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doi:10.2204/iodp.proc.343343T.103.2013 LithologySeven lithologic units were recognized during examination of cores from Site C0019 (Fig. F22; Table T2). The units were differentiated on the basis of color, composition (siliciclastic, volcaniclastic, or siliceous microfossil components), grain size, and presence of minor lithologies. We also used X-ray computed tomography (CT) images to identify characteristic bioturbation and structural styles where possible for each unit. Unit boundaries were only sampled for the contacts between Units 5 and 6 and 6 and 7 because of the operational constraints and incomplete recovery. In addition, Units 1 and 2 were restricted to one and two cores respectively. Unit 1 (slope facies or wedge sediments)
The dominant lithology of Unit 1 is medium olive-gray siliceous mudstone (Figs. F23, F24), which extends from 176.5 to 185.2 mbsf. A secondary lithology is defined by isolated, few-millimeter to 2 cm thick layers of yellowish gray ash with gradational contacts. The siliceous mudstone is dominated by abundant siliceous microfossils (diatoms, sponge spicules, and radiolarians), abundant clay- to silt-sized siliciclastic material, and includes common ash shards. Biogenic silica, terrigenous silt grains, and angular ash/glass fragments are generally 10–100 µm in size. Unit 1 also contains decimeter-spaced, millimeter to centimeter thick, dark gray to black laminae that are locally laterally discontinuous. Smear slide observations indicate that the composition is 22%–70% biogenic silica, 0%–45% siliciclastic mineral grains, and 10%–33% volcanic ash. Light yellowish gray to light olive-gray ash with 60%–97% volcaniclastic grains is a secondary lithology within Unit 1 (Figs. F23, F24). Ash occurs in several centimeter-thick wavy layers or discontinuous elongate patches up to a few centimeters long with interfingering and gradational boundaries with surrounding siliceous mud. The edges of ash-rich layers are mixed with the adjacent siliceous mudstone and contain many burrows, suggesting they have been preferentially bioturbated. Ash layers typically appear brighter in X-ray CT images than the surrounding siliceous mudstone. The mudstone adjacent to ash layers is generally darker gray than surrounding mudstone. Three bulk mudstone samples (177, 183, and 185 mbsf) were analyzed with X-ray diffraction (XRD), and qualitative mineral mode by weight was estimated (see “Lithology” in the “Methods” chapter [Expedition 343/343T Scientists, 2013]). The three samples gave nearly identical results: ~¾ total clay, <¼ quartz, <¼ plagioclase, and negligible calcite. This analysis is approximate due to the limitations of the method and does not include estimation of other phases, including amorphous volcanic ash, opaline silica, and accessory minerals. Mineral grains may be part of the siliciclastic fraction, but volcanic ash is also a potential contributor, so these estimates are not directly comparable to the grain component estimates from smear slide observations. Subrounded pumice clasts (<<1%) up to 4 mm in diameter are sporadically distributed throughout Unit 1 (Fig. F23). These clasts are white-gray and very friable (easily crushed and smeared when the core was split with a wire). Pumice fragments occur in isolation, concentrated (locally up to 5%) in short intervals, or in pod-shaped clusters (probably relict fragments from the in situ weathering of larger clasts) with a mantle of darker gray mudstone. No obvious burrows were observed on the split core surface, but subtle mottling and patches of different color ranging from olive-brown to dark gray are common. The mottled areas commonly obliterate the primary layering and correspond to areas where discontinuous tubular structures on a range of scales, interpreted as burrows, are identified in X-ray CT images. Burrows are subhorizontal to vertical and filled with material either slightly brighter or slightly darker in the X-ray CT image than the surrounding siliceous mudstone. Cross sections of the burrows observed in the X-ray CT image are round to elliptical. Smaller burrows that are very bright in the X-ray CT image are common throughout Unit 1. These 0.5–1 mm wide traces follow circuitous, random paths through