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doi:10.2204/iodp.proc.333.104.2012 LithologyIn Hole C0011B, five lithologic units were identified during Expedition 322 on the basis of sediment composition, sediment texture, and sedimentary structures (see fig. F2 and table T3 in Expedition 322 Scientists, 2010). Lithologic Unit I was not cored during Expedition 322 (see Expedition 322 Scientists, 2010); however, based on LWD (Hole C0011A) and core-log-seismic integration, Unit I was assigned to cover the interval from 0 to 340 mbsf (see fig. F2 and table T3 in Expedition 322 Scientists, 2010) and has an estimated age of Holocene to late Miocene (Expedition 322 Scientists, 2010). Lithologic Unit I and the uppermost part of Unit II was the drilling target for Expedition 333. A total of 380 m of strata was drilled in Holes C0011C and C0011D. The cored succession recovered sediments belonging to Unit I and the upper part of Unit II with very good recovery (Fig. F3; Table T2). The new coring also rectified the low recovery rates achieved when drilling the upper part of Unit II during Expedition 322 (Expedition 322 Scientists, 2010). Two lithologic subunits were interpreted within Unit I during the examination of cores. Cored lithologies in Holes C00011C and C0011D include silty clay, clayey silt, and clay interbedded with volcanic ash (Figs. F3, F4). Below 347.82 mbsf the observed lithologies shift abruptly into coarse-grained tuffaceous sandstone and heterolithic gravel and sand. The uppermost sandstone bed represents the top of Unit II. The silty clay, clayey silt, and clay consist mainly of clay minerals with quartz, feldspar, abundant calcareous nannofossils, and some diatoms. Discoaster sp. is common below 130 mbsf. Sponge spicules, radiolarians, and silicoflagellates are relatively rare and are nearly absent in the 243.7–349 mbsf interval (see Site C0011 smear slides in “Core descriptions”). Opaque minerals are generally rare (see Site C0011 smear slides in “Core descriptions”). However, biotite, orthopyroxene, and hornblende are common below 250 m (see Site C0011 smear slides in “Core descriptions”). A likely equivalent to the onland Azuki volcanic ash bed (0.85 Ma; Hayashida et al., 1996) occurs at 21.18 mbsf (Section 333-C011C-3H-10, 10–27 cm), and a probable correlative to the onland Pink volcanic ash bed (1.05 Ma; Hayashida et al., 1996) occurs at 31.30 mbsf (Section 333-C0011D-2H-1, 84–86 cm). Both events are tentatively recognized in the cores from Holes C0011C and C0011D as thin discrete accumulations of ash and by characteristic microscopic features in smear slides, which are distinct from all other ash layers. The Azuki volcanic ash is composed of abundant bubble wall type glass shards with a few obsidian fragments and orthopyroxene and clinopyroxene crystals. The Pink volcanic ash contains fibrous bubble wall type glass shards and abundant hornblende minerals. In addition, a probable correlative to the onland Ohta volcanic ash bed (4.0 Ma; Satoguchi et al., 2005) is present in Core 333-C0011D-17H-9, 2–5 cm, at 157.26 mbsf (Fig. F3), and a possible match to the Habutaki I volcanic ash bed (2.8–2.9 Ma; Nagahashi and Satoguchi, 2007) is located at 80.56 mbsf (Section 333-C0011D-8H-2, 80 cm, through 8H-CC, 8.5 cm). This interpreted correlation is based on positive identification of characteristic microscopic features for these volcanic ashes in smear slides: the Ohta volcanic ash is characterized by bubble junction type glass shards and biotite; the Habutaki I volcanic ash is characterized by bubble wall type glass shards and beta quartz. Lithologic Unit I (hemipelagic/pyroclastic facies)
Two depositional subunits are distinguished within Unit I based on the degree of bioturbation, the degree of induration, and the prevalence and thickness of ash layers. Subunit IA is composed of soft greenish gray to grayish silty clay, clayey silt, and clay with abundant thin (<50 cm) intercalations of volcanic ash (Fig. F5) and common thin, few-millimeter thick green bands showing higher Fe content in X-ray fluorescence (XRF) scan data (Fig. F6). At 251.52 mbsf, the mud becomes stiffer (designated as mudstone), bioturbation intensifies, and ash layers are comparatively scarce (Fig. F7). This change marks the top of Subunit IB. It correlates to the shift in LWD response in Hole C0011B at 251.5 m LWD depth below seafloor (LSF) (Expedition 322 Scientists, 2010) and in physical properties (see “Physical properties”). A more complete description of these subunits is presented as follows. Subunit IA
