Site 1252 is located on the eastern flank of Hydrate Ridge between Sites 1244 and 1251 (see Figs. F1 and F8 both in the "Leg 204 Summary" chapter). One hole (Hole 1252A) was drilled at Site 1252, reaching a maximum penetration of 259.8 mbsf. Recovery was very good (97.6%), which provided a nearly complete sedimentary sequence at Site 1252. On the basis of visual core description (VCD) and smear slides (e.g., variations in sedimentary structure and grain size or biogenic and lithologic components) and in correlation with the results from neighboring sites, three main lithostratigraphic units (Units I-III) were distinguished (Fig. F2).
We also compare and correlate our results with the three-dimensional (3-D) multichannel seismic reflection data (see Fig. F8 in the "Leg 204 Summary" chapter), downhole wireline logging data (gamma ray, density, and resistivity), as well as physical property measurements (magnetic susceptibility [MS] and gamma ray attenuation [GRA] density) to better constrain the lithostratigraphic units (Fig. F3). 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 consists of dark greenish gray (5GY 4/1) diatom-rich to diatom-bearing homogeneous silty clay, locally interbedded with silt to fine sand lenses and including a thick debris flow deposit toward the base of the lithostratigraphic unit (Fig. F2). The lower boundary of lithostratigraphic Unit I coincided with by a regional unconformity (Fig. F3), defined based on in the 3-D seismic reflection data from Site 1251 (see Fig. F8 in the "Leg 204 Summary" chapter) and in the cores as an increase in grain size and calcareous biogenic components. Based on the changing abundance of biogenic components, grain size, and correlation with subunits defined at Site 1251, lithostratigraphic Unit I is divided into three subunits (Subunits IA-ID) above the aforementioned seismic unconformity. Lithostratigraphic Subunit IA correlates with lithostratigraphic Subunit IA at Site 1251, and lithostratigraphic Subunit IB-IC is coeval to Subunits IB and IC identified at Site 1251. Lithostratigraphic Subunit IB-IC cannot be distinguished as two separate units at Site 1252, possibly a result of variations in sediment distribution and accommodation to the preexisting topography at the time of deposition but is named in such a way as to be consistent with the unit names at Site 1251. Lithostratigraphic Subunit ID at Site 1252 encompasses a seismically defined debris flow deposit not drilled at Site 1251, which lies above the regional seismic unconformity (see details in "Environment of Deposition") (Fig. F3) (see Fig. F8 in the "Leg 204 Summary" chapter).
Lithostratigraphic Subunit IA (Hole 1252A; 0-7 mbsf) consists of diatom- and nannofossil-rich silty clay, locally interbedded with fine to very fine foraminifer-bearing sands. The bases of the sand layers are sharp and the tops are gradational, suggestive of turbidite sequences. These layers are thin (2-4 mm thick), frequent (every 10 to 20 cm), and mostly concentrated in Sections 204-1252A-1H-1 and 1H-2.
Smear slide analyses indicate that the major lithology of lithostratigraphic Subunit IA is composed of sediment with ~70% clay, ~25% silt, and <1% sand (Fig. F2). Sulfides were not observed. The minor lithology is dominated by coarse grains, with an average distribution of 28% silt and 23% sand, locally reaching up to 60% at the top of the subunit (Section 204-1252A-1H-1). Bioturbation is common, especially at the base of minor lithologies. Biogenic content is high throughout the subunit, with an average composition of 7% calcareous components and 9% siliceous components (Fig. F4). Diatoms dominate the top of the lithostratigraphic subunit, reaching up to 15% (e.g., Samples 204-1252A-1H-1, 2 cm, and 1H-1, 22 cm).
Ten distinctive light greenish gray (5GB 7/1), thin (0.5-2 cm thick) clay layers are observed in Section 204-1252A-1H-2 (bases at 33, 37, 44, 49, 61, 67, 81, 82, 94, and 105 cm) at ~2 mbsf (Fig. F5). These layers appear to correlate with similar intervals at Site 1251 that may represent the Holocene-upper Pleistocene transition. The placement of the boundary between lithostratigraphic Subunits IA and IB-IC was based on the last occurrence of sand layers in Section 204-1252A-2H-2 (7 mbsf).
