Site 1245 is located just west of the crest of southern Hydrate Ridge (see Figs. F1 and F5 both in the "Leg 204 Summary" chapter). Five holes (Holes 1245A-1245E) were drilled and four holes (Holes 1245B-1245E) were cored at Site 1245. Hole 1245B was cored to 471.7 mbsf, Hole 1245C was cored to 198.7 mbsf, Hole 1245D was cored to 24 mbsf, and Hole 1245E was cored from 473.7 to 540.30 mbsf (for 66.6 m). Recovery was generally good (Hole 1245B = 88.7%; Hole 1245C = 93.3%; Hole 1245D = 103.4%; and Hole 1245E = 27.7%).
We divide the sedimentary sequence recovered at Site 1245 into five lithostratigraphic units (Units I-V) (Figs. F2, F3) based on sedimentological criteria (e.g., variations in sedimentary structure, grain size, and biogenic and lithologic components) and other parameters, such as calcium carbonate content (expressed as CaCO3 weight percent), total organic carbon (TOC), and mineralogy from X-ray diffraction (XRD) (Figs. F2, F4). We also compare and correlate our results with the 3-D seismic data, downhole LWD data, and physical property measurements (magnetic susceptibility [MS] and gamma ray attenuation [GRA] density) to better define the entire stratigraphic sequence (Fig. F3). Based on the above criteria, we further divide lithostratigraphic Unit III into two subunits (Subunits IIIA and IIIB) and lithostratigraphic Unit IV into two subunits (Subunits IVA and IVB). Correlation of the lithostratigraphic units defined here with the other Leg 204 sites is summarized in Figure F10 in the "Leg 204 Summary" chapter.
Good core recovery and correlation of lithostratigraphic Unit I in Holes 1245B and 1245C suggests that we recovered a complete record of lithostratigraphic Unit I at Site 1245. The majority of the sediments recovered in Hole 1245D were sampled for microbiology, but the sections we described do correlate with Holes 1245B and 1245C. Lithostratigraphic Unit I consists of dark greenish gray (5GY 4/1) nannofossil-bearing clay to silty clay with some foraminifer-bearing zones and a few thin (<1 cm) graded silt layers, which we interpret as turbidites. Coarser silt is present in the top 30 cm of Hole 1245C as a thick layer; however, it is homogenized with the surrounding clay as a result of coring-related disturbance and, thus, may be reworked. Authigenic carbonates are common, and both sulfide mineralization and bioturbation increase toward the base of lithostratigraphic Unit I.
The top of lithostratigraphic Unit I contains several clay zones with authigenic carbonate precipitation. Semiconsolidated to solid carbonate nodules were found in Hole 1245B in Sections 204-1245B-1H-4, 1H-5, and 2H-4 (Fig. F5), as well as in Hole 1245C in Section 204-1245C-1H-4. Sulfide mineralization increases toward the base of the unit and is observed as (1) an increase in the dark gray (N3) color of the sediment (e.g., intervals 204-1245B-3H-4 and 204-1245C-4H-3) and (2) an appearance of sulfide nodules (e.g., Sections 204-1245B-3H-6, 9-10 cm; 204-1245C-4H-3, 74 cm; and 204-1245C-4H-4, 121 cm).
The lithostratigraphic Unit I/II boundary is marked by a general downhole increase in the frequency of turbidites and the onset of coarser material (sand-size fraction) as well as by a slight increase in the relative abundance of siliceous microfauna (Figs. F2, F4).
Lithostratigraphic Unit II consists of dark greenish gray (5GY 4/1) diatom-bearing clay and silty clay commonly interbedded with fine to very fine sand layers (Fig. F6). The sharp bases and graded nature of the sands suggest they were deposited by turbidity currents. The upper boundary of lithostratigraphic Unit II is defined by an increase in mean grain size (from clay to silty clay) as well as an increase in biogenic components (mainly siliceous). This boundary also correlates with an increase in the LWD resistivity and density (Fig. F3). The lower boundary of lithostratigraphic Unit II is defined by a decrease in grain size from sand to silt, accompanied by a slight decrease in biogenic opal (Figs. F2, F4). This decrease occurs at 76 mbsf in Hole 1245B and at 77 mbsf in Hole 1245C, which correlates well with the estimated depth of Horizon Y, a distinct discontinuity imaged in the 3-D seismic data (see Fig. F5 in the "Leg 204 Summary" chapter). However, the west-dipping strata above Horizon Y that downlap onto the flat, possibly erosional surface of Horizon Y (Fig. F3) were not detected in the stratigraphy recovered at Site 1245.
