We drilled six holes, four of which (Holes 1250C-1250F) were cored at Site 1250, located at the southern summit of Hydrate Ridge on the carbonate apron known as the Pinnacle identified as an authigenic carbonate mound on the seafloor (Bohrmann et al., 2002). It is characterized by high acoustic backscatter on side-scan sonar imagery (Johnson et al., in press) (see Figs. F1 and F7 both in the "Leg 204 Summary" chapter). Hole 1250C was cored to 145 mbsf, with good recovery (~82%). Hole 1250D was cored to 145 mbsf. Hole 1250E was only cored to 16 mbsf but provided additional data for the top part of the sedimentary sequence. Hole 1250F was cored from 103 to 180 mbsf, extending 35 m deeper than Hole 1250C.
Three lithostratigraphic units (Units I-III) (Figs. F2, F3) were defined on the basis of visual core description, smear slide and thin section analyses, and other parameters such as calcium carbonate content (expressed as weight percent CaCO3), total organic carbon (TOC), and mineralogy from X-ray diffraction (XRD). Using these criteria, correlation between the four holes cored at Site 1250 was made. We also compare our results with the 3-D multichannel seismic reflection data, downhole LWD data (resistivity and density), and physical property measurements (magnetic susceptibility [MS]) to better constrain the lithostratigraphic units (Fig. F4). 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 is composed mainly of dark greenish gray (5GY 4/1) diatom-bearing clay to diatom-rich silty clay near its base (Fig. F2). Although the recovery in this lithostratigraphic unit was poor and substantial sampling of gas hydrate occurred on the catwalk prior to description, a clay sequence containing two distinct volcanic glass-rich layers was observed in both Holes 1250C and 1250D. Smear slide estimates indicate the dominant lithology of lithostratigraphic Unit I contains ~70% clay, ~20% silt, and ~10% sand (Fig. F5). Biogenic opal (mainly diatoms) composes ~5% of the total sedimentary components of lithostratigraphic Unit I (Fig. F2). Calcareous components (calcareous nannofossils and foraminifers) are rare, except near the base of lithostratigraphic Unit I in Hole 1250D, where they reach up to 3%. Rare bioturbation is recorded only in the lower part of lithostratigraphic Unit I, above the volcanic glass horizons in Holes 1250C and 1250D (Fig. F6). The middle and lower part of this unit contain sparse sulfide precipitates, giving the core a mottled appearance in places.
Authigenic carbonate precipitates are present as small lenses at depths of 0.53 and 0.74 mbsf in Section 204-1250D-1H-1. Smear slide observations suggest that these lenses are composed of 80% calcite needles. XRD analysis of Sample 204-1250D-1H-1, 72-76 cm, also suggests a high calcite content based on the d(104) peak height (Fig. F7). Calcium carbonate content ranges from 2 to 5 wt% throughout lithostratigraphic Unit I and does not show the presence of the carbonate precipitate layers because the sample spacing was too large.
Two volcanic glass-rich silty clay and silt layers (5-6 cm thick), interbedded with a diatom-rich silty clay layer, are present in the lowest part of lithostratigraphic Unit I (intervals 204-1250D-2H-4, 47-69 cm, and 2H-4, 110-130 cm) (Fig. F6). These layers were not observed in Hole 1250C because of poor recovery. The color of the volcanic glass-rich silty clay layers grades from 10Y 6/1 to 10Y 5/1 with variations in glass content from 40% to 80% of the total sedimentary components (Fig. F6). The boundary between lithostratigraphic Units I and II (9.5 mbsf) was placed at the base of the lower volcanic glass-rich layer (Fig. F2).
Lithologic characteristics of lithostratigraphic Unit II are based on the sequences recovered in Holes 1250C and 1250D, as Hole 1250E recovered only the uppermost 4.5 m of this lithostratigraphic unit. Lithostratigraphic Unit II is a sequence of diatom-bearing to diatom-rich silty clay to layers interbedded with thin silt and fine sand, which we interpret as turbidites. The frequency of turbidites varies between holes at Site 1250, with sand layers present from 9.5 to 13.5 mbsf in Holes 1250C and 1250E and from 15.5 to ~43.0 mbsf in Hole 1250D.