Lithostratigraphic Subunit IB-IC (Hole 1252A; 7-69.5 mbsf) consists of homogeneous diatom-bearing to diatom-rich silty clay. Iron sulfides, observed as black to dark gray (N3) color precipitates, nodules, and mottles, are common throughout this lithostratigraphic subunit, particularly in Cores 204-1252A-4H, 5H, and 7H, and correlate with three peaks recorded in the MS data at ~26, 45, and 58 mbsf (see "Magnetic Susceptibility" in "Physical Properties"). Bioturbation is also abundant, often highlighted by the presence of iron sulfides. The lower boundary of this lithostratigraphic subunit is marked by the onset of a debris flow deposit located at 69.5 mbsf (DF2 on Fig. F3) and characterized by both a transparent facies in the 3-D seismic profiles (see Fig. F8 in the "Leg 204 Summary" chapter) and by a large peak in the MS data (see "Magnetic Susceptibility" in "Physical Properties").
Smear slide analyses indicate that the major lithology of lithostratigraphic Subunit IB-IC is typically composed of 69% clay, 30% silt, and 1% sand (Fig. F2). The major mineral components of the subunit are feldspar, quartz, and clay and opaque minerals. Biogenic components, particularly biogenic opal (mostly diatoms and sponge spicules) are abundant in this subunit, composing 12% of the sediment on average and reaching 25% locally (Samples 204-1252A-2H-3, 96 cm, and 2H-6, 42 cm). Calcareous components, mostly nannofossils, are abundant (up to 12%) in the upper part of this lithostratigraphic subunit (7-34 mbsf) and decrease drastically to <1% from 34 to 71.5 mbsf (Figs. F2, F4).
Lithostratigraphic Subunit ID (Hole 1252A; 69.5-96.5 mbsf) is characterized seismically as a distinct wedge of incoherent reflectors (see Fig. F8 in the "Leg 204 Summary" chapter) common in sediment lacking bedding (e.g., large debris flow deposits). In the cores, soft-sediment deformation features, convoluted bedding, and clay clasts, which we interpret as evidence of such a debris flow deposit, are found between 69.5 and 71.5 mbsf (see Sections 204-1252A-8H-6 to 8H-CC). Mud clasts, ranging from 1 to 3 cm in diameter, are embedded in a clay-rich convoluted matrix (Fig. F6). Below the prominent soft-sediment deformation features and convoluted bedding, lithostratigraphic Subunit ID is composed of a homogeneous sequence of diatom-rich to diatom-bearing silty clay, typically dark greenish gray (5GY 4/1) to very dark gray (5GY 4/1) where sulfide precipitates are present. The lower boundary of this subunit is marked by the regional seismic unconformity and corresponds to an abrupt change in sedimentation rate (see "Summary" in "Biostratigraphy").
The clay-size fraction of the major lithology of lithostratigraphic Subunit ID composes, on average, 70% of the sedimentary components. Silt-size grains compose the remaining 30% of the major lithology, with sand-size grains entirely absent from the lithostratigraphic subunit (Fig. F2). Quartz is the dominant mineral identified in smear slide analyses, ranging from 10% to 20% of the mineral constituents, with feldspar, opaque minerals, and glauconite composing ~1%-5% of the remaining coarse fraction. The minor lithologies analyzed in smear slides rarely differ greatly from the major lithology, except in Sample 204-1252A-11H-1, 128 cm, taken from an authigenic carbonate-rich cement surrounding the only carbonate nodule in lithostratigraphic Unit I.
Lithostratigraphic Subunit ID is rich in biogenic components. Smear slide analyses of both the major and minor lithologies indicate that calcareous components (calcareous nannofossils and foraminifers) typically compose 3%-7% of the sedimentary components (Fig. F2). Siliceous microfossils (diatoms, radiolarians, and siliceous sponge spicules) compose 8%, on average, of the major and minor sedimentary components, with the average siliceous microfossil content of the major lithology ~11%.
Bioturbation and sulfide precipitates are abundant in lithostratigraphic Subunit ID. Cores 204-1252A-10H and 12H are particularly rich in sulfide. The high sulfide precipitate content of the lithostratigraphic subunit corresponds to a 25-m-thick high in the MS data (see "Magnetic Susceptibility" in "Physical Properties"). Correlation between MS highs and sulfide precipitates was seen at several sites during Leg 204, and at this site the best correlation occurred in the zone containing pyrrhotite nodules in Section 204-1252B-10H-CC of lithostratigraphic Subunit ID.