the mud and concentrate within the traces of the larger burrows. They also form useful one-dimensional offset markers for identifying shear surfaces (see “Structural geology”). The origin of Unit 1 material is difficult to determine because siliceous mudstone is widespread along the Japan Trench slope, both seaward and landward of the trench (see DSDP Leg 56/57 [Scientific Party, 1980]), and we did not observe the nature of the boundaries of this unit. This unit is therefore interpreted as representing either the upper part of wedge sediments or as slightly compacted and deformed forearc slope deposits. The abundant volcaniclastic material probably originates from the volcanic arc associated with the Japan Trench subduction system, but any further interpretation of its significance will require dating of the volcaniclastic component (by geochemical correlation to ash stratigraphy already established throughout Japan). Only one core has been sampled from this unit, therefore its upper and lower boundaries cannot be established. It has been differentiated from Unit 2 on the basis of contrasting color, grain size, siliceous material content, and other macroscopic features. Unit 2 (brown and bluish gray mudstone)
Unit 2 comprises bluish gray and grayish brown ashy mudstone and extends throughout 648.0–659.7 mbsf (equivalent to Cores 343-C0019E-2R and 3R). This interval is characterized by good recovery but pervasive drilling- or recovery-induced brecciation. As a result, the two lithologies within Unit 2 are represented primarily by a few 5–20 cm clasts and biscuits with rounded ends (Fig. F25). The relative proportion of the two main lithologies was not measured because they were represented by so little intact core. The remainder of the recovered material was in ~1 cm to sand-sized cuttings. In Core 2R, the drilling breccia was sorted with the coarsest particles in the center axis of the core liner, gradually fining to the outside. In Core 3R, the different sizes of cuttings were well-mixed with no sorting trends noted. No sorting is apparent between lithologies, with roughly equal proportions of each throughout the cores. The bluish gray ashy mudstone contains abundant siliciclastic grains (50%–55%) with common to abundant volcaniclastic (30%–35%) and common biogenic components (10%). Overall, this ashy mudstone has a speckled gray/black appearance at the grain scale. Paler areas where the dark grains are absent define mottled patches. Rare elliptical, pale gray patches correspond to worm burrows. Thin (1–2 mm), black, discontinuous laminations are interbedded in the mudstone. Semiquantitative analysis of bulk powder XRD spectra showed the bluish gray ashy mudstone contains ~¾ clay, <¼ quartz, <¼ plagioclase, and negligible calcite. The grayish brown ashy mudstone is fossiliferous and contains dominant siliciclastic grains (60%–70%, with silt slightly more abundant than clay) and common volcaniclastic grains (~25%–30%) with biogenic fragments present and occasionally common (2%–10%). All components are fine grained (<63 µm). Semiquantitative XRD interpretation suggests the grayish brown ashy mudstone is composed of ~¾ clay, <¼ quartz, <¼ plagioclase, and negligible calcite. Bedding is poorly defined and both calcite-cemented burrows and mud-filled burrows indicate extensive bioturbation. Macrofossils are mostly disaggregated fragments (undifferentiated), but two whole gastropod shells were observed. Calcite cement occurs in irregular patches that may correspond to brighter X-ray CT areas compared to noncemented areas. In the X-ray CT images, the calcite cement patches are weakly aligned. Cuttings are predominantly composed of the two main lithologies. The reddish brown ashy mudstone cuttings are angular to subangular, and the bluish gray cuttings are subangular to subrounded. Possible sublithologies identified from isolated fragments within the cuttings only included carbonate veins and a medium-pale gray ashy mudstone. The mixing of gray and brown mudstone throughout the recovered drilling breccia in Cores 343-C0019E-2R and 3R may indicate that Unit 2 is composed of repetitive interbeds of the two lithologies. The overall scarcity of biogenic components in the two lithologies differentiates Unit 2 from Unit 1 (in which biogenic microfossils are abundant). Silt in Unit 2 indicates a relatively proximal sediment source. Macrofossils suggest a shallow depositional environment compared to the trench axis. Unit 3 (gray mudstone)
