Site 1244 is located on the eastern flank of southern Hydrate Ridge (see Figs. F1 and F5 both in the "Leg 204 Summary" chapter). Six holes were drilled at Site 1244, and five of these (Holes 1244A-1244C, 1244E, and 1244F) were cored. Hole 1244C was cored to 333.5 mbsf (142.5 m with the APC and 191 m with the XCB), and Hole 1244E was cored to 135.8 mbsf (APC). These two holes provide the primary data for this chapter. Holes 1244A, 1244B, and 1244F were cored to 9.5, 54.1, and 24.1 mbsf, respectively (primarily using the APC), and provide some additional data for the shallow part of the lithostratigraphic sequence at this site.
The sedimentary sequence at Site 1244 is dominated by clay and silty clay, with intervals of silt to very fine sand layers (Fig. F2). Recovered sediments were divided into three lithostratigraphic units based on the visual description of sedimentary structure, grain size, sediment color, smear slide analyses, and comparison with the 3-D seismic data. Other parameters, such as calcium carbonate content (expressed as CaCO3 weight percent), total organic carbon (TOC), mineral information from X-ray diffraction (XRD), and physical property measurements of magnetic susceptibility (MS) and bulk density (see "Sediment Density from Multisensor Track and Moisture and Density" in "Physical Properties") were also used to characterize lithologic changes. Correlation of the lithostratigraphic units defined here with the other Leg 204 sites is summarized in Figure F10 in the "Leg 204 Summary" chapter.
The major lithology of lithostratigraphic Unit I consists of dark greenish gray (5GY 4/1) clay, with scattered thin layers of silty clay and fine silt (minor lithology) 3-17 cm thick (Fig. F2). The biogenic content of lithostratigraphic Unit I varies from 5% to 40% in the recovered sedimentary section. The sedimentary sequence observed in the cores is extensively fractured, likely the result of gas expansion upon core recovery. Cracks and voids increase throughout the lithostratigraphic unit from top to bottom and are particularly prominent features in Cores 204-1244C-3H through 7H. This gas expansion can increase the recovered length of the cores by as much as ~50 cm. Extensive whole-round sampling of the first three cores in Hole 1244C resulted in the removal of a substantial portion of the recovered section before it reached the core description table. However, Cores 204-1244A-1H through 3H and 204-1244E-1H through 3H were used to fill in the sampled intervals so that a complete description could be made.
Smear slide analysis of the mineralogic components of lithostratigraphic Unit I indicates that the major lithology (dark greenish gray clay) typically consists of 5% quartz, 10% feldspar, and 70% clay minerals (Fig. F2). Some silty clay samples (minor lithology) contain a minor amount of sand-sized grains (2%). Glauconite (2%), pyrite (3%), and volcanic glass (3%) are present as accessory minerals in these layers. Biotite, muscovite, and micronodules are also present in trace amounts (~1%-3%) throughout lithostratigraphic Unit I. Authigenic calcite needles, ranging in length from 1 to 5 mm, were estimated to compose ~80% of Sample 204-1244E-4H-7, 45 cm, and were identified during visual observation by the presence of light-colored patches and bands (1-4 cm thick) within the sediment in the upper 20 mbsf in both Holes 1244C and 1244E. In certain, but not all, cases the light-colored authigenic carbonate-rich sediments surrounded a cemented carbonate nodule that ranged in diameter from 1 to 4 cm (Fig. F3).
Dispersed patches of slightly lighter-colored sediment, not associated with carbonate precipitates, were observed in the middle of lithostratigraphic Unit I (Sample 204-1244C-4H-3, 35-38 cm, and interval 4H-4, 17-47 cm). Based on smear slide analyses, these color changes do not reflect a major change in mineralogy. However, color changes from dark greenish gray (5GY 4/1) to dark gray (N3) do occur throughout the core because of variations in the amount of sulfides present. The presence of sulfide precipitates (<1 mm) also helps to identify bioturbated zones within the sedimentary section. Nodular sulfide precipitates from Sections 204-1244C-5H-4 and 204-1244B-3H-5 were analyzed by XRD and found to be composed of pyrrhotite.