Subunit IA comprises a 251 m thick succession of greenish gray silty clay, clayey silt, and clay with minor amounts of grayish silty clay and <50 cm intercalations of volcanic ash layers. Bioturbation is particularly observed in the upper part of the unit. Zoophycos and Chondrites burrows are abundant. Patches of ash and pumice are observed throughout the succession as <1 cm silt-sized intervals with mottled color and as scattered grains or clasts (see Site C0011 smear slides in “Core descriptions”). Silt- and sand-sized ash layers are more common in the uppermost half of the subunit to ~100 mbsf (Figs. F3, F4). Notable is also a marked increase in sedimentation rate below 80 mbsf (see “Biostratigraphy”). Deposition in Subunit IA was dominated by hemipelagic settling. However, several sand-sized ash layers display turbidite characteristics (basal lamination and upward fining) (Fig. F5) and contain significant amounts of reworked volcaniclastic grains as observed in smear slides (Fig. F7; see also Site C0011 smear slides in “Core descriptions”), suggesting these were remobilized. The occurrence of fresh volcanic ash is important throughout the succession, gradually decreasing toward the Subunit IB boundary (Fig. F3). A small mass transport deposit (MTD) characterized by mixed sediments containing mud clasts is observed in the uppermost few meters of Hole C0011C, between 0.85 and 2.77 mbsf (Core 333-C0011C-1H). Subunit IB
The Subunit IA/IB boundary is marked by the abrupt appearance of abundantly burrowed and moderately consolidated mudstone (Section 333-C0011D-34X-1, 3 cm) (Fig. F3). In addition, a sharp gradient of ash alteration is observed on smear slides from discrete ash layers (Fig. F8; see also Site C0011 smear slides in “Core descriptions”). In an ash layer at Section 333-C0011D-35X-1, 72 cm, glass shards have no sharp edges and present dissolution pits. In an ash layer at Section 333-C0011D-35X-CC, 12 cm, glass appears almost entirely replaced by clay. From this level down to near the transition to lithologic Unit II, all ash layers observed on smear slides are altered with no or very little remaining glass. The dominant lithology of Subunit IB is a heavily bioturbated greenish brown to dark gray mudstone with only minor occurrence of altered volcanic ash layers (Figs. F3, F7). Clay minerals and altered volcanic glass are the most abundant particles on smear slides, with accessory percentages of nannofossils, quartz, and opaque minerals (Figs. F8, F9; see Site C0011 smear slides in “Core descriptions”). Heavy minerals appear as trace percentages in most of the subunit. Deposition of Subunit IB was dominated by hemipelagic settling with minor contribution of volcanic ash and low-energy volcaniclastic turbidity currents that become more abundant toward the base of the subunit. Other than the differences in the degree of bioturbation (higher in Subunit IB) and occurrence of volcanic ash layers (lower in Subunit IB), there is no obvious primary lithology-based observation that could be related to the level of induration and volcanic ash alteration within Unit I. The sudden change in physical properties and correlated change in ash alteration state may thus be related to postdepositional diagenetic processes. Lithologic Unit II (volcanic turbidite facies)
