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doi:10.2204/iodp.proc.311.105.2006

Lithostratigraphy

Site U1327 is located on the mid-continental slope off Vancouver Island (~1309 mbsl), several hundred meters from Site 889. Four of the five holes (Holes U1327B–U1327E) at this site were cored. Hole U1327B was cored to 9.5 mbsf, Hole U1327C was advanced to a TD of 300 mbsf (297 m cored), Hole U1327D was advanced to a TD of 300 mbsf (75.9 m cored), and Hole U1327E was advanced to a TD of 300 mbsf (12.5 m cored). Recovery was good to very good (Hole U1327B = 103.1%, Hole U1327C = 88.3%, Hole U1327D = 71.1%, and Hole U1327E = 91.4%).

We divided the 300 m thick sedimentary section recovered at Site U1327 into three lithostratigraphic units (Fig. F10) based on visual inspection of the recovered 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 2-D seismic data, downhole LWD/MWD data, and physical property measurements.

Lithostratigraphic units

Lithostratigraphic Unit I

  • Intervals: Sections 311-U1327B-1H-1 through 1H-CC, 311-U1327C-1H-1 through 11H-4, and Cores 311-U1327E-1H through 3P
  • Depths: Hole U1327B: 0.00–9.79 mbsf, Hole U1327C: 0.00–90.10 mbsf, and Hole U1327E: 3.00–12.50, 40.00–41.00, and 80.00–81.00 mbsf
  • Age: Quaternary

Unit I is composed of dark greenish gray (5GY 4/1) and dark gray (N4) clay and silty clay (Fig. F11) sometimes showing parallel lamination. The clay and silty clay is often interbedded with silt, clayey silt, sandy silt, sand, and gravel layers that have mostly sharp and sometimes erosional contacts (Figs. F11, F12, F13). The single layers show varying thickness from a few millimeters to several centimeters. Often, fining-upward sequences were observed (Figs. F11, F12). The major nonbiogenic components of Unit I are quartz, feldspar, volcanic glass (as much as 20%), clay minerals, opaque minerals, accessory minerals (mainly amphibole), and some glauconite. A 2 cm thick layer rich in zeolites was identified from XRD analyses (Sample 311-U1327C-2H-5, 69–70 cm), indicating highly altered volcanic ash (Fig. F14). Rare sulfide mottling, appearing as dark gray (N4) stains, occurs especially at the top and the bottom of Unit I. Sulfide concretions were observed in Cores 311-U1327C-1H, 3H, 4H, 5H, and 9H. Mostly rare to moderate bioturbation, often indicated by iron sulfide precipitates, occurs in Cores 311-U1327C-2H, 3H, 4H, 5H, 7H, and 9H.

The biogenic components in the major lithology have mostly very low concentrations (Fig. F10). In the minor lithology of Core 311-U1327C-1H, an abundance of diatoms (as high as 45%) was estimated in a smear slide. Some bivalve shell fragments are present in Cores 311-U1327C-1H, 2H, 3H, 4H, 9H, 10H, and 11H, and one gastropod is seen in Section 4H-6.

Greenish gray (5GY 5/1) unlithified authigenic carbonate cement is found in Section 311-U1327C-10H-1 (Fig. F15). XRD analyses confirm a high-Mg calcite (HMC) composition (5 mol% MgCO3). No evidence of dolomite precipitation is found, although it is inferred from interstitial water (IW) chemistry results (see "Interstitial water geochemistry").

Rocks, millimeters to centimeters in size, are present in Cores 311-U1327C-1H, 4H, 7H, and 8H. Some of them are carbonates, but we also observed other lithologies (e.g., granite) (Fig. F13). The distinction between authigenic and detrital carbonates is not possible without further analyses, so they are plotted as "nodule/concretion, general" in the visual core descriptions.

Soft-sediment deformation is present in Cores 311-U1327C-4H, 7H, 8H, 9H, and 11H. The deformation mostly shows in distorted sand layers (Fig. F16). Sections 311-U1327C-11H-1, 11H-3, and 11H-4 have extensive soft-sediment deformation structures, including a large, more greenish mud clast (56 cm). This seems to indicate a major slump event, whereas the deformational features observed in the other cores are probably only small-scale gravitational slides.