The boundary between lithostratigraphic Units I and II is defined by the regional unconformity observed in the 3-D seismic data (Fig. F3), also seen at Site 1251 below Subunit IC (see Fig. F8 in the "Leg 204 Summary" chapter). Lithostratigraphic Unit II is primarily composed of foraminifer-rich silty clay that is dark greenish gray (5GY 4/1). This silty clay is interlayered with lighter-colored layers of fine sand and coarse silt, especially at the base of the lithostratigraphic unit. The major sedimentary components of lithostratigraphic Unit II are clay minerals, quartz, and feldspar. The biogenic components consist of mainly calcareous microfossils (foraminifers and calcareous nannofossils) with some siliceous microfossils (sponge spicules, silicoflagellates, and diatoms) (Fig. F4).
High-frequency fining-upward turbidites, the bases of which contain fine to very fine sand that grades to silt and clay, characterize lithostratigraphic Unit II (Figs. F2, F4) and differentiate it from lithostratigraphic Unit I. The sand and silt layers that form the sharp erosional bases of the turbidite sequences are commonly ~1 cm thick, although occasionally they reach 3 cm thick (Fig. F7B). These layers typically contain macroscopic foraminifers (e.g., Cores 204-1252A-11H and 12H) and grade upward into sulfide-rich, bioturbated, homogeneous silty clay and clay (Fig. F7A).
The mineralogy of the major lithology of lithostratigraphic Unit II is similar to that of lithostratigraphic Subunit ID. Quartz is, again, the dominant mineral constituent, composing 10%, on average, of the coarse fraction. Silt- and sand-size grains compose 28% and 5%, respectively, of the sedimentary components, with the remainder composed of clay-size minerals. The primary textural difference between lithostratigraphic Units I and II occurs in the minor lithology. Sand composes as much as 70% of the mineral components in the minor lithologies near the base of lithostratigraphic Unit II (e.g., Sample 204-1252A-13H-2, 67 cm) and typically composes 30% of the minor lithology throughout the rest of the unit (Fig. F2).
Lithostratigraphic Unit II is composed of as much as 18% biogenic components, and Sample 204-1252A-12H-7, 86 cm, contains 38% calcareous components (35% foraminifers and 3% calcareous nannofossils). This sample contains the highest percentage of calcareous microfossils in lithostratigraphic Unit II. However, lithostratigraphic Unit II lacks significant siliceous microfossils. Smear slides indicate <3% of the sediment is composed of siliceous microfossils (Fig. F2).
The boundary between lithostratigraphic Units II and III is present at 114 mbsf, where a 5-m-thick series of glauconite-rich sand layers is interbedded with silty clay hemipelagic sediment (Figs. F8, F9). This depth corresponds to the depth of the western limb of anticline B on the 3-D seismic data (see Fig. F8 in the "Leg 204 Summary" chapter) as well as to prominent changes in the well logging and physical property characteristics of the section (see "Downhole Logging" and "Physical Properties"). 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. Additionally, lithostratigraphic Unit III contains abundant carbonate precipitates in the form of both cements and nodules (Fig. F10), mousselike textured sediments (Fig. F11) above the base of the gas hydrate stability zone (GHSZ), and debris flow deposits at 125.5 and 197 mbsf. Clay and silty clay composed of <10% biogenic components make up the major lithology of lithostratigraphic Unit III, and the minor lithologies lack the silt and sand turbidites typical of lithostratigraphic Unit II (Fig. F2).
Lithostratigraphic Unit III is a sequence of silty clay horizons punctuated at irregular and infrequent intervals by graded sandy silt turbidites. The major lithology of the lithostratigraphic unit comprises 73% clay-, 24% silt-, and 3% sand-size grains. The minor lithologies range in composition from clay, composed of 5% silt and 95% clay, to sand-silt-clay, composed of 40% sand, 30% silt, and 30% clay (Fig. F2). Quartz, feldspar, and opaque minerals compose the majority of the minerals in the major lithology, and carbonate is abundant in the minor lithologies. Cores 204-1252A-14H, 19X, and 21X each contain multiple carbonate-cemented layers, identified in the cores by their lighter color (Fig. F10) and visible as carbonate precipitates. The precipitates tend to be clustered between 121 and 127 mbsf in Core 204-1252A-14H, just below the glauconite sand layers that define the upper boundary of the lithostratigraphic unit, and deeper in Unit III (between 179 and 192 mbsf). Smear slide analyses of the carbonate cements surrounding the authigenic carbonates between 121 and 127 mbsf indicate that these cements contain similar amounts of authigenic carbonate, typically 60% to 80%, to those cements sampled between 179 and 192 mbsf. Silt composes as much as 30% of the sediments in these cements, although it is typically only 10%-15% of the sample.