Expedition 322 Scientists (2010) defined the top of Unit II at 340 mbsf (i.e., starting depth of RCB coring in Hole C0011B), referring to LWD data that show the first occurrence of sand layers at 337 m LSF and define the top of logging Subunit 2B. Detailed comparison between correlated lithology, natural gamma measured on whole-round multisensor core logger (MSCL-W) from the two expeditions, and LWD data (Fig. F10), however, reveals that this sand layer at 337 m LSF is not one of the characteristic coarse-grained tuffaceous sandstones that define the volcanic turbidite facies (see Expedition 322 Scientists [2010] and detailed description below). Instead, almost complete recovery over the critical interval during Expedition 333 allows for confident identification of the first occurrence of a coarse-grained tuffaceous sandstone and thus the top of Unit II at 347.82 mbsf (Section 333-C0011D-46X-4, 78.4 cm). The base of Unit II was not cored during Expedition 333 but extends to 479.06 mbsf as defined in Hole C0011B (Expedition 322 Scientists 2010) (Table T2). Below the Unit I/II boundary, mudstone (silty claystone to clayey siltstone) is less indurated than in Subunit IB and fresh volcanic glass is abundant (Fig. F8). The upper part of Unit II is dominated by coarser grained tuffaceous sandstone, heterolithic gravel, gravelly sand, and sand beds with sharp and well-defined upper and lower boundaries (Figs. F11, F12). These beds are separated by mud very similar to that of Subunit IB with respect to mineralogy, texture, and bioturbation intensity. The upper sandstone beds show normal grading with some scattered pumice. The beds in the middle part of the unit are also normally graded, and intervals of heterolithic gravel are dominated by pumice and mudstone clasts. In some cases, the upper and lower boundaries of these gravelly intervals are gradational. In other cases, there are sharp upper boundaries (Fig. F11). The lowermost bed cored in Hole C0011D shows normal grading and comprises some scattered pumice. Biotite, orthopyroxene, and hornblende are common in the tuffaceous sandstones (Fig. F12; see Site C0011 smear slides in “Core descriptions”). Throughout the deposition of Unit II, the paleoenvironment was dominated by sediment transport within a sandy turbidite system. Miocene sandy turbidites have previously been identified in the Shikoku Basin; the Miocene siliciclastic turbidites at Ocean Drilling Program (ODP) Site 1177 were derived from a relatively large land mass, most likely southern Japan, and the dispersal system spread terrigenous sediment over a broad area of the Shikoku Basin (Shipboard Scientific Party, 2001a; Fergusson, 2003). However, the deposits of Unit II were derived from a different source. See Expedition 322 Scientists (2010) for more detailed description of sandstone and inferred interpretation of provenance and of the channel system through which the turbidity currents moved. X-ray diffraction dataAccording to the X-ray diffraction (XRD) data (Fig. F13; Table T3), the relative abundance of total clay minerals for the entire drilled succession averages 63 wt%. There is a general trend of increasing relative abundance of clay minerals with depth: an average of 57 wt% at the top of the hole, increasing to an average of 67 wt% (ranging between 74 and 46 wt%) in Subunit IB and in background mudstones within Unit II. The relative clay mineral abundance in Unit II, however, drops within (tuffaceous) sandstone beds. Quartz is relatively stable in its relative abundance throughout Holes C0011C and C0011D (Fig. F13; Table T3), with only a slight decreasing trend occurring between ~200 and 250 mbsf (~2 wt% relative change), marking a slight but detectable difference in mineralogical composition between Subunit IA (average = 19 wt%) and Subunit IB (average = 17 wt%) and Unit II (average = 16 wt%). Similarly, feldspar abundance at the top of Unit I reaches an average of 18 wt% compared to an average of 12 wt% in Subunit IB. The relative amount of feldspar then increases downhole and reaches a maximum of 46 wt% within (tuffaceous) sandstone layers in Unit II. Calcite is only a trace constituent between 150 and 215 mbsf and again below 318 mbsf (Fig. F13; Table T3). However, calcite maxima are recorded close to the seafloor, at 250 mbsf (i.e., at the top of the highly bioturbated Subunit IB), and at ~310 mbsf. Calcite abundance decreases steadily with depth below these maxima. Thick beds of tuffaceous sandstone and gravel are depleted in calcite. X-ray fluorescence dataXRF analyses were performed on 57 samples from Holes C0011C and C0011D to estimate the bulk chemical composition of