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Site U1325 is located within the first slope basin in the southwestern part of the Expedition 311 transect. Three of the four holes (Holes U1325A–U1325D) at this site were cored: Hole U1325B was cored to 206.5 mbsf (205.5 m cored), Hole U1325C to 304.3 mbsf (115.5 m cored), and Hole U1325D to 4.7 mbsf (4.7 m cored). Recovery was excellent in Hole U1325D (99.8%), good in Hole U1325B (68.3%), but more limited in Hole U1325C (53.3%).
We divided the 304.3 m thick sedimentary section recovered from Holes U1325B and U1325C into four 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. The results were also compared and correlated with seismic data, downhole LWD/MWD data, and physical property measurements.
The sediments recovered in Unit I were divided into two subunits based on grain-size distribution and occurrence of bioturbation, sulfide mottles, and sponge spicules. Compared with Subunit IB, the sediments of Subunit IA are dominated by thick sand layers (as thick as ~2.5 m).
Subunit IA is mainly composed of dark greenish gray (5GY 4/1) silty clay with diatoms, diatom silty clay, silty clay, and black (N2.5) sand. Sand layers have mostly sharp (Fig. F8) and some erosional contacts. The single layers show varying thickness from a few millimeters to several meters (Fig. F9). The layers show fining-upward sequences in Sections 311-U1325B-1H-5, 1H-7, and 2H-5. The major nonbiogenic components of Subunit IA are clay minerals, quartz, feldspar, opaque minerals, accessory minerals (mainly amphibole), biotite, and some rock fragments. Rare to abundant sulfide mottling, appearing as dark gray (N4) stains, occurs in Sections 311-U1325B-1H-2, 1H-4, 2H-4, 2H-5, 2H-7, and 3H-1.
The major biogenic components of Subunit IA are diatoms, as much as 30% in Sample 311-U1325B-1H-2, 30 cm. Bivalve shell fragments are visually observed in Sections 311-U1325B-1H-4, 1H-2, and 1H-7; sponge spicules in Section 1H-2; and foraminifers in Section 3X-1.
Sedimentary structures related to soft-sediment deformation are observed in interval 311-U1325B-2H-5, 84–110 cm. Dropstones are present in Sections 311-U1325B-2H-2, 2H-6, and 2H-7 (Fig. F10). Some small concretions (<3 mm) are present in Sections 311-U1325B-2H-5 and 2H-7.
Subunit IB is mainly composed of dark greenish gray (5GY 4/1) silty clay with diatoms, diatom silty clay, and silty clay. Minor lithologies are silt and sand-silt-clay. The major nonbiogenic components of Subunit IB are clay minerals, quartz, feldspar, opaque minerals, some accessory minerals (mainly amphibole), and biotite. A volcanic ash lens is present in Section 311-U1325B-6H-3 (Fig. F11). This lens might correlate to a volcanic ash layer found in Hole U1328B (interval 311-1328B-9H-5, 142–144 cm). Using the given sedimentation rates (see "Biostratigraphy"), the ash in Hole U1325B has an approximate age of 108 ka, whereas the layer in Hole U1328B has an approximate age of 122 ka. Note that these ages are rough estimates and give only the time of deposition, not the time of eruption, because these are most likely turbiditic, reworked deposits.
Subunit IB shows abundant sulfide mottling (Fig. F12), which often coincides with abundant bioturbation. Abundant iron sulfide concretions are observed in Sections 311-U1325B-4H-1 through 4H-4, 5H-1 through 5H-4, 5H-7, 6H-2, 6H-3, 6H-6, and 6H-7. Greenish spots of glauconite are present as dispersed grains in Sections 311-U1325B-5H-2, 6H-1, and 6H-CC.
In the major lithology, diatoms (as much as 30%) and sponge spicules (Figs. F13, F14) (Sections 311-U1325B-4H-1 through 5H-2 and 6H-1 through 6H-6) are abundant, whereas calcareous fossils are almost absent in smear slides. A wood fragment was observed in Section 311-U1325B-4H-7, visible foraminifers in Sections 4H-7 and 5H-2, and bivalve shell fragments in Section 5H-2.
Fractures in the core caused by gas expansion are common in Sections 311-U1325B-5H-1 through 5H-3 and 5H-6. Mousselike texture related to the presence of gas hydrate is present in intervals 311-U1325B-5H-7, 26–26.5 cm, and 5H-7, 29.5–30.5 cm. The Unit I/II boundary is marked by an absence of diatoms, bioturbation, less intense sulfide mottles, and a sharp decrease in the occurrence of sponge spicules.
