Eleven holes (Holes 1249B-1249L) were cored at Site 1249. Table T2 records the depth of penetration and core recovery in each hole drilled. All of the recovered sediments from Holes 1249G-1249L were placed directly into either pressure vessels or liquid nitrogen to be preserved for future studies. Thus, these cores were not, therefore, described or logged according to standard ODP procedures.
Site 1249 is located near the crest of southern Hydrate Ridge in close proximity to Sites 1248 and 1250 (see Figs. F1 and F7 both in the "Leg 204 Summary" chapter). We were able to distinguish two separate lithostratigraphic units (Units I and II) above Horizon Y at Sites 1245, 1247, and 1250 but were unable to do so at Site 1249 because of poor recovery (~56%). However, because of the proximity of the sites at the ridge crest, we call our first lithostratigraphic unit at this site Unit I-II, for better correlation with the other sites and do not distinguish between a clay-dominated Unit I and a turbidite-dominated Unit II. The lithostratigraphic sequence recovered at Site 1249 was divided into two lithostratigraphic units (Units I-II and III) based on variations in sedimentary structure and grain size as well as changes in the biogenic and lithogenic components (Fig. F2).
Calcium carbonate content (expressed as weight percent CaCO3) and mineralogy from X-ray diffraction (XRD) were also used to delineate the lithostratigraphic unit boundaries of Site 1249. Additionally, we compare and correlate our results with the three-dimensional (3-D) seismic data, downhole LWD data, and physical property measurements (magnetic susceptibility [MS] and GRA density) to better define the entire stratigraphic sequence (Fig. F2). 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 composed of clay and silty clay with a varying biogenic component that ranges from nannofossil- to diatom-bearing and diatom-rich clay. The sediments of lithostratigraphic Unit I-II tend to be dark greenish gray (5GY 4/1), although very dark gray (N3) sediments are also found in zones containing sulfide precipitates. These precipitates are absent from the top 15 mbsf in every hole cored and tend to increase with depth in the hole (Fig. F3). The homogeneous nature of the recovered sediments, combined with the poor recovery (~56%) of lithostratigraphic Unit I-II (Table T2), made correlation between holes difficult. However, the location of the lower boundary of lithostratigraphic Unit I-II, which was placed above the first occurrence (FO) of fining-upward silty to sandy interlayers in lithostratigraphic Unit III, agrees to within 8 m in all the holes cored and ranges in depth from 51.5 mbsf in Hole 1249F to 59.3 mbsf in Hole 1249C.
The primary texture of the mineralogic components of lithostratigraphic Unit I-II ranges from clay to silty clay. On average, ~70% of the total components are clay and ~30% are silt. The silt fraction of the sediment decreases slightly with depth from ~37% to 20% at the lithostratigraphic Unit I-II/III boundary in Hole 1249F. This trend, however, is not observed in either Holes 1249B or 1249C, the only other holes in which the lithostratigraphic boundary was recovered. Sand grains can be found in smear slide samples taken from both Holes 1249C and 1249F, though sand does not exceed 5% of the total components of lithostratigraphic Unit I-II in any hole.
Biogenic components typically compose 5%-15% of the sediments in lithostratigraphic Unit I-II. Calcareous nannofossils compose 2%-5% of the major lithology and tend to be more abundant in the top 30 mbsf than below. Siliceous microfossils, primarily diatoms, compose 2%-15% of the sedimentary components in both the major and minor lithologies of the lithostratigraphic unit. An increase in the siliceous microfossil content occurs at 24 mbsf in Hole 1249C and continues to 37 mbsf.
Perhaps the most distinguishing feature of lithostratigraphic Unit I-II is the ubiquitous presence of mousselike and soupy textures. Core 204-1249F-9H was the only core recovered from lithostratigraphic Unit I-II that did not contain either texture. The disruption of the original sedimentary fabric is presumed to have resulted from the dissociation of gas hydrate. This hypothesis is supported by the abundant gas hydrate specimens sampled at this site, the multiple cold anomalies observed with the infrared (IR) camera (see "Physical Properties"), chloride anomalies in the geochemical data (see "Chloride Concentration and the Presence of Gas Hydrate" in "Interstitial Water Geochemistry") and, most especially, by the resistivity log data (see "Gas Hydrate" in "Interpretation of LWD Logs" in "Downhole Logging").
