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doi:10.2204/iodp.proc.338.107.2014

Lithology

One lithologic unit with two lithologic subunits are recognized in Hole C0022B (Fig. F7; Table T1). Coring and logging at this site targeted a slope basin interval that was previously described at Sites C0004 and C0008 (Expedition 316 Scientists, 2009a, 2009b; Strasser et al., 2009, 2011; Kimura et al., 2011). A total of 41 cores yielded 316 sections (Table T2), with recovery ranging from 154% to ~20% (average = ~82%) (see “Background and objectives”).

Unit designations applied here are adopted with minor modification from Kimura et al. (2011). Units are differentiated based on contrasts in major and minor lithologies, grain size, mineralogy, age (see “Biostratigraphy” and “Paleomagnetism”), and log character (see “Logging while drilling”). Caution is required in the assessment of overall sand content and sand/mud ratios, as many of the cores, notably Cores 338-C0022B-12X through 34X, display prominent coring disturbance with heavy biscuiting and fracturing with admixing of drilling slurry. It is likely that much of the sand washed out during coring (see “Operations”).

Lithologic Subunit IA (slope sediment)

• Interval: Sections 338-C00022B-1H-1, 0 cm, to 38X-2, 56 cm
• Depth: 0–383.47 mbsf

The dominant lithology of lithologic Subunit IA is dark olive-gray silty clay (2.5GY 4/1) with variable components of calcareous nannofossils, foraminifers, siliceous biogenic debris (sponge spicules, diatoms, silicoflagellates, and radiolarians), and volcanic ash. Calcite, determined by X-ray diffraction (XRD), ranges from the detection limit (~0.5%) to ~38%, with an average of ~7%. A trend toward diminishing carbonate content with depth is observed (Fig. F8; Tables T3, T4, T5). The ratio of feldspar to quartz + feldspar trends toward larger values across the depth range of this subunit (Fig. F8).

Minor lithologies include sand, sandy silt, silt, and volcanic ash, which vary significantly in thickness and frequency throughout the section (Fig. F9). A few beds with recognizable fining-upward successions are observed. Such beds typically begin with a sharp-based ~2 cm dark gray sand layer, grading upward through burrowed clayey silt into silty clay that typically contains more pelagic debris (coccoliths and siliceous skeletal elements) (Fig. F10). Many sands contain benthic foraminifers in amounts up to several volume percent (Fig. F11B). Blebs of volcanic ash (possible lapilli), discrete pumice fragments (granule to pebble size), and thin ash layers are widely distributed through most of the core sections.

Faint green color banding and mottling is present throughout most of the subunit (Fig. F10). Bioturbation is common and includes Chondrites, Zoophycos, possible Trichichnus, and other discrete burrows, many of which are pyritized. Bioturbation is especially apparent in the X-ray computed tomography (CT) images in the shallowest cores. Rare tubes of a sponge spicule agglutinating organism (possible benthic foraminifer) are observed throughout the hole.

The silt and sand fraction is dominated by quartz, feldspar, and metasedimentary lithic fragments and locally includes abundant clear volcanic glass and pumice fragments (Fig. F11E, F11F). Volcanic ash layers are distributed throughout lithologic Subunit IA (Fig. F7). Many of the sands are dark gray to nearly black, a coloration that arises from the high content of authigenic pyrite (Fig. F12) rather than from a mafic volcaniclastic component. In the most pyrite-rich sand, pyrite takes the form of microcrystalline grain coatings that greatly hinder grain identification (Fig. F12B). In other sands, pyrite is in the form of disseminated framboids (Fig. F12A), in some cases localized along burrows that survive as pyrite-cemented aggregates in smear slides (Fig. F12C).

