Three holes were drilled at Site 1248, located to the west of the southern summit of Hydrate Ridge (see Figs. F1 and F7 both in the "Leg 204 Summary" chapter). Holes 1248B and 1248C were both cored, although Hole 1248B was abandoned at 17 mbsf as a result of poor recovery (~43%). Similarly, Hole 1248C had poor recovery from 0 to 40 mbsf (~26%), but by combining the data from both holes, it was possible to identify the complete sedimentary sequence at Site 1248.
On the basis of visual core descriptions (VCDs), smear slide and thin section analyses, and other parameters such as calcium carbonate content (expressed as weight percent CaCO3) and mineralogy from X-ray diffraction (XRD), two main lithostratigraphic units (Units I-II and III) were distinguished (Fig. F2). We also compare and correlate our results with the 3-D multichannel seismic reflection data, downhole LWD data (resistivity at the bit [RAB] and density-porosity), and physical property measurements (magnetic susceptibility [MS] and gamma ray attenuation [GRA] density) to better constrain the lithostratigraphic units (Fig. F3).
The sedimentary section recovered at Site 1248 is primarily composed of diatom-bearing to diatom-rich clay and silty clay above the Horizon Y unconformity identified on the 3-D seismic images at 39 mbsf and diatom- and nannofossil-bearing silty and sandy turbidites below it (Figs. F2, F3). We were able to define two lithologic units above Horizon Y at Sites 1245, 1247, and 1250. However, poor recovery (~34%) at this site made identification of two distinct units impossible. For the sake of consistency between the sites and based on both the 3-D seismic data and the proximity of the sites to each other, we decided to call this uppermost unit lithostratigraphic Unit I-II. This same upper unit definition above Horizon Y was applied at Site 1249. 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-II is a sequence of dark greenish gray (5GY 4/1), homogeneous fine-grained sediments that are mainly hemipelagic diatom-bearing to diatom-rich clay and silty clay. Most of the recovered sediments show mousselike and soupy textures (see "Lithostratigraphy" in the "Explanatory Notes" chapter) that obscure original sedimentary structure and are typically found in gas hydrate-bearing sediments. Authigenic carbonate precipitates related to the presence of gas hydrate are common in the upper part of this unit. Mottles and patches of dark gray (N3) iron sulfides and bioturbation are abundant. The lower boundary of lithostratigraphic Unit I-II is defined by the first occurrence (FO) of coarse grain-size sediments at ~39 mbsf in Core 204-1248C-5X (Fig. F2).
Smear slide descriptions indicate that lithostratigraphic Unit I-II is composed of sediments that are typically ~80% clay, ~19% silt, and ~1% sand. The major nonbiogenic components of the unit are quartz, feldspar, and clay and opaque minerals. Opaque grains, mostly sulfides in irregular and framboidal forms, are common in lithostratigraphic Unit I-II, especially between 20 and 30 mbsf, where they sometimes compose >10% of the total sediment (e.g., Samples 204-1248C-3X-1, 60 cm, and 4X-1, 20 cm). Sulfides appear as dark gray (N3) mottles or stained surfaces. Bioturbation, often evidenced by iron sulfide precipitates, is moderate to abundant throughout the lithostratigraphic unit.
The biogenic components (mainly siliceous microfossils) compose 2% to 13% of the total sedimentary components. Diatoms are abundant (
12%) in the top 20 mbsf of lithostratigraphic Unit I-II (e.g., Samples 204-1248B-2H-1, 20 cm; 2H-2, 70 cm; and 204-1248C-3X-1, 60 cm) and are typically part of the silt fraction. Calcareous microfossils are rare, with foraminifers and nannofossils present at the base of lithostratigraphic Unit I-II in Sections 204-1248C-4X-1 and 5X-1 (Fig. F2).