Unit II is composed of dark gray (N4) clay, dark gray (N4) and dark greenish gray (5GY 4/1) silty clay, and clayey silt locally interbedded with frequent dark gray (N4) quartz sand, very dark gray (N3) silty sand, and dark gray (N4) sandy silt layers (Fig. F15). Silty or sandy layers show fining-upward sequences in Sections 311-U1325B-7H-4 and 8H-1 through 8H-4 (Fig. F15). The frequency of the coarser-grained layers is about every 5 to 10 cm in Sections 311-U1325B 7H-3 through 7H-5, 8H-1 through 8H-4, and 12H-1. The major nonbiogenic components of Unit II are clay minerals, quartz, feldspar, accessory minerals (mainly amphibole), opaque minerals, and biotite. Rare to moderate sulfide mottling is observed throughout Cores 311-U1325B-7H to 13X. Small sulfide concretions are present in Section 311-U1325B-10X-1, and a glauconite-rich patch is seen in Section 10X-2. A rounded dropstone (0.8 cm) is observed in Section 311-U1325B-7H-4. A white piece of pumice (1 cm) was found in an interstitial water (IW) sample (Sample 311-U1325B-7H-4, 150–165 cm).
Biogenic components, observed in smear slides, are almost absent in Unit II. Visually, we observed aggregates of sponge spicules in Sections 311-U1325B-10X-1 through 10X-3 and bivalve shell fragments in Section 10X-2.
A small-scale sedimentary structure related to soft-sediment deformation is observed in interval 311-U1325B-7H-4, 67–70 cm. Major core disturbance is present in Sections 311-U1325B-8H-6 and 8H-7, with vertically arranged sand layers. This was probably related to the stiff nature of the sediment and resulting partial stroke of the APC corer, at which point coring was changed to the XCB system. Mousselike texture related to the presence of gas hydrate is present in intervals 311-U1325B-8H-1, 91–93 cm; 8H-2, 36–38 cm; and 11P-1, 0–20 cm. The Unit II/III boundary is marked by a strong increase in diatoms (see "Biostratigraphy") and an increase in induration of the sediments, which shows in the occurrence of drilling biscuits and a sudden increase in aggregates of sponge spicules.
Unit III is mainly composed of dark greenish gray (5GY 4/1) and dark gray (N4) clay, silty clay, silty clay with diatoms, diatom silty clay, nannofossil silty clay, clayey silt with diatoms, silty clay, diatom ooze, and nannofossil ooze. In the upper part of Unit III, from Section 311-U1325B-14X-1 to 19X-CC (102.30–145.98 mbsf), dark greenish gray (5GY 4/1) and dark gray (N4) silty clay, clayey silt with diatoms, and diatom clayey silt are the main lithologies. Nannofossil ooze and nannofossil silty clay occur between 140.50 and 145.98 mbsf. The lower part of Unit III, from Section 311-U1325B-20X-1 to 25X-CC (150.20–197.40 mbsf), is mainly dominated by dark gray (N4) silty clay and silty clay with diatoms associated with dark greenish gray (5GY 4/1) silty diatom ooze in Sections 311-U1325B-24X-4 and 24X-5 (183.27–185.72 mbsf). Coarser-grained silt, sandy silt, sand, quartz sand, and foraminifer silty sand occur as minor lithologies in Cores 311-U1325B-14X through 16X, 20X through 24X, and 311-U1325C-1X. Fining-upward sequences and planar laminations (Fig. F16) are observed in Sections 311-U1325B-14X-4 through 14X-6, 15X-1, 15X-3, 16X-4, 19X-3, 19X-5, 20X-4, 21X-CC, 24X-1, 24X-2, 24X-4, and 24X-5.
The major nonbiogenic components of Unit III are clay minerals, quartz, feldspar, accessory minerals, opaque minerals, and biotite. Sulfide mottling is present in Sections 311-U1325B-14X-1, 14X-4, 14X-5, 20X-2, 20X-4, 21X-1, 21X-2, and 24X-1. Glauconite-rich patches are observed in Sections 311-U1325B-16X-2, 16X-3, 16X-6, and 16X-7. A dark greenish gray (5GY 4/1), round rock fragment (2 cm x 3 cm), probably a dropstone, is present in Section 311-U1325B-14X-5.
