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doi:10.2204/iodp.proc.311.106.2006

Lithostratigraphy

Site U1328 is located ~3.7 km southeast of Site U1327 on the mid-continental slope off Vancouver Island (~1267.9 mbsl) and has been identified as an active cold vent. Four of the five holes (Holes U1328B–U1328E) at this site were cored. Hole U1328B was cored to 56.5 mbsf (54.5 m recovered), Hole U1328C to 300 mbsf (242.5 m recovered), Hole U1328D to 15 mbsf (15 m recovered), and Hole U1328E to 235 mbsf (49 m recovered). Recovery was generally good, but lower in Holes U1328D and U1328E (Hole U1328B = 71.3%, Hole U1328C = 80.3%, Hole U1328D = 66.9%, and Hole U1328E = 25.3%).

We divided the 300 m thick sedimentary section recovered at Site U1328 into three lithostratigraphic units (Fig. F11) based on visual inspection of the recovered cores and analysis of smear slides. Other parameters, such as mineralogy data from 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-U1328B-1H-1 through 10H-CC and 311-U1328C-1H-1 through 10X-2; Cores 311-U1328D-1H through 3P and 311-U1328E-1X through 10P
• Depths: Hole U1328B: 0.00–56.98 mbsf, Hole U1328C: 56.50–132.60 mbsf, Hole U1328D: 0.00–15.00 mbsf, and Hole U1328E: 0.00–92.26 mbsf
• Age: Pleistocene (<1.0 Ma)

Unit I is mainly composed of dark gray (N4) and dark greenish gray (5GY 4/1) clay and silty clay (Fig. F12). Fossils are very rarely observed in the major lithology; however, exceptions include diatoms in Core 311-U1328C-2H and 23% calcareous fossils (15% foraminifers and 7% nannofossils) in Section 311-U1328B-10H-1. The clay and silty clay is often interbedded with clayey silt, silt, sand-silt-clay, silty sand, and sand layers that have mostly sharp and sometimes erosional contacts (Figs. F12, F13). The single layers show varying thickness from a few millimeters to several centimeters. Sand layers are extremely abundant from Section 311-U1328B-8H-3 to the base of Unit I, often showing fining-upward sequences (Fig. F12). The major nonbiogenic components of Unit I are quartz, feldspar, clay minerals, opaque minerals, and accessory minerals (mainly amphibole). In Section 311-U1328E-8X-3 (29.54 mbsf), a 4 cm thick, glauconite-rich greenish layer is observed. A 2 cm thick layer of almost pure volcanic ash is observed in Section 311-U1328B-9H-5 (Figs. F14, F15).

Mostly rare sulfide mottling, appearing as dark gray (N4) stains, occurs in Unit I in Holes U1328B–U1328D. In Hole U1328E, however, we observe moderate to abundant sulfide mottles in Sections 311-U1328E-8X-2 and 9X-3 through 9X-5 (Fig. F16). Some sulfide concretions were observed at the bottom of Unit I in Cores 311-U1328C-1H, 3H, 6H, 7X, and 9X. Cores 311-U1328B-1H and 311-U1328E-8X are moderately to extremely bioturbated.

The biogenic components in the major lithology (clay and silty clay) have very low concentrations, if at all present, based on smear slide analyses (Fig. F11). From visible observations, some large foraminifers were observed in Cores 311-U1328B-9H, 10H, and 311-U1328C-7X. Some bivalve shell fragments are present in Cores 311-U1328B-10H, 311-U1328C-6H, 7X, and 311-U1328E-9X. A wood fragment was observed in Section 311-U1328C-3H-1.

Lithified and/or partly lithified carbonates are present in Sections 311-U1328B-1H-1, 1H-3, and 9H-CC, and unlithified carbonate cement is present in Section 311-U1328B-1H-2. A notable observation is that we often found lithified and/or partly lithified carbonates associated with soupy and mousselike sediment textures in Cores 311-U1328B-6X, 311-U1328D-1X, 2X, 311-U1328E-2X, 8X, and 9X (Fig. F17). Some of the carbonates seem to be cemented sediments, but we also observed some whitish crusts or coatings composed of pure carbonate, possibly calcite or aragonite. Sample 311-U1328-6X-1, 21–22 cm, revealed a mixed mineralogy of high-Mg calcite (17 mol% MgCO₃) and aragonite. These precipitates indicate carbonate formation directly related to the presence of gas hydrate (Bohrmann et al., 1998; Teichert et al., 2005).

Angular and rounded rock pebbles, 1–6 cm in size, are present in Cores 311-U1328B-1H, 10H, and 311-U1328C-4X. The pebble from Core 311-U1328B-10H is a well-rounded olivine basalt, which we interpret as a dropstone.

Soft-sediment deformation is present in Sections 311-U1328C-8X-1, 8X-4, and 9X-1 (Fig. F18), with only slightly tilted silt or sand layers, and in Sections 311-U1328C-2H-1, 2H-2, and 9X-2, with intermingled sand/silt and clay/silty clay layers.

