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doi:10.2204/iodp.proc.311.104.2006

Lithostratigraphy

Site U1326 is located at the far western downslope end of the Expedition 311 transect, on the first uplifted ridge of accreted sediments. Three of the four holes (Holes U1326A–U1326D) at this site were cored; Hole U1326B was cored to 1.5 mbsf (1.5 m cored), Hole U1326C to 86.7 mbsf (85.7 m cored), and Hole U1326D to 300 mbsf (192.6 m cored). Recovery was excellent in Hole U1326B (103.3%) but limited in Holes U1326C (63.5%) and U1326D (63.3%).

We divided the 271.4 m thick sedimentary section recovered from Holes U1326C and U1326D into three lithostratigraphic units (Fig. F7) based on visual inspection of the cores and analysis of smear slides. Other parameters, such as mineralogy data from X-ray diffraction (XRD) analyses, helped to better define the entire stratigraphic section. The results were also compared and correlated with seismic data, downhole LWD/MWD data, and physical property measurements.

Lithostratigraphic units

Lithostratigraphic Unit I

• Intervals: Sections 311-U1326B-1H-1 through 1H-CC and 311-U1326C-1H-1 through 4H-1
• Depths: Hole U1326B: 0.00–1.55 mbsf and Hole U1326C: 0.00–24.13 mbsf
• Age: Holocene–Pleistocene (<0.3 Ma)

Unit I is mainly composed of dark gray (N4) and dark greenish gray (5GY 4/1) silty clay and clay, and clay with diatoms (Fig. F8). Gradual color changes from dark gray to dark greenish gray are observed especially in Core 311-U1326C-3H (Fig. F8). Minor lithologies are silty clay with foraminifers, dark gray foraminifer sand, dark gray silt, silty sand, and sand layers or lenses (Fig. F9). The silt and sand layers in Sections 311-U1326C-2H-2, 2H-5 through 2H-7, 3H-1, 3H-3, 3H-4, and 4H-1 are all tilted in the same direction. The single layers show varying thickness from a few millimeters to ~10 cm. In Section 311-U1326C-2H-2, layers show coarsening-upward sequences. The major nonbiogenic components of Unit I are clay minerals, quartz, feldspar, opaque minerals, accessory minerals, biotite, and some rock fragments. Rare to moderate sulfide mottling, appearing as dark gray (N4) stains, occurs in Sections 311-U1326C-1H-2, 2H-2, 2H-4, 2H-5, 3H-1, and 3H-3 through 3H-6. Small sulfide concretions (<2 mm) are present in Sections 311-U1326C-1H-2, 2H-5, and 2H-6.

The major biogenic components of Unit I are diatoms (as much as 10% in Sample 311-U1326C-3H-6, 90 cm). Foraminifers occur as well and are especially abundant (35%) in Sample 311-U1326C-1H-2, 40 cm. Visibly observed foraminifers occur in Section 311-U1326C-1H-CC. Bivalve shell fragments are observed in Sections 311-U1326C-1H-2 (Fig. F10) and 3H-5, and sponge spicules are seen in Sections 311-U1326C-3H-4 and 3H-5.

Some small sedimentary structures related to soft-sediment deformation are observed in Section 311-U1326C-2H-5. Greenish patches, rich in glauconite, are present in intervals 311-U1326C-1H-2, 30–65 cm; 2H-4, 15–65 cm; and 2H-5, 0–10 cm. Unlithified carbonate cement, as well as partly lithified carbonate, is observed in intervals 311-U1326B-1H-1, 95–102 cm (nonstoichiometric dolomite; 46 mol% MgCO₃) (Fig. F11); 1H-1, 149–150 cm (Fig. F12); and 1H-2, 62–64 cm. Some small concretions (millimeter scale) are present in Section 311-U1326C-3H-1. The Unit I/II boundary is marked by the disappearance of diatoms and calcareous fossils and an increase in silty and sandy layers in the minor lithology.

Lithostratigraphic Unit II

• Intervals: Sections 311-U1326C-4H-1 through 10X-CC and 311-U1326D-2X-1 through 7X-CC
• Depths: Hole U1326C: 24.13–82.70 mbsf and Hole U1326D: 88.40–146.30
• Age: Pleistocene (>0.3 Ma)

Unit II is composed of dark gray (N4) clay and silty clay and dark olive-gray (5Y 3/2) clay locally interbedded with dark gray (N4) silt, silty sand, and sand layers (Fig. F13). In the lower part of Unit II, some sand patches occur within clay or silty clay (Fig. F14). Some silty or sandy layers show fining-upward sequences in Sections 311-U1326C-6X-4, 7X-2, 9X-2, 311-U1326D-2X-4, and 7X-3. Parallel laminations are present in Sections 311-U1326C-6X-5 and 311-U1326D-3X-5. Silt laminae are observed in Sections 311-U1326D-9X-4 and 10X-1.

The major nonbiogenic components of Unit II are clay minerals, quartz, feldspar, accessory minerals (mainly amphibole), opaque minerals, and biotite. Rare sulfide mottling is observed in the upper (Sections 311-U1326C-5X-1 and 5X-3) and lower (Sections 311-U1326D-5X-1, 5X-4, 6X-3, 6X-CC, and 7X-3) part of this unit. Sulfide concretions are present in Sections 311-U1326D-5X-1 and 5X-2. Dropstones (<1.5 cm) are found in the lower part of Unit II (Sections 311-U1326D-4X-4, 4X-5, 5X-4, and 6X-3) (Fig. F14). Unlithified carbonate cements are present in Sections 311-U1326C-6X-1, 8X-CC, and 311-U1326D-5X-2 (Fig. F15). A lithified carbonate is found in Section 311-U1326C-9X-2.