The biogenic components are distinctly different from Unit II. The major lithology shows a high content of diatoms, as high as 50% in Sample 311-U1327B-24X-4, 34 cm. The most important carbonate fossils are nannofossils (Fig. F17). Sample 311-U1325B-19X-1, 100 cm, has the composition of a nannofossil ooze (60% nannofossils). Visually observed aggregates of sponge spicules are present in Cores 311-U1325B-14X through 20X. Bioturbation is only present in Sections 311-U1325B-14X-1 and 14X-2.
A layer of carbonate cement occurs in interval 311-U1325B-16X-6, 27–33 cm. XRD analysis confirmed a combination of high-Mg calcite (12 mol% MgCO3) and a nonstoichiometric dolomite (44 mol% MgCO3). Soupy and mousselike sediment textures related to the presence of gas hydrate are present in Sections 311-U1325B-16X-1, 18X-1, and 19X-5 (Fig. F18). The Unit III/IV boundary is marked by the sudden absence of diatoms (see "Biostratigraphy").
Unit IV is mainly composed of dark gray (N4) and dark greenish gray (5GY 4/1) clay and silty clay, locally interbedded with dark gray (N4) sand, quartz sand, silty sand, sandy silt, silt layers, dark greenish gray (5GY 4/1) silt layers, and dark greenish gray (5GY 4/1) and olive-gray (5Y 3/1) clayey silt. Silty or sandy layers show fining-upward sequences and sharp bottom contacts in Sections 311-U1325C-6X-4, 7X-2, 7X-3, and 7X-7 (Fig. F19). Drilling biscuits and other coring-related disturbance features are present almost in the whole unit.
The major nonbiogenic components of Unit IV are clay minerals, quartz, feldspar, biotite, accessory minerals, and opaque minerals. Sulfide mottling is mainly observed in Cores 311-U1325C-8X and 11X, and small sulfide concretions are present in Section 311-U1325C-8X-5 (Fig. F20).
Biogenic components identified from smear slide analyses are almost absent. In IW Sample 311-U1325C-15X-3, 60–65 cm, a well-preserved brittle star is present (Fig. F21).
A lithified carbonate piece (4 cm) occurs in Section 311-U1325C-11X-1.
Mousselike textures typical of the presence of gas hydrate are present in intervals 311-U1325C-7X-3, 72–78 cm, and 8X-5, 80–90 cm (Figs. F19, F22), even though they are below the stability field for gas hydrate. This discrepancy, which is observed in other measurements as well (see "Interstitial water geochemistry" and "Physical properties"), will be addressed postcruise.
Drilling at Site U1325, located within the first slope basin of the northern Cascadia accretionary prism (~2193 mbsl), recovered a sequence that corresponds to slope basin sediments (see "Background and objectives").
Lithostratigraphic Unit IV is characterized by fine-grained (clay to silty clay) detrital sediments with few silty/sandy interlayers from turbiditic deposits. The poor recovery at the top and bottom of this unit, as well as the biscuiting of the sediments, limit the observation of sedimentary structures. The few coarser grained layers, however, indicate low-energy turbidity currents.
Lithostratigraphic Unit III is characterized by fine-grained (clay to silty clay) detrital sediments with abundant sponge spicule remains and marine and nonmarine diatoms (see "Biostratigraphy"), as well as abundant nannofossils. We interpret this interval as mixed hemipelagic-turbiditic sediments. The presence of authigenic carbonate cement suggests 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. Their frequent occurrence might indicate times of active tectonism and uplift or maybe earthquakes.
Lithostratigraphic Subunit IB is characterized by fine-grained (clay to silty clay) detrital sediments with few thin silty/sandy interlayers from turbidites. The occurrence of only marine diatoms (see "Biostratigraphy") suggests that the terrigenous sediment flux was somehow restricted. In addition, the occurrence of abundant bioturbation and sulfide mottles suggests more hemipelagic-dominated sedimentation.
Lithostratigraphic Subunit IA is characterized by very abundant, thick, coarse-grained (sand) layers within fine-grained (clay and silty clay) detrital sediments. The thickness of these layers suggests the depositional environment of tributary channels within the slope basin with mass transport parallel to the first ridge. This facies is unique to Site U1325; it was not cored at any of the other sites. Sedimentation rates are expected to be very high. From the diatom biostratigraphy (see "Biostratigraphy") we have only an estimated sedimentation rate for the upper 129 m of sediments (Units I, II, and upper part of Unit III) of 43 cm/k.y. This is the highest sedimentation rate of all Expedition 311 sites. If we assume that the sedimentation rate for Units II and III is lower than that of Unit I, then the rate for Unit I is probably even higher than 43 cm/k.y.