Site 1251 is located on the eastern flank of southern Hydrate Ridge, where sediments from the eastern slope basin were recovered (see Figs. F1 and F8 in the "Leg 204 Summary" chapter). Eight holes were drilled at Site 1251 (Holes 1251A-1251H), and six of these (Holes 1251B-1251G) were cored. Hole 1251A was drilled using LWD to collect a suite of downhole measurements prior to coring. Hole 1251B was cored to 442.1 mbsf (the upper 194.6 mbsf using the APC and the next 247.5 m using the XCB). Hole 1251C was cored to 17.6 mbsf (the upper 8.1 mbsf using the APC and the next 9.5 m using the XCB). Hole 1251D was cored to 230.5 mbsf (the upper 26.9 m using the XCB, followed by 147.5 m using the APC, and then 56.1 m using the XCB). Holes 1251E and 1251F were each cored to 9.5 mbsf (APC) for high-density sampling. Hole 1251G was slightly offset from Hole 1251F and washed to 2.5 m before coring to 12 mbsf (using the APC). Hole 1251G was then washed to 20 mbsf and a 1-m-long pressure core was taken (Core 204-1251G-2P). Hole 1251H was drilled to 445 mbsf TD and wireline logged.
We divide the sedimentary sequence at Site 1251 into three lithostratigraphic units (Units I-III) (Figs. F2, F3). This division is based on sedimentological criteria (e.g., variations in sedimentary structure and grain size or biogenic and lithologic components) and other parameters such as calcium carbonate content (expressed as weight percent CaCO3), total organic carbon (TOC), and mineralogy determined from X-ray diffraction (XRD). We also compare and correlate our results with the three dimensional (3-D) seismic data, downhole LWD data, and physical property measurements (magnetic susceptibility [MS]) to better define the entire stratigraphic sequence (Figs. F4, F5). Based on the above criteria, we further divide lithostratigraphic Unit I into three subunits (Subunits IA-IC) and lithostratigraphic Unit II into two subunits (Subunits IIA and IIB) (Figs. F2, F3). Correlation of the lithostratigraphic units defined here with the other Leg 204 sites is summarized in Figure F10 in the "Leg 204 Summary" chapter.
Lithostratigraphic Unit I (0-130 mbsf) is composed of silty clay and clay with interlayered sand and silt layers and mud clast deposits, which we interpret as turbidites and debris flows, respectively. Lithostratigraphic Unit I is divided into three subunits, defined as lithostratigraphic Subunits 1A (Holes 1251B [0-23 mbsf] and 1251D [0-28 mbsf] and Holes 1251C and 1251E [all cores]), IB (Holes 1251B [23-34 mbsf] and 1251D [28-34 mbsf]), and IC (Holes 1251B [34-130 mbsf] and 1251D [34-130 mbsf]), based on several sedimentological criteria.
Core recovery in the uppermost portion of lithostratigraphic Subunit IA was poor, especially in Holes 1251B and 1251D. There was no recovery for Core 204-1251B-2H and poor recovery in Cores 204-1251D-1X (39.9%) and 2X (35.3%). Core recovery improved in Cores 204-1251C-1H (101%) and 2X (57.4%) and in Cores 204-1251E-1H (104.1%), 204-1251F-1H (104.4%), and 204-1251G-1H (106.4%), but only one core was obtained from each hole.
Lithostratigraphic Subunit IA consists of dark greenish gray (5GY 4/1) clay with thin (1-5 cm thick) silt to silty sand turbidites that grade to diatom-bearing silty clay and clay (Fig. F6A) These graded beds increase in frequency and thickness toward the base of lithostratigraphic Subunit IA. We place the boundary between lithostratigraphic Subunits IA and IB at 23 mbsf, the top of a seismically and lithologically distinct series of debris flow deposits (Fig. F5).
