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doi:10.2204/iodp.proc.311.107.2006

Lithostratigraphy

Site U1329 is the easternmost and shallowest (~946 mbsl) site of the transect drilled during Expedition 311. Four of the five holes (Holes U1329B–U1329E) at this site were cored. Hole U1329B was cored to 9.5 mbsf, Hole U1329C was cored to 189.5 mbsf (187.5 m recovered), Hole U1329D was cored from 201 to 210.5 mbsf (9.5 m recovered), and Hole U1329E was cored to 34.5 mbsf and then from 54.5 to 126 mbsf (total recovery of 48.0 m). Recovery was generally good (Hole U1329B = 105.5%, Hole U1329C = 99.3%, Hole U1329D = 10.8%, and Hole U1329E = 78.6%).

We divide the 187.5 m thick sedimentary section recovered at Site U1329 into three lithostratigraphic units (Fig. F7) 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. Results were also compared and correlated with two-dimensional seismic data, downhole LWD/MWD data, physical property measurements, and color reflectance spectrophotometry from the archive multisensor track.

Lithostratigraphic units

Lithostratigraphic Unit I

• Intervals: Sections 311-U1329B-1H-1 through 1H-CC; 311-U1329C-1H-1 through 5H-1; and 311-U1329E-1H-1 through 4H-CC
• Depths: Hole U1329B: 0.00–10.01 mbsf; Hole U1329C: 0.00–37.18 mbsf; and Hole U1329E: 0.00–33.31 mbsf
• Age: Holocene?–Pleistocene

Unit I is composed of dark greenish gray (5GY 4/1) and dark gray (N4) clay and silty clay (Fig. F8) sometimes showing parallel lamination. The clay and silty clay are locally interbedded with silt, clayey silt, and sand layers and lenses. The major nonbiogenic components of Unit I are quartz, feldspar, clay minerals, opaque minerals, accessory minerals (mainly amphibole), and some glauconite. Rare sulfide mottling, appearing as dark gray (N4) stains, occurs in all cores from Unit I. Sulfide concretions were observed in all cores and are especially frequent in Core 311-U1329C-2H. Bioturbation is rare to abundant in Cores 311-U1329C-1H, 3H, and 4H.

The biogenic components have mostly very low concentrations in the major lithology (Fig. F7). In the minor lithology of Hole U1329B, as much as 60% diatoms was estimated by smear slide analysis. Some rare bivalve shell fragments are present in Cores 311-U1329-3H and 4H, and one gastropod is present in Section 311-U1329-2H-6. A 5 cm long piece of wood is present in Section 311-U1329E-1H-2 (Fig. F8).

Unlithified authigenic carbonate cements (Fig. F9A) and partly lithified carbonates (Fig. F9B), as well as lithified carbonates (Fig. F9C) are abundant in intervals 311-U1329C-4H-3, 90–142 cm; 311-U1329E-4H-5, 46–122 cm; and 4H-6, 30–90 cm. XRD analyses confirmed a dolomitic composition for all these intervals (Fig. F10). The occurrence of dolomites in Holes U1329C and U1329E at about the same depth suggests that dolomite precipitation is proceeding in a widespread "layer." Ca and Mg concentrations (see "Interstitial water geochemistry") seem to confirm ongoing dolomite precipitation at this level. Some dispersed, millimeter-scale carbonate concretions are present in Section 311-U1329C-2H-1.

The Unit I/II boundary is marked by the first occurrence of diatom ooze in Section 311-U1329C-5H-1 at 58 cm, which can clearly be seen as a distinct change in color.

Lithostratigraphic Unit II

• Intervals: Sections 311-U1329C-5H-1 through 16H-CC and 311-U1329E-5P-1 through 10P-1
• Depths: Hole U1329C: 37.18–135.60 mbsf and Hole U1329E: 33.31–125.95 mbsf
• Age: Pleistocene (>0.3–2.0 Ma)

Unit II is composed of dark greenish gray (5GY 4/1) and dark olive-gray (5Y 3/2) clay, silty clay (Fig. F11), silty clay with diatoms (Fig. F12), and diatom ooze (Fig. F11) locally interbedded with silt, clayey silt, sand, and foraminifer ooze 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 (Fig. F11). The major nonbiogenic components of this unit are quartz, feldspar, clay minerals, opaque minerals, accessory minerals (mainly amphibole), some glauconite, and volcanic glass. Rare sulfide mottling occurs throughout Unit II. Sulfide concretions were especially abundant in Cores 311-U1329C-10H and 11H. Rare to moderate bioturbation occurs in this unit.

The composition of the biogenic components is characteristic of Unit II, in particular the high content of diatoms forming diatom ooze (Fig. F13) with a biogenic opal content of ~78%, as estimated from smear slides. The biogenic carbonate fraction is low, on average 2%–8%. In the minor lithologies, the biogenic content is even higher, with as much as 87% biogenic opal and as much as 60% biogenic carbonate. Bivalve shell fragments are very abundant in Unit II, especially from Core 311-U1329C-10H to the bottom of the unit. A piece of wood is present in Section 311-U1329C-12H-1.

Unlithified, partly lithified, and lithified carbonates are dispersed throughout Unit II in Cores 311-U1329C-5H, 8H, 10H, 12H, and 16H. The composition varies from low-Mg calcite (2 mol% MgCO₃) to high-Mg calcite (6 and 13 mol% MgCO₃), which was confirmed by XRD analyses.

The Unit II/III boundary is marked by the occurrence of conglomerate (Fig. F14) (intervals 311-U1329C-16H-6, 120–150 cm, and 16H-7, 0–69 cm). This conglomerate is composed of partly lithified and lithified rounded clasts supported by a silty clay matrix. Various-colored clasts are observed (Fig. F14), such as light greenish gray, dark greenish gray, light dark gray, and dark gray. The size of individual clasts is ~2–4 mm.

