Site 1246 is located between Sites 1245 and 1244 just east of the crest of southern Hydrate Ridge (see Fig. F1 in the "Leg 204 Summary" chapter). Seismic reflection profiles suggest that Horizon B and B´, cored at Site 1244 below the BSR, are present at Site 1246 above the BSR (see Fig. F5 in the "Leg 204 Summary" chapter). Two holes were drilled at Site 1246. One hole was logged with LWD (Hole 1246A), and one was cored (Hole 1246B) using the APC to 136.70 mbsf with 99% core recovery. Two lithostratigraphic units were defined at Site 1246 (Figs. F2, F3) based on sedimentological criteria (variations in sedimentary structure and grain size or biogenic and lithologic components) and correlation with other parameters. Lithostratigraphic Unit I is composed of dark greenish gray diatom-bearing clay. Lithostratigraphic Unit II contains dark greenish gray diatom-bearing clay with abundant silt and sand turbidites. We compare and correlate our results with the downhole LWD data (density and resistivity) and physical property measurement (magnetic susceptibility [MS]) to better define the entire stratigraphic sequence. We also discuss the 3-D seismic reflection data, which were used to correlate the stratigraphy at this site to that at Site 1244. 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 mainly composed of dark greenish gray (5GY 4/1) clay with diatom-rich silty clay and nannofossil and diatom-bearing silty clay near the base of the unit (Fig. F2). Smear slides show >80% clay in samples above 15 mbsf in lithostratigraphic Unit I; as much as 20%-40% silt was observed in samples below 15 mbsf to the base of the unit. Biogenic components (mainly diatoms and calcareous nannofossils) in smear slides compose between 3% and 16% of the major and minor lithologies (Fig. F2). The abundance of siliceous components increases throughout lithostratigraphic Unit I, reaching 12% at 21.7 mbsf. Calcareous nannofossils are present in small amounts (2%-5%) and vary in abundance only slightly, except in a bioturbation burrow at 2.9 mbsf (Sample 204-1246B-1H-1, 140 cm), which contained up to 50% calcareous nannofossils (Fig. F2).
Bioturbation is more common in the upper 15 mbsf of lithostratigraphic Unit I, whereas sulfide patches or mottles increase in abundance toward the base of the unit; large patches (up to 3 cm2 on the split-core surface) or irregular layers (up to 2 cm thick) of iron sulfide were observed in Sections 204-1246B-3H-3 to 3H-5.
Authigenic carbonates are present in Core 204-1246B-1H (e.g., at 2.65 and 3.65 mbsf) as light-colored crumbly patches. Smear slide observation suggests that these carbonates are dominantly composed of 90% calcite needles (Fig. F4).
Lithostratigraphic Unit II is composed of a series of fining-upward sequences of silt to silty clay. The sediment is dark greenish gray (5GY 4/1) to very dark gray (N3) where sulfides are present; the color varies to dark gray (N4) in 10- to 100-cm-thick intervals between 50 and 70 mbsf (Cores 204-1246B-7H through 8H). The top of lithostratigraphic Unit II is defined by the onset of graded silt and sand layers, interpreted as turbidites (at 21.7 mbsf) (Fig. F2). Lithostratigraphic Unit I lacks these sequences.
Grain size and mineralogy of both the major and minor components of lithostratigraphic Unit II were determined from smear slide analysis (Fig. F2). The dominant lithology is silty clay with variations in the amount of biogenic opal and few biogenic calcareous components. Silt to silty sand is the typical minor lithology observed in lithostratigraphic Unit II. These coarser layers commonly grade upward into silty clay from an erosional basal contact. The interval from 90 to 120 mbsf (Cores 204-1246B-11H through 13H) is slightly more clay rich than the rest of the lithostratigraphic unit, although silt and sand interlayers are still present throughout, as seen in the minor lithologies (Fig. F2). Prominent increases in the sand fraction (where >50% of the total sedimentary components is composed of sand) are observed at 62, 71, and 136 mbsf (Fig. F2).
Thin layers with high silt content (>60%) are present throughout lithostratigraphic Unit II and, in all cases, mark the base of a fining-upward sequence. Sections 204-1246B-7H-3 and 8H-3 contain two distinct >30-cm-thick silt horizons that correlate well with peaks in the MS data (see "Magnetic Susceptibility" in "Physical Properties"). Each MS peak associated with the silt layers has a width close to 2 m and is cut off at its upper boundary by another thinner silt layer. The sequences are bound by erosional contacts at both their upper and lower contacts. The thickness of each of these MS peaks is indicative of the total thickness of the fining-upward sequence. Interestingly, smear slide analyses show that these high-susceptibility intervals are quartz rich and do not contain an unusual abundance of iron-bearing minerals.