The major lithology of lithostratigraphic Unit III is diatom-bearing to diatom-rich clay and silty clay with nannofossil-bearing to nannofossil-rich zones. Clay and silty clay are punctuated by minor lithologies of silt, silty sand, and sand (1 mm to 10 cm thick), which compose the bases of the turbidite sequences. The majority of the lithostratigraphic unit is dark greenish gray (5GY 4/1) but varies to very dark gray (N3) in the presence of sulfide. The upper boundary of lithostratigraphic Unit III with Unit I-II is defined by (1) the presence of visible turbidites in the core, (2) an increase in grain size (major lithology sand fraction), (3) a slight increase in calcareous components, and (4) a slight decrease in biogenic opal (Fig. F2). The boundary between these lithostratigraphic units is also coincident with seismic Horizon Y (see Fig. F7 in the "Leg 204 Summary" chapter). Although Horizon Y appears conformable at Site 1249, seismic correlation between sites suggests it may be a regional unconformity, supporting a unit contact at its interface (see Figs. F5 and F7 both in the "Leg 204 Summary" chapter). Recovery of lithostratigraphic Unit III was much improved in Holes 1249C (99.1%) and 1249F (94%) compared to the recovery of lithostratigraphic Unit I-II in the same holes, 66.0% and 66.6%, respectively (Table T2).
Grain size of both the major and minor mineral components and the biogenic components of lithostratigraphic Unit III was determined by smear slide analyses (Fig. F2). The character of individual turbidites varied from thin (1-2 mm) layers to closely spaced (1-2 cm) silts and sands (Fig. F4A) to thicker (2-3 cm) single graded beds (Fig. F4B). Sulfide mottles and dark gray (N3) zones are common throughout the unit. Abundant iron sulfide nodules were observed near the bottom of Holes 1249C (Section 204-1249C-16H-2) and 1249F (Sections 204-124FC-15H-5, 15H-6, and 15H-7). Moderate to rare bioturbation is also common throughout the unit and is best preserved beneath the bases of the turbidites.
Although Holes 1249C and 1249F reached similar TDs, Hole 1249F recovered a sedimentary sequence that contained soft-sediment deformation features and clay clasts (1-2 cm in diameter) (intervals 204-1249F-16H-4, 10-65 cm, and 16H-5, 0-56 cm). We interpret this matrix-supported clay-clast deposit as a debris flow (similar to those seen at other sites). An abundance of (authigenic) carbonate-rich clay was seen as a lens in Sample 204-1249F-16H-1, 18-20 cm, and glass-rich silty clay (30% glass) was observed as a lens or thin clast in both Samples 16H-3, 148-149 cm, and 16H-4, 2-5 cm. In addition, glauconite is present as a concentrated lens (~25% glauconite) in interval 204-1249F-16H-4, 33-50 cm.
A total of 57 gas hydrate samples were taken at Site 1249 between 0 and 75 mbsf. Six samples were taken from Hole 1249B, 28 samples from Hole 1249C, 5 samples from Hole 1249D, 5 samples from Hole 1249E, and 13 samples from Hole 1249F. Soupy and mousselike textured sediments related to the in situ presence and subsequent dissociation of gas hydrates were observed in all holes cored at Site 1249 (Fig. F5). These textures were mainly observed in lithostratigraphic Unit I-II (Fig. F6). Please note that Figure F5 does not necessarily show the overall distribution of gas hydrate because bad recovery and extensive whole-round sampling prior to core description limited our ability to completely report all of the hydrate-related sediment disruption occurrences in the stratigraphy.
Based on observations of dissociating gas hydrate, we presume the soupy textures result from the dissociation of massive gas hydrate and mousselike textures result from the dissociation of disseminated gas hydrates. Soupy textures are present in the upper <25 cm of the first section of Cores 204-1249B-2A through 5A, 8A, and 9A. In the first core sections of Cores 204-1249C-1H and 7H, soupy textures are present from 0 to 95 cm as well as from 0 to 75 cm in the first section of Cores 204-1249F-1H and 5H. Some of these textures present within the first 10 cm of a core might be caused by coring disturbance. Mousselike textures are observed in much higher abundance than soupy textures (Fig. F5). They are present throughout the sections and are not limited to the upper 25 cm of a section (Figs. F6, F7). The longest interval of sediment exhibiting mousselike textures is 1.95 m long and is present in Sections 204-1249F-3H-1 through 3H-2 (Fig. F5).