Smear slide analyses indicate that lithostratigraphic Unit II is primarily composed of 78% clay, 19% silt, and 3% sand (Fig. F4). The major nonbiogenic components of lithostratigraphic Unit II are feldspar, quartz, and clay and opaque minerals. Opaque grains, mostly sulfides in irregular and framboidal forms, are common in all grain sizes and typically comprise ~3% of the major and minor lithologies, although locally they reach 10% (e.g., Sections 204-1245B-4H-1 and 7H-4). Glauconite composes >2% of the sediment, and it reaches up to 15% in a 7-cm-thick sand layer located at 67.2 mbsf (Sample 204-1245B-8H-1, 112 cm) (Figs. F7A, F8). Volcanic glass-rich sand, containing 15%-30% glass, is present in Section 204-1245B-6H-3 at ~51.2 mbsf.
The total biogenic component of the sediment, which is composed of foraminifers, nannofossils, diatoms, radiolarians, and sponge spicules, ranges from 3% to 13% of the total sediment. Biogenic opal reaches up to 11%, is composed primarily of diatoms (Sample 204-1245B-8H-2, 67 cm), and is more abundant than the calcareous components (Figs. F2, F4).
Lithostratigraphic Unit II contained one authigenic carbonate nodule in interval 204-1245C-6H-4, 103-105 cm (~53 mbsf). Thin section analysis of the nodule indicates that it is primarily micritic carbonate, containing sparse foraminifers and nonbiogenic components (e.g., quartz, feldspar, and opaques). Two phases of calcite with differing Mg contents were recognized based on the XRD analyses.
Lithostratigraphic Unit III consists of dark greenish gray (5GY 4/1) clay and silty clay to nannofossil-rich and diatom-rich silty clay. The major silty clay lithology is commonly interbedded with fining-upward sandy silt and silt turbidites (Fig. F9). Most of the recovered cores preserve original sedimentary structures, with the exception of the gas hydrate-bearing sediments, which are characterized by mousselike textures (Fig. F10) (see also "Sedimentary Evidence of Gas Hydrate"). Sediments are slightly fractured, with subhorizontal cracks that are presumably caused by gas expansion. Core recovery was high in lithostratigraphic Unit III (90.3% in Hole 1245B and 89.1% in Hole 1245C), making it possible to correlate between the sequences recovered in Holes 1245B and 1245C.
The lower boundary of lithostratigraphic Unit III was placed above the first glauconite-bearing sand layers at ~212.7 mbsf in Core 204-1245B-24X (Figs. F7, F8). This boundary also corresponds to an increase in the MS of sediments (see "Physical Properties"). Lithostratigraphic Unit III was subdivided into two subunits (Subunits IIIA and IIIB), based on the change in the abundance of biogenic components, variations in grain size, and correlation with 3-D seismic data.
Lithostratigraphic Subunit IIIA (76-183 mbsf in Hole 1245B and 77-189.5 mbsf in Hole 1245C) consists of nannofossil- and diatom-rich clay and silty clay (Figs. F2, F11) finely interbedded with thin (2-4 mm to <1 cm thick) sandy silt and silt layers, which we interpret as turbidites (Fig. F9). Mottles, patches, and nodules of dark gray (N3) iron sulfides and bioturbation are common to rare throughout the subunit. The base of lithostratigraphic Subunit IIIA is defined by an increase in mean grain size and the presence of volcanic glass-rich sediments and ashes, identified as Horizon A on the 3-D seismic data (Fig. F3) (see Figs. F5 and F6 both in the "Leg 204 Summary" chapter).