the sediments and to characterize compositional trends with depth and/or lithologic characteristics (Fig. F14; Table T4). Lithologic Unit I (hemipelagic/pyroclastic facies)As with the underlying units (Underwood et al., 2009), major element contents in the hemipelagic mud of Unit I span a relatively small range of values that resemble those of the upper continental crust as defined by Taylor and McLennan (1985). No significant change in major element contents can be observed over the Subunit IA/IB boundary (Fig. F14). The bulk composition of the MTD occurring next to the surface is similar to that of mud, whereas sand and volcanic ash layers show significantly different compositions. With the exception of few outlying samples, SiO2, Fe2O3, MgO, and TiO2 contents of mud appear fairly uniform throughout the entire unit (Fig. F14), with average values of 62.1 wt% for SiO2, 6.52 wt% for Fe2O3, 2.46 wt% for MgO, and 0.690 wt% for TiO2. Above ~80 mbsf, Al2O3 content shows slightly lower values than the underlying sediments whereas CaO content appears to be higher and more scattered. This may reflect the lower proportion of clay minerals together with the higher and more variable proportion of calcite observed by XRD in the upper part of Unit I. From the seafloor to ~80 mbsf, K2O content in mud shows an overall decreasing trend and appears constant throughout the rest of the unit. This may reflect variations of clay mineralogy in the uppermost 80 m of sediments. With depth, Na2O and P2O5 contents gently decrease throughout Unit I and MnO values increase and scatter below ~150 mbsf. One outlying sample of hemipelagic mud at 238.44 mbsf shows very high Fe2O3 concentration (up to 22.6 wt%) together with high MnO and CaO contents and low SiO2 and Al2O3 concentrations (Fig. F14). This high Fe2O3 content, associated with very high sulfur concentration (up to 11 wt%, see “Organic geochemistry”), likely accounts for the presence of pyrite (FeS2), a mineral that is scattered in this unit (see Site C0011 smear slides in “Core descriptions”). Volcanic materials sampled in the upper part of Unit I have high SiO2 and Na2O concentrations together with low Al2O3, Fe2O3, MgO, P2O5, and TiO2 contents compared to the background composition of the hemipelagic mud (Fig. F14). The volcanic sand and the ash layer sampled at 45.02 and 125.08 mbsf show significantly higher K2O content than the ash layer sampled at 134.20 mbsf (4.90 wt% versus 2.15 wt%). Such a difference in composition may suggest either a change or a chemical evolution of the volcanic sources in the upper part of Unit I. Compared to the average composition of Unit I, the sample of sand analyzed at 338.54 mbsf shows relatively high CaO, MnO, and P2O5 concentrations (Fig. F14). This difference is likely related to the presence of apatite (Ca[PO4]3[F,Cl,OH]), commonly observed in sandstone (see Site C0011 smear slides in “Core descriptions”), and rhodochrosite (MnCO3) as suggested by Underwood et al. (2009) to explain the very high MnO content in the lime mudstones of the underlying Units II and III. Lithologic Unit II (volcanic turbidite facies)Major element compositions of mudstones and tuffaceous sandstones analyzed in the upper part of Unit II are consistent with the data of Underwood et al. (2009) as shown in Figure F14. When compared to the background composition of mudstone, tuffaceous sandstones show relatively high SiO2 and Na2O contents together with low Al2O3, Fe2O3, MgO, TiO2, and K2O concentrations. Around the Unit I/II boundary, the overall increasing with depth trend in Al2O3 concentration is followed by an abrupt downward decrease between 340 and 380 mbsf and an even more abrupt return to background values. This transition may result from a major volcanic event that dramatically changed the chemical composition of the sediment input at ~380 mbsf. Such a volcanic event constitutes a potential source for the tuffaceous material found in the overlying sandstones and may account for the increasing trend in SiO2 content as well as the second-order decreasing Fe2O3, MgO, and TiO2 values observed at the same depths. |
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