Lithologic Subunit IB

• Interval: Sections 338-C0022B-38X-2, 56 cm, through 41X-CC
• Depth: 383.47–415.90 mbsf

The top of lithologic Subunit IB is placed at the first occurrence of a discrete bed of mud clast gravel (granule size) at 383.5 mbsf (Sample 338-C0022B-38X-2, 56 cm) (Fig. F13). At approximately the same depth, the gamma ray log suggests a substantial increase in sand below this level (see “Logging while drilling”). The upper portion of the subunit (within Core 38X) consists of a series of interbedded mud clast gravels (Figs. F7, F13), with layers of thin sand, clayey silt, and silty clay. Gravel beds range from 2 to 30 cm thick and are composed of well-rounded greenish gray to dark greenish gray slightly indurated silty clay clasts, which range in size from <1 mm to 5 cm, in an olive to greenish gray silty clay matrix (Fig. F13). The gravel is mostly unconsolidated, but within the consolidated sections gravel shows fabric variation from matrix supported within a silty clay matrix to clast supported. The great majority of clasts are rounded to subrounded, with rare subangular pieces. In smear slides, an abundance of possible sand-sized clay clasts is observed (Fig. F14).

Below Core 338-C0022B-39X, the dominant lithology within Subunit IB is dark olive-gray silty clay with significant interbeds of clayey silt and dark gray fine- to coarse-grained sandstone. Sand contains subordinate quartz, abundant plagioclase, and as much as 2% or more of dense mafic minerals (mostly brown amphibole and pyroxene) (Fig. F15) with common brown to black semiopaque grains, which could be intermediate mafic volcanic rock fragments, clay clasts, or glauconite. A notable crystal ash occurs in Sample 39X-5, 40 cm. This ash contains an abundance of clear glass with a significant component of euhedral brown hornblende, euhedral zoned plagioclase, and brown glass (Fig. F15E, F15F).

X-ray fluorescence core scanning

Section 338-C0022B-8H-4 shear bands

The sediment in Section 338-C0022B-8H-4 has distinct shear bands (see “Structural geology”). The section interval between 5 and 90 cm was scanned using the TATSCAN-F2 core scanner (Sakamoto et al., 2006) (Fig. F16; Tables T5, T6, T7). A scanning resolution of 0.5 cm was used with a measurement time of 200 ms per measurement. Within this section, an area 60 cm² (40–55 cm) was rescanned using the mapping utility of the TATSCAN-F2. A scanning resolution of 0.5 cm and a measurement time of 200 ms was used again, but this time elemental composition was measured over a 15 cm × 4 cm area.

In the line scan, we see an increase in Al₂O₃ around the shear bands (Fig. F16). The increase in Al₂O₃ is less clear in the lowest shear band at 65 cm, but this might be due to the crack present in this area, which hampers the collection of the X-ray signal. In general, K₂O is higher around the shear bands. Fe₂O₃ is rather low in the upper part of the section across the first two shear bands, except for the Fe₂O₃ peak at 20 cm, which can be linked to a patch of ash. At the shear band at 65 cm, Fe₂O₃ is high and K₂O is low.

CaO variability corresponds to the presence of patches of white coloration (Fig. F16). In areas with more prominent white coloration, CaO is higher. The increase in CaO content is even more obvious in the X-ray fluorescence (XRF) mapping (Fig. F17). CaO concentrations are clearly higher in the whiter areas, whereas SiO₂ concentrations are lower. Al₂O₃ is also lower in whiter areas. Al₂O₃ concentrations are higher in the shear band area. This could indicate that more clay minerals are present in this area, although it could also be linked to reduced porosity (Milliken and Reed, 2011). The XRF mapping also indicates that the increase in Fe₂O₃ is not associated with the darkest coloration but, rather, is linked to areas with a green color. Possibly the green color indicates a higher glauconite or chlorite content.

Section 338-C0022B-38X-5 mud clasts

The sediment in Section 338-C0022B-38X-5 contains mud clast intervals within a clearly deformed zone (Figs. F13C, F18). Unfortunately, this section has a lot of drilling disturbance (biscuiting) and cracks, hampering collection of a good XRF measurement.

