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doi:10.2204/iodp.proc.333.105.2012 LithologyIn Holes C0012C and C0012D, 180 m of lithologic Unit I and the upper part of lithologic Unit II were drilled during Expedition 333. Holes C0012E, C0012F, and C0012G aimed at recovering the boundary between sediments in the Shikoku Basin and the igneous oceanic crust and deepening penetration into the basalt. The age of these units and their boundaries are known from Expedition 322, which cored Hole C0012A from 60 to 576.0 mbsf (Expedition 322 Scientists, 2010). However, the quality of RCB cores and core recovery in the upper part of Hole C0012A were not good enough to provide a complete and reliable description of Units I and II. During Expedition 333, sediments belonging to Unit I and the upper part of Unit II were cored with the HPCS with very good recovery. Thus, the new holes added information on upper Shikoku Basin strata not acquired during Expedition 322 (Expedition 322 Scientists, 2010). Three lithologic subunits were interpreted within Unit I during the examination of cores (Fig. F3). Lithologic Unit I (hemipelagic/pyroclastic facies)
Subunits in Unit I are distinguished based on the abundance and thickness of volcanic ash layers within a background of hemipelagic mud. Lithologies in Holes C0012C and C0012D include dark greenish gray clay, silty clay, and clayey silt interbedded with generally fine volcanic ash (Figs. F3, F4; see Site C0012 smear slides in “Core descriptions”). A major change in the frequency of the occurrence of ash layers is recorded around 71.5 mbsf, marking the Subunit IA/IB boundary (Fig. F3). Underneath Subunit IA, ash layers are scarce to ~123 mbsf (Subunit IB) (Fig. F3). Below 123 mbsf, another interval of dark greenish gray silty clay with abundant ash layers extends to the base of Unit I at ~149.77 mbsf. This lower interval is assigned as Subunit IC. A more detailed description of these subunits is presented below. Subunit IA
Subunit IA comprises a 71.5 m thick succession dominated by dark greenish gray clay to silty clay. The top 3 m is light brown–yellowish gray. Intercalated are 1–2 cm thick green bands enriched in iron, analogous to what has been observed and analyzed by means of X-ray fluorescence (XRF) core logging at Site C0011 (see Fig. F6 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]), and <40 cm thick ash layers (Figs. F4, F5). The subunit is moderately or heavily bioturbated, and the ichnofossil genus Chondrites is abundant. Scattered ash is observed throughout the succession within intervals of <1 cm to several tens of centimeters in thickness (Fig. F5). Clay minerals and nannofossils are the most abundant particles on smear slides, with accessory percentages of volcanic glass and quartz. Siliceous fossils such as sponge spicules, diatoms, and radiolarians occur as a rare or trace component in most of Subunit IA, being relatively more abundant at the top of this subunit (see Site C0012 smear slides in “Core descriptions”). Wood fragments occur in interval 333-C0012C-2H-2, 48–59 cm, at 6.3–6.4 mbsf. An ash layer that appears to be equivalent to the onland Azuki volcanic ash bed (0.85 Ma; Hayashida et al., 1996) was identified at 5.70 mbsf (interval 333-C0012C-2H-1, 118–132 cm), based on positive identification of characteristic microscopic features such as abundant bubble wall type glass shards with a few obsidian fragments and orthopyroxene and clinopyroxene crystals (Fig. F6; see Site C0012 smear slides in “Core descriptions”). A tentative correlative to the onland Pink volcanic ash bed (1.05 Ma; Hayashida et al., 1996) occurs at 7.70 mbsf (interval 333-C0012C-2H-3, 40–73 cm) (Fig. F6; see Site C0012 smear slides in “Core descriptions”), as inferred from the occurrence of characteristic microscopic features such as fibrous bubble wall type glass shards and abundant hornblende minerals, which are distinct from other ash layers. A third major volcaniclastic event deposit, the Ohta volcanic ash bed (4.0 Ma; Satoguchi et al., 2005), appears to match a cored ash layer at 44.95 mbsf (interval 333-C0012C-6H-2, 120–124 cm), which contains bubble junction type glass shards and biotite (see Site C0012 smear slides in “Core descriptions”) typical of this volcanic marker bed. In addition, large pumice pebbles (>2 