The stratigraphy of lithostratigraphic Subunit IB is dominated by zones of convoluted clay with abundant sulfide material containing unsorted clay-rich granules and pebble-sized silty clay clasts (intervals: Sections 204-1251B-3H-4 through 3H-6 and 4H-3 [all] and Sections 201-1251D-4H-1 through 4H-5) (Fig. F6B). The clasts have a noticeably different color than the matrix material (e.g., lighter and darker shades of dark greenish gray). Clasts are composed of clay and silty clay and range from 1 to 20 cm in diameter. Bioturbation and mottled sulfides are common in the clay matrix and generally absent from the clasts. Some silt layers are also present just above and within the silty clay clast layers (Section 204-1251B-2H-2). Zones of convoluted bedding are common but most often observed in the clays underlying the more clast-rich intervals.
Lithostratigraphic Subunit IC is characterized by interbedded clays and silty clays, with bioturbation and sulfides commonly observed. The relative amounts of coarse sand and silt decrease throughout the subunit, whereas the relative abundances of diatom-, foraminifer-, and nannofossil-bearing clays and silty clays increase toward the base of the subunit (Fig. F3); the frequency of visible sponge spicule clusters also increases (see "Site 1251 Visual Core Descriptions"). Glauconite was observed in greater amounts in Subunit IC than in the previous subunits (Fig. F7A). The percentage of sand observed in smear slides decreases throughout lithostratigraphic Unit I (e.g., is highest in lithostratigraphic Subunit IA, decreases in lithostratigraphic Subunit IB, and is lowest in lithostratigraphic Subunit IC) (Figs. F2, F3). The boundary between lithostratigraphic Subunit IC and lithostratigraphic Unit II occurs at 130 mbsf in Holes 1251B and 1251D, where an abrupt angular unconformity is seen in the 3-D seismic reflection data.
The boundary between lithostratigraphic Units I and II was placed at 130 mbsf in both Holes 1251C and 1251D. This boundary corresponds to an abrupt angular unconformity, which is seen in the 3-D seismic data (Figs. F4, F5) (see Fig. F8 in the "Leg 204 Summary" chapter). An increased frequency of turbidites and an abundance of biogenic opal in lithostratigraphic Unit II also supports a unit boundary at this location (Figs. F2, F3).
Lithostratigraphic Unit II is composed of silty clay and clay and is divided into Subunits IIA and IIB at ~220 mbsf (Section 204-1251B-27X-5 and near the base of Hole 1251D). Below ~220 mbsf (in lithostratigraphic Subunit IIB), there is also an increase in the calcareous component of the sediment (Fig. F3). Core recovery in lithostratigraphic Unit II was 80.9% in Hole 1251B and 84.9% in Hole 1251D. The base of lithostratigraphic Unit II is only present in Hole 1251B at 300 mbsf, where the character of the sediment changes and a distinct seismic reflection (the top of the accretionary complex) is observed (Figs. F4, F5) (see Fig. F8 in the "Leg 204 Summary" chapter).
In the presence of sulfide, which is commonly found as burrow infills in highly bioturbated intervals or as disseminated horizons 1 to 5 cm thick, the color changes to very dark gray (N3). Sulfide mottles are more abundant in lithostratigraphic Subunit IIA than in Subunit IIB. A change in the character of the MS data at ~220 mbsf may reflect the change in the sulfide abundance between Subunits IIA and IIB (see "Environment of Deposition"). Where present, bedding is defined by either erosional surfaces capped with coarse silt (turbidites) or by dark sulfide horizons and is typically subhorizontal throughout both subunits. The state of lithification changes within lithostratigraphic Subunit IIB, with the base being firmer and more indurated than the top. The change from APC to XCB coring occurred at 194 mbsf in Hole 1251B and at 175 mbsf in Hole 1251D; therefore, most of the characterization of lithostratigraphic Subunit IIB is based on the examination of intact drilling biscuits from the XCB cores.
Lithostratigraphic Subunit IIA differs from IIB in both grain size and microfossil content in smear slides, with a more abundant coarse fraction (silt and sand) in Subunit IIA and a significant increase in calcareous components in Subunit IIB (Figs. F2, F3).
