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doi:10.2204/iodp.proc.314315316.133.2009 LithologyFour lithologic units were recognized during examination of cores from Site C0004 (Fig. F3; Table T3). Two subunits are recognized in Unit II. Units and subunits are differentiated based on contrasts in grain size, mineralogy, composition, and presence (and thickness) of minor lithologies. In choosing unit boundaries, we also considered biostratigraphic information, paleomagnetic data, X-ray computed tomography (CT) images, observations of structural style, and interpretations of LWD and seismic results obtained during Expedition 314 (see the “Expedition 314 Site C0004” chapter). Unit I (slope facies)
The dominant lithology of Unit I is greenish gray silty clay with a substantial component of calcareous nannofossils (up to ~25%) and a lesser amount of siliceous biogenic debris (sponge spicules, diatoms, and radiolarians) (Fig. F3; Table T3). In LWD data, the comparable base of logging Unit I was placed at 67.9 m CSF. Green color banding is present throughout most of the unit (Fig. F4) along with isolated glauconite grains. Secondary lithologies include thin interbeds and irregular patches of sand, sandy silt, silt, and volcanic ash. An additional minor lithology is synsedimentary breccia (silty clay clasts in a silty clay matrix; ranging from ~4 to 30 cm thick) recognized in X-ray CT images (Section 316-C0004C-1H-1) (Fig. F5). The breccia is cryptic in the split core. Sediment at the top of the core has suffered drilling-induced disturbance, including drag folding along the core liner, vertical rotation of disrupted stratum, and flow texture with liquefaction. Despite this disturbance, some horizontal layers are preserved, and it is clear that intervals of sedimentary breccia are interspersed with horizontally bedded sediment. Some trace fossils penetrate both clasts and horizontal layers, clear evidence that the brecciation predates coring and is not a drilling-induced artifact. Within the breccia layers, dark gray (low CT number) clasts are mixed within a light gray (high CT number) matrix. Within Unit I there is a consistent trend in bulk powder X-ray diffraction (XRD) data toward diminishing calcite content with depth (Fig. F6; Tables T4, T5), ranging from ~20% near the top of the cores to below XRD detection at the lower unit boundary. The silt and sand fraction is dominated by quartz and plagioclase (averaging 21% and 19% of the bulk sediment, respectively) and locally includes abundant clear volcanic glass and pumice fragments. The Unit I/II boundary is an angular unconformity that dips ~50° across the core (Fig. F7). The unconformity is associated with a significant stratigraphic hiatus, as indicated by both paleomagnetic and nannofossil data (see “Biostratigraphy” and “Paleomagnetism”). X-ray fluorescence (XRF) scanning data show significant variation in rock composition in the vicinity of the unconformity (Fig. F8); CaO declines uniformly from ~7 wt% in the silty mud 10 cm above the unconformity to ~2 wt% immediately above the unconformity. Iron displays an ~2 wt% increase in the 2 cm below the unconformity. Unit I was deposited from early to late Pleistocene as a sediment blanket on the trench slope mainly by hemipelagic settling with minor volcanic ash and sand-silt input. The gradual increase in carbonate content upsection is consistent with greater carbonate dissolution at depth and implies that the unit was slowly uplifted to its present water depth of 2632 m. Gradual tectonic uplift is consistent with accretion and thickening of the underlying accretionary prism during the Pleistocene. A similar history was recorded at IODP Site C0001 (see the “Expedition 315 Site C0001” chapter) and Ocean Drilling Program (ODP) Sites 1175, 1176, and 1178 of the Muroto transect (Shipboard Scientific Party, 2001a, 2001b, 2001c; Underwood et al., 2003; Moore, Taira, Klaus, et al., 2001; Moore et al., 2001), ~200 km west-southwest of Site C0004. Unit II (accretionary prism)
Strata below the unconformity at the top of Unit II extend from 78.06 m CSF in Hole C0004C to 258.01 m CSF (Fig. F3). Within this interval, Expedition 314 shipboard scientists recognized four subunits in the logs (see the “Expedition 314 Site C0004” chapter). Lithologic variations in the cores, however, do not precisely correspond to these divisions. Two subunits are recognized in the cores with a boundary at 117.72 m CSF. Subunit IIA (mass-transport complex)