In comparison to lithostratigraphic Unit I, the abundance of biogenic opal in lithostratigraphic Unit II increased from ~5% to ~15% of the sedimentary components in Hole 1250C and from ~3% to ~20% of the sedimentary components in Hole 1250D. The abundance of calcareous nannofossils and foraminifers also increased slightly, reaching up to ~3% in the lower part of lithostratigraphic Unit II in both holes (Fig. F2). Bioturbation varies in intensity but is present throughout lithostratigraphic Unit II, and sulfide patches or mottles were observed throughout the recovered sequence. Iron sulfide nodules or lenses are present in the upper part of Holes 1250C and 1250D.
Authigenic carbonate is present as ~3- to 8-cm-thick layers in Sections 204-1250C-4H-3 and 4H-4. Only one carbonate-bearing clay lens was observed in Section 204-1250D-3H-CC. Smear slide observations indicate these precipitates are dominantly composed of 60%-80% calcite needles.
Lithostratigraphic Unit III consists of diatom-bearing to diatom-rich silty clay to nannofossil-rich clay interbedded with abundant layers of silt to fine sand (Figs. F2, F3, F5). A fine-grained thick debris flow deposit, dominated by calcareous biogenic components, disrupts the cycle of turbidites toward the middle of the lithostratigraphic unit (Fig. F8). Seismic Horizon A, a regional stratigraphic marker mainly composed of volcanic glass-rich sands, was also identified within lithostratigraphic Unit III (Figs. F4, F5, F8) (see Fig. F6 in the "Leg 204 Summary" chapter). Core recovery in lithostratigraphic Unit III was high, which provided a good stratigraphic correlation between the three deep holes at Site 1250. The boundary of lithostratigraphic Unit III is located at ~45 mbsf and is defined by (1) the onset of high-frequency turbidite sequences, (2) an increase in the grain size of minor lithologies, and (3) an increase in biogenic, especially calcareous, components (Figs. F2, F3). The boundary between lithostratigraphic Units II and III (Fig. F2, F3, F4) correlates with seismic Horizon Y, a regional unconformity also identified at Sites 1245 and 1247-1249 (see Figs. F5, F7, F10 all in the "Leg 204 Summary" chapter). The lower boundary of lithostratigraphic Unit III is present at the base of Holes 1250C and 1250D, which both reached similar TDs, as well as in Hole 1250F, which reached to 180 mbsf. Based on the variations in abundance of biogenic calcareous components, lithostratigraphic Unit III was divided into two subunits (Subunits IIIA and IIIB).
Lithostratigraphic Subunit IIIA (Hole 1250C: 43.75-145 mbsf; Hole 1250D: 39.5-145 mbsf; and Hole 1250F: 103-153.5 mbsf) consists of dark greenish gray (5GY 4/1) diatom-bearing to diatom-rich clay and silty clay interbedded with thin (2-4 mm to 1 cm), closely spaced (centimeter) graded sandy silt and silt turbidites. The turbidites have an average frequency of one turbidite per meter. Near the top of lithostratigraphic Subunit IIIA (from 44 to 50 mbsf), coarser-grained, higher-frequency (50 cm), and thicker (up to 20 cm thick) single graded sands are observed (Fig. F9). This interval (Sections 204-1250C-6H-2 through 6H-4) corresponds to the location of Horizon Y on the 3-D seismic data. This interval is associated with a density increase recorded in the LWD data and a large positive excursion of the whole-core MS (see "Magnetic Susceptibility" in "Physical Properties"). Opaque grains, mostly sulfides in irregular and framboidal forms, are common in lithostratigraphic Subunit IIIA. They compose between 5% and 10% of the sediment, locally reaching from 25% to 40% (opaque-rich sandy silt) in Cores 204-1250C-12H and 204-1250D-11H through 12H (92.1-97.2 mbsf). Bioturbation, often highlighted by iron sulfide precipitates, is moderate to abundant. Biogenic components (calcareous and siliceous) are common in Subunit IIA, ranging between 5% and 10% (rarely, 20%-30%) (Fig. F8). The lower boundary of lithostratigraphic Unit IIIA is defined by an increase in the abundance of calcareous nannofossils below Horizon A in Section 204-1250F-11X (~153.5 mbsf) (Fig. F3).