Lithostratigraphic Subunit IIA consists of silty clay, which typically contains 30%-35% silt and 65%-70% clay (Figs. F2, F3). The silt fraction consists of quartz (3%-15%) and feldspar (0%-3%). Calcareous components in the sediments are rare in lithostratigraphic Subunit IIA. Only Sample 204-1251B-16H-1, 54-54 cm, contains significant amounts of calcite, as determined by XRD analysis. The lack of calcareous constituents is also mirrored in the composition of the biogenic fraction (Figs. F2, F3). Overall, biogenic components compose no more than 20% of the major and minor lithologies in lithostratigraphic Subunit IIA, and these are dominated by siliceous rather than carbonaceous components (Fig. F3). Diatoms compose up to 15% of the major and minor lithologies in Section 204-1251B-22H-5, 71 cm, whereas calcareous components compose ~3% to 9%.
Lithostratigraphic Subunit IIB is composed of primarily clay-sized grains. The major lithology contains 15% to 20% silt and 80% to 85% clay. The quartz content of the silt-size fraction ranges from 3% to 30% and is slightly higher overall than the quartz content of lithostratigraphic Subunit IIA. The feldspar content of lithostratigraphic Subunit IIB ranges from 2% to 15% and is consistently higher than that of lithostratigraphic Subunit IIA, in which feldspar is often absent.
The biogenic components of lithostratigraphic Subunit IIB are predominantly calcareous (Fig. F3). Foraminifers first appear in smear slides in Section 204-1251B-25X (~195 mbsf) and reach a maximum of 8% in Core 28X. In Section 204-1251B-36X-CC, foraminifers are even visible without the aid of a microscope (Fig. F8A). Nannofossils, which are rare in lithostratigraphic Subunit IIA, are present in excess of 30% in lithostratigraphic Subunit IIB. The first appearance of calcareous nannofossils occurs in Section 204-1251B-25X-CC (~195 mbsf) (Fig. F8B). The maximum abundance of calcareous nannofossils is present in Core 204-1251B-28X, where they compose 40% of the total sediment. In Cores 204-1251B-28X through 33X (between 225 and 271 mbsf), calcareous nannofossils compose from 15% to 18% of the sediment (Fig. F3). Below 271 mbsf to the base of lithostratigraphic Unit II, they continue to compose 5%-10% of the sediment. Diatom abundance ranges from 2% to 8% throughout the subunit, which is similar to the diatom abundance observed in lithostratigraphic Subunit IIA (Fig. F3). In addition to the high biogenic carbonate content of lithostratigraphic Subunit IIB, Core 204-1251B-32X contains a carbonate-rich horizon composed of 70% authigenic calcite crystals that range in size from 1 to 5 µm in diameter. Other than this exception, both lithostratigraphic Subunits IIA and IIB lack carbonate nodules.
Pyrrhotite nodules, which are associated with the presence of sulfide, are more abundant in lithostratigraphic Subunit IIA and are absent from lithostratigraphic Subunit IIB in Hole 1251D. These nodules range in size from <0.1 to 0.2 cm in diameter. Pyrrhotite nodules are also found in lithostratigraphic Unit II in both holes (e.g., in Cores 204-1251B-15X, 20X, 29X, and 204-1251D-16H). These nodules often take the shape of burrow infilling, are attracted to a magnet, and are nonreactive in HCl.
The only gas hydrate recovered at Site 1251 was from Cores 204-1251D-22X and 24X in lithostratigraphic Subunit IIA above the BSR (Fig. F4).
Lithostratigraphic Unit III was only drilled in Hole 1251B. In contrast to lithostratigraphic Unit II, it is composed of hard indurated clay, silty clay, and claystone with reduced calcareous components and increased biogenic opal and glauconite (Figs. F3, F7). The clay and silty clay are predominantly dark greenish gray (5GY 4/1) with lighter variations from olive to olive gray (5Y 5/3 to 5Y 5/2) in Cores 204-1251B-41X through 50X. Core recovery in lithostratigraphic Unit III was good (~88%), except in Core 204-1251B-48Y (FPC), which did not recover any sediment. All of the cores exhibited drilling disturbance; even the drilling biscuits were heavily fractured in some cases (see "Site 1251 Visual Core Descriptions").