Unit 3 consists of four dominant lithologies (Fig. F26):
These units are interbedded on the meter scale with occasional centimeter-scale interlayers and intermittent centimeter-scale secondary clay and silt beds. All lithologies are dominated by siliciclastic material and contain trace or present siliceous microfossils and fossil fragments, with rare horizons containing up to 15% siliceous fragments. Ash is abundant in the ashy mudstone, but otherwise ash layers are rare, and ash content decreases significantly downhole below the ashy mudstone. XRD data show the overall quartz, feldspar, and clay content of Unit 3 is generally consistent throughout the section, except for discrete clay and silt beds. Unit 3 has a much higher component of siliciclastic material and a lesser component of volcanic and siliceous grains than Unit 1. Unit 3 contains none of the red to red-brown and bluish gray mudstone of Unit 2. The dominant lithology in Unit 3 consists of a dark gray to black fine-grained mudstone with bedding defined by thin interlayers of black, very fine grained clay-rich laminations (Figs. F26A, F27A). Siliclastic material (>70%) is dominant, with common volcaniclastic (≤30%) and common or present siliceous material (≤15%). Dark gray to black laminations are common, are often wavy, and have laterally variable thickness. Discontinuous beds and clay lenses indicate soft-sediment deformation. The mudstone is commonly strongly mottled, with mixing of gray and black materials. Ash-rich layers are rare, and isolated pumice clasts are present. The ashy mudstone is greenish gray to black, ultrafine-grained, and laminated and contains dominant siliciclastic (≤70%), abundant volcaniclastic (10%–45%), and common siliceous components (Figs. F26B, F27B). Grain size is characterized by approximately equal clay-sized (50%–60%) and silt-sized (40%–50%) fractions, with silt occurring as black and colorless grains. Bedding is defined by medium gray to black compositional bands consisting of dominant siliclastic (70%) and volcaniclastic (30%) grains, with trace siliceous microfossil grains. Laminations range from submillimeter to several millimeters thickness and locally contain discontinuous lenses of pyrite. These layers frequently occur as streaky or wavy laminae. Thin, planar, coherent, laminated beds are present but less common. Mottled areas, rare elliptical black smudgy patches, and submillimeter gray mottling are common. Ashy mudstone includes light greenish gray claystone (>90% clay) and occasional sandy horizons, trace pumice fragments, and occasional pyrite in burrows. Pyrite-rich mudstone has a similar composition to the dominant mudstone but with abundant black, elliptical, occasionally pyrite-bearing dark spots and numerous pyritized burrows and pyritized laminations (Fig. F26C). The elliptical spots have aligned long axes parallel to bedding where bedding is represented by laminations and define the layering where laminations are absent. Clay-rich mudstone is the least common of the lithologies, occurring predominantly in Cores 343-C0019E-7R and 8R (Fig. F26D). Clay-rich mudstone is very similar to the dark mudstone (described above), with >70% siliciclastic material, <30% volcaniclastic material, and a minor siliceous component. Of the siliciclastic material, >90% is clay. Unit 3 reflects a more terrigenous environment of accumulation than Units 1 and 2. Unit 3 accumulated at a time of lesser siliceous microfossils accumulation and increased distance from, or lesser activity of, volcanic sources. Unit 4 (sheared clay)Unit 4 was sampled only in one 97 cm long core (Section 343-C0019E-17R-1) and mostly consists of strongly deformed clays (Figs. F28, F29). The clay is pervasively sheared, forming sharp, strongly aligned phacoids with polished, striated surfaces, defining a scaly fabric (see “Structural geology” for a description). From visual inspection and smear slide analysis we can recognize two main components: (1) red-brown clay and (2) dark brown to black clay. At the top of the core, 0.5–4 cm bands of the two clays are intercalated. A clast