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doi:10.2204/iodp.proc.338.106.2014

Lithology

Hole C0021B was drilled to 194.5 mbsf. A total of 14 cores were recovered in two intervals: 0–5.9 and 80–194.5 mbsf. Core recovery was high, often exceeding 100% because of core expansion. Core quality was generally high, although pervasive core disturbance (flow-in and liner jamming in the core barrel) occurred in Cores 338-C0021B-11H and 12H, which led to a change in coring system to acquire Cores 13T and 14T (see “Operations”).

The recovered sedimentary succession is divided into one lithologic unit (I), including two subunits (IA and IB), on the basis of visual core descriptions (including smear slides) and X-ray computed tomography (CT) (Fig. F9; Table T1). These subunits are consistent with LWD data from Hole C0021A (see “Logging while drilling”) and regional seismic stratigraphy (Strasser et al., 2011). The naming convention used here is consistent with nearby Site C0018 (Expedition 333 Scientists, 2011). However, Subunits IA and IB defined here and at Site C0018 are equivalent to Subunit IA at Site C0022 (see “Lithology” in the “Site C0022” chapter [Strasser et al., 2014d]), according to regional seismic stratigraphic interpretation (Strasser et al., 2011).

Lithologic Subunit IA is composed of mottled greenish gray silty clay with local thin interbeds of fine sand and ash layers. Subunit IA also contains intervals with chaotic and distorted bedding interpreted to be evidence of MTDs. The Subunit IA/IB boundary is defined at 176.16 mbsf. Subunit IB is composed of a succession of thin sand beds interbedded with silty clay and local ash layers. Individual sand beds are generally 2–10 cm thick (35 cm thick maximum) (Fig. F10) and spaced ~20–30 cm apart.

Subunit IA (slope basin)

• Interval: Sections 338-C0021B-1H-1, 0 cm, to 13T-1, 66 cm
• Depth: 0–176.16 mbsf

The dominant lithology of Subunit IA is mottled greenish gray silty clay that contains a variable amount of calcareous nannofossils, foraminifers, siliceous biogenic debris (sponge spicules, diatoms, silicoflagellates, and radiolarians), and volcanic ash (Fig. F11; see Site C0021 smear slides in “Core descriptions”). Within the silty clay, minor interbeds of fine sand and volcanic ash vary significantly in thickness and frequency (Figs. F10, F11A). A few beds with recognizable fining-upward successions are observed. Such beds typically begin with sharp-based ~2 cm dark gray sand, grading upward through burrowed silty clay. Dispersed 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–brown color banding and mottling is common throughout most of Subunit IA, interpreted to be a signature of bioturbation. Bioturbation includes Chondrites, Zoophycos, possible Trichichnus, and other discrete burrows, many of which are pyritized. Bioturbation is particularly apparent in CT images as thin, subvertically oriented ribbons with high CT values.

Bulk mineral compositions (total clay, quartz, feldspar, and calcite) were estimated by X-ray diffraction (XRD). Calcite content ranges between 4 and 35 wt%, with an average of ~18 wt%. A trend toward diminishing carbonate content with depth is observed (Fig. F12; Table T2). Total clay, quartz, and feldspar contents are variable across the depth range of this subunit around average values of 44.6, 20.7, and 16.5 wt%, respectively (Fig. F12).

Two MTD intervals, including mud clasts, chaotic bedding, and steeply dipping beds, occur within Subunit IA (Table T3; Fig. F13) (see “Mass transport deposits”). The style of deformation is distinct from drilling/coring/handling and flow-in disturbance observed in Cores 338-C0021B-11H and 12H. The first interval (MTD A) occurs between 94.16 and 116.75 mbsf. A second, thicker MTD interval (MTD B) occurs between 133.76 and 176.16 mbsf.

Subunit IB (sand-rich slope basin)

• Interval: Sections 338-C0021B-13T-1, 66 cm, through 14T-CC
• Depth: 176.16–194.5 mbsf (end of core)

Subunit IB consists of interbedded fine-grained sand and silty clay (Fig. F11). The top of Subunit IB is defined at 176.16 mbsf at the first occurrence of repeating sand beds with spacing of 20–30 cm. This depth marks the boundary between the predominantly silty clay above (Subunit IA) and a succession of thin, frequent sand beds with minor interbeds of silty clay below (Subunit IB). This depth also corresponds to a prominent decrease in shear strength and increase in porosity (“Physical properties”).