The Unit I/II boundary is marked by a sharp decrease of sand and silt layers and by an increase in the abundance of diatoms in Section 311-U1327C-11H-5 (30% diatoms estimated at Section 11H-5, 6 cm), which can clearly be seen as a change in sediment color.

Lithostratigraphic Unit II

  • Intervals: Sections 311-U1327C-11H-5 through 20X-CC, 311-U1327D-4E-1 through 11X-CC, and 311-U1327E-4E-1
  • Depths: Hole U1327C: 90.10–170.40 mbsf, Hole U1327D: 125.30–170.50 mbsf, and Hole U1327E: 128.00–129.00 mbsf
  • Age: Pleistocene (>0.3–1.0 Ma)

Unit II is composed of dark greenish gray (5GY 4/1) and dark gray (N4) clay, clay with diatoms, silty clay, silty clay with diatoms, and diatom silty clay, all locally interbedded with sandy silt and sand layers and lenses. Often the diatom-rich sections display a more brownish color (dark olive-gray, 5Y 3/2), whereas the sections barren in diatoms are more grayish. The major nonbiogenic components of Unit II are quartz, feldspar, accessory minerals (mainly amphibole), clay minerals, opaque minerals, some glauconite, and volcanic glass. Rare to moderate sulfide mottling and rare sulfide concretions are present throughout Unit II. No bioturbation is observed in this unit.

The biogenic components are distinctly different from those of Unit I. The major lithology shows a high content of diatoms (as high as 40% in Sample 311-U1327C-20X-3, 90 cm). The biogenic carbonate fraction is low, on average 1%. Bivalve shell fragments are present throughout this unit, in particular in Cores 311-U1327C-18X and 19X.

A layer composed of different-colored mud clasts 1–2 mm in diameter is present in interval 311-U1327E-4E-2, 41–48 cm (128.41–128.48 mbsf). The light-colored clasts are carbonate rich (show reaction with HCl). This layer probably correlates to a similar layer observed in Hole U1327C (interval 311-U1327C-16X-3, 119–141 cm; 127.89–128.11 mbsf).

Very few unlithified carbonate cements, visible as faint light spots, occur in Unit II. Lithified carbonates of different mineralogy and origin are abundant in Cores 311-U1327C-16X, 18X, and 19X and 311-U1327D-5X, 6Y, and 9X through 11X (Fig. F17). The interval between 142 and 148 mbsf in Hole U1327C and between 151 and 157 mbsf in Hole U1327D (Fig. F17) is especially rich in rocks as large as 4 cm in diameter. Though there is a small offset, these two intervals might correlate to each other. Some pieces are rounded, and some are angular. Based on visual observation they are of very different origin. Intervals 311-U1327C-18X-2, 130–150 cm and 18X-3, 105–106 cm and 311-U1327D-11X-1, 7–8 cm, are of dolomitic composition (46–48 mol% MgCO3), whereas interval 311-U1327C-18X-5, 120–150 cm, is a siderite. The nature of this deposit is ambiguous. Further shore-based studies on the composition of the different clasts will be carried out to better describe this deposit.

At this site, we observed soupy and mousselike sediment textures related to the presence of gas hydrate. Soupy textures are present in intervals 311-U1327C-16X-1, 10–20 cm; 17X-1, 0–20 cm; 18X-2, 20–72 cm; and 20X-1, 123–155 cm. Mousselike textures are present in intervals 311-U1327D-5X-1, 0–30 cm; 7X-3, 0–40 cm; 7X-4, 0–75 cm; 9X-1, 0–43 cm; and 11X-1, 0–25 cm. Our observations are in agreement with the gas hydrate–bearing interval identified by infrared (IR) camera temperature anomalies (see "Physical properties"). In interval 311-U1327D-7X-4, 22–31 cm, we observed a probably fracture-related soupy texture surrounded by mousselike sediments (Fig. F18A). Another probably fracture-related temperature anomaly was sampled in this core on the catwalk (Fig. F18B). This might indicate that gas hydrate formation in this section is fracture controlled.