The biogenic content of lithostratigraphic Unit III lies in stark contrast to the overlying lithostratigraphic Unit II. Below the glauconite sand layers at 114 mbsf, the calcareous microfossil content of the sediment drops to 0%, though it increases slightly below this depth (Fig. F2). Foraminifers are found in trace amounts (1%-2%) in Samples 204-1252A-19X-2, 26 cm; 20X-3, 9 cm; and 21X-1, 22 cm, but are otherwise absent from lithostratigraphic Unit III. Calcareous nannofossils can compose as much as 20% of the major lithology of lithostratigraphic Unit III (Sample 240-1252A-23X-2, 60 cm), although typically, they are absent. The biogenic opal content of lithostratigraphic Unit III is similar to that of the overlying lithostratigraphic units at the top, but increases (up to 35%) with depth (Fig. F4).
The two most striking features of lithostratigraphic Unit III are the glauconite sand layers (Figs. F8, F9) and the carbonate precipitates immediately below (Fig. F10). Two debris flow deposits are also present within the lithostratigraphic unit but show no major deviations from the dominant lithology in smear slide analyses. Unfortunately, total organic carbon (TOC) and X-ray diffraction analyses to determine the composition of the carbonate samples were not done on board the ship but will be conducted postcruise. However, as a result of the high density of these samples (~2.0 g/cm3) (see "Physical Properties"), the carbonate precipitates are likely composed of dolomite.
The clay content of the major lithology in lithostratigraphic Unit III slightly increases below 211 mbsf, in conjunction with the increase in biogenic opal content (Fig. F2). Clay composes 78% of the major lithology, silt composes 20%, and sand 3%, on average (Fig. F2). Two glauconite-bearing sands are visible in lithostratigraphic Unit III at 232 and 234 mbsf in Core 204-1252A-26X. The sampled minor lithologies were similar to the major lithology in grain size and mineral composition, although their biogenic components differed from those of the major lithologies (Fig. F2). Calcareous microfossils are entirely absent from smear slide samples taken near the base of lithostratigraphic Unit III (211-259 mbsf). Biogenic opal content, on the other hand, increases in this interval relative to the overlying sediments of lithostratigraphic Unit II. Scaly fabric, caused by the alignment of clay minerals under an oriented stress, was also observed in lithostratigraphic Unit III. However, the orientation of the fabric could not be determined because the core likely rotated within the liner during recovery.
Lithostratigraphic Unit III lies within the sediments of the deeper accretionary complex (sediments known to contain gas hydrate at Ocean Drilling Program Site 892; see Fig. F1 in the "Leg 204 Summary" chapter) and contains abundant disrupted sedimentary textures indicative of the dissociation of gas hydrate (particularly in Cores 204-1252A-14H through 18X and 20X) (Fig. F11). These soupy and mousselike textures were encountered between 126 and 179 mbsf at Site 1252; however, the only samples presumed to contain gas hydrate, as identified by cold anomalies with the infrared (IR) camera (see "Infrared Scanner" in "Physical Properties"), were taken from ~83 and ~99 mbsf. Initial attempts to correlate the disrupted sedimentary features with the IR images and well logging data were unsuccessful at sea, but additional postcruise examination may reveal some relationships. Additionally, chloride samples were taken from one soupy layer and will be analyzed postcruise. Thus, there is textural sedimentological evidence for the dissociation of gas hydrate in the cores described at Site 1252; however, there is insufficient supporting evidence, at this time, for the presence of gas hydrate in these cores to definitively rule out core-related disturbance as the cause of the disrupted sedimentary fabric.
Site 1252 is located on the eastern flank of southern Hydrate Ridge between Sites 1244 and 1251. The stratigraphy at this site is best correlated to that at Site 1251 by direct comparison between the recovered sedimentary section and the stratigraphic geometries observed in the 3-D seismic reflection data (see Figs. F8 and F10 both in the "Leg 204 Summary" chapter).