cm in diameter) are present in interval 333-C0012C-8H-3, 22–32 cm, at ~62.8 mbsf. The deposition of Subunit IA occurred over a basement high (Kashinosaki Knoll) dominated by hemipelagic settling and ash from volcanic eruptions. The Subunit IA/IB boundary is marked by the transition to an interval where ash layers become scarce and is defined at the base of the last downsection occurrence of a thick ash layer at 71.54 mbsf (Section 333-C0012C-9H-2, 62 cm). Remarkably, there is a significant age gap in the upper part of Subunit IA at 14 msbf (see “Biostratigraphy” and “Paleomagnetism”), which is overlain by a 4 m thick interval showing evidence for disturbance from both drilling and in situ deformation (Fig. F5). Below this is a 62.5 m thick interval with tilted strata (Fig. F5; see also “Structural geology”). This suggests that a significant portion of the stratigraphic succession of Subunit IA and the upper part of Subunit IB is affected by slumping (see “Structural geology”). Subunit IB
Subunit IB is a heavily bioturbated dark greenish gray silty clay with minor contributions of volcanic ash (Figs. F3, F4). Tilted colored bands and ash layers are seen in the uppermost part of the subunit. Below ~81.63 mbsf is a ~4.5 m thick zone of chaotic bedding (see “Structural geology”). Clay minerals, nannofossils, and altered volcanic glass are the most abundant particles on smear slides, with accessory percentages of quartz and opaque minerals (see Site C0012 smear slides in “Core descriptions”). Other microfossils are absent from the subunit. The subunit is dominated by hemipelagic settling with rare ash intercalations from volcanic eruptions. The upper part of this unit has been affected by slumping (see “Structural geology”). A smear slide from 91.19 mbsf (Section 333-C0012C-11H-5, 30 cm) provides the first downsection observation of altered volcanic ash (Fig. F7). The last smear slide observation of fresh glass is from Section 333-C0012C-7H-8, 17 cm (59.24 mbsf, within the lower part of Subunit IA) (Fig. F7). Due to a gap in shipboard observation of volcanic ash in smear slides between 60 and 90 mbsf (i.e., the interval also characterized by the base of the slumped section [see “Structural geology”]), the exact depth of ash alteration remains undefined and may or may not correlate with (1) the Subunit IA/IB boundary, as a similar change in the degree of alteration of ash has been recorded at the Subunit IA/IB boundary in Hole C0011C at 251.52 mbsf (see “Lithology”), and/or (2) changes in physical properties (see “Physical properties”) and inorganic geochemistry (see “Inorganic geochemistry”) that occur more progressively from the top of Subunit IB to ~100 mbsf. The Subunit IB/IC boundary is marked by more frequent appearance of ash layers and is defined at the top of an ash layer at 123.31 mbsf (Section 333-C0012D-2H-7, 87 cm). Subunit IC
Subunit IC comprises heavily bioturbated dark greenish gray silty clay. The characteristic feature of this unit is the relative increase in the content of volcanic glass, which is higher than within the overlying subunit. Intercalated in the subunit are <20 cm thick ash layers, which contain altered glass as observed in smear slides (see Site C0012 smear slides in “Core descriptions”) and green bands inferred to be enriched in iron analogous to what has been observed and analyzed by means of XRF core logging at Site C0011 (see Fig. F6 in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). Scattered ash is observed throughout the succession (see Site C0012 smear slides in “Core descriptions”). Smear slides indicate that clay minerals remain the most abundant particles. Accessory components include quartz and nannofossils (see Site C0012 smear slides in “Core descriptions”). The other detrital constituents appear as trace percentages in most of the subunit. Deposition of Subunit IC was dominated by hemipelagic settling with contribution from a volcanic source. The Unit I/II boundary is marked by the first occurrence of a semiconsolidated sandy volcanic turbidite at 149.77 mbsf (Section 333-C0012D-6H-4, 98 cm) and correlates to the Unit I/II boundary cored in Section 322-C0012A-12R, 43cm (150.86 mbsf) (Expedition 322 Scientists, 2010). Below this depth, background sediment in Unit II is finer than that recorded over the same depth interval by Expedition 322 in Hole C0012A, with coarser grained material (including calcite-rich gravel) appearing below 173.9 mbsf. Lithologic Unit II (volcanic turbidite facies)