The interval between ~10 and 20 cm, where most of the mud clasts occur, shows higher Fe₂O₃ and K₂O values and lower Al₂O₃ values at the depth where the clasts are present (orange dashed line in Fig. F18). Overall, the interval shows a general increase in Al₂O₃ (yellow band in figure). Al₂O₃ also shows a decreasing trend in regions where the sediment surface has a green color (green bands in figure). The fining-upward interval between ~80 and 65 cm has rather variable Fe₂O₃ content but shows a clear increase in CaO as grain size decreases (gray band in figure). The deformed interval between ~85 and 120 cm is rather homogeneous (orange band in figure), except for the occurrence of some very small mud clasts at ~105 cm (orange dashed line in figure). At intervals where distinct burrows are visible, indicating the homogenization of the sediment by bioturbation, the XRF line scan results show very little variability (black dashed/dotted lines in figure).

Comparison to lithologic variations at Sites C0004 and C0008

The stratigraphic interval sampled in Hole C0022B was previously cored at Sites C0004 and C0008 (Expedition 316 Scientists, 2009a, 2009b) (Fig. F19). The slope basin interval cored at Site C0018 (Expedition 333 Scientists, 2011) is also analogous but cannot be easily correlated to Hole C0022B without biostratigraphic data because an anticlinal structure separates the stratigraphic successions at these two sites (Kimura et al., 2011). Hole C0022B, as seen in seismic sections (Fig. F20), penetrated Unit I as defined by Kimura et al. (2011). Based on interpretation of the seismic section, lithologic Subunit IB at Site C0022 is interpreted to be within the lower slope basin interval that onlaps onto the underlying prism, thinning in the direction of Site C0008. Age determinations (see “Biostratigraphy”) show that the lowermost portion of Hole C0022B (>400 m; 2.3–2.5 Ma) is approximately correlative with sediment just above the upper boundary of the accretionary prism in the lowermost portion of Subunit IB of Hole C0008A.

Mineralogical composition observed in Hole C0022B has similarities and contrasts with compositions observed at Sites C0004, C0008, and C0018. The feldspar content (Table T3; Fig. F19), together with the contents of mafic glass, heavy minerals (abundant brown hornblende and pyroxenes), and intermediate to basaltic lithic fragments, displays a trend suggesting increasing contribution of mafic volcanic material with depth that begins in the lower portion of lithologic Subunit IA, becoming quite pronounced within the coarser materials of lithologic Subunit IB. The presence of mafic material displays a similar pattern at all three sites, although a portion of the trend is missing at Site C0004, as a consequence of faulting along the megasplay.

The overall decline in carbonate content with depth in lithologic Subunit IA is not uniform across the sites. All four sites have the highest carbonate content (Fig. F19) in the shallowest portion of lithologic Subunit IA and display a trend of declining carbonate content with depth. The dominant form of carbonate at all three sites is coccolith debris, although calcareous benthic foraminifers are also reported as important components within sand (Expedition 316 Scientists, 2009a; Expedition 333 Scientists, 2011). At Site C0018, the decline in carbonate occurs at the Subunit IA/IB boundary. At Site C0004, carbonate is much diminished in the lower portion of Subunit IC sampled below the fault, but the transition from high to low carbonate has been removed by faulting. In Hole C0022B, the decline in carbonate occurs between ~230 and 270 mbsf (Figs. F19, F20), possibly corresponding to reflector “A” of Kimura et al. (2011) within the lower portion of their Subunit IC. At Site C0008, however, the carbonate content remains high until the appearance of the mud clast gravels (Fig. F19) (placed within Subunit IC by Kimura et al. [2011] but placed here within lithologic Subunit IB), very similar to the situation at Site C0018.

Summary

Lithologic Subunit IA is interpreted as a slope basin succession in agreement with previous work (Expedition 316 Scientists, 2009a, 2009b; Strasser et al., 2009, 2011; Kimura et al., 2011). Within this subunit, there is an overall trend of increasing siliciclastic composition, both with depth and across the sites, that is balanced by an associated decrease in the content of pelagic biogenic debris. Unit I at Site C0008 contains more carbonate overall and retains more carbonate to a greater depth than Unit I at Sites C0004 and C0022. We tentatively propose that this trend relates to a relatively more distal depositional position at Site C0008 at the more seaward edge of the slope basin. Lithologic Subunit IB, with more and coarser sand, may represent the earliest stages of the slope basin fill.

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