Site 1247 is located between Sites 1245 and 1248 just east of the crest of southern Hydrate Ridge (see Figs. F1 and F7 both in the "Leg 204 Summary" chapter). Two holes were drilled at Site 1247. One hole was logged by LWD (Hole 1247A), and one was cored (Hole 1247B) to a depth of 220.0 mbsf. Seismic reflection profiles show that Horizon A, cored at Sites 1245, 1248, and 1250, is present at Site 1247 as well (see Fig. F6 in the "Leg 204 Summary" chapter). Although the seismic amplitude and LWD response of Horizon A in Hole 1247A are similar to those exhibited at Sites 1245, 1248, and 1250, the lithologic manifestation of the horizon differs significantly in Hole 1247B from that observed at the other sites. Note that the seismic character of this horizon also differs significantly between the two holes drilled at this site (Fig. F2).
Three lithostratigraphic units were defined at Site 1247 (Figs. F3, F4) based on variations in sedimentologic criteria such as grain size and biogenic components, as well as on geochemical parameters, such as calcium carbonate content (expressed as CaCO3 weight percent), total organic carbon (TOC), and mineralogy from X-ray diffraction (XRD). Lithostratigraphic Unit I is composed of a section of uninterrupted dark greenish gray diatom-bearing clay. Lithostratigraphic Units II and III are composed of dark greenish gray diatom-bearing clay with abundant silt and sand turbidites. Lithostratigraphic Unit III was further subdivided into two subunits (Subunits IIIA and IIIB) on the basis of biogenic content. We also compare and correlate our results with the 3-D seismic reflection data, downhole LWD data (density and resistivity), and physical property measurement (magnetic susceptibility [MS]) to better define the entire stratigraphic sequence (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.
Good core recovery suggests that a complete record of lithostratigraphic Unit I was recovered at Site 1247 and can be correlated to the uppermost lithostratigraphic units recovered at Sites 1245 and 1250 (see Fig. F10 in the "Leg 204 Summary" chapter). Lithostratigraphic Unit I consists of dark greenish gray (5GY 4/1) hemipelagic clay and silty clay with a low total biogenic content (<10%) as determined by smear slide analysis, although diatoms are present throughout. Authigenic carbonate precipitates are common, primarily as light-colored cements in Sections 204-1247B-1H-1 and 2H-4 (Fig. F5); sulfide mineralization and bioturbation increase toward the base of lithostratigraphic Unit I. The lower boundary of lithostratigraphic Unit I is defined by the first occurrence of coarse grain-size sediments at ~27 mbsf in Core 204-1247B-5H (Fig. F3).
The clay content of lithostratigraphic Unit I varies between 70% and 80% in the major lithology. Silt-size components compose the remaining 20%-30% of the lithology, with sand-size particles present only in trace amounts (0%-3%) (Fig. F3). The predominant minerals identified from smear slides are quartz, feldspar, and opaque and clay minerals. Opaque grains, associated with zones of sulfide precipitates, are commonly irregular and framboidal forms. The onset of the first zone of major sulfide precipitates and bioturbation in lithostratigraphic Unit I occurs in Core 204-1247B-3H and corresponds to a peak in the MS data at ~21 mbsf (see "Magnetic Susceptibility" in "Physical Properties").
The calcium carbonate content (see "Carbon Analyses, Elemental Analyses, and Rock-Eval Characterization") of lithostratigraphic Unit I remains relatively constant despite the presence of authigenic carbonate found in both Cores 204-1247B-1H and 2H (Fig. F6); however, this may be an effect of sparse sampling. The carbonate-rich zones were initially identified by their light color in the cores (Fig. F5). Smear slides analyzed from these patches indicate these zones contain 70%-90% authigenic carbonate in needlelike grains that are typically 1-5 µm long, and XRD analysis indicates that the carbonates are primarily calcitic, though both Samples 204-1247B-2H-1, 43-44 cm, and 2H-4, 55-56 cm, have two phases (Fig. F7). Of the XRD samples analyzed shipboard from Site 1247, only Sample 204-1247B-2H-4, 124-125 cm, contains carbonate of a dolomitic composition (Fig. F7).
The total biogenic component of lithostratigraphic Unit I, consisting of diatoms, siliceous microfossils, foraminifers, and calcareous nannofossils, does not exceed 10% of the total sediment (Fig. F3). Cores 204-1247B-1H, 3H, and 5H contain the highest percentage of biogenic components, with diatoms composing 5%-8% of the major lithology in Cores 204-1247B-1H and 5H. Carbonaceous nannofossils and foraminifers compose 10% of the major lithology in Core 204-1247B-3H.