The upper part of Unit II comprises semiconsolidated sandy volcanic turbidites with sharp and well-defined upper and lower boundaries. In the lower part of the unit they appear as more consolidated volcanic sandstone turbidites, including fresh and altered glass with some associated calcite (see Site C0012 smear slides in “Core descriptions”). Commonly, beds have normal grading, but some intervals are massive and may contain clay clasts 1–10 cm in diameter (Fig. F8). The lowermost conglomerate bed in Core 333-C0012D-13H is composed of disaggregated pieces of volcaniclastic sandstone and bioturbated silty claystone showing evidence for sediment remobilization (see also “Structural geology”), likely correlating to a chaotic interval in Hole C0012A (178–181.1 mbsf) (Expedition 322 Scientists, 2010). Some of the sand layers in Unit II present fractures filled with magnesium-rich calcite and traces of barite. During the deposition of the upper part of Unit II, the paleoenvironment was dominated by deposition from a sandy system as described and interpreted in more detail by the Expedition 322 Scientists (2010). The probable source of the volcanic sand was the Izu-Bonin volcanic arc. The top of the volcanic turbidite facies (Unit II) cored in Holes C0012D (at 149.77 mbsf) and C0012A (at 150.86 mbsf) (Expedition 322 Scientists, 2010) is ~7.8 Ma in age (see “Biostratigraphy” and “Paleomagnetism”) and thus is time correlated to the top of the volcanic turbidite facies (Unit II) cored in Holes C0011B and C0011D (see “Lithology,” “Biostratigraphy,” and “Paleomagnetism” in the “Site C0011” chapter [Expedition 333 Scientists, 2012b]). However, in comparison to Site C0011 the Miocene sandy volcanic turbidites are finer grained at Site C0012, likely because the site of deposition rests on the crest of the Kashinosaki Knoll. The bathymetric high was not large enough at the time of deposition, however, to prevent the transport and deposition by turbidity currents (see also the detailed discussion in Expedition 322 Scientists, 2010). Sediment/Basement contactTwo cores recovered from Hole C0012E (Cores 333-C0012E-1X and 2X) were greenish silty claystone intercalated with thin sandstone layers from 500 to 510.5 mbsf. This interval corresponds to the base of lithologic Unit V (volcaniclastic-rich facies) defined during Expedition 322 (see Expedition 322 Scientists, 2010, for detailed description and interpretation of this facies). The first consistent occurrence of reddish brown calcareous claystone corresponding to lithologic Unit VI (pelagic claystone) (Expedition 322 Scientists, 2010) occurs at the top of Core 333-C0012E-3X (519 mbsf), overlying the sediment/basaltic basement contact cored in Section 333-C0012E-3X-7, 114 cm (525.815 mbsf) (see also “Igneous petrology”). Unit VI pelagic claystone facies was also recovered as broken, drilling-disturbed pieces in Hole C0012F, from the top of the cored section to the sediment/basaltic basement contact in Section 333-C0012F-1R-1, 46 cm, and as a continuous section in Hole C0012G, from the top of the cored section to the sediment/basement contact in Section 333-C0012G-2R-2, 80 cm (see also “Igneous petrology”). The calcareous claystone is rich in nannofossils with minor amounts of radiolarian spines and is interpreted to represent a thin veneer of pelagic sediments overlying the basement (see also Expedition 322 Scientists, 2010). Locally, the claystones have a mottled red-green coloration. They also hold veins of calcite with traces of barite as well as several layers with accumulations of manganese oxide forming millimeter- to centimeter-sized lumps. XRF core logger data show relative variations of calcium anti-correlated with silicium and aluminium variations (Fig. F9). These likely represent variations in the biogenic calcium carbonate input. The sediment immediately above the basalt appears relatively calcium rich and aluminium and magnesium poor, suggesting a lower clay content. The basalt appears, unsurprisingly, to have higher titanium, magnesium, and iron and lower potassium compared to the sediment. An interval of manganese oxide