of gray mudstone (length = 13 cm) is enclosed inside the clay. Downhole from the mudstone interval, dark brown to black clay is dominant. Smear slide analysis suggests that the red-brown clay is composed predominantly of clay minerals, with rare coarser grains and vitric grains, whereas the dark clay is composed mainly of dark brown clay minerals mixed with dark grains (probably Fe or Mn oxides/hydroxides). The specific clay phases in either clay were not identified by shipboard XRD analysis. However, the composite clay peak (at 19.4°–20.4°2θ) shape changes in Core 343-C0019E-17R compared to the surrounding mudrock, suggesting the clay composition in this layer is distinct (Fig. F30). This is supported by X-ray fluorescence (XRF) major element analyses that show Core 17R is relatively rich in aluminum, potassium, and manganese but relatively depleted in calcium and sodium (Fig. F29). The black clay and red-brown clay are intermixed in the predominantly black clay regions. Red-brown clay boundaries in these regions are sharp but do not correspond to phacoid boundaries. Red clay patches vary from submillimeter to centimeter size. Smearing of the red-brown component along the shearing scaly fabric surfaces is common. In one place, a clast of dark material surrounded by a thin rim (<1 mm) of red-brown material was observed, suggesting a diagenetic origin. The contrast in X-ray CT images between clasts of red-brown and black clay is negligible. In the split core we observed phacoids of fractured black material with a metallic luster just above the interval of high magnetic susceptibility identified by the whole-round multisensor core logger (MSCL-W). The 13 cm long mudstone clast is composed predominantly of siliceous minerals; volcaniclastic grains and biogenic grains are also present. The boundaries between the clay and the mudstone clasts have probably been reworked during drilling, so it is not possible to directly observe the bounding surface; however, no evidence was found of a sedimentary transition to the surrounding clay. Unit 5 (brown mudstone)
Unit 5 is a yellowish to grayish brown mudstone composed of dominant (>85%) siliciclastic grains, minor (5%–10%) volcaniclastic grains, and trace (0%–5%) siliceous microfossils (radiolarians) (Fig. F31). The siliciclastic component of the rock is predominantly clayey in the upper part of the unit (Sections 343-C0019E-18R-1, 18R-CC, and 19R-1; >60% clay and <40% silt) and becomes siltier in the lower part (Sections 343-C0019E-19R-1 and 19R-2; <60% clay and >40% silt) with trace sand (≤3%). Elongate lenses of various colors of mudstone characterize Unit 5. Grayish brown, dark gray-brown, yellowish brown, and red-brown lenses occur in a discontinuous matrix of pale brown mudstone with abundant black silt grains that are likely manganese minerals. Bedding is commonly indistinct but is loosely defined by the alignment of the long axes of the elliptical lenses of all colors. Dark silty beds are commonly concentrated in laterally discontinuous horizons. In addition, centimeter-scale clay interbeds are present in which clay is dominant. Decimeter-scale fining-upward sequences from a basal, silty, manganese-rich mudstone to a more homogeneous clay-rich mudstone occur frequently in the lower 5 m of Unit 5. Irregular, white, sugary-textured patches of ash ≤1 cm thick are present throughout the unit. Burrows and mottling from bioturbation are common throughout the unit. Biogenic material is rare, and discrete ash layers and pumice fragments are common. The overall nature of the unit indicates a significant influx of siliciclastic and terrigenous material to the section, dominating over the biogenic source. The contact with Unit 6 at the base of Unit 5 is most likely stratigraphic. As Units 6 and 7 represent the incoming plate sediments (see “Discussion”), Unit 5 also likely represents the underthrust incoming plate. Unit 6 (pelagic clay)The lower 60 cm of Core 343-C0019E-20R (interval 20R-2, 53–114 cm) consists of centimeter-scale laminar, dark brown, yellow, pink, and green clays, with occasional white laminae (Fig. F28A). Unit 6 consists of >90% siliclastic grains, with trace volcaniclastic grains and siliceous microfossils. Diatoms are not observed in this interval; however, radiolarians are