Smear slide analyses indicate that the major lithology of lithostratigraphic Subunit IIIA is typically composed of ~77% clay and ~23% silt. The texture of the minor lithology is dominated by sand and silt, and this grain size becomes more abundant with depth in this subunit (Fig. F2). The major mineral components of the subunit are feldspar, quartz, and clay and opaque minerals. Biogenic components (calcareous and opal) vary from 0% to 25% of the total sediments (Fig. F2).
Soft-sediment deformation features are locally observed in lithostratigraphic Subunit IIIA (e.g., Section 204-1245B-14H-2). Mousselike textures were observed near intervals where gas hydrate was sampled between 126 and 143 mbsf near the depth of the BSR (see "Sedimentary Evidence of Gas Hydrate"). Iron sulfide precipitates and bioturbation range from common to rare throughout this subunit. A sequence of volcanic glass-rich sediments, located at ~180 mbsf in Hole 1245B, appears to correspond to Horizon A, a bright reflector identified in the 3-D seismic data.
Five distinct light-colored intervals containing volcanic glass-rich sediments and ash were found between 176 and 183 mbsf (Sections 204-1245B-21X-2 to 21X-4) (Fig. F12). These five intervals correlate with the approximate depth of Horizon A in the 3-D seismic data (Fig. F3) (see Figs. F5 and F6 both in the "Leg 204 Summary" chapter). A detailed analysis of more than 32 smear slides was conducted in conjunction with high-resolution color reflectance (b*) and MS measurements acquired at 1-cm spacing across these intervals (Fig. F13). Smear slide results are described on the barrel sheets and tabulated in smear slide summaries (see "Site 1245 Smear Slides"). The volcanic glass-rich sediment and ash sequences, ranging from 6 to 23 cm thick, are typically composed of volcanic glass-rich sands (dark beige color) that grade upward into sandy-silty volcanic ash (light beige color) and clayey-silty ash (pure white). The glass content of each layer increases in the sequence from the basal sand containing 0%-20% glass, to the sandy to silty ash containing up to 80% glass, to the uppermost clayey-silty ash containing 80%-96% glass (Figs. F13, F14, F15). Both color reflectance data (b*; ranging from blue to yellow) and MS values are inversely correlated across the glass-rich sediment and ash intervals, where the sands at the bases of the ash layers have high MS and low color reflectance values compared to the surrounding major lithology (silty clay), and the intervals with the highest concentrations of ash appear to have low MS and high color reflectance values (Fig. F13). The intervals containing the volcanic glass-rich sediments and ashes are considered to represent the primary source of Horizon A in the 3-D seismic reflection data (Fig. F3).
Lithostratigraphic Subunit IIIB (183-212.7 mbsf in Hole 1245B and 189.5-200.9 mbsf in Hole 1245C) consists of both silty clay and nannofossil-rich silty clay interbedded with graded silt layers (see "Site 1245 Visual Core Descriptions"). Near the top of Subunit IIIB, biogenic opal, predominantly in the form of diatoms, composes up to 45% of the sediment (Fig. F4). Calcareous components increase slightly toward the base of the subunit (Figs. F2, F7B).
Lithostratigraphic Unit IV is primarily composed of dark greenish gray (5GY 4/1) to very dark gray (N3) indurated claystone and silty claystone. This unit has been subdivided into two lithostratigraphic subunits at 320 mbsf (Subunits IVA and IVB) on the basis of changes in both grain size and biogenic content (Figs. F2, F16, F17). Above 320 mbsf, the major lithology of lithostratigraphic Subunit IVA typically contains coarse-grained silt layers that fine upward into hemipelagic clay containing >10% nannofossils. Below 320 mbsf, lithostratigraphic Subunit IVB lacks silt horizons and typically contains <10% total biogenic components.
Glauconite-bearing sands, induration, and scaly clay fabric at 212.7 mbsf mark the upper boundary of lithostratigraphic Unit IV (Figs. F2, F4, F7A) and differentiates this unit from the graded sand-silt-clay sequences of lithostratigraphic Subunit IIIB. The base of lithostratigraphic Unit IV is defined by another shift in grain size and biogenic composition at 420 mbsf (Fig. F2). This shift correlates well with a change in the lithium profile at Site 1245 (see "Interstitial Water Geochemistry," Fig. F28).
