatoms and diatomaceous sponge spicules), and between 10% and 20% tephra. Fragmented and poorly preserved foraminifers occur occasionally. The composition of sandy beds is observed to be slightly different from the bottom to the top of Unit II. The sand layers of the lower part of Unit II are mostly siliciclastic in bulk composition (up to 80%) with a tephra fraction of ~5%. In the upper part of Unit II, the tephra fraction was estimated to reach as high as 10%. These estimates are semiquantitative, based on shipboard comparison to composition charts, and should not be taken as absolute. The composition and grain size typically change vertically in a turbidite bed (Fig. F15) indicating grain sorting during its deposition. Sandy beds are often associated with brown silty clay and greenish white silty clay. In smear slides, the brown silty clay contains a mixture of siliciclastic and biosiliceous grains (mainly diatoms, up to 20%). The greenish white silty clay typically contains fewer siliciclastic silt grains and a larger biosiliceous component, though diatom preservation can be poor. Bulk mineralogyThe results of XRD analyses conducted on Hole U1422C sediment are listed in Table T3. In general, the bulk mineral composition of Unit II sediment is similar to that of Unit I. Figure F7 indicates a slight downhole decrease in quartz, plagioclase, illite, kaolinite and/or chlorite, and pyrite contents, countered by a long-term increase in opal-A. However, the differences are subtle and may be biased by the relatively low resolution of the shipboard sampling interval. The most significant occurrence of calcite in Unit II was observed in the form of a yellow-brown concretion and cemented layer found at 105.55 m CSF-A in Hole U1422C. DiscussionOverall, the sedimentary succession at Site U1422 records a history of largely terrigenous deposition since the Pliocene (<3.9 Ma). Over this interval, sediment is predominantly silty clay and clay with less abundant diatomaceous sediment. Nonetheless, the downhole distribution of biosiliceous components provides a useful means of dividing the section into units, following the convention previously used at nearby Site 795, with Unit II defined by a significant increase in diatom-rich strata as compared to Unit I (Tada and Iijima, 1992; Tada, 1994). Unit II sediment was probably deposited during a period of elevated biological productivity and good overall circulation in the basin. Well-oxygenated conditions and a steady food supply for the benthos in the deep basin are indicated by the moderate to heavy bioturbation of Unit II sediment. A significant difference between the Unit II sequence recovered at Sites 795 and U1422 is the prevalence of turbidites in the latter, which suggests either a more proximal source of terrigenous materials or a location more susceptible to downslope movements. The turbidites continue up to the lower Subunit IB. Site U1422 is located in a basin west of the Okushiri Ridge, which is thought to be a young and immature plate boundary. Initiation of subduction along the boundary has been inferred to be 1.8 Ma (Tamaki et al., 1992). Turbidite deposition is possibly related to tectonic movements along the Okushiri Ridge. Further shore-based research may test this hypothesis. In contrast, Unit I sediment at both sites is very similar and lithologic variations appear to be highly correlative. The Pleistocene–Holocene sediment that makes up Unit I is characterized by lower siliceous content and rhythmic alternation in colors marked by light colored (gray to light greenish gray) silty clay to clay intervals interbedded with darker colored (dark olive-gray to black) layers. Although smear slides are only semiquantitative, their content suggests that the dark layers are often richer in some combination of organic debris, pyrite, or biogenic skeletal material (usually diatoms). The pronounced lack of bioturbation through much of Unit I and the gray to greenish hue of much of the sediment are consistent with lower deepwater oxygen levels to, at times, anoxic seafloor conditions. Within Unit I, an increase in the frequency of dark colored sediment intervals is used to distinguish between two subunits, with Subunit IA marked by decimeter-scale dark layers that are more frequent, more distinct, and typically better laminated than in Subunit IB. This pattern parallels previous observations from Site 795. Tephra layers are present but are a relatively minor constituent of Subunit IA sediment, increasing in frequency in Subunit IB and the upper part of the Unit II sequence. Discrete tephra layers decrease downsection in Unit II, a trend apparent at Site 795 as well. In contrast, turbidite deposition in Unit II is much more prevalent at Site U1422 than at Site 795. Turbidite composition varies within Unit II, with a larger fraction of reworked tephra grains incorporated in turbidite deposits in the upper part of Unit II and a greater proportion of siliciclastic grains in turbidites from deeper in the Unit II sequence. This is consistent with observations of in situ tephra distribution that suggest greater regional volcanism during the time period represented by upper Unit II/Subunit IB sediment. |
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