Lithostratigraphic Unit III

  • Intervals: Sections 311-U1327C-21X-1 through 35X-CC and 311-U1327D-12X-1 through 17P-1
  • Depths: Hole U1327C: 170.40–300.00 mbsf and Hole U1327D: 170.50–247.00 mbsf
  • Age: Pleistocene (>1 Ma)

Unit III is mainly composed of dark greenish gray (5GY 4/1) silty clay and dark gray (N4) silty clay and clay in the upper part of the unit. In the lower part of the unit, below Section 311-U1327C-29X-2, dark greenish gray (5GY 4/1) silty clay with diatoms is present. Coarser grained clayey silt (with diatoms) and sandy silt occur as minor lithologies in Cores 311-U1327C-22X, 26X, 27X, and 29X.

The major nonbiogenic components of Unit III are quartz, feldspar, accessory minerals (mainly amphibole), clay minerals, and opaque minerals. Almost no sulfide mottling or sulfide concretions are present. Bioturbation is only observed in Section 311-U1327C-30X-4. Glauconite-rich layers are visually observed in Section 311-U1327C-25X-4 and 26X-3. Some rare bivalve shell fragments are present in Cores 311-U1327C-21X, 22X, 26X, 27X, and 31X through 35X.

Unit III is distinguished from Unit II by the sudden absence of diatoms in smear slides (see also "Biostratigraphy"), as well as the degree of induration of the sediments, which shows in the occurrence of drilling biscuits (Fig. F19). Diatoms occur again at the bottom of the unit (<25%).

Few unlithified carbonate cements, visible as faint light spots, occur in Unit III in Cores 311-U1327C-23X, 29X, 30X, and 31X and 311-U1327D-16X. XRD analyses show a combination of HMC (8–11 mol% MgCO3) with nonstoichiometric (Ca rich) dolomite in most of the cores, or a combination of rhodochrosite with HMC and Ca-rich dolomite in Samples 311-U1327C-29X-4, 59–60 cm (Fig. F20), and 30X-3, 80–81 cm.

Interval 311-U1327C-24P-1, 0–69 cm, from a pressure core, contained numerous lithified carbonate rocks (Fig. F21). Three different carbonate mineralogies can be differentiated by XRD analyses: dolomite (47–49 mol% MgCO3), HMC (14 mol% MgCO3), and a mixture of HMC and dolomite or nonstoichiometric (Ca or Fe rich) dolomite (7–10 mol% MgCO3 and 36–48 mol% MgCO3, respectively). Lithified carbonate rocks of different mineralogy and origin are also present in intervals 311-U1327C-32X-1, 11–12 cm; 34X-CC, 2–3 cm; and 35X-3, 70–71 cm.

Environment of deposition

Coring at Site U1327 recovered a sequence that corresponds to slope and slope basin sedimentation (see "Background and objectives").

Lithostratigraphic Unit III is characterized by fine-grained (clay to silty clay) detrital sediments with few silty interlayers from turbiditic deposits and by rare to absent fossils. 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. Between 197.30 and 197.99 mbsf, a conglomerate layer is present, suggesting a major event like a debris flow. The sedimentation rate (12 cm/k.y.), inferred from diatom studies (see "Biostratigraphy"), is the lowest rate at this site, consistent with a more "basin plain" setting. The presence of unlithified authigenic carbonate cements (some of them rhodochrosite rich) shows that diagenetic processes, likely driven by organic matter remineralization (see "Interstitial water geochemistry"), 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 some siliceous fossils (mainly diatoms; see also "Biostratigraphy"). The sedimentation rate is slightly higher (16 cm/k.y.) and intervals with a high nonmarine/marine diatom ratio are more abundant, showing the increasing importance of intermittent terrigenous input of detrital sediments from land by turbidites (see "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. The mud clast level (Cores 311-U1327E-4E and 311-U1327C-16X at ~128 mbsf) is interpreted as reworking and redeposition of sediments following uplift of the sedimentary column at the deformation front. Below this level, between ~142 and 148 mbsf in Hole U1327C and 151 and 157 mbsf in Hole U1327D, numerous lithified carbonate rocks of different origin are very abundant. Some of them may be dolomite/siderite.

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 greater input of turbidite sedimentation, compared to lithostratigraphic Unit II, shows in the increased sedimentation rate (from 16 to 22 cm/k.y.) as well. Rocks found in Cores 311-U1327C-1H, 4H, 7H, and 8H are sometimes granitic, sometimes carbonate. Scattered nonsedimentary rocks in fine-grained sedimentary facies are interpreted as dropstones (ice-rafted debris), indicating sea ice cover in the Pacific Ocean, similar to the well-known Heinrich events in the North Atlantic Ocean.