Lithostratigraphic Subunit IVA (Hole 1245B; 212.7-320 mbsf) is composed of indurated silty claystone with a total biogenic content >10%. Minor lithologies are silt- to sand-sized interlayers that are unevenly spaced throughout the subunit at intervals ranging from 1 to 15 m. Glauconite-bearing sands are found at 213, 218, 226, and 256 mbsf, respectively (Fig. F7A). At other Leg 204 sites, the onset of the glauconite-bearing sand correlates with the top of the deeper accretionary complex. However, the glauconite found in the sand fraction of the fining-upward sequences at this site may have been transported to this location via turbidity currents and, therefore, may not reflect the original in situ diagenetic conditions. Sulfide and bioturbation are also common in lithostratigraphic Subunit IVA. Sulfide is found both as smeared interlayers <0.2 cm thick, which extend across the entire core, and as infill in bioturbated horizons.
Planar and subhorizontal bedding is visible throughout lithostratigraphic Subunit IVA. Scaly clay fabric was observed from 225 to 320 mbsf. This fabric was most commonly found in fractured drill biscuits and does not have a uniform orientation.
Calcareous nannofossils are the dominant biogenic component of lithostratigraphic Subunit IVA. The calcareous nannofossil content of the major lithology is >10% and >15% in Cores 204-1245B-31X, 32X, and 34X (Figs. F2, F4). Nannofossil abundance in the minor lithology is typically higher than that in the major lithology. Foraminifers are also present but typically compose 5%-15% of the minor lithologies from 263 to 320 mbsf and rarely exceed 10% of the major lithology (e.g., Core 204-1245B-34X). A slight increase in biogenic opal, from the background content of 1%-3% to 8% in Core 204-1245B-35X, occurs between 295 and 300 mbsf.
Lithostratigraphic Subunit IVB (Hole 1245B; 320-419.3 mbsf) is composed of indurated claystone and silty claystone, which is the major lithology of lithostratigraphic Subunit IVA. The primary difference between these two lithostratigraphic subunits lies in the composition of the minor lithology. Lithostratigraphic Subunit IVB contains significantly fewer graded silt turbidite sequences than were found in lithostratigraphic Subunit IVA. The minor lithologies of lithostratigraphic Subunit IVB are typically composed of 30% silt and 70% clay; glauconite is absent, as is most sulfide and bioturbation. Where bioturbation is present, pyrrhotite and dispersed (millimeter scale) white precipitates, likely gypsum, are also found. These precipitates are found in Cores 204-1245B-39X, 40X, and 42X, are present as an inner lining on preserved burrows, and commonly surround sulfide-rich infilled burrows.
Planar bedding is only found at the top of lithostratigraphic Subunit IVB in Core 204-1245B-37X (Fig. F17). Scaly fabric is more common in lithostratigraphic Subunit IVA than lithostratigraphic Subunit IVB, although it was observed in Cores 204-1245B-37X through 41X as well as in Core 47X.
Both calcareous and siliceous biogenic components are less common in lithostratigraphic Subunit IVB than in IVA (Figs. F2, F4). Foraminifers are absent from the major lithology of lithostratigraphic Subunit IVB, but are present in sandy interlayers (minor lithology) in Cores 204-1245B-37X and 39X. Calcareous nannofossil content drops dramatically in lithostratigraphic Subunit IVB, with only Cores 204-1245B-37X and 43X composed of nannofossil-rich major lithologies; all other cores contain <6% nannofossils. Biogenic opal is slightly more abundant at the top of lithostratigraphic Subunit IVB. A smear slide analysis of Sample 204-1245B-37X-1, 38 cm, indicates that the dominant lithology contains 6% diatoms. Throughout the rest of lithostratigraphic Subunit IVB, the biogenic opal content ranges from 0% to 3% in both the major and minor lithologies (Fig. F4).