The sediments of lithostratigraphic Unit I-II are generally disturbed and contain both soupy and mousselike textures that preserve no original sedimentary structures (Fig. F4). Gas hydrate was detected and sampled on the catwalk in several core sections between 0 and 10 mbsf. The observed sediment textures in this interval are spatially correlated with negative temperature anomalies in the infrared (IR) images and are, therefore, presumed to result from the dissociation of gas hydrate. The distribution of soupy and mousselike intervals and their relationship to the presence of gas hydrate is discussed further in "Sedimentary Evidence of Gas Hydrate".
Authigenic carbonates found at the top of lithostratigraphic Unit I-II are associated with mousselike intervals in the cores (Figs. F4B). Several large carbonate nodules (~2-5 cm in diameter) were found in every section of Core 204-1248B-1H, from 0 to 2.3 mbsf (Table T2; Fig. F5). Thin section analysis of carbonate nodules indicates that the concretions consist of micritic carbonate containing biogenic and nonbiogenic components and clasts (Fig. F6). Carbonate clasts are predominantly rich in quartz, feldspar, and glauconite grains. Bioclasts are mainly foraminifers (Fig. F6). Smaller carbonate precipitates (~0.2 cm in diameter) are scattered within the sediment in Sections 204-1248B-1H-1 and 1H-2 (Fig. F4), and carbonate-rich patches are present in interval 204-1248C-1X-1, 116-118 cm. Two phases of calcite of different Mg content were recognized in the upper 20 mbsf, based on the XRD analyses; their abundance seems to decrease downhole from Section 204-1248B-1X-1 to 204-1248C-4X-1 (Fig. F7).
Lithostratigraphic Unit III is mainly composed of diatom- and nannofossil-bearing silty clay interbedded with thin layers of graded silt and fine sand, which we interpret as turbidites (Fig. F2). A fine-grained, thick debris flow deposit dominated by biogenic components disrupts the cyclicity of turbidite frequency toward the middle of the lithostratigraphic unit. Seismic reflector Horizon A, a regional stratigraphic marker predominantly composed of volcanic glass-rich sands and ashes, was also identified at Site 1248 within this lithostratigraphic unit. The upper boundary of lithostratigraphic Unit III is located at ~39 mbsf and is defined by (1) the onset of silt and sand turbidites, the frequency of which varies downhole, and (2) the increase in biogenic components, especially calcareous components (Fig. F2). The boundary between lithostratigraphic Units I-II and III correlates with Horizon Y, a seismic reflector that corresponds to the unconformity identified on the 3-D seismic data and drilled at Sites 1245, 1247, and 1250 (Fig. F3) (see Figs. F5, F6, and F7 all in the "Leg 204 Summary" chapter). Based on changes in the abundance of biogenic calcareous components, lithostratigraphic Unit III was subdivided into two subunits (Subunits IIIA and IIIB) (Fig. F2).
Lithostratigraphic Subunit IIIA (Hole 1248C; 39-134 mbsf) consists of dark greenish gray (5GY 4/1) diatom- and nannofossil-bearing silty clay. Based on smear slide analyses, this subunit is primarily composed of sediment with ~75% clay, ~25% silt, and <1% sand (Fig. F2). This major lithology is interbedded with thin (2 mm to 1 cm), closely spaced sandy silt and silt turbidites (Fig. F2). These turbidites dominate the intervals from 39 to 75 mbsf and from 110 to 133.5 mbsf, showing a maximum-event frequency of 50 cm in Cores 204-1248C-7H and 8H (see "Site 1248 Visual Core Descriptions"). At the bases of the turbidites, intense bioturbation of the hemipelagic clay is commonly observed (e.g., Core 204-1248C-16H). Iron sulfides are common throughout lithostratigraphic Subunit IIIA, especially in Core 204-1248C-8H, from 67 to 75 mbsf, showing abundant mottles, patches, and iron sulfide nodules of dark gray (N3) color. Mousselike textures were observed near intervals where gas hydrate was sampled at 87 and 105 mbsf (see "Sedimentary Evidence of Gas Hydrate").