The dominant lithology of Subunit IIA is greenish gray synsedimentary breccia made of rounded to subangular clasts of pebble size with subsidiary silty clay horizons (Figs. F7, F8). Both the clasts and the matrix in the breccia are composed of dark greenish gray silty clay, and no discernible compositional difference exists between the two parts of the sediment. The breccia is clast-supported in places and matrix-supported in others, and there are horizons in which no obvious clasts can be identified in either visual core description (VCD) or X-ray CT scans. We interpret the mud intervals as products of hemipelagic settling between episodes of mass wasting and formation of mass-transport deposits. Locally, patches of hemipelagic mud clasts occur within the base of overlying breccia beds, indicating reworking and a rather catastrophic style of mass wasting deposition (e.g., Hampton et al., 1996). The immediate upslope source of the mass-transport deposits, slope sediments versus reworked prism material versus some combination of these sources, cannot be discriminated on the basis of shipboard observation. Thin layers of silt or silty sand are rare and are restricted to the upper part of the unit (Fig. F3), although their absence in the lower part of the unit cannot be proven indisputably because of poor core recovery. Volcanic ash layers are also rare and thin (Fig. F9). Calcareous nannofossils are significantly less abundant in this unit than within comparable mud deposits of Unit I as shown by the calcite XRD values (Fig. F6), which are generally below detection (0%–5%; average = 0.8%). Quartz and plagioclase content in the bulk sediment (averaging 21% and 19%, respectively) display no trend within the unit. Preliminary observations in core and X-ray CT images suggest that sediments immediately below the unconformity (i.e., at the top of Unit II) preserve a complex paragenetic sequence that potentially encompasses burial, chemical alteration and deformation, uplift, and seafloor exposure. Pyrite mineralization is clearly visible in the core in the top 5 cm below the unconformity (Fig. F7B, F7C). Pyrite in proximity to the unconformity forms cubes and aggregates of cubes (Fig. F10) that contrast with the mostly framboidal form of pyrite that is widely distributed through the muddy lithologies throughout the core. In X-ray CT images, pyrite fills near-vertical fractures that end abruptly at the upper surface of the unconformity and do not transect the overlying hemipelagic mudstones (Fig. F7B). Subunit IIA has only been positively identified in Hole C0004C and does not appear to be present in Hole C0004D. However, drilling disturbance (by rotary drilling) in Hole C0004D in particular has caused widespread brecciation of Unit II, which may have masked much of the original texture of the rock, including the sedimentary breccias observed in Hole C0004C. The base of Subunit IIA is picked partially on the basis of a discernible drop in magnetic susceptibility (see “Paleomagnetism”) and partially on the basis of small changes in XRD mineralogy (Fig. F6). Subunit IIB
The dominant lithology in Subunit IIB is dark greenish gray silty clay. Minor breccias may also occur, but these are difficult to identify because of the extensive drilling-induced brecciation of the core. Sand and silt beds were not observed, though their absence could be ascribed to the poor core recovery throughout much of the interval because LWD in Expedition 314 suggests the existence of sand/silt beds in this unit (see the “Expedition 314 Site C0004” chapter). Minor ash (Fig. F9) and carbonate (0%–6%; average = 1%) occurrences are observed in the upper portions of this subunit. Quartz and plagioclase content in the mud average 18% and 17%, respectively. Overall, assessment of this subunit was hampered by poor core recovery. Summary for Unit IILogging Unit II was interpreted during Expedition 314 as the accretionary prism based on seismic data and log characteristics (see the “Expedition 314 Site C0004” chapter). Coring during Expedition 316 indicates that the uppermost part of Unit II consists mainly of sedimentary breccia with silty clay clasts that most likely resulted from deposition of slumps and mass wasting along an unstable slope. The original environment of deposition is conjectural. The low carbonate content of Unit II is consistent with deposition at much greater depths than its present setting and was presumably formed either close to or below the calcite compensation depth. Unit III (structurally bounded package)