Calcitic components were also identified in XRD analyses of Samples 204-1244C-1H-3, 28-29 cm; 1H-3, 74-75 cm; 2H-4, 34-35 cm; and, especially, 4H-3, 26-27 cm. Although XRD analyses are less accurate for quantitative comparisons, variations in the height of the d(104) calcite spacing documents the pattern of downhole changes in the abundance of biogenic and authigenic carbonate constituents. Changes in the peak position of d(104) values of 3.035 to 3.018 indicate low-Mg calcite. Calcite was identified in Samples 204-1244B-1H-3, 73-74 cm; 1H-3, 137-138 cm; and 204-1244C-6H-4, 74-75 cm, where it is abundant (23%) in the silt fraction of silty clay and silt layers.
The biogenic microfossil content of lithostratigraphic Unit I is dominated by diatoms that compose ~5% of the major lithology. Silicoflagellates, radiolarians, sponge spicules, and organic debris are typically only present in trace amounts (~1%) but can exceed 5% of the sedimentary components, locally (Fig. F4). Calcareous nannofossils and foraminifers compose, on average, 10%-20% of the minor lithology in the upper 20 mbsf of lithostratigraphic Unit I in Hole 1244E and are all but absent between 20 and 77 mbsf (Fig. F2). Shell fragments and sponge spicules were noted in the visual core descriptions throughout lithostratigraphic Unit I and are present in all holes at Site 1244.
The distinction between lithostratigraphic Units I and II is based primarily on the increased presence of very fine sand layers in Unit II (Fig. F2). Lithostratigraphic Unit II is composed of dark greenish gray (5GY 4/1) silty clay. This silty clay is interlayered with lighter-colored layers of fine sand and coarse silt, which we interpret as turbidites. The major sedimentary components of lithostratigraphic Unit II are clay minerals, quartz, feldspar, and muscovite. The biogenic component, which consists of mainly siliceous microfossils (sponge spicules, silicoflagellates, and diatoms), typically composes 3%-8% of the total sedimentary components in Holes 1244C and 1244E (Fig. F2). Extensive degassing cracks and voids, often associated with silt or sand layers, are observed throughout the upper 50 m of lithostratigraphic Unit II.
High-frequency turbidites, the bases of which contain fine to very fine sand that grades to silt and clay, characterize the upper 77 m of lithostratigraphic Unit II (69-146 mbsf) (Fig. F5). These turbidites range in total thickness from 0.2 to 2.1 m (coarse base to fine tail of turbidite), with an average thickness of 0.8 m. Between 146 and 245 mbsf, the frequency of the turbidites drops significantly. Within this interval, the turbidites range from 1 to 20 cm thick and the bases of individual events are separated by ~8 m on average. The sand and silt layers that form the sharp erosional bases of the turbidite sequences are commonly 1 to 3 cm thick, although occasionally, they reach 30 cm thick. The turbidites grade upward to become dominated by homogeneous silty clay and clay. Above many of the sand and silt bases, the silt and silty clay layers occur in thin (~1 mm) finely spaced layers over intervals that span as much as 15 cm. Sulfide mottles and halos, which may indicate a chemical-reaction front linked to bioturbation intensity, are preserved in the hemipelagic sediments between turbidites and help to distinguish these sediments from the graded turbidite tails. The turbidite tail in Section 204-1244C-13H-4 is characterized by a light gray homogeneous silty clay ~70 cm thick, whereas the uppermost hemipelagic layer, which consists of highly bioturbated clay with dark gray sulfide mottles, is ~30 cm thick (Fig. F5). MS data also show these graded turbidites throughout this interval in lithostratigraphic Unit II (see "Magnetic Susceptibility" in "Physical Properties"). Of particular interest are the large MS spikes near ~170 mbsf, which correspond to seismic Horizon B and turbidites in the core.