Lithostratigraphic Unit II is composed of dark greenish gray (5GY 4/1) diatom-bearing to diatom-rich silty clay with graded silt and sand turbidites. Authigenic carbonates in lithostratigraphic Unit II are few and are present only in Core 204-1247B-6H. Lithostratigraphic Unit II also contains more sulfide precipitates than lithostratigraphic Unit I. The top of lithostratigraphic Unit II is defined by the first occurrence of a fining-upward clayey silt layer in Section 204-1247B-5H-2 at 27 mbsf. The frequency of turbidites increases toward the base of the lithostratigraphic unit (~60 mbsf). This depth corresponds to the location of seismic Horizon Y, a regional unconformity best identified on the 3-D seismic reflection data (see Fig. F6 in the "Leg 204 Summary" chapter).
The major lithology of lithostratigraphic Unit II is diatom-bearing to diatom-rich silty clay (contains 20%-40% silt-size grains) punctuated by fining-upward sequences of clayey silt to silt to silty sand (Fig. F3). The coarse-grained minor lithologies typically have erosional basal contacts. They are often <0.5 cm thick but can grade upward over 10 cm or more and are interpreted as turbidites (Cores 204-1247B-5H and 6H). Soft-sediment deformation features and mud clasts, representative of a debris flow deposit, were observed at ~29 mbsf, just below the onset of the silt and sand interlayers.
Both the major and minor lithologies of lithostratigraphic Unit II are composed of quartz, feldspar, and opaque and clay minerals. The grain size of the minor lithologies in lithostratigraphic Unit II ranges from clayey silt (containing 40% clay-size grains) to sandy silt (containing 40% sand-size grains). Trace quantities of glauconite (<5%) were observed in the minor lithology of Cores 204-1247B-5H, 6H, and 8H. This glauconite is associated with the coarse-grained material at the bases of turbidites and is likely detrital rather than authigenic.
The biogenic content of lithostratigraphic Unit II, composed of diatoms, siliceous microfossils, calcareous nannofossils, and foraminifers, ranges from 5% to 35% of the total sedimentary components and is consistently higher than that of lithostratigraphic Unit I (Fig. F3). Diatoms were observed in every smear slide from the major lithology of lithostratigraphic Unit II, although they were typically absent from the minor lithology (Fig. F3). Calcareous nannofossils, unlike diatoms, are limited to the minor lithology of lithostratigraphic Unit II. Macroscopic foraminifers are also found predominantly in the minor lithology, although microscopic foraminifers compose 3% of the major lithology in Core 204-1247B-5H.
Sulfide precipitates are common in lithostratigraphic Unit II and correlate well with high values in MS (see "Magnetic Susceptibility" in "Physical Properties"). A 10-cm-long vein of pyrrhotite dipping at 45° on the core face was observed in Section 204-1247B-5H-6 at ~31 mbsf (Fig. F8) and was recorded in a high-resolution MS scan, performed every 1 cm (10-s count interval). The result of this study confirmed the influence of sulfide precipitates on full-core MS records. Below the pyrrhotite vein, the whole-core MS data show more frequent and higher-amplitude peaks than those above 33 mbsf. The change in magnitude and frequency of the magnetic highs probably is a result of the combined effect of frequent turbidite layers and increased sulfide precipitates, both of which characterize lithostratigraphic Unit II.
Lithostratigraphic Unit III is composed of dark greenish gray (5GY 4/1) diatom-bearing to diatom-rich clay and silty clay (Fig. F3). The core recovery of the unit was very good (96%). Although lithostratigraphic Unit III is dominantly composed of diatom-bearing to diatom-rich clay and silty clay, it also contains abundant layers of fining-upward parallel-bedded silt to sand, interpreted as turbidites, some sulfides, and rare to moderate bioturbation. The major components of the clay and silty clay, as determined by XRD analyses, are quartz, feldspar, muscovite, illite, other clay minerals, and probably biogenic calcite in varying amounts.