Volcanic ash layers appear to be more common below 250 m CSF than in the overlying but poorly recovered Subunit IIB (Fig. F9). Dispersed glass and pumice clasts/microclasts are also common in the silty clay in this interval. Vitric grains are typically more abundant than quartz and feldspar grains (averaging 20% and 19%, respectively) in these sediments. XRD results show that calcite is also slightly more abundant (0%–9%; average = 2.7%) than in the overlying subunit. The exact position of the Unit II/III boundary is difficult to identify from lithologic information alone. However, a small biostratigraphic age reversal above and below 258.01 m CSF (Cores 316-C0004D-25R and 26R) coincides with a lithologic change to slightly more calcitic and ash-bearing sediments across a fault contact between Units II and III. A larger age reversal is found at the Unit III/IV boundary at 307.52 m CSF. Unit III is a structurally bounded package that hosts most of the prominent brittle deformation in the hanging wall of the thrust fault zone (see “Structural geology”). Its original stratigraphic affinity is ambiguous. The Pliocene-age fault-bounded sedimentary package may be related to the overlying slightly older Pliocene-age prism sediments (a repetition of the overlying age-equivalent upper part of Subunit IIB). Alternatively, a fault-transported sliver may have been derived from the underlying underthrust sequence (a unit that would be encountered beneath Unit IV). Unit IV (underthrust slope facies)
The dominant lithology of Unit IV is dark olive-gray silty clay with a moderate amount of calcareous nannofossils and a lesser amount of calcareous and siliceous microfossils (foraminifers, diatoms, radiolarians, and sponge spicules). The calcite content ranges from 0% to 39% and averages 3%. The mud is typically parallel laminated, although divergent or even wavy laminations can be observed locally. Bioturbation, by the Zoophycos-Chondrites association, is common throughout much of the section. Thin sand and silt beds are common in this unit, particularly in the upper part (Fig. F11), and some are graded. Grain size of the silty clay is typically coarser than in the overlying units. Volcanic ash beds are rare, along with a moderate amount of dispersed lapilli and glass shards. The low abundance of volcaniclastic grains as observed in smear slides, slightly greater carbonate content (as mentioned above), higher plagioclase content (average = 26%), and relative abundance of red-brown organic matter (Fig. F12) are some of the compositional attributes that have been used to distinguish these sediments from Unit III. Unit IV is considered to have formed in a lower trench-slope environment, dominated by fine-grained hemipelagic deposition with relatively minor input of sand and silt turbidites. The environment was similar to that of Unit I, although the inclination of the seafloor may have been flatter, thereby allowing for more turbidite deposition. The lower carbonate content compared to Unit I (Fig. F5) may indicate that deposition was closer to the carbonate compensation depth (CCD), or alternatively, the pelagic calcite has been diluted by a larger siliciclastic influx. X-ray CT-defined lithologiesX-ray CT images were used extensively for evaluation of structural features in Units II and III. Figure F13 shows the depth trends for CT number (averaged pixel intensity for a 1 mm2 area) determined for coherent rock pieces and tectonic breccia clasts. CT number reflects average sample density at the scale of the CT observation, a value that is affected both by sample composition (mineral composition and density) and porosity (bulk density); in these porous samples it is likely to be dominated by porosity. Across Units I, II, and III, CT number varies widely with no apparent discontinuities or depth trend. At the Unit III/IV boundary, CT number distinctly increases, which is consistent with the abrupt compositional change observed at this boundary. Average CT numbers of Unit I, Subunit IIB, and Units III and IV are 1223, 1235, 1208, and 1327, respectively. CT number of Unit III is notably lower than the others, especially in the lower part of Unit III below 274 m CSF, where it averages 1197. Attempted correlation of CT numbers to specific lithologies at the core scale was not successful. This result is not surprising, given the relatively high porosity in these samples and the relatively narrow range of densities characteristic of the main rock-forming minerals (clay minerals, quartz, feldspar, and calcite). |
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