Smear slide analyses indicate that the coarse fraction (major and minor lithologies) of the sampled sediments is dominated by silt-sized grains and typically composes 10%-35% (rarely from 50% to 70%) of the total sediment. Sands, when present, compose only a few percent (1%-2%) of the major lithology, although locally (e.g., in the minor lithology of Hole 1244E) they were observed to compose as much as 30%-50% (e.g., Samples 204-1244C-9H-6, 143 cm, and 204-1244E-13H-1, 51 cm, respectively). The mineralogy of the silt- and sand-sized fraction in both the major and minor lithologies is mainly dominated by quartz, feldspar, muscovite, and biotite. Glauconite (up to 8%) is observed in the silt layers of smear slide Samples 204-1224C-8H-6, 147 cm; 9H-6, 143 cm; 9H-7, 81 cm; and 13H-4, 115 cm. Pyrrhotite-rich layers are often present from 75 to 110 mbsf (e.g., Cores 204-1224C-9H, 10H, and 12H), occasionally representing up to 3% of the coarse fraction. Dark gray (N3) sulfide-rich bands (10-40 cm thick) with pyrrhotite nodules (up to 0.4 cm in diameter) are often observed in Cores 204-1244C-23X through 29X.
Core 204-1244C-27X contains a 60-cm-thick volcanic ash horizon at 216 mbsf (Fig. F6), with 50% glass content identified in the smear slide (Sample 204-1224C-27X-1, 8 cm). Most glass shards are very fine sand size and are present as both pristine, unaltered shards and altered shards (now clay minerals) (Fig. F7). XRD analyses show that glass, quartz, feldspar, muscovite, and clay minerals are present throughout this layer (Fig. F8). The ash layer displays a sharp subhorizontal base and is clearly correlated with a prominent peak on the MS and density data (see "Magnetic Susceptibility" and "Sediment Density from Multisensor Track and Moisture and Density" both in "Physical Properties"). This layer corresponds to the B´ reflector identified in the 3-D seismic reflection data and is also found at Site 1246 west of Site 1244.
Lithostratigraphic Unit III is composed of hard, indurated silty clay and clayey silt with scattered glauconite sand layers. The silty clay is predominantly dark greenish gray (5GY 4/1); however, lighter variations in color are present in Cores 204-1244C-32X through 35X and typically indicate the presence of calcareous sediments (positive reaction to HCl) (Fig. F9). Core 204-1244C-36X exhibits a range of colors, from very dark gray (N3) to dark olive gray (5Y 3/2) to light bluish gray (5B 5/1). Lithostratigraphic Unit III is distinguished from lithostratigraphic Unit II by its general lack of sulfide precipitates, bioturbation, and silt layers as well as its higher state of lithification. The Unit II/III boundary lies directly below a silt layer at 244.5 mbsf in Hole 1244C, which represents the lowermost major turbidite in lithostratigraphic Unit II. Additional evidence for this unit boundary is seen in the bulk density data, which indicate a significant shift in physical properties below 245 mbsf (see "Sediment Density from Multisensor Track and Index Properties" in "Physical Properties"). There is also a change to lower total calcium carbonate content at the same depth and a correlation with the onset of borehole breakouts imaged with the LWD-RAB tool (see "Downhole Logging").
Core recovery in lithostratigraphic Unit III was high (90%), although the cores were generally highly disturbed, with drilling biscuits heavily fractured, making primary sedimentary structures difficult to identify. Bedding laminations, delineated by subtle color and grain size changes, were observed within intact drilling biscuits in Cores 204-1244C-32X, 35X, and 36X, and zones of bioturbation ranging from 3 to 10 cm thick were identified in intervals 204-1244C-32X-1, 44-54 cm; 32X-6, 114-118 cm; and 33X-6, 95-98 cm. Only one interval of mottled sulfide was identified in the entire unit (interval 204-1244C-30X-4, 65-145 cm).
The major sedimentary components of lithostratigraphic Unit III (only cored in Hole 1244C) are primarily clay minerals, quartz, and feldspar. Clay-size grains typically compose 70% of the sedimentary components, although local variations occur throughout the lithostratigraphic unit (Fig. F2). Pyrrhotite, biotite, and muscovite occur as accessory minerals in Cores 204-1244C-31X, 32X, 33X, and 35X. The upper sequence (from 255 to 299 mbsf) is rich in calcite, which constitutes as much as 61% of the sediment in Section 204-1244C-35X-5. The calcite is present as uniform needles (5 to 10 mm long) in smear slides and is interpreted to be authigenic from the regularity of its shape and size (Fig. F10). The carbonate silt is present in patches rather than layers.