The upper boundary of lithostratigraphic Unit III is defined by an increase in the sand fraction of the major lithology of up to 10% (~60-80 mbsf) as well as an increase in biogenic calcareous and siliceous components of up to 20% (~60-70 mbsf) (Fig. F3). The boundary between lithostratigraphic Units II and III correlates with seismic Horizon Y. This reflector is seen in the 3-D seismic data and is interpreted to be an unconformity at this site.
Lithostratigraphic Unit III is divided into two subunits (Subunits IIIA and IIIB), based on the increase in calcareous biogenic components in Subunit IIIB and the correlation with 3-D seismic data. The boundary between the two subunits lies at 163.50 mbsf (in Section 204-1247B-22X-1) and corresponds to the first occurrence of nannofossil-rich clay.
The major lithology of lithostratigraphic Unit IIIA (60.00-163.50 mbsf) is diatom-bearing to diatom-rich clay and silty clay (Fig. F3). Cores 204-1247B-9H and 10H are also nannofossil bearing and foraminifer bearing to nannofossil rich. Smear slide analyses indicate that lithostratigraphic Subunit IIIA is composed of up to 97% clay, though more typically it contains ~20% silt and >10% sand. The major nonbiogenic components of lithostratigraphic Subunit IIIA are feldspar, quartz, and clay and opaque minerals.
The diatom-bearing to diatom-rich clay and silty clay is punctuated by 1- to 10-cm-thick turbidites composed of foraminifer-bearing to foraminifer-rich and diatom-bearing to diatom-rich silty-sandy clay and clayey-sandy silt to sand. These turbidites are present as planar-laminated fining-upward sequences, with event spacing varying from 2 cm to 2 m.
The minor lithology in Sections 204-1247B-20X-2 through 20X-4 contains minor amounts of volcanic glass (<5%) in thin layers or lenses (1 mm). Wood fragments were found in interval 204-1247B-8H-6, 140-142 cm. Two hydrate samples were recovered in lithostratigraphic Subunit IIIA in Sections 204-1247B-12H-2 and 14H-5 and correspond to an observed increase in biogenic calcareous and siliceous components at 100-120 mbsf (Fig. F3). Macroscopic foraminifers were found in the coarse fraction (minor lithology) of Cores 204-1247B-23X and 26X. The moderate to rare abundance of sulfides in Core 204-1247B-18X responsible for the dark gray (N3) color of the sediments, seems to correlate with a high in the MS data (see "Magnetic Susceptibility" in "Physical Properties"). A series of clay clast-rich deposits we interpret as a debris flow deposit is present between 156 and 161 mbsf (Sections 204-1247B-21X-2, 72 cm, through 21X-5, 140 cm) at the base of Subunit IIIA (Fig. F9). This debris flow consists of five layers ranging from 0.1 to 2 m thick and is correlated with Horizon A, a bright regional seismic reflector. The clayish matrix of the debris flow contains clasts (1-5 cm) of a slightly lighter color and different composition (foraminifer- and diatom-bearing clay) that lack volcanic glass. The upper and lower contacts of the debris flow are sandy-silty turbidites that also lack volcanic glass. At Sites 1245, 1248, and 1250, however, Horizon A is present as a series of volcanic glass-rich and ash sequences and is a bright reflection on the 3-D seismic section. The difference in the lithologic manifestation of Horizon A between Site 1247 and all other sites at which it was drilled corresponds to a change in the character of the seismic horizon in Hole 1247B where the reflector is more washed out (Fig. F2).
Lithostratigraphic Subunit IIIB is distinguished from Subunit IIIA by a distinct increase in biogenic calcareous components, which reach up to 50% in the minor lithology (Section 204-1247B-23X-4 [177.6 mbsf]) (Fig. F3). However, the major lithology of lithostratigraphic Subunit IIIB (163.50-220.62 mbsf) is nannofossil-bearing to nannofossil-rich clay and silty clay (Fig. F3). Cores 204-1247B-22X and 23X are foraminifer bearing to foraminifer rich, and Cores 24X through 26X are diatom bearing. Smear slide analyses indicate that lithostratigraphic Subunit IIIB is composed of up to 90% clay, though more typically it contains 20%-30% silt components and up to 10% sand. The major nonbiogenic components of lithostratigraphic Subunit IIIA are feldspar, quartz, and clay and opaque minerals.