In Hole 1249F, we were able to obtain a whole-round core sample (Sample 204-1249F-8H-2, 117-127 cm) that displayed a thin gas hydrate vein (at the top of the interval; Fig. F8A) in order to examine the relationship between the location of gas hydrate in a core and the resulting response of the sediment fabric during dissociation. The sample was immediately placed in liquid nitrogen after it was removed from the core on the catwalk and was later transferred to the -80°C freezer for several days. The whole-round sample was then split along multiple perpendicular planes inside a walk-in -4°C freezer. Photographs were taken, and notes were made on textural and structural relationships.
An initial split of the whole-round sample perpendicular to the axis of the core revealed that the hydrate vein, far from being tabular, was nonhomogeneous in both its breadth and width within the sample (Fig. F8B). The vein was thicker at depth in the sample than it was as seen on the face exposed on the catwalk (Fig. F8B). Additionally, the first cut made in the freezer revealed several thinner subparallel veins that were closely associated with sulfide precipitates (yellow arrow in Fig. F8B). In order to determine the 3-D characteristics of the hydrate vein, a second split was made parallel to the first (perpendicular to the axis of the core), through the lower half of the sample. The primary vein was seen to span almost the entire width of the sample, and a second smaller vein crosscutting the first at ~20° became visible. This split also revealed nodular hydrate around the main vein. Black sulfide precipitates were visible adjacent to the gas hydrate vein parallel to the core axis (yellow arrows in Fig. F8C). A third cut was made parallel to the main gas hydrate vein along the core axis (Fig. F9) and revealed that the vein spanned the width of the core and appeared nodular at the base of the sample. Closely spaced vertical cracks or cleavage planes were also visible with ~1-mm spacing (red lines in Fig. F10). Though these features appear to be original, we cannot rule out their formation upon core recovery and refreezing in liquid nitrogen. Figure F10 also shows that the gas hydrate vein was not preserved along the outside edges of the core, possibly as a result of friction and/or dissociation during the sampling process. Finally, we allowed the hydrate to dissociate completely and observed the formation of the mousselike textures characteristic of hydrate-bearing sediments (Fig. F11).
Once the gas hydrates had completely dissociated in the whole-round sample, we took smear slides from the soupy and mousselike textured regions of the sample in order to examine the potential differences in grain size in the sediments that hosted the hydrate. These slides, along with slides taken from sediment unaffected by gas hydrate indicate that the soupy textured sample (Sample 204-1249F-8H-2, 123 cm) is a silty clay with a marginally higher percentage of coarser grains (2% sand and 40% silt), whereas the mousselike textured sample (Sample 8H-2, 119 cm) and the hydrate-unaffected sample (Sample 8H-2, 122 cm) are both silty clay (1% sand and 24% silt). The percentage of microfossils is highest in the soupy and mousselike textured samples, with ~22% of biogenic opal vs. 11% in the unaffected sample and 6% calcareous biogenic components vs. 4%, respectively.
The lithostratigraphy at Site 1249 is very similar to that at Site 1248 (see "Lithostratigraphy" in the "Site 1248" chapter). Lithostratigraphic Units I-II and III have the same sedimentological characteristics, and seismic Horizon Y can be identified at each site (see Fig. F7 in the "Leg 204 Summary" chapter). Lithostratigraphic Unit III is characterized by the presence of fining-upward turbidite sequences with silt, silty sand, and sand layers at their bases. The presence of these turbidites coincides with an increase in sedimentation rate toward the base of the hole (see "Summary" in "Biostratigraphy"). The lowest part recovered of lithostratigraphic Unit III at Site 1249 is marked by a matrix-supported conglomerate with clay clasts. These clasts are interpreted to be a debris flow deposit and would also be consistent with higher sedimentation rates. The debris flow deposit is thinner at this site and is present slightly higher in the section than at Site 1248, which was expected from the 3-D seismic data (see Fig. F7 in the "Leg 204 Summary" chapter).
In lithostratigraphic Unit I-II, coarse-grained layers are lacking, suggesting that deposition is dominated by hemipelagic clay accumulation. The sedimentation rates (see "Summary" in "Biostratigraphy") range from 2 to 9 cm/k.y. for lithostratigraphic Unit I-II and decrease toward the base of the unit.