Bulk mineralogy of the silty clay, determined by XRD, shows higher feldspar and quartz values in this subunit compared to Subunit IA (Fig. F12; Table T2), whereas the ratio of feldspar to quartz + feldspar remains constant across the subunit boundary. Mean values for calcite are low in Subunit IB, ranging from 3.1 wt% to below detection limit (<0.5 wt%).

Individual sand beds in Subunit IB are subhorizontal and typically a few centimeters to a few decimeters thick; these beds have a sharp base and a fining-upward grading (Fig. F11B). Sand bed spacing is ~20–40 cm. Sand mineralogy is dominated by quartz, feldspar, and metasedimentary lithic fragments (Fig. F11; Site C0021 smear slides in “Core descriptions”). Many of the sands are dark gray to nearly black, a coloration that arises from the high content of authigenic pyrite, similar to what has been described at Site C0022 (see “Lithology” in the “Site C0022” chapter [Strasser et al., 2014d]).

Mass transport deposits

MTD intervals (MTDs A and B) were defined based on the supporting information of visual core descriptions, X-ray CT images, physical properties, measurements of structural geology elements, and seismic data interpretation (Fig. F13; Table T3). Each MTD corresponds to

• The occurrence of mud clasts, tilted bedding, and/or chaotic bedding,

• An increase in shear strength and a decrease in porosity (see “Physical properties”),

• The occurrence of shear zones/faults (see “Structural geology”), and

• A semitransparent seismic facies (see “Core-log-seismic integration”).

The tops of MTDs A and B are defined as zones of mud clasts capped by thin, draping sand. Mud clasts are round and are 1–3 cm in the longest dimension. Below the mud clasts, a zone of variable chaotic/tilted/homogeneous bedding occurs. The bases of the MTDs are not obvious visually or in CT images. Therefore, the bases of the MTDs are defined based on the supporting data of structural elements and physical properties. Specifically, the base of MTD A is defined as the last occurrence of a shear zone in Section 338-C0021B-5H-8, 6 cm (116.75 mbsf). This also corresponds to the base of a zone with relatively high shear strength. The base of MTD B (176.16 mbsf) is defined as the last occurrence of a shear zone in Section 13T-1, 80 cm (see “Structural geology”). This also corresponds to the base of a zone of relatively high shear strength (see “Physical properties”). Preliminary core-log-seismic integration suggests that MTD B is correlative to MTD 6 at Site C0018 (see “Core-log-seismic integration”).

X-ray fluorescence analyses

X-ray fluorescence (XRF) analyses were performed on 24 samples from Hole C0021B to estimate the bulk chemical composition of the sediment and to characterize compositional trends with depth and/or lithologic characteristics (Fig. F14; Table T4).

The silty clay of Subunit IA is characterized by relatively low SiO₂ and Al₂O₃ content (~60 and 15 wt% on average, respectively). Below ~150 mbsf, these increase to higher values reaching 63.3 and 16.8 wt%, respectively, in bulk silty clay samples from sand-rich Subunit IB (Fig. F14). This downhole increase may be explained by a higher proportion of siliciclastic grains in Subunit IB. In contrast, Subunit IA contains somewhat higher CaO content. The marked decrease in CaO from ~9 wt% in Subunit IA to ~3 wt% in Subunit IB is consistent with the very low calcite content observed by XRD below ~176 mbsf and CaCO₃ determined by CARB measurements (see “Geochemistry”).

Large variations observed for almost all the major elements at various depths (e.g., at 91.8 mbsf; Fig. F14) likely result from the sampling of specific lithologies such as volcaniclastic sand beds and volcanic ash layers.

In general, the lower part of Subunit IA (i.e., where data are available below 80 mbsf) shows rather homogeneous composition of Al₂O₃, Fe₂O₃, K₂O, P₂O₅, MgO, and TiO₂ (Fig. F14; Table T4). The bulk composition of the MTDs does not differ from the composition of the homogeneous overlying strata. However, below 150 mbsf, and thus within the lower part of MTD B, systematic trends toward Subunit IB potentially suggest that the MTD partly incorporated the Subunit IB strata.

Summary

Sediment in Hole C0021B was drilled to 194.5 mbsf within the slope basin seaward of the megasplay fault. Cores were collected in two zones: 0–5.9 and 80–194.5 mbsf. Two lithologic subunits are defined: Subunits IA and IB. Subunit IA is dominated by silty clay and contains two MTDs. Subunit IB contains thin, frequent sand interbedded with silty clay. Volcaniclastic ash layers are present in both units. This succession is lithologically similar to previously drilled holes in this basin (Expedition 333 Scientists, 2011; Expedition 316 Scientists, 2009a, 2009b; Strasser et al., 2009, 2011; Kimura et al., 2011).

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