The nannofossil-bearing to nannofossil-rich clay and silty clay is interbedded with 1- to 12-cm-thick layers of foraminifer-bearing to foraminifer-rich clay, nannofossil-bearing and diatom-bearing to diatom-rich silty clay, and clayey-sandy silt to silty sand. These coarser-grained layers are present as planar-laminated fining-upward sequences (turbidites) and contain macroscopic foraminifers (Fig. F10) in Sections 204-1247B-23X-1, 23X-4, 26X-2, 26X-4, and 26X-5. The erosional bases of turbidites are visible in Core 204-1247B-23X. The turbidites of lithostratigraphic Subunit IIIB are present at a lower frequency than those in lithostratigraphic Subunit IIIA. Single turbidite interlayers have event spacing between 0.20 and 6 m. Cores 204-1247B-23X and 24X (180-186 mbsf) show abundant bioturbation and sulfides. The high sulfide content generally correlates with highs in the MS (see "Magnetic Susceptibility" in "Physical Properties").
There was little evidence for the presence of gas hydrate at this site. No mousselike or soupy textures were observed, although hydrate samples suspected to contain hydrate were taken on the catwalk from Cores 204-1247B-12H and 14H in lithostratigraphic Subunit IIIA, just above depth of the BSR.
Consistent with the lithostratigraphic unit boundaries at Sites 1245, 1248, and 1250, Unit III at Site 1247 is placed below the unconformity and Units II and I above the unconformity at Horizon Y (Figs. F3, F4). Lithostratigraphic Unit III was subdivided at Horizon A into Subunits IIIA and IIIB (Figs. F3, F4). The stratigraphy present at this site is similar to that at the other sites cored near the southern summit of Hydrate Ridge, with variations in grain size and biogenic content suggestive of an active slope basin or abyssal plain environment of deposition.
Although Horizon A has a strong seismic amplitude in Hole 1247A and was also detected as a density low on the LWD density log (see "Downhole Logging"), the location of Hole 1247B was west of Hole 1247A, in a region where the amplitude of Horizon A decreases abruptly (Fig. F2). The depth of Horizon A in Hole 1247B is ~158 mbsf. Interestingly, the volcanic glass-rich horizons, which are present at Horizon A at Sites 1245, 1248, and 1250, were not observed here. Instead, a debris flow, bound by turbidites free of volcanic glass, was identified at 156-161 mbsf; a debris flow lacking volcanic glass is most likely the cause of the apparent decrease in seismic amplitude of Horizon A in Hole 1247B. A lateral variation of sedimentary facies resulting from local factors (e.g., original topography and/or turbidite-channel sedimentary pathways) may explain why a debris flow was deposited at Site 1247, whereas an ash-rich sequence was deposited at Sites 1245, 1248, and 1250.
A good correlation between MS and turbidite sequences, sulfide precipitates, or both is also observed at Site 1247. Because most, if not all, of the high MS peaks at Site 1247 are coincident with either sands and silts bearing magnetic minerals transported via turbidity currents and/or clays rich in magnetic iron sulfide precipitates formed in situ (pyrrhotite); therefore, changes in their magnitude and recurrence interval must be interpreted carefully. The turbidites above and below the debris flow at Horizon A, for example, are seen as high-amplitude spikes in the MS data, whereas the clay-rich low-sulfide debris flow shows low MS (see "Magnetic Susceptibility" in "Physical Properties"). The high susceptibility from 210 mbsf to the end of the core is also caused by turbidites. In contrast, the high susceptibility seen from 180 to 190 mbsf is caused by the presence of abundant sulfide.
At Site 1245, Horizon Y represents an angular unconformity or a thrust fault (see "Environment of Deposition" in "Lithostratigraphy" in the "Site 1245" chapter); however, at Site 1247, lithostratigraphic Units II and III are disconformable at Horizon Y (60 mbsf) thereby making it more difficult to detect within the stratigraphic record. A slightly higher frequency of turbidites is observed above Horizon Y than below. This change is associated with a decrease in sulfide abundance across the boundary and the onset of foraminifers within the turbidites at 54 mbsf. These observations may indicate that there is a higher sedimentation rate above Horizon Y and that the sediments are more susceptible to sulfide mineralization. The onset of foraminifers within the turbidites at 54 mbsf at Site 1247 may indicate a change in the source area or in the productivity of this microfossil group, either of which can vary in space and time and thus are not diagnostic evidence for the presence of an unconformity. The best evidence for an unconformity or thrust fault origin for Horizon Y comes from the seismic profiles that cross Site 1245, which depict the angular bedding relationships (see Fig. F5 in the "Leg 204 Summary" chapter).