The lowermost 43 m of lithostratigraphic Unit III contains glauconite in two discrete silty layers and dispersed among the dominant silty clay lithology. The glauconite grains, which are present as both pristine, unaltered green and oxidized brownish forms, tend to be rounded and range in size from <0.0039 mm (clay fraction) to 0.275 mm (medium sand) (Fig. F11).
The biogenic components (diatoms, foraminifers, nannofossils, silicoflagellates, sponge spicules, and organic debris) of lithostratigraphic Unit III compose 1%-30% of the total sedimentary components (Fig. F2). Diatoms are abundant, composing as much as 25% of the total sediment, and are found in association with authigenic carbonate silt layers in Samples 204-1244C-35X-4, 19 cm, and 35X-5, 16 cm. Foraminifers are only found in Cores 204-1244C-35X and 36X.
Quartz, feldspar, and mica are all present in varying amounts throughout the lithostratigraphic unit, depending on the overall grain size. Quartz content ranges from 1% in a clay horizon at 264 mbsf (Sample 204-1244C-32X-1, 44 cm) to 30% in a silt layer directly below at 265.04 mbsf (Section 204-1244C-32X-1). Feldspar follows the same trend and ranges from 1% to 14% of the total sediment.
Carbonate and gypsum nodules are also found in lithostratigraphic Unit III. Carbonate nodules are present in Sections 204-1244C-34X-1 and in 35X-1 (283.9 and 293.2 mbsf, respectively) in clay-rich layers with higher biogenic carbonate contents. The surrounding sediment is highly fractured and disturbed from the core-splitting process, which drags the rigid carbonate nodules through the surrounding sediment. Unlike the carbonates, gypsum nodules are small (<2 mm) and are typically found within hard clay fragments (Sections 204-1244C-35X-6, 38X-1, 38X-4, and 38X-CC).
A hard silty clay breccia layer containing angular clasts up to 2 cm in size is present toward the base of lithostratigraphic Unit III at 307.65 mbsf (interval 204-1244C-36X-4, 55-100 cm). The change in lithification from clay to claystone and the shift in the bulk density data at the lithostratigraphic Unit II/III boundary suggest a transition into the deeper accretionary complex, although alternately, this layer may simply represent an unconformity.
Gas hydrate was sampled in cores from lithostratigraphic Unit I in both Holes 1244C and 1244E. The uppermost hydrate sample taken from Hole 1244C was at 63 mbsf (Core 204-1244C-8H); that from Hole 1244E was at 50 mbsf (Core 204-1244E-7H). However, Core 204-1244C-7H (at ~59 mbsf) contains a linear zone of mousselike textured sediment that crosscuts bedding at an angle of ~45° (Fig. F12). A review of the infrared (IR) thermal images from this interval shows a thermal anomaly of -5.1°C (see "Physical Properties") (Table T2), which suggests that gas hydrate may have dissociated within this interval prior to splitting the core liner. Similar indications of dissociated gas hydrate are present in Cores 204-1244C-6H and 7H. Gas hydrate was also sampled from intervals 204-1224C-8H-1, 47-52 cm; 8H-5, 50-80 cm; and 10H-2, 70-110 cm, in lithostratigraphic Unit II. Additionally, thermal anomalies were matched to disrupted sedimentary structures in lithostratigraphic Unit II, indicating that not all the gas hydrate recovered in this lithostratigraphic unit was sampled. Smear slide analyses of the mousselike textures attributed to gas hydrate-related disruption of the sediment do not indicate that the hydrate prefers to form in sediment with a different composition than that of the dominant lithology. However, it should be noted that the mousselike texture associated with hydrate dissociation may itself be biased toward clay sediments.