Lithostratigraphic Unit III is characterized by its high state of lithification, diatom-rich clay, authigenic carbonate in the clay fraction, carbonate nodules, and increasing scaly clay fabric. The scaly fabric is first present in Core 204-1251B-49X and is observed in broken drilling biscuits. The boundary between lithostratigraphic Units II and III is placed at 300.6 mbsf, directly above a 120-cm-thick glauconite-bearing to glauconite-rich (<30%) silt and silty clay layer (Fig. F9A) and marks the first significant occurrence of glauconite in this hole (Fig. F7A). This concentrated presence of glauconite extends for 4.3 m to 304.9 mbsf and may correspond to an unconformity (see "Environment of Deposition").
Borehole breakouts observed during LWD confirm the transition from clay to claystone at ~304 to 316 mbsf (see "Logging While Drilling" in "Downhole Logging"). Below 334 mbsf, borehole breakouts dominate the RAB images and affect the log data.
Rare bioturbation and mottled fabric, resulting from black (N3) sulfide precipitates, are present only in Sections 204-1251B-44X-4 to 46X-1 and observed as small patches in Sections 49X-2 to 49X-6. Macroscopic aggregates of sponge spicules are observed in Cores 204-1251B-49X to 50X. Clam-shell fragments are found in Cores 204-1251B-52X and 53X.
The upper part of lithostratigraphic Unit III (Sections 204-1251B-37X-1 to 43X-CC) is dominated by silty claystone. Based on smear slide analyses (major lithology), the silty claystone contains up to 5% sand and 30% silt (Fig. F2). The major components of the silty claystone, as determined by XRD analyses, are quartz, feldspar, muscovite, illite, other clay minerals, and calcite in varying amounts. The silty claystone, as well as the claystone deeper in the hole, is diatom bearing to diatom rich (Fig. F9C). The siliceous microfossil content steadily increases from the top of lithostratigraphic Unit III to ~20% diatoms at 328.4 mbsf (Fig. F3). The biogenic carbonate fraction, consisting of foraminifers and nannofossils, decreases (Fig. F3) proportionally. Below 401.79 mbsf, biogenic carbonate is absent from the sediments.
Glauconite appears not only at the top of lithostratigraphic Unit III (Fig. F9A) but is also dispersed through the whole unit. Glauconite is much more abundant in lithostratigraphic Unit III than in the overlying units, and its concentration in the sediments of Unit III increases with depth (Fig. F7A). The percentage of glauconite also increases in comparison to the other lithostratigraphic units (Figs. F7A, F9A). In lithostratigraphic Unit III, it composes up to 20% of the sediment in Sections 204-1251B-36X-4 through 36X-6, 37X-1, 46X-3, 47X-CC, 49X-1 through 49X-3, 49X-6, and 53X-3. In Sections 204-1251B-51X-1 through 51X-4, glauconite is a major lithology (e.g., glaucony).
In the lower part of lithostratigraphic Unit III (Sections 204-1251B-44X-1 through 53X-CC), the major lithology is claystone, containing up to 20% silt (Fig. F2). The major components of the claystone, as determined by XRD analyses, are similar to the composition of the silty claystone discussed above.
Substantial amounts of authigenic micritic carbonate were observed in lithostratigraphic Unit III as a minor, slightly indurated (or nonlithified) lithology (Fig. F9B). These carbonate-rich zones, which are observed in Cores 204-1251B-41X, 44X through 47X, 49X, 50X, and 52X, appear as olive-gray (5Y 5/2) to olive (5Y 5/3) patches between 1 and 60 cm thick. Clay-sized carbonate needles compose ~90% of the sediment components seen in smear slides. XRD analyses show a mixed calcite composition in Samples 204-1251B-41X-3, 18-19 cm, and 44X-6, 30-31 cm (Fig. F10). Chemical measurements of CaCO3 (Fig. F7A) show that these are areas of high calcium carbonate content (Samples 204-1251B-41X-3, 18-19 cm [333.78 mbsf], 47X-5, 124-127 cm [394.5 mbsf], 49X-1, 86-90 cm [398.77 mbsf], and 50X-6, 53-55 cm [414.55 mbsf]). These samples have a CaCO3 content of 17.6-50.4 wt%. For comparison, the estimated authigenic carbonate content in a nearby smear slide at 333.69 mbsf is ~80%.