The Unit I/II boundary is marked by a sharp decrease of sand and silt layers and by the onset of diatom-bearing intervals in Section 311-U1328C-10X-3 (20% diatoms estimated in Sample 311-U1328C-10X-3, 62 cm).

Lithostratigraphic Unit II

• Intervals: Sections 311-U1328C-10X-3 through 16X-CC and 311-U1328E-11Y-1
• Depths: Hole U1328C: 132.60–197.10 mbsf and Hole U1328E: 197.00–198.00 mbsf
• Age: Pleistocene (>0.3 and <1.0 Ma)

Unit II is composed of dark greenish gray (5GY 4/1) and dark gray (N4) clay, clay with nannofossils, silty clay, silty clay with diatoms, diatom silty clay, and diatom ooze locally interbedded with sand and sandy silt layers or lenses. Some silty or sandy layers show fining-upward sequences. The amount of silty and sandy layers or lenses rapidly decreases in this unit compared to Unit I. The diatom-rich sections often display a dark greenish gray (5GY 4/1) color, whereas the sections barren in diatoms are dark gray (N4). The major nonbiogenic components of Unit II are quartz, feldspar, accessory minerals (mainly amphibole), clay minerals, opaque minerals, biotite, glauconite, and volcanic glass. Rare to abundant sulfide mottling is observed throughout Unit II. Rare sulfide concretions occur in Cores 311-U1328C-14X and 15X. Visible glauconitic green patches are found in Sections 311-U1328C-14X-1 and 16X-5. Rare to moderate bioturbation is observed in Sections 311-U1328C-14X-5 and 16X-3.

The presence of biogenic components characterizes Unit II, based on smear slide observations. The major lithology shows a high content of diatoms, on average 10% and as high as 45% in Sample 311-U1328C-14X-2, 81 cm (Fig. F19). The biogenic carbonate fraction is low, on average 4%. Locally high concentrations of foraminifers (25%) and nannofossils (18%) are found in Samples 311-U1328C-16X-3, 139 cm; and 16X-5, 90 cm, respectively. From visible observations, foraminifers are present in Cores 311-1328C-10X and 12X through 16X. Rare bivalve shell fragments are present in Cores 311-U1328C-14X and 16X. A wood piece is observed in Section 311-U1328C-14X-1. Siliceous spicules are present in Sections 311-U1328C-16X-1 and 16X-2. No unlithified carbonate cements or lithified carbonates occur in Unit II.

Mousselike textures related to the presence of gas hydrate are present in intervals 311-U1328C-11X-4, 37–46 cm; 13X-1, 34–39 cm; 13X-2, 6–7 and 26–27 cm; and 14X-3, 28–40 and 70–80 cm, although infrared (IR) camera temperature anomalies are not clear in those cores (see "Physical properties").

Lithostratigraphic Unit III

• Intervals: Sections 311-U1328C-17X-1 through 27X-CC
• Depths: Hole U1328C: 197.10–300.00 mbsf
• Age: Pleistocene (>0.3 to <1.6 Ma)

Unit III is mainly composed of dark greenish gray (5GY 4/1), dark gray (N4), and dark olive-gray (5Y 3/2) clay, silty clay, silty clay with diatoms, and diatom silty clay. In the upper part of Unit III, Sections 311-U1328C-17X-1 through 20X-CC (197.10–235.60 mbsf), dark greenish gray (5GY 4/1) silty clay and clay dominate. The middle part of the unit, Sections 311-U1328C-21X-1 through 25X-CC (235.60–283.70 mbsf), consists mainly of dark gray (N4) silty clay and clay, associated with dark greenish gray (5GY 4/1) clay in Sections 311-U1328C-23X-1 and 23X-2 and dark olive-gray (5Y 3/2) silty clay in Sections 311-U1328C-25X-1, 25X-2, 25X-6, and 25X-CC. Dark greenish gray (5GY 4/1) silty clay, silty clay with diatoms, and diatom silty clay are the main lithologies in the lower part of Unit III, Sections 311-U1328C-26X-1 through 27X-CC (283.70–300.00 mbsf). Coarser grained clayey silt, sandy silt, quartz sandy silt, and sand occur as minor lithologies in Cores 311-U1328C-18X through 22X, 26X, and 27X.

The Unit II/III boundary is distinguished by the sudden absence of diatoms in smear slides (see also "Biostratigraphy"), as well as the increase of induration of the sediments, characterized by the occurrence of drilling biscuits in Cores 311-U1328C-20X through 27X. Diatoms occur again (<30%) at the bottom of Unit III in Core 311-U1328C-27X. The major nonbiogenic components of Unit III are quartz, feldspar, accessory minerals, clay minerals, and opaque minerals. Sulfide concretions or sulfide mottling structures are present in Sections 311-U1328C-17X-2, 17X-5, 18X-1, 20X-2, 22X-6, 22X-7, 25X-2, and 27X-2. Glauconite-rich layers are observed in Sections 311-U1328C-17X-1, 17X-2, 17X-4, 17X-6, 18X-1, 19X-2, 20X-2, 20X-3, 25X-1, 27X-4, and 27X-7. A dark gray (N4) angular rock fragment (0.6 cm x 1.2 cm) is present in Section 311-U1328C-27X-1 and is probably a dropstone.