The biogenic components in lithostratigraphic Unit II were divided according to chemical composition. Calcareous biogenic components consistently compose <10% of the total sedimentary components in this unit (Fig. F2). Macroscopic foraminifers are found in Cores 204-1246B-6H, 7H, 8H, and 12H, but foraminifers only exceed 5% of the sedimentary components in Samples 204-1246B-6H-1, 74 cm, and 14H-2, 113 cm. Calcareous nannofossils do not exceed 4% of the sedimentary components in any sample from lithostratigraphic Unit II.
Siliceous biogenic components, unlike their calcareous counterparts, commonly compose >5% of the sediment and in Cores 204-1246B-9H, 10H, and 12H exceed 10% of the total sedimentary components. The overall siliceous microfossil content is slightly lower in the uppermost 50 m (from 21.7 to 71.7 mbsf) of lithostratigraphic Unit II (Fig. F2) than in the lower section of the unit. The sediments of seismic Horizon B (Fig. F3) (see "Logging Units" in "Downhole Logging"), which extends from 55.5 to 76 mbsf, are distinguished from those of the rest of lithostratigraphic Unit II by their lack of biogenic components. Horizon B is composed of quartz-rich dark gray silt and silty clay layers.
Seismic Horizon B´, which is also sampled in lithostratigraphic Unit II (Fig. F3), is thought to be associated with a gas-charged interval of increased porosity caused by the presence of volcanic glass that is disseminated throughout Cores 204-1246B-10H and 11H. Two discrete volcanic glass-bearing layers (<1 and <0.1 cm thick, respectively) were found at 88 and 95 mbsf; the glass-bearing layer at 88 mbsf is directly above an interstitial water (IW) sample location and may be thicker than what was described in the core (Fig. F5). Samples 204-1246B-11H-2, 87 cm, and 11H-3, 43 cm, both contain >10% volcanic glass in their silt size fraction (Fig. F6).
Five gas hydrate samples were taken from lithostratigraphic Unit II at 66.5, 96.6, 104.9, 105.1, and 109.5 mbsf (Samples 204-1246B-8H, 25-30 cm; 11H-5, 93-126 cm; 12H-4, 87-102 cm; 12H-4, 102-127 cm; and 13H-1, 0-30 cm). However, mousselike textures were observed only in Sections 204-1246B-7H-1 and 14H-3, above and below the depths where hydrate was sampled. Additionally, the mousselike texture that is present in Section 204-1246B-14H-3 is below the projected depth of the BSR at Site 1246 (see "Introduction") and is likely a product of coring-related disturbance rather than gas hydrate dissociation. Thermal anomalies indicative of the presence of gas hydrate correspond well to the bases of turbidites in Cores 204-1246B-7H and 8H (see "Magnetic Susceptibility" in "Physical Properties"), although no textural evidence for the dissociation of gas hydrate was observed in these cores. If hydrate is disseminated in the pore space of the coarse-grained base of turbidites, soupy and mousselike textures typical of fine-grained sediment may not form and evidence for the presence of hydrate may be obscured.
Correlation between Sites 1246 and 1244 is possible using Horizons B and B´ (in lithostratigraphic Unit II) observed in the 3-D seismic data and the stratigraphic similarities observed in the cores from each site (e.g., Horizon B´ contains volcanic glass at both sites and Horizon B is characterized by a thick turbidite couplet). Generally, lithostratigraphic Unit II, at both Sites 1244 and 1246, (1) is dominated by turbidites and (2) intersects the Horizon B and B´ reflectors, which can be traced from site to site on the seismic data (see Fig. F5 in the "Leg 204 Summary"). Horizons B and B´ are offset by numerous normal faults, and the sediments at Site 1246 have been uplifted, and likely eroded, resulting in a much shallower position for Horizons B and B´ at Site 1246. This evidence for uplift and erosion suggests that part or all of lithostratigraphic Unit I and perhaps the uppermost portion of lithostratigraphic Unit II from Site 1244 is likely not preserved at Site 1246.
Based on the above correlation, we infer that the turbidites observed in lithostratigraphic Unit II at Sites 1246 and 1244 were deposited in the same depositional environment and that subsequent uplift and erosion has since separated these once-continuous stratigraphies. Lithostratigraphic Unit I has less coarse-fraction sediment than lithostratigraphic Unit II at Site 1246. This suggests that sedimentation rates were higher during the deposition of lithostratigraphic Unit II than they were during the deposition of lithostratigraphic Unit I. Perhaps this change in sedimentation rate occurred in response to the uplift of Hydrate Ridge beneath Site 1246, which may have isolated the summit, allowing turbidites to pond only around its base and hemipelagic sediment to accumulate on its crest (as observed in the lithology of lithostratigraphic Unit I). Alternatively, erosion near the crest of a growing Hydrate Ridge may have removed a previously deposited stratigraphic sequence and only relatively recently replaced it with a younger drape of hemipelagic sediment. In either case, the fine-grained nature of lithostratigraphic Unit I compared to Unit II suggests a change in depositional environment. Whether this change represents a response to tectonic uplift or changes in the source region for the coarse material remains to be resolved.