The presence of carbonate nodules is characteristic of lithostratigraphic Unit III (Table T2). A distinction can be made between solid lithified nodules and semilithified nodules. Solid nodules are present at ~305.4 mbsf (Samples 204-1251B-37X-4, 30-32 cm, and 37X-4, 32-33 cm), 322.08 mbsf (Sample 204-1251B-39X-2, 68-69 cm), 336.12 mbsf (Sample 204-1251B-41X-4, 102-104 cm), and 427.12 mbsf (Sample 204-1251B-52X-1, 132-135 cm). The mineralogy of the solid carbonate samples, determined by XRD analyses, varies. Samples 204-1251B-37X-4, 30-32 cm, and 37X-4, 32-33 cm, are of pure dolomitic composition (Fig. F10; Table T2). Sample 204-1251B-39X-2, 68-69 cm, shows calcitic and dolomitic carbonate phases, and Sample 204-1251B-52X-1, 132-135 cm (Fig. F10), is purely calcitic. Thin sections reveal a micritic matrix of dolomite, with 9% foraminifer tests, 5% opaque minerals, and 1% glauconite in Sample 204-1251B-37X-4, 31-32 cm. Sample 204-1251B-39X-2, 68-69 cm, shows a micritic matrix (calcite/dolomite?) as well, with only 3% foraminifers, 5% diatoms, 15% opaque minerals, 2% glauconite, and 3% quartz.
Semilithified nodules are present from 378.56 to 398.76 mbsf. Based on their d(104) spacings, Samples 204-1251B-46X-1, 96-98 cm, and 46X-CC, 7-9 cm, are of calcitic composition. Sample 204-1251B-47X-5, 121-126 cm, shows two different calcite phases, and Sample 204-1251B-49X-1, 86-90 cm, is composed of three different carbonate phases (Fig. F10) that are formed by substitution of calcium by major cations like iron, magnesium, and manganese in the crystal lattice.
The sediments of the eastern slope basin lack substantial textural evidence for the dissociation of gas hydrate. Mousselike textures were found in cores from Hole 1251B (Sections 204-1251B-10H-1 and 10H-5) and four cores from Hole 1251D (Sections 204-1251D-2X-1, 3X-1, 5X-1, and 18H-1). Thermal and chloride anomalies indicate hydrate may have been present in Cores 204-1251D-22X and 24X (see Fig. F20 in the "Leg 204 Summary" chapter) just above the BSR, although no corresponding lithologic evidence was found.
Site 1251 is located on the eastern flank of southern Hydrate Ridge. At this site, we recovered the stratigraphic sequence of the eastern slope basin (Fig. F5). The stratigraphy preserved in this slope basin environment is characterized by hemipelagic sedimentation and episodic coarse- and fine-grained turbidite and debris flow deposition.
Lithostratigraphic Unit III is composed of indurated scaly clay (claystone). Biostratigraphic analyses indicate an age between 2.0 and 1.6-1.0 Ma for this unit, which suggests an average sedimentation rate of ~33 cm/k.y. (see "Summary" in "Biostratigraphy"). The presence of glauconite-rich intervals and authigenic carbonate nodules and cements (Figs. F7A, F9A, F9B) suggests that diagenetic processes are active throughout the unit. High values on the LWD gamma ray density data correlate with the presence of some of the glauconite layers, possibly resulting from the high K content of glauconite clay minerals.
Authigenic formation of green glauconite grains is documented by submarine weathering of phyllosillicates, which were modified as a result of an adequate supply of both potassium and iron from seawater. In general, glauconization forms a diagenetic sequence ranging from glauconitic smectite to glauconitic mica (Odin and Matter, 1981). Since glauconite contains both ferric and ferrous iron, its formation is limited to a specific level of oxidation potential close to the seafloor, which needs to be maintained over long timescales (1-1000 k.y.). These distinct glauconite layers often represent time intervals of very low or zero sediment accumulation and result in unconformities in the sediment sequence.