Smear slide analyses indicate that the major lithology of lithostratigraphic Subunit IIIA is primarily composed of sediment, with 80% clay, 20% silt, and <1% sand. The texture of the minor lithology is dominated by sand and silt, which, on average, composes 34% and 32% of the sediments, respectively (Fig. F2). The major nonbiogenic components of this lithostratigraphic subunit are clay minerals, feldspar, quartz, and opaque minerals.
In both Holes 1250C and 1250D, we recovered a sedimentary sequence characterized by soft-sediment deformation features, convoluted bedding, and mud clasts that we interpret to be a debris flow deposit (see Sections 204-1250C-11H-2 to 12H-4 [86.5-100 mbsf] and 204-1250D-10H-2 to 11H-6 [75-96 mbsf]) (Figs. F10, F5). Mud clasts, ranging from 1 to 5 cm in diameter, are embedded in a clay-rich convoluted matrix (Fig. F10). Clasts are rich in glauconite (up to 40% in Sample 204-1250C-10H-1, 25 cm [73.25 mbsf]), sulfides, and volcanic glass (20% glass), which is present as a lens or thin clast (Sample 204-1250D-12H-1, 53 cm [94.5 mbsf]). Biogenic components (both calcareous and siliceous) are abundant throughout the debris flow interval (Fig. F8). In particular, nannofossils compose up to 30% of the sedimentary components observed in Sample 204-1250C-11H-4, 55-56 cm (87 mbsf).
Several light-colored volcanic glass-rich layers were observed between 146.5 and 149.5 mbsf (intervals 204-1250C-19X-7, 7-28 cm, and 64-68 cm, and Sections 204-1250D-10H-3 to 10H-5) defining the base of lithostratigraphic Subunit IIIA (Fig. F11) and seismic Horizon A. The glass-rich sequences are mostly composed of thick layers (up to 10 cm-thick) of volcanic glass-rich clayey to sandy silt (10%-30% glass) that grade to volcanic ash deposits a few centimeters thick containing ~60% glass (Figs. F11, F12). A high-resolution color reflectance (b*, from blue to yellow) and MS (spacing = 1 cm, count time = 10 s) study was conducted over the glass-rich and ash intervals. This study showed a negative correlation between the high b* values and the low MS values. Three thick layers of ash, located at 147.5, 148.15, and 149.5 mbsf, respectively, in Hole 1250F, are clearly identified in both the MS and reflectance data (Fig. F13). The uppermost interval (located between 146.6 and 147.2 mbsf in Holes 1250C and 1250F) is characterized by thinly interbedded glass-rich silty sand and ash layers (0.5 cm thick) (Fig. F11A). This is expressed as small positive amplitude anomalies in the reflectance data and negative excursions in the MS data (Figs. F11A, F13). The glass-rich layers also correlate with a low density anomaly in the LWD data and with the bright Horizon A seismic reflector on the 3-D seismic data (Fig. F4) (see Fig. F6 in the "Leg 204 Summary" chapter).
Lithostratigraphic Subunit IIIB is only present in Hole 1250F, from 149.5 to 180 mbsf, and consists of dark greenish gray to greenish gray nannofossil-rich clay and silty clay. (Fig. F3). Smear slide analyses indicate that sediment is primarily composed of 80% clay, 20% silt, and <1% sand. The major lithology is interbedded with thin (2-5 mm thick) sandy silt layers. Dark gray (N3) glauconite-rich sandy silt is present in Sections 204-1250F-13X-2 to 13X-6. Sulfide precipitates are rare. Calcareous biogenic components are abundant throughout this lithostratigraphic subunit. Macroscopic foraminifers compose up to 15% of the sedimentary components and nannofossils up to 30% in Sample 204-1250F-11X-1, 31 cm (Fig. F3).
Seventeen gas hydrate samples (7 samples from Hole 1250C and 10 samples from Hole 1250D) were taken at Site 1250 between 0 and 95 mbsf. Soupy and mousselike textures related to the presence of gas hydrate (Fig. F14) were observed in the sediments of Site 1250, although these textures are present less frequently than at the neighboring Site 1249.