Toward the middle of the subunit, from 82 to 112 mbsf, a 25-m-thick sedimentary sequence characterized by soft-sediment deformation features, convoluted bedding, and clay clasts was identified and interpreted to represent multiple debris flow deposits (Sections 204-1248C-9H-4 through 9H-5; 10H-2 to 10H-5; 11H-2 to 11H-5; and 12H-4 to 12H-5). Mud clasts, ranging from 1 to 5 cm in diameter on average, are embedded in a clay-rich convoluted matrix (Fig. F8). Biogenic components are common throughout the debris flow interval, composing on average 13% of the sedimentary components. Locally, they compose up to 15% (nannofossils) and 20% (diatoms) of both the major and minor lithologies (Fig. F2).
High concentrations of beige- to white-colored volcanic glass-bearing sands and volcanic ash (20%-60% glass content) were observed between 126 and 132 mbsf and correspond to the approximate depths of Horizon A in the 3-D seismic and LWD density data (Sections 204-1248C-14H-3 to 14H-CC) (Figs. F3, F9). The volcanic glass-rich sediment and ash sequences, typically ~2-3 cm thick, are composed of dark-greenish gray volcanic glass-rich sands that grade into sandy to silty ash containing up to 60%-80% glass. The glass is generally concentrated near the top of a fining-upward sequence because its low density, caused by the presence of vesicles, the amorphous mineral composition, and the platy shape cause it to settle more slowly through the water column than other mineral grains (Fig. F9).
Prior to splitting Core 204-1248C-14H, a 5-cm-thick layer of light-colored "sand" was observed through the liner of the core on the catwalk. Subsequent drilling into the core liner to release excess gas pressure destroyed this light-colored layer, spreading a thin (<1 mm thick) coating of volcanic glass around the surface of the core. Examination of the core confirmed the existence of the glass; although physical evidence for the original mode of emplacement of the 5-cm-thick layer was not preserved.
Lithostratigraphic Subunit IIIB (Hole 1248C; 134-149 mbsf) consists of nannofossil-rich and diatom-bearing clay to silty clay. Smear slide analyses indicate that the presence of fine-grained sediment increases in lithostratigraphic Subunit IIIB (76% clays, 24% silts, and no sands in major lithology). Iron sulfide precipitates and bioturbation are moderate to rare along this subunit. The calcareous biogenic components, primarily nannofossils, increase considerably with respect to lithostratigraphic Subunit IIIA. They locally compose up to 30% of the sediment. Biogenic opal, predominantly in the form of diatoms, composes 5%-10% of the sediment (Figs. F2). Near the bottom of lithostratigraphic Subunit IIIB, glauconite-rich sandy silts (up to 11% of glauconite) are present as minor lithologies (e.g., Sections 204-1248C-17X-3 and 17X-5).
Gas hydrate was sampled from one core in Hole 1248B and from seven cores in Hole 1248C. Although gas hydrate samples were removed from cores on the catwalk prior to sedimentological description, a disruption of the remaining sediment fabric, interpreted to be induced by hydrate dissociation, was described for all cores above the BSR in Holes 1248B and 1248C.
Soupy sediment textures were described in Cores 204-1248B-1H and 2H and 204-1248C-2X (Fig. F4B). These soupy layers range in thickness from 5 to 25 cm. In all cases, the soupy layer was bordered on both the top and bottom by sediment exhibiting mousselike textures. Carbonate nodules ranging in size from <0.1 to 0.5 cm in diameter were also found in the mousselike layer bordering the soupy sediment in Core 204-1248B-1H. Core 204-1248B-1H was the only core in Hole 1248B in which carbonate nodules were found and the only one, which these nodules were associated with sediment disruption related to gas hydrate dissociation (Fig. F5A).
Side-scan sonar imagery collected prior to this leg (Johnson et al., in press) show an apron of high backscatter surrounding Site 1248, which is interpreted as carbonate near or just beneath the seafloor. Therefore, the presence of carbonate nodules at this site was not unexpected. In addition, from 0 to 50 mbsf, the alkalinity of interstitial water (IW) samples is generally high, suggesting chemical conditions are favorable for authigenic carbonate precipitation at Site 1248 (see "Carbon Cycling" in "Interstitial Water Geochemistry").