Seismic reflection data across Site 1252 indicate that the deeper accretionary complex sediments of lithostratigraphic Unit III are folded and that the sediments of lithostratigraphic Unit II were deposited against the western limb of fold at this site. The contact between lithostratigraphic Units III and II is truncated in the seismic data and, thus, represents a buttress unconformity at ~114 mbsf. Just below the unconformity at Site 1252, the uppermost portion of lithostratigraphic Unit III is composed of clay and silty clay with very few calcareous components. However, above the unconformity, calcareous components and foraminifer-bearing silty turbidites are present and there is an abundance of glauconite from ~114 to 120 mbsf (Fig. F8). Abundant glauconite, which was also detected on the well logging density and gamma data (see "Downhole Logging"), is present within the silty clay and clay of Unit III. These fine-grained sediments are suggestive of a lower sedimentation rate and perhaps a lower-energy environment conducive to glauconite formation (Leeder, 1999). Below the glauconite-rich zone, there is a zone of abundant authigenic carbonate. Perhaps the unconformity serves as a regional conduit for fluids and, thus, promotes carbonate precipitation.
Lithostratigraphic Unit II, as defined at Site 1252, is correlated with Lithostratigraphic Unit II at Site 1251, although the lithostratigraphic unit is much thinner at Site 1252 than Site 1251. The apparent buttress unconformity that separates the two lithostratigraphic units at Site 1252 is not present on the eastern limb of Anticline B, drilled at Site 1251, although regional mapping in the 3-D seismic data suggests the sediments of Unit II at Sites 1251 and 1252 may have been deposited at the same time (see Fig. F8 in the "Leg 204 Summary" chapter).
A regional unconformity is observed at both Sites 1252 and 1251 above Unit II and provides a good tie point for intersite correlation (horizon labeled "U" on Fig. F8 in the "Leg 204 Summary" chapter). This unconformity separates lithostratigraphic Unit II from lithostratigraphic Subunit ID at Site 1252 and separates lithostratigraphic Unit II from lithostratigraphic Subunit IC at Site 1251.
Site 1244 lies upslope from Site 1252, and because of stratigraphic pinchouts, only the uppermost stratigraphy correlates between Sites 1244, 1252, and 1251 (see Figs. F5 and F10 both in the "Leg 204 Summary" chapter). We recovered cores from the eastern slope basin sedimentary section, which contains at least two large and seismically distinct submarine landslides or debris flows (DF1 and DF2) within lithostratigraphic Unit I at both Sites 1252 and 1251 (see Fig. F8 in the "Leg 204 Summary" chapter). Because of the good correlation with the younger regional unconformity discussed above, it is possible to correlate all of the stratigraphy of Unit I at Sites 1252 and 1251. The older of the two debris flows (DF2) is present only at Site 1252 (see Fig. F8 in the "Leg 204 Summary" chapter), where it is identified as lithostratigraphic Subunit ID.
Lithostratigraphic Subunit ID at Site 1252 is interpreted as a submarine landslide or debris flow deposited on the unconformity surface that serves as the upper boundary for lithostratigraphic Unit II (at both Sites 1251 and 1252). Although lithologic evidence for the landslide only appears as a 2-m-thick zone of clasts inferred as a debris flow capping an ~10-m-thick zone of calcareous- and siliceous-rich clays and silty clays, the seismic signature is distinct, forming a wedge-shaped lens with chaotic reflectivity. This wedge of sedimentation can be traced regionally on the 3-D seismic data, except in the immediate area of Site 1251, where Subunit IC is deposited directly on the unconformity surface. Continuation of the landslide (see DF2 on Fig. F8 in the "Leg 204 Summary" chapter) on the unconformity surface east of Site 1251 can be identified in the deeper part of the eastern slope basin, however, suggesting that its absence at Site 1251 resulted from either sediment bypass or erosion associated with emplacement.
The uppermost lithostratigraphic unit preserved at both Sites 1252 and 1251 is lithostratigraphic Subunit IA. Although the youngest Holocene record is preserved at Site 1251, it is absent at Site 1252. The uppermost stratigraphy recovered at Site 1252, however, does contain bands of gray clay (Fig. F5) and turbidites, which were also observed at Site 1251. A color change similar to this is regionally identified throughout the central Cascadia margin and typically signifies the transition from Pleistocene to Holocene sedimentation (see "Environment of Deposition" in "Lithostratigraphy" in the "Site 1251" chapter).