Lithostratigraphic Unit III

• Intervals: Sections 311-U1329C-17H-1 through 23P-1 and 311-U1329D-1X-1
• Depths: Hole U1329C: 135.60–189.50 mbsf and Hole U1329D: 201–202.03 mbsf
• Age: late Miocene (>6.7 Ma)

Unit III is mainly composed of dark greenish gray (5GY 4/1), dark olive-gray (5Y 3/2), and dark gray (N4) clay (Fig. F15) and silty clay (Fig. F16). Coarser grained, interbedded silt and sand layers are very rare and occur only in Cores 311-U1329C-17H and 21X. The major nonbiogenic components of Unit III are quartz, feldspar, clay minerals, opaque minerals, accessory minerals (mainly amphibole), some glauconite, and volcanic glass. Almost no sulfide mottling or sulfide concretions are present. Moderate bioturbation is present only in Core 311-U1329C-18X.

Most smear slide samples were barren of biogenic components. Only Sample 311-U1329C-20X-1, 77 cm, in the major lithology contains ~10% diatoms. Some rare bivalve shell fragments are present in Cores 311-U1329C-17H, 18X, and 22X, and a wood fragment is present in Section 21X-2.

Unlithified carbonate cements, partly lithified carbonates, and lithified carbonates are abundant in Unit III in Cores 311-U1329C-18H through 23X. The composition is variable and shows no zonation with depth. Dolomite, siderite (Fig. F17), low-Mg calcite, and high-Mg calcite (Fig. F16) are present and were identified by XRD (Fig. F18).

A minor component in Unit III is glauconite, which is present in Sections 311-U1329C-18X-2, 18X-3, 22X-1, and 22X-2. Smear slide Sample 311-U1329C-18X-2, 126 cm, contains ~97% glauconite grains that have a diameter of ~100 µm (Fig. F19). Another minor component is very small volcanic ash lenses (<1 cm) composed of >90% volcanic glass shards (intervals 311-U1329C-18X-3, 1–2 cm; 18X-4, 50 cm; 19X-3, 6–68 cm; 19X-5, 95 cm; and 19X-6, 5–23 cm).

The bottom of Unit III consists of an accumulation (Core 311-U1329C-23P) of a wide variety of different clasts as large as 4 cm in diameter (sandstone/quartzite with a pyrite vein, different carbonate rocks, cemented bioturbation burrows, and different mud clasts of varying color). This conglomerate-type deposit is pronounced in the pressure core gamma ray log (see "Pressure coring") and probably corresponds to the depth of increasing density and electrical resistivity in the LWD data of Hole U1329A (see "Downhole logging") and the wireline logging data of Hole U1329D (see "Downhole logging"). A change in consolidation of the sediments is also visible through drilling biscuits that are present at the bottom of Unit III, starting with Core 311-U1329C-21X. The conglomerate probably marks a lithostratigraphic boundary, but because there was only one core recovered (Core 311-U1329D-1X) deeper in the section, it is difficult to describe this lithostratigraphic unit underneath the conglomerate. Core 311-U1329D-1X is composed of silty clay without any biogenic components.

Environment of deposition

Drilling at Site U1329, the shallowest site drilled during Expedition 311 on the continental slope off Vancouver Island, recovered a sequence dominated by slope sedimentary processes (see "Background and objectives").

Lithostratigraphic Unit III is characterized by fine-grained (clay to silty clay) detrital sediments with only a few coarser interlayers from turbiditic deposits. The input of detrital sediments from turbidites is supported by the presence of nonmarine diatoms in the interval 180.5–186.39 mbsf (see "Biostratigraphy"). At the lowermost part of lithostratigraphic Unit III (188.5–189.5 mbsf) a conglomerate deposit, which was found in pressure Core 311-U1329C-23P, indicates a major event, possibly a debris flow. The total thickness of this deposit is unknown. The overall sedimentation rate of lithostratigraphic Unit III, which has a large uncertainty because of difficulties with the age constraints (see "Biostratigraphy"), indicates slow sedimentation of ~0.8–2.8 cm/k.y. The presence of abundant unlithified carbonate cements and lithified carbonates suggests that diagenetic processes were and are still active. The upper boundary of this unit corresponds to an upper Miocene/Pleistocene unconformity.

Lithostratigraphic Unit II is characterized by a high abundance of biogenic silica (mainly diatoms), which also corresponds to higher sedimentation rates. Although there are some uncertainties in the age constraints (see "Biostratigraphy"), the sedimentation rate seems to increase within lithostratigraphic Unit II from ~4 to 10 cm/k.y. The conglomerate at the base of the unit (134.80–135.79 mbsf) follows an important stratigraphic hiatus (no sediments preserved from 2 to 6.7 Ma) and probably illustrates uplift and erosion of Pliocene sediments. Assuming similar sedimentation rates in the Pliocene as in adjacent sediments, a sediment column of ~188 m thickness may have been eroded. The great abundance of diatoms along with resting spores of Chaetoceros within lithostratigraphic Unit II suggest blooming in a shallow-water shelf environment and coastal upwelling (see "Biostratigraphy"). The presence of some dispersed unlithified carbonate cements and lithified carbonates suggests that diagenetic processes are active.

Lithostratigraphic Unit I is characterized by fine-grained detrital sediments (clay and silty clay) locally interbedded with coarse-grained sediments. The elevated sedimentation rates from the top of lithostratigraphic Unit II continue, but the absence of diatoms indicates a major change in the environmental conditions. The presence of dolomite at a depth of 31.00–31.52 mbsf in Hole U1329C and 30.46–32.40 mbsf in Hole U1329E indicates a local diagenetic process.

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