Lithostratigraphic Unit III encompasses the oldest sediments recovered at Site 1244 and is composed of well-lithified claystone. Examination of the 3-D seismic data indicates that this unit lies within the deeper accretionary complex beneath Hydrate Ridge (see Fig. F5 in the "Leg 204 Summary" chapter). This indurated claystone may have originated from either a slope basin or abyssal plain environment of deposition and has since been uplifted and deformed during the tectonic evolution of the ridge. The upper boundary of lithostratigraphic Unit III is well characterized on the 3-D seismic profiles as an abrupt change in reflectivity and reflects the change from the deeper accretionary complex sediments, which form the core of Hydrate Ridge, to the overlying slope basin sediments of lithostratigraphic Units I and II.
A thick ash layer (>60 cm thick), bearing >40% detrital volcanic glass shards, is present near the base of lithostratigraphic Unit II (Horizon B´) and indicates a volcanic influence near the source area for this deposit. The detrital nature of the glass in this deposit suggests that the ash, which was probably derived from the Cascade volcanic arc, was delivered to the continental shelf by rivers and, subsequently, transported to the lower slope and/or abyssal plain by turbidity currents. The mid-Holocene Mt. Mazama (Crater Lake) eruption, air fall ash distribution, and spatial distribution of volcanic glass-bearing turbidites across the Cascadia abyssal plain (Nelson et al., 1968) may serve as an analog for this type of process.
Overlying the volcanic glass in lithostratigraphic Unit II is a series of turbidites that are typically fine grained (silt to fine sand) and thin (1-3 cm) at their bases and contain a thicker (up to 80 cm) clay tail near their upper extent. These characteristics are suggestive of a distal turbidite facies, which could have originated from either (1) the continental shelf, resulting in fine-grained deposition near the lower slope and abyssal plain, or (2) the continental slope, where slumping or turbidity currents are sourced in the local slope region and fed by bathymetric highs composed of uplifted and accreted fine-grained abyssal plain deposits. Because both mechanisms result in fine-grained turbidite deposition, determining the source region for these deposits within the accretionary wedge is difficult. However, because of the lack of modern or ancient submarine canyons along this part of the margin, a local source region for these turbidites seems more likely. That is, the uplifted and folded sediments of the lower slope must have shed their sediments during slope failure and deposited these recycled sediments into an actively forming slope basin.
The boundary between lithostratigraphic Units I and II is characterized by a dramatic decrease in the presence of turbidites in the younger lithostratigraphic unit. This difference in turbidite frequency suggests that one or both of the above mechanisms may have operated during the deposition of Unit II (turbidites common), whereas only one, or potentially neither, was dominant during the deposition of Unit I, resulting in mainly hemipelagic clay accumulation. Alternatively, the higher frequency of turbidites in lithostratigraphic Unit II could represent increased slumping along the lower slope during a sea level lowstand. Lower sea level would result in more sediment delivery to the middle and lower slope, potentially causing oversteepening of slopes and sediment failure. A lower sea level might also destabilize gas hydrate on the slope, causing sediments to fail. Ultimately, better age control within this sedimentary section may help determine which mechanism is responsible for the observed change in turbidite frequency.
Three-dimensional seismic reflection data suggest that at Site 1244 both lithostratigraphic Unit I and II lie within the same deformed and uplifted slope basin sedimentary sequence that was cored at Site 1246 and potentially at Site 1245 (above Horizon Y) (see Fig. F5 in the "Leg 204 Summary" chapter). Seismic Horizons B and B´ are present within lithostratigraphic Unit II at both Sites 1244 and 1246 (see Fig. F11 in the "Leg 204 Summary" chapter) and have the same lithologic signatures; Horizon B contains a thick turbidite couplet and Horizon B´ is a volcanic glass-rich horizon. Neither Horizon B nor B´ is present at Site 1245; however, on the 3-D seismic data, Horizon Y is nearly coincident with the BSR at the bottom of Site 1246, suggesting deeper coring at Site 1246 might have recovered it and provided a marker horizon to tie Sites 1245 and 1246. The stratigraphy at Sites 1245 and 1246 in both lithostratigraphic Units I and II is also very similar, adding support for the correlation among Sites 1245, 1246, and 1244.