Lithostratigraphic Unit V, which was cored in Holes 1245B and 1245E, is composed of dark greenish gray (5GY 4/1) claystone and silty claystone (Fig. F2). Cores were highly disturbed by drilling, such that the drilling biscuits were heavily fractured in some intervals. Lithostratigraphic Unit V cannot be correlated between Holes 1245B and 1245E (Fig. F2) because the recovered intervals do not overlap.
Lithostratigraphic Unit V is composed of a highly lithified, nannofossil-rich clay and silty claystone (Fig. F18) that displays dispersed moderate bioturbation, parallel-bedded, fining-upward silt and sand turbidites, and macroscopic foraminifers (Fig. F19). The lithostratigraphic Unit IV/V boundary is placed at 419.3 mbsf, directly above the first occurrence (FO) of frequent thick turbidites that are rich in biogenic calcareous components (nannofossils and foraminifers) (Fig. F2). An increase in the pore water lithium concentration to significantly higher values at ~430 mbsf (see "Interstitial Water Geochemistry") also correlates with the lithostratigraphic Unit IV/V boundary.
The major lithology of lithostratigraphic Unit V in Hole 1245B is clay and silty claystone that is nannofossil rich from 426 to 462 mbsf (Cores 204-1245B-49X and 52X) (Fig. F2). Based on smear slide analyses, the claystone contains up to 98% clay and the silty claystone up to 3% sand and 27% silt (Fig. F4). The major components of the clay and silty claystone, as determined by XRD analyses, are quartz, feldspar, muscovite, illite, other clay minerals, and minor amounts of calcite.
The major lithology of lithostratigraphic Unit V is further characterized by rare to moderate bioturbation, absent sulfides, macroscopic foraminifers, mollusk shell fragments, and wood fragments. Macroscopic foraminifers (Cores 204-1245B-49X and 204-1245E-2R, 5R, 6R, and 8R), mollusk shell fragments (Cores 204-1245B-49X and 52X and 204-1245E-2R, 6R, and 7R), and wood fragments (Sections 204-1245B-49X-3, 110 cm, and 204-1245E-6R-1, 64 cm) were all noted in visual core descriptions. Observed diagenetic features include dispersed white precipitates, likely gypsum, in Cores 204-1245E-4R through 5R and a sulfide precipitate (probably pyrrhotite) in Section 6R-2, 85 cm.
Minor lithologies in lithostratigraphic Unit V are highly varied and are typically found as interlayers within the major lithology (see Table T2). These interlayers are typically turbidites (4-30 cm thick) characterized by grading (Fig. F20) and planar laminations (Fig. F19A). Dark grains (~1 mm) are visible in Sections 204-1245E-3R-1, 48 cm; 3R-2, 13 and 106 cm; and 3R-CC, 14 cm. Foraminifer-rich clay and silty claystone are also minor lithologies within lithostratigraphic Unit V from 492 to 531 mbsf.
Intervals 204-1245B-51X-1, 54-60 cm, and 100-115 cm; and 53X-2, 95-99 cm; 53X-4, 8-7 cm; 53X-5, 66-150 cm; and 53X-6, 45-60 cm, contain mud clast layers ranging in thickness from 4 to 84 cm (Fig. F19B). The clasts are composed of well-rounded, unsorted mudstone and are slightly greener and browner than the surrounding lithology. A diatom-bearing silty claystone clast is found in Section 204-1245B-53X-4 (smear slide Sample 53X-4, 30 cm). The size of the different clasts varies from 0.5 to 6 cm in diameter.
A total of 11 gas hydrate samples were taken from Site 1245 (Table T3). Mousselike textures were observed within lithostratigraphic Units II and III and correspond to intervals near the locations of gas hydrate samples. The observed disruption to the sedimentary structures is presumed to have been caused by the dissociation of gas hydrate and correlates well with cold anomalies detected in the infrared (IR) images (see "Physical Properties").
Below 134 mbsf, the depth of the BSR at Site 1245, mousselike texture was observed in Cores 204-1245B-16H and 204-1245C-24H. However, no low chlorinity anomalies were detected in interstitial water (IW) geochemistry data, and no cold anomalies were observed with the IR thermal camera at these depths (see "Interstitial Water Geochemistry" and "Physical Properties"). Therefore, the presence of mousselike texture below 134 mbsf is probably due to coring-related disturbance rather than to sediment disruption related to gas hydrate dissociation.
