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doi:10.2204/iodp.proc.314315316.136.2009 LithologyTwo lithologic units were recognized during examination of cores from Site C0008 (Fig. F3; Tables T3, T4). Two subunits are recognized in Unit I. 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 CT images, observations of structural style, and interpretations of seismic data. Unit I (slope sediments)
Unit I has been divided into two subunits with a boundary between the two at 234.55 m CSF in Hole C0008A (Fig. F3; Tables T3, T4) and at 170.90 m CSF in Hole C0008C. Subunit IA
The dominant lithology of Subunit IA is greenish gray silty clay with a substantial component of calcareous nannofossils, siliceous biogenic debris (sponge spicules, diatoms, silicoflagellates, and radiolarians), and volcanic ash. Calcite content is a distinctive feature of this subunit. Calcite determined by X-ray diffraction (XRD) ranges from 0% to 37% and averages ~7%. A consistent trend toward diminishing carbonate content with depth is observed (Fig. F4; Tables T5, T6). Faint green color banding is present throughout most of the unit, which is often disturbed by mottling or bioturbation. Secondary lithologies include sand, sandy silt, silt, and volcanic ash, which vary significantly in thickness and frequency throughout the section. The section between 0.00 and 63.00 m CSF in Hole C0008A and 0.00 and 40.00 m CSF in Hole C0008C is dominated by mud with very thin (up to 5 cm) sand interbeds. The section between 63.00 and 116.55 m CSF in Hole C0008A and 40.00 and 65.00 m CSF in Hole C0008C consists of graded sand/silt to mud beds containing ~40% sand/silt and 60% mud. These beds are cyclic, typically with a sharp-based 2 cm dark gray sand, grading upward into 8 cm of burrowed clayey silt and 12 cm of silty clay that typically contains more pelagic debris (coccoliths and siliceous skeletal elements) (Fig. F5). Clusters of volcaniclastic debris and/or ash layers are common near the top of the clay-rich intervals, typically 1–3 cm beneath the base of the next sand bed (Fig. F5B). From 116.55 m CSF in Hole C0008A and 65.00 m CSF in Hole C0008C to the base of the subunit, these graded beds persist, but the sand and silt beds are thinner and less frequent. Some caution is required in interpretation, however, as many of the cores in this section suffered from extensive flow-in structures. The silt and sand fraction is dominated by quartz and feldspar and metasedimentary lithic fragments and locally includes abundant clear volcanic glass and pumice fragments. Many of the sands are dark gray to nearly black, a coloration that arises from the high content of authigenic pyrite (Fig. F6) rather than from an abundant mafic volcaniclastic component. In the most pyrite-rich sands, pyrite takes the form of microcrystalline grain coatings that greatly hinder grain identification (Fig. F6A). In other sands, pyrite is in the form of disseminated framboids (Fig. F6B) and also framboids that are localized on fragments of terrigenous organic matter (Fig. F6C). Volcanic ash layers are distributed throughout Subunit IA (Fig. F7). A distinctive 5 m thick volcaniclastic sand bed is found at 185 m CSF in Hole C0008A with a very high volcaniclastic component (including basaltic fragments with brown glass) with abundant pumice and glass particularly concentrated near the top of the bed. The true thickness of this bed might be smaller as coring disturbance (flow-in structure) and/or gas expansion add uncertainty to the stratigraphic representation in the lower part of cores (see “Operations”). A thick ash layer encountered at 203 m CSF in Hole C0008A and 130 m CSF in Hole C0008B may serve as a regional stratigraphic marker (Fig. F7). Locally the coarser lithologies (sand, silty sand, sandy silt, and ash) of Subunit IA contain subequant cavities 1–5 cm in diameter that are interpreted as voids left by the decomposition of methane hydrates (see “Organic geochemistry”). The lower part of Subunit IA is a coarsening-upward sequence. We recognize a substantial decrease in sand and silt content in the upper 63 m CSF of Hole C0008A and above 40 m CSF in Hole C0008C. Subunit IA is interpreted as a slope succession. Reduced sand and silt input affected the upper part, possibly reflecting a change upslope that modified the supply of coarser sediment. Subunit IB
Subunit IB consists of a series of interbedded mudclast gravels and silty clay beds. The upper boundary of Subunit IIB is based on the first appearance of gravel-size mudclasts in the core and, compared to Subunit IA, displays lesser calcite content (average = 1%) and greater plagioclase content (average = 32%) in the mud as shown in XRD analysis (Table T6). Gravel beds range from 2 to 80 cm thick and are composed of greenish gray to dark greenish gray slightly indurated silty clay clasts and pumice pebbles, which range in size from <1 mm to 5 cm, in an olive to greenish gray silty clay or silt matrix (Fig. F8). The gravel is mostly unconsolidated, but where more highly consolidated sections do exist, they show a variation in clast-matrix fabric from matrix-supported to clast-supported. The great majority of clasts are rounded to subrounded, with rare subangular pieces (Fig. F8). In smear slides, an abundance of possible sand-size clay clasts is observed (Fig. F8C). The degree of clast induration and rounding is markedly greater than that observed in the sedimentary breccias at Site C0004. Another contrast with the sedimentary breccias at Site C0004 (Subunit IIA) is the lack of a pronounced unconformity capping the coarse units. As at Site C0004, no shipboard observations address the proximate source of these gravels. Subunit IB is interpreted as a mass-transport complex that accumulated in the lower slope environment, possibly during an early stage of postaccretion sedimentation. Unit II (sand-rich turbidites)
The dominant lithology is dark gray fine- to coarse-grained and locally pebbly sandstone with minor silty clay interbeds. The three mud samples analyzed by XRD contain amounts of calcite near the detection limit for the XRD method (0.5% in one sample). Sands are polymictic, and smear slide observations reveal abundant quartz, feldspar, and heavy minerals (mostly pyroxene) with common brown to black semiopaque grains, which could be mafic-intermediate volcanic rock fragments, clay clasts, or glauconite. Pebbles include rounded fragments of basalt, plutonic rocks, schist, vein quartz, chert, sandstone, and mudstone. This detrital grain assemblage is similar to that observed in the sands and associated gravels encountered in Unit II at Site C0007. As at Site C0007, the sands correspond to a zone of high drilling penetration rate and limited core recovery. The coarse-grained nature of this unit is consistent with the more axial portion of a trench wedge deposit. X-ray CT numberX-ray CT images were used extensively by the science party for evaluation of sedimentological and structural features. Figure F9 shows the depth trends for CT number (averaged pixel intensity for a 1 mm2 area) determined for coherent rock pieces and tectonic conglomerate clasts of silt clay. CT number reflects the average sample density at the scale of the CT observation, a value that is affected by sample composition (mineral composition and density) and porosity (bulk density), although in these porous samples it is largely dominated by the porosity. Across Units I and II CT number increases with depth. |
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