present. More than 75% of the material falls in the clay-size fraction. Some grain boundaries show evidence of siliceous mineralization. The clay contains minor amounts of pyrite and/or manganese oxides and possible zeolites. The unit is poorly lithified and was muddy when the core was split. The bottom section consists of green to light green clays and gradationally transforms into chert (Unit 7). Radiolarian molds are present in the basal section of Unit 6. Unit 6 is interpreted as pelagic clay accumulated on the incoming Pacific plate. Similarity in color and lamination to that observed in the underlying cherts and the intermixing of clay and chert observed in Cores 343-C0019E-20R and 21R suggest that the underlying chert is formed by silicification of the clay. The transition from pelagic clays in Unit 6 to chert in Unit 7 is indicated by the presence of chert nodules and intercalated chert layers within Unit 6, suggesting the transition is a diagenetic front. Unit 7 (chert)Yellow-brown and chocolate-brown laminar chert was recovered as fragments at the base of Core 343-C0019E-20R and in Core 21R, corresponding to 833–847 mbsf (Fig. F28B). Yellow-brown chert consists of millimeter- to submillimeter-scale laminae of light yellow-brown (5Y 7/2) and dark yellow-brown (5Y 6/3) chert with occasional translucent subcentimeter-scale bands of amber (10YR 5/6) chert. The chert fragments in Core 20R occur in the lowermost 3 cm of Section 20R-2 (interval 115–118 cm) as well as in the 8 cm recovered in the core catcher. The fragments in Section 20R-2 occur directly below a predominantly intact interval (20R-2, 103–105 cm, see Fig. F28A) of layered grayish yellow to greenish yellow clay. The chert fragments in Core 21R occur in the lowermost 8 cm of Section 21R-1 (interval 22–30 cm) as well as the 17 cm recovered in the core catcher. The uppermost 22 cm of Section 21R-1 is composed of brown and dark gray to black mudstone fragments, which are interpreted to be fragments of Units 2 and 3 that fell to the bottom of the borehole and were recovered in our last core. The chert found in Cores 20R and 21R is interpreted to represent a silicification of the laminated pelagic clays recovered in Core 20R. This unit is correlated to the chert recovered in the incoming Pacific plate by DSDP Leg 56 Site 436 (Shipboard Scientific Party, 1980). Similar to Leg 56, the fine-scale laminations within in the chert represent the only primary sedimentary structures, implying minimal transport and reworking of the clays prior to silicification. DiscussionCorrelation of lithologic units to Site 436Site 436 is located off Northern Honshu and represents the most proximal drilling site that can be used as a reference for the section cored at Site C0019 (Shipboard Scientific Party, 1980). Site 436 consists of three lithologic units: Unit 1, vitric diatomaceous silty clay and claystone; Unit 2, radiolarian diatomaceous claystone; and Unit 3, pelagic clay with chert and porcellanite. Site 436 Units 1 and 2 are lithologically similar to Site C0019 Units 1–3, which comprise mudstone, siliceous mudstone, and locally ashy mudstone. This lithologic description is very general and does not necessarily imply a direct correlation between the units in Hole C0019E and the incoming plate strata. Site C0019 Units 4, 5, 6, and 7 correlate to Site 436 Unit 3, with direct matches between the brown clay at Site 436 and the multicolored clays and the chert at Site C0019. For example, chert in Site C0019 Units 6 and 7 closely resembles descriptions of chert at 378.5–397.5 mbsf at Site 436, in which scarce Albian–Cenomanian radiolarians indicate a Cretaceous depositional age (Shipboard Scientific Party, 1980). However, an equivalent of Site C0019 Unit 5 (brown mudstone) was not identified in Site 436 Unit 3. At Site C0019, the significant silt component, bioturbation, and volcaniclastic content of Unit 5 suggest a proximal terrigenous source compared to Unit 6. The lithology of Site C0019 Unit 5 could represent a stratigraphic variation between the two holes or could be structurally emplaced from the upper plate at Site C0019. The former interpretation is favored because of the absence of a sheared basal contact of Unit 5 at Site C0019. |
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