Visually observed foraminifers are very abundant in Cores 311-U1328C-17X through 20X, 21X, and 25X. Some bivalve shell fragments are present in Cores 311-U1328C-17X, 25X, 26X, and 27X. Bioturbation is not observed in Unit III.

Lithified carbonates and unlithified carbonate cements occur in Cores 311-U1328C-19X, 22X, and 25X. XRD analyses show a combination of high-Mg calcite (14–15 mol% MgCO₃), nonstoichiometric (probably Ca rich) dolomite (31–33 mol% MgCO₃) in Section 311-1328C-19X-4 (Figs. F20, F21), and almost stoichiometric dolomite (47 mol% MgCO₃) in Section 311-1328C-25X-2.

Sedimentary evidence of gas hydrate

At Site U1328, gas hydrate was sampled and evidence of gas hydrate was found in the recovered cores, in IR temperature anomalies in the unsplit cores on the catwalk (see "Physical properties"), and in the composition of the interstitial water (see "Interstitial water geochemistry"). It must be noted, however, that many intervals with gas hydrate were sampled and saved as whole rounds for postcruise analysis and could not, therefore, be observed in split core.

The presence of gas hydrate at Site U1328 is indicated in the split-core sediments by soupy and mousselike textures (Fig. F22). The term "soupy" refers to a sediment structure caused by dissociation of larger nodules or veins of gas hydrate. During dissociation, water is released, which liquifies the sediment (Figs. F17, F23). At Site U1328, soupy texture is only present within lithostratigraphic Unit I (<100 mbsf) in Sections 311-U1328B-2H-2, 6X-1, 6X-2, 311-U1328C-2H-1, 4X-3, 311-U1328D-1X-1 through 1X-6, 311-1328E-2X-1, 8X-1, and 9X-1. All interstitial water (IW) samples recovered from Hole U1328D indicate contamination with drilling fluid and seawater (see "Interstitial water geochemistry"). Analyses of soupy material by inorganic geochemists revealed seawater-like salinities, suggesting that a contribution of drilling fluid (seawater) may be causing some of the soupy appearance of the sediments. Nevertheless, IR temperature data clearly document the presence of gas hydrate in these sections (see "Physical properties").

The term "mousselike" refers to a sediment structure caused by dissociation of disseminated gas hydrate. The dissociation of small grains of gas hydrate gives the sediment a mousselike texture with many small, roundish voids (Figs. F23, F24).

At Site U1328, mousselike textures are present occasionally within lithostratigraphic Units I–III (Fig. F22) in Sections 311-U1328B-1H-4, 2H-2, 6X-1, 6X-2, 311-U1328C-2H-1 through 2H-3, 4X-3, 7X-1, 10X-1, 11X-4, 13X-1, 13X-2, 14X-3, 18X-1, 20X-2, 311-U1328D-1X-6, 2X-1, 2X-2, 311-U1328E-2X-1, 8X-1, 9X-1, and 9X-2.

Authigenic carbonates are abundant within the soupy and mousselike intervals in Sections 311-U1328B-6X-1, 311-U1328D-1X-2 through 1X-6, 311-U1328E-2X-1, 8X-1, and 9X-1 (Fig. F17). These authigenic carbonates display varying shapes and colors: roundish shapes with light to dark gray color; thin gray chips; thin, even layers (sometimes showing layering); thin crusts with a rough, irregular surface and gray color; and thin crusts with a very irregular surface and a white coating. Further postcruise analyses will constrain the origin of the different shapes and might confirm formation related to the presence of gas hydrate.

Environment of deposition

Site U1328 recovered a sequence that corresponds to slope and slope basin sediments (see "Background and objectives").

Lithostratigraphic Unit III is characterized by fine-grained (clay to silty clay) detrital sediments with very few silty interlayers from turbiditic deposits. We suggest that the sediments were deposited in an abyssal plain environment, as the agents of transport and deposition are distal, low-energy turbidity currents. The presence of authigenic unlithified carbonate cements shows that diagenetic processes are active in lithostratigraphic Unit III.

Lithostratigraphic Unit II is characterized by fine-grained (clay to silty clay) detrital sediments with some silty interlayers from turbiditic deposits and siliceous fossils (mainly diatoms; see also "Biostratigraphy"). The great abundance of marine diatoms along with resting spores of Chaetoceros within lithostratigraphic Unit II suggest blooming in a shallow-water shelf environment and coastal upwelling (see "Biostratigraphy") followed by reworking by turbidity currents.

Lithostratigraphic Unit I is characterized by fine-grained detrital sediments (clay and silty clay), with abundant coarse-grained layers as thick as 6 cm, indicating turbiditic deposits. The large input of turbiditic deposits, as compared to lithostratigraphic Unit II, is reflected in an increase in sedimentation rate (from 24.6 to 37.5 cm/k.y.).

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