Biogenic components are almost absent in Unit II. There are no diatoms. Foraminifers, nannofossils, and organic fragments are rarely present on the basis of smear slide observations. Pieces of brownish wood fragments are observed in Section 311-U1326D-5X-5.

Soft-sediment deformation is observed over an interval of two cores in the upper part of Unit II (Cores 311-1326C-5X through 6X) (Fig. F16). A fault displacing silty layers in a normal sense is present at the bottom part of the zone of soft sediment deformation (interval 311-1326C-6X-5, 24–26 cm) (Fig. F17).

In Unit II, we observed soupy and mousselike textures. Soupy textures are present in Sections 311-U1326C-7X-3 and 8X-1. Mousselike textures are observed in Sections 311-U1326C-7X-3 and 311-U1326D-2X-1, 2X-4, 3X-1, and 7X-1. The Unit II/III boundary is marked by the appearance of diatoms (see "Biostratigraphy"), although their abundance is low and at the beginning of coring-related biscuiting.

Lithostratigraphic Unit III

• Interval: Sections 311-U1326D-8X-1 through 20X-CC
• Depth: 146.30–271.40 mbsf
• Age: Pleistocene (>0.3 Ma)

Unit III is mainly composed of dark greenish gray (5GY 4/1) and dark gray (N4) clay, clay with diatoms, silty clay, and silty clay with diatoms. Coarser grained dark gray (N4) and very dark gray (N3) clayey silt, sandy silt, silt, sand-silt-clay, silty sand, and sand occur as minor lithologies in Unit III (Fig. F17). Fining-upward sequences are observed in Sections 311-U1326D-10X-3 and 20X-5. Drilling biscuits and other coring-related disturbance features are present throughout most of the unit.

The major nonbiogenic components of Unit III are clay minerals, quartz, feldspar, accessory minerals, opaque minerals, and biotite. Rare to moderate sulfide mottles are only present in the upper part of Unit III in Sections 311-U1326D-8X-3, 9X-2, 11X-2, 11X-4, and 20X-7. Small (<5 mm) concretions are present in Sections 311-U1326D-11X-5 and 12X-1. A dark, round, volcanic rock fragment (1.5 cm x 2 cm) is observed in Section 311-U1326D-17X-1, and an olive colored rock fragment (1 cm x 1.5 cm) is seen in Section 311-U1326D-20X-5.

The biogenic components differ from Unit II. The major lithology contains some fossils, diatoms, foraminifers, siliceous spicules, and organic debris. The most important fossils are diatoms (as much as 37% in Sample 311-1326D-20X-5, 87 cm).

Angular, partly lithified carbonates occur in intervals 311-U1326D-9X-1, 0–2 cm, 16 cm, 18–19 cm, 25 cm (Fig. F18), and 46–47 cm. XRD analysis confirms a combination of high-Mg calcite (8 mol% MgCO₃) and nonstoichiometric dolomite (34 mol% MgCO₃). Unlithified carbonate cements are present in Sections 311-U1326-12X-5, 12X-CC, and 20X-3. A lithified carbonate nodule is present in Section 311-U1326D-16X-1.

Soupy sediment textures related to the presence of gas hydrate are present in Sections 311-U1326D-11X-2, 13X-1, and 17X-1. Mousselike sediment textures are present in Sections 311-U1326D-10X-1, 11X-2, 11X-4, 13X-1, 14X-1, 14X-3, 16X-1, 16X-3, 16X-4, 17X-1, 17X-5, 18X-2, and 18X-4 (Figs. F19, F20).

Environment of deposition

Site U1326 recovered a sequence that corresponds to slope basin sediments (see "Background and objectives").

Lithostratigraphic Unit III is characterized by fine-grained (clay to silty clay) detrital sediments with few, thin silty/sandy interlayers from turbidites. Generally abundant, mostly marine diatoms (see "Biostratigraphy") occur throughout this unit. We interpret this interval as mixed hemipelagic-turbiditic deposition. The presence of authigenic carbonate cement shows that diagenetic processes are active in Unit III.

Lithostratigraphic Unit II is characterized by fine-grained (clay to silty clay) detrital sediments with intervals of frequent silty/sandy interlayers. We interpret the coarser interlayers as turbiditic deposits, the frequent occurrence of which might indicate times of active tectonism. Below 30.4 mbsf, abundant soft-sediment deformation and dipping strata (also inferred from logging results; see evidence of layers dipping to the northeast in "Downhole logging") show that tectonism is more active at this westernmost end of the transect drilled during Expedition 311. The presence of unlithified authigenic carbonate cement and a lithified carbonate shows that diagenetic processes are active in Unit II (see also "Interstitial water geochemistry").

Lithostratigraphic Unit I is characterized by fine-grained (clay to silty clay) detrital sediments with thin silty/sandy interlayers from turbidites. Soft-sediment deformation observed in Section 311-U1326C-2H-5 (~10 mbsf) may be potentially related to a slump (see also "Interstitial water geochemistry"). Foraminifers, bivalve shell fragments, sponge spicule remains, mottling, and bioturbation, together with the high marine/nonmarine ratio of diatoms suggest hemipelagic sedimentation mixed with turbiditic inputs (coarse-grained facies). The presence of unlithified carbonate cement shows that diagenetic processes are active in Unit I.

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