Eleven gas hydrate samples were recovered from 0 to 5 mbsf in lithostratigraphic Unit I in Sections 204-1250C-1H-1 through 2H-CC, 204-1250D-1H-1 to 1H-CC, and 204-1250E-2H-1, and evidence of gas hydrate at the top of the stratigraphic sequence was indicated prior to coring by a large resistivity peak in the LWD data (Fig. F4). Few soupy and mousselike textures were observed, however, and these were mainly limited to interval 204-1250C-1H-1, 49-115 cm. The lack of soupy and mousselike textures in the sediment may be explained by the sampling procedures for gas hydrate on the catwalk, which removes 5-10 cm of sediment on either side of each gas hydrate sample taken. The sediments immediately bordering and hosting the gas hydrate are the sediments in which soupy and mousselike textures are expected to form upon dissociation of the gas hydrate. Gas hydrate sampling as whole rounds prior to core description also limits the identification of all soupy and mousselike textures.
Only one gas hydrate sample was taken from lithostratigraphic Unit II, at a depth of 27.5 mbsf (Core 204-1250C-3H). Mousselike textures were observed in Core 204-1250D-5H at a depth of 35.5 mbsf, although no gas hydrate was recovered.
Four gas hydrate samples were recovered from the top of lithostratigraphic Subunit IIIA between 42 and 50 mbsf (Cores 204-1250C-6H and 204-1250D-6H and 7H). Mousselike and soupy textures were observed in intervals 204-1250C-13H-2, 56-66 cm, and 13H-4, 87-120 cm (106-108 mbsf), just above the depth of the BSR (Fig. F14). No gas hydrates were sampled from this interval, although sediment observations together with negative chloride anomalies (see "Hydrocarbon Gases" in "Inorganic Geochemistry") indicate that gas hydrate was likely present in situ and dissociated upon core retrieval.
The sedimentary succession recovered at Site 1250 is primarily composed of diatom-bearing clay and silty clay to nannofossil-rich clays near the base of the section. These fine-grained hemipelagic sediments are frequently interbedded with abundant silt and sand turbidites. The boundaries between the lithostratigraphic units at Site 1250 correspond to changes in the environment and style of deposition.
Lithostratigraphic Subunit IIIB is characterized by a high proportion of biogenic components (especially nannofossils) relative to terrigenous material in both the coarse and fine fractions. This suggests either a change in the biogenic content of the source area or a change in overall biogenic productivity prior to the deposition of Horizon A. Horizon A corresponds to the boundary between lithostratigraphic Subunits IIIB and IIIA and is a bright seismic reflector composed of several volcanic glass-rich layers interbedded with sand intervals. It was encountered at Sites 1245, 1247, 1248, and 1250, although the lithologic manifestation of this horizon varies between sites. Horizon A at Site 1250 typically contains interlayered volcanic glass-rich silts that grade upward to volcanic ash. At Sites 1250, 1245, and 1248, we interpret the ash in the sequence to be detrital, emplaced by turbidity currents, rather than a repeated air fall deposit.
A large matrix-supported clast deposit (up to 20 m thick), which we interpret as a debris flow, is found between 75 and 100 mbsf at Site 1250. This event correlates with an increase in the sedimentation rate in lithostratigraphic Unit III below Horizon Y (see "Summary" in "Biostratigraphy"). A thinner debris flow was also observed at Site 1249 between 87 and 89 mbsf. The boundary between lithostratigraphic Units III and II corresponds to a regional unconformity showing lateral variability in the 3-D seismic reflection data. The interpretation of Horizon Y as a regional unconformity is supported by subtle changes in composition of sediments across the boundary and, at this site, by the abrupt change in sedimentation rate from 15 to 2.4 cm/k.y. at ~45 mbsf (see "Summary" in "Biostratigraphy").
Though it is unresolved in the biostratigraphy, an additional decrease in sedimentation rate likely occurred between lithostratigraphic Units II and I. The presence of high-frequency turbidites in lithostratigraphic Unit II and lack of turbidites in lithostratigraphic Unit I suggests sedimentation rates were higher during the deposition of Unit II. The lack of turbidites and the relatively thin (~9.5 m) unit thickness also may indicate that lithostratigraphic Unit I is simply hemipelagic drape that has been slowly accumulating on the crest of Hydrate Ridge.