Site 1245 is located on the western flank of the southern summit of Hydrate Ridge, where we recovered a thick (~540 m) sedimentary sequence of folded and uplifted Quaternary age strata (see Fig. F5 in the "Leg 204 Summary" chapter). We divide the sequence into five lithostratigraphic units (Figs. F2, F3, F4), of which three (Units I-III) correlate with Sites 1250, 1249, 1248, and 1247 and two (Units I and II) with Site 1246 (see Fig. F10 in the "Leg 204 Summary" chapter).
Lithostratigraphic Unit V is characterized by mud clast deposits and thick turbidites within a major lithology of nannofossil-rich clay and silty claystone and may include the top of the deeper accretionary complex of southern Hydrate Ridge (see Fig. F5 in the "Leg 204 Summary" chapter). The abundant macrofossils and wood fragments in the turbidites of this unit suggest a shelf rather than a slope basin origin for these sediments. The proximal position of these sediments to the deformation front also suggests they were originally deposited on the abyssal plain and have since been accreted and uplifted during accretionary wedge formation.
Lithostratigraphic Unit IV consists of claystone and silty claystone with abundant calcareous nannofossils. Biostratigraphic data indicate a nannofossil zone boundary at ~280 mbsf (see "Biostratigraphy"), which may be reflected in the stratigraphy as a decrease in calcareous nannofossils (below ~280 mbsf). The reduced abundance of calcareous nannofossils in lithostratigraphic Subunit IVB could be caused by either dilution of biogenic materials by terrigenous sediment or a change in nannofossil productivity at the time of deposition.
The distinguishing features of lithostratigraphic Unit III are the abundance of biogenic opal, the volcanic glass-rich sediment and ash sequences, and frequent turbidites. The detailed record of volcanic glass-rich sediment and ash sequences around 180 mbsf in Hole 1245B is well correlated to Horizon A in the 3-D seismic data. The repeated association of graded sand- and silt-bearing volcanic glass and the high concentration of volcanic ash near the upper limit of the graded sequences suggests the volcanic glass and ash were deposited by turbidity currents. High-frequency turbidites throughout Unit III correspond to the slightly higher average sedimentation rate of 13 cm/k.y. compared to lithostratigraphic Unit II, which contains less coarse material and has a sedimentation rate of 10 cm/k.y. (see "Biostratigraphy"). The increased abundance of biogenic opal in lithostratigraphic Unit III with depth may suggest a high level of biological productivity, perhaps contributing to the higher sedimentation rate for this unit.
The boundary between lithostratigraphic Units III and II corresponds to a distinct seismic discontinuity (Horizon Y) at 75 mbsf. Horizon Y appears to be an angular unconformity (see Fig. F5 in the "Leg 204 Summary" chapter); however, the stratigraphy above it is folded and the unconformity surface itself is not. The folded geometry above Horizon Y would be more consistent with that produced during folding above a ramp-flat thrust fault system. Although there is no evidence of fault gouge in the recovered cores, a fault origin for Horizon Y cannot be excluded.
The decrease in coarse material (sand) toward the base of lithostratigraphic Unit II suggests a decrease in the sedimentation rate, which is consistent with the biostratigraphically determined average sedimentation rate of 10 cm/k.y. just above Horizon Y at Site 1245 (see "Biostratigraphy").
The presence of turbidites at the top of lithostratigraphic Unit II suggests that sedimentation rates were higher during deposition of the upper portion of lithostratigraphic Unit II, which is consistent with the last increases in sedimentation rate (23 cm/k.y.; from 0 to 60 mbsf) determined from the biostratigraphy (see "Summary" in "Biostratigraphy"). The lack of obvious turbidites in lithostratigraphic Unit I may indicate that it was deposited as hemipelagic drape over an uplifted Hydrate Ridge. Alternatively, it may represent an older clay-rich section of the stratigraphy now exposed near the crest of Hydrate Ridge because of erosion.