Mousselike textures associated with sediment disruption and the dissociation of gas hydrate are present in all of the cores recovered above 125 mbsf from Hole 1248C and in all cores recovered from Hole 1248B (Fig. F4A); this was noted in the VCDs. IR thermal images helped distinguish ambiguous zones of stiffer mousselike textures at intervals along the edges of the core or next to a void from mousselike textures within the center of the core (see "Physical Properties"). The stiffer disrupted textures, however, are still thought to be the result of hydrate dissociation rather than disturbance related to coring (also see "Lithostratigraphy" in the "Explanatory Notes" chapter).
Smear slide analyses of the sediments within and surrounding these disrupted zones do not reveal any characteristics that distinguish these zones from the dominant lithologies of lithostratigraphic Unit I-II. The soupy and mousselike layers are typically present in the predominant silty clay and diatom-bearing silty clay. At this site, silty and sandy interlayers that tend to preserve original bedding contacts are present within 0.1 to 1 m of the disrupted layers but do not show evidence of the dissociation of gas hydrate.
Our observations of the relationship between core disruption and the dissociation of gas hydrate are supported by both the IW geochemistry and LWD data from Site 1248. The chemical composition of IWs (see "Interstitial Water Geochemistry") indicates freshening relative to seawater values from 0 to 20 mbsf and also at discrete intervals to the estimated depth of the BSR at 115 mbsf. These low chlorinity anomalies are likely related to the addition of freshwater to the system from the dissociation of hydrate. In addition, from 0 to 20 mbsf, high LWD resistivity values indicate the presence of hydrate in the pore space of the sediment (see "Downhole Logging").
The hemipelagic intervals between turbidites in lithostratigraphic Unit III typically vary in thickness from 5 to 20 cm, which is comparable in thickness to the adjacent turbidites. The largest clay-dominated zone, from 80-112 mbsf (i.e., Section 204-1248C-9H-3 through Core 12H), is actually composed of unsorted clay clasts within a clay to silty clay matrix that is high in biogenic opal and calcareous components. Soft-sediment deformation and load structures delineated by thin sulfide-rich laminae are observed within this interval as well. We interpret this interval as a series of debris flow deposits. Similar deposits at Sites 1249 and 1250 were also identified near the middle of lithostratigraphic Unit III and may correlate with the deposits seen here. The higher abundance of biogenic components in the debris flows at 80-112 mbsf, their unsorted nature, and their fine grain size suggest that they were derived from a proximal source area rich in hemipelagic clays.
Seismic Horizon A was recovered at Site 1248, thus providing a deep tie point for intersite correlation. Similar to Sites 1245 and 1250, Horizon A at Site 1248 consists of volcanic glass-rich turbidite sequences. The highest glass concentrations are present at the top of the sequences, where the less dense grains (like volcanic glass shards) settle out from suspension. The inverse grading of ash concentration within the normal graded sands and silts and the multiple presence of such events over a short period of time (i.e., they are separated by a few centimeters) suggest the volcanic ash was deposited by turbidity currents rather than repeated air fall ash deposition. However, future detailed sedimentologic work will better define the mode of emplacement.
The lack of coarse-grained material in the upper portion of lithostratigraphic Unit I-II, in addition to the high biogenic opal content, is indicative of sediment deposition dominated by hemipelagic clay accumulation. In contrast, coarser-grained sediments and graded beds indicate deposition dominated by periodic turbidity currents, which at Sites 1250, 1247, and 1245 are observed in lithostratigraphic Units II and III and at Site 1248 in the lower part of Units I-II and III. High-frequency turbidites usually also indicate a higher sedimentation rate than deposition dominated by hemipelagic clay accumulation. Based on the observed stratigraphy here, this would imply a higher sedimentation rate for lithostratigraphic Unit III vs. Unit I-II.