The 3-D seismic data suggest a major discontinuity exists at ~300 mbsf, between irregular, discontinuous, and chaotic reflections (lithostratigraphic Unit III) and well-stratified seismic reflectors (lithostratigraphic Unit II). Visual core descriptions, examinations of seismic reflection profiles, and the presence of borehole breakouts on LWD resistivity data all suggest that lithostratigraphic Unit III is part of the older accretionary complex of Hydrate Ridge.
Lithostratigraphic Unit II is composed of hemipelagic silty clays and clays ranging from ~1.0 to 0.3 Ma in age, yielding a sedimentation rate of ~17 cm/k.y. (see "Summary" in "Biostratigraphy"). In the 3-D seismic data, lithostratigraphic Unit II is characterized by a series of well-stratified east-dipping seismic reflectors. The reflectivity changes from low above the BSR to high below the BSR, delimiting the boundary between a possible free gas zone beneath the BSR and gas hydrate-bearing region above. Hydrate was sampled from Unit II near the BSR (190 mbsf) and is associated locally with coarse-grained sediments (Figs. F2, F4).
Lithostratigraphic Subunit IIB is clay dominated and characterized by the highest abundance of calcareous microfossils at Site 1251 (Fig. F3). The homogeneous fine-grained sediments and lack of significant iron sulfides within lithostratigraphic Subunit IIB correlate well with the uniform low signature of MS data (Fig. F4) (see "Magnetic Susceptibility" in "Physical Properties"). Because lithostratigraphic Subunit IIA contains few sand and silt turbidites, a distinct abundance of iron sulfide material (pyrrhotite) may help explain the large excursions recorded in MS data (see "Magnetic Susceptibility" in "Physical Properties").
The boundary between lithostratigraphic Units II and I corresponds to a major angular unconformity at ~130 mbsf best seen in the 3-D seismic data (Fig. F5). This discontinuity, however, is not obvious in the recovered core from Site 1251 because the lithologies of lithostratigraphic Subunit IC are similar to those of Unit II and bedding planes and attitudes are difficult to identify in the stratigraphy. Nevertheless, there is a noticeable lack of the large MS excursions in Subunit IC (Fig. F2). Lithostratigraphic Subunit IC is also characterized by the onset of pelagic sediment components, including diatoms, foraminifers, and nannofossils, and an abrupt decrease in terrigenous coarse components (Figs. F2, F3). An observed decrease in sedimentation rate may be consistent with this style of sedimentation (see "Summary" in "Biostratigraphy"). In the 3-D seismic data, Unit III corresponds to a 100-m-thick sequence characterized by subparallel, well-stratified, and slightly folded seismic reflectors (Fig. F5).
Lithostratigraphic Subunit IB is composed of large unsorted clasts of heterogeneous compositions and convolute bedding, suggestive of debris flow deposition. The lithology of this subunit correlates with a large positive peak (up to 100 SI units) in the MS data (see "Magnetic Susceptibility" in "Physical Properties"). In the 3-D seismic data, lithostratigraphic Subunit IB is characterized as a lens-shaped sediment package with a chaotic internal reflection pattern and an erosional base. This depostional unit reaches a thickness of up to 70 m at the center of the eastern basin (Fig. F5) and covers an area ~2.5 km wide x 8 km long. Similar deposits are observed on the Mediterranean margin and are interpreted to have resulted from slope instability initiated by the sea level lowstand during the late Pleistocene (Rothwell et al., 1998). Perhaps the sediments of lithostratigraphic Subunit IB have a similar origin.
Lithostratigraphic Subunit IA is composed of a series of turbidite deposits that increase in frequency and size downhole. Fine-grained, low-frequency turbidites are present within the upper 11 mbsf of this subunit, whereas coarser and larger events dominate the lower part. An increase in siliceous microfossil abundance in the upper part of lithostratigraphic Subunit IA (Fig. F4) may represent the late Pleistocene/Holocene boundary, as documented in several cores throughout the northern and central Cascadia margin (Duncan et al., 1970).