accumulation in the sediment is found between 523.2 and 525.5 mbsf. X-ray diffractionX-ray diffraction (XRD) data for Subunit IA (Fig. F10; Table T2) indicate an average clay mineral content of 63 wt%, with the lowest content characterizing the uppermost 15 m. Quartz content shows uniform values throughout the subunit with an average of ~19 wt%. Calcite content is highest in the upper 15 m, where it reaches ~25 wt%. Below 15 m, calcite content of Subunit IA is very low, dropping the average for the subunit to 4 wt%. In Subunit IB, clay mineral content is relatively stable and averages 66 wt% (Fig. F10; Table T2). Quartz and feldspar contents are also stable and average 17 and 12 wt%, respectively. Calcite is present in the subunit, varying from 0 to 20 wt% in relative abundance. However, it averages only 6 wt% in the entire Subunit IB. XRD data for Subunit IC indicate an average clay mineral content of 72 wt%. There is on average ~18 wt% quartz and feldspar. Calcite content varies from 0 to 5 wt% in relative abundance (Fig. F10; Table T2). The relative clay mineral abundance in Unit II remains stable relative to that of Subunit IC (Fig. F10; Table T2). Quartz content is also similar to Subunit IC at 17 wt% on average. Feldspar values vary from 10 to 22 wt%, averaging 13 wt% in Unit II, which is also similar to Subunit IC. Calcite is residual at only 1 wt% on average. X-ray fluorescenceDiscrete XRF analyses were performed on 28 samples from Holes C0012C and C0012D to estimate the bulk chemical composition of the sediments and to characterize compositional trends with depth and/or lithologic characteristics (see Fig. F11; Table T3). The major element composition of Units I and II is fully consistent with the XRF results obtained during Expedition 322 at Site C0012 (Expedition 322 Scientists, 2010) (Fig. F11). As for the underlying units, major element concentrations of Units I and II span a relatively small range of values and resemble those of the upper continental crust as defined by Taylor and McLennan (1985). Unit I: hemipelagic/pyroclastic faciesIn the uppermost 15 m of Unit I, CaO content significantly decreases with depth while SiO2, Al2O3, Fe2O3, K2O, MgO, Na2O, and TiO2 contents show an overall increasing trend (Fig. F11; Table T3). These variations may reflect a dilution effect by carbonates as suggested by the decreasing proportions of calcite and increasing proportions of clays observed by XRD (Fig. F10). Excluding one outlier sample at 105.33 mbsf, K2O, TiO2, and P2O5 show no significant variation with depth below ~20 mbsf, with average values of 3.07 wt% for K2O, 0.67 wt% for TiO2, and 0.09 wt% for P2O5. SiO2 content is slightly scattered but gently increases throughout Unit I, whereas MgO and Na2O concentrations show a gradually decreasing trend (Fig. F11; Table T3). CaO, Al2O3, and Fe2O3 contents are relatively variable throughout Unit I, possibly due to a dilution effect. The higher CaO content observed in the lower part of the unit can be related to higher abundances of calcite as suggested by XRD data (Fig. F10). MnO concentration appears fairly uniform in the upper part of Unit I but significantly scatters below ~60 mbsf where CaO content is higher. Such a scattering in MnO concentration may account for the presence of rhodochrosite (MnCO3) as suggested by Expedition 322 Scientists (2010) to explain the high MnO content measured in a few calcareous sediments at Site C0012. In the same depth interval, at 105.33 mbsf, one outlying sample shows higher CaO (12.1 wt%) and P2O5 (0.28 wt%) concentrations together with relatively low SiO2, K2O, and MgO contents (Fig. F11; Table T3). This difference can be related to the presence of apatite (Ca[PO4]3[F,Cl,OH]). Unit II: volcanic turbidite faciesNo significant difference in major element composition is observed between Units I and II (Fig. F11; Table T3). As in the overlying unit, one sample of hemipelagic mud shows high CaO, MnO, and P2O5 contents at 174.45 mbsf, a feature that may be related to the presence of apatite and rhodochrosite (Table T3). The sand layer sampled at 174.0 mbsf shows significantly lower K2O and MgO concentrations than the background composition of the sedimentary pile but has relatively high Na2O content (Fig. F11). |
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