Site 1245 (proposed Site HR3a) is located in 870 m of water on the western flank of Hydrate Ridge, ~3 km northwest of the southern summit (Fig. F1). The 3-D seismic data show that the BSR is at a depth of ~134 mbsf at this site. As at all sites drilled during Leg 204, the temperature and pressure at the seafloor at Site 1245 are well within the GHSZ, indicating that gas hydrates can exist within the entire stratigraphic section above the BSR if hydrate-forming gases are available in concentrations that exceed their in situ solubility. A faint reflection that underlies the BSR and is approximately parallel to it has tentatively been interpreted to be a second BSR that may indicate the base of the stability zone of hydrates that include higher-order hydrocarbons. This site also samples Horizon A. Horizon A can be mapped from the northern boundary of the seismic survey (where it clearly follows stratigraphic boundaries) to the summit (where it appears as a "bright spot" beneath the BSR). On its downdip edge, it appears to lap onto the boundary between coherent folded strata and the seismically incoherent facies interpreted to represent highly deformed sediments of the accretionary complex. It has been interpreted to be a "conduit" that transports fluids from the accretionary complex to the summit. Several unconformities, referred to as Horizons Y and Y', overlie Horizon A and appear to represent discontinuities in sediment accumulation in a slope basin that was formed during growth of an underlying accretionary anticline.
Primary objectives at Site 1245 were to (1) determine the distribution, composition, and concentration of gas hydrate in the sediments on the western flank of Hydrate Ridge and contrast these parameters with those on the eastern flank of the ridge and in the adjacent slope basin where the sub-BSR fluid migration pathways inferred from seismic data are distinctly different; (2) sample sediments and fluids from seismic Horizon A; and (3) sample the sedimentary section of the western flank of Hydrate Ridge below the BSR to provide constraints for interpreting variations in BSR strength across the western flank. Site 1245 is also a reference site for a north-southtrending transect that extends from Site 1245 to the summit and includes Sites 1247, 1248, 1249, and 1250.
Hole 1245A was drilled to a depth of 380 mbsf (without coring) to obtain the initial LWD data for this site. Hole 1245B was cored to 473.7 mbsf using the APC and XCB. Holes 1245C and 1245D were cored to 201.7 and 24 mbsf, respectively, for extensive high-resolution geochemical and microbiological sampling. Hole 1245E was drilled to 473.7 mbsf and then cored to 540.3 mbsf using the RCB. Coring in Hole 1245E stopped short of the originally planned depth of 700 mbsf because of deteriorating hole conditions. The hole began to collapse, trapping the BHA. Fortunately, it was not necessary to sever the pipe, although preparations were made to do so. The upper 300 mbsf of Hole 1245E was used for wireline logging. Plans for conventional, offset, and walk-away seismic lines were abandoned when the downhole seismometer would not clamp in Hole 1245E and the hole continued to collapse. The APCT was run eight times, the DVTP was run three times, and the PCS was run five times at this site. There were also two runs each of the HYACINTH HRC and FPC devices. Eleven whole-round samples of sediment thought to contain gas hydrate were preserved in liquid nitrogen or in pressure vessels for postcruise studies.
Biostratigraphic observations from the 540 m of core recovered at Site 1245 indicate that the entire 540-m-thick sequence is younger than 1.65 Ma (Fig. F10). Distinct changes in sedimentation rate occurred at 55 and 150 mbsf. Sediments deeper than 150 mbsf were deposited from 1.0 to 1.65 Ma at a rate of ~62 cm/k.y., whereas the overlying strata were deposited at a slower rate of 1023 cm/k.y. (Fig. F11). Lithostratigraphic analysis indicates that the dominant lithologies in the upper 031 mbsf are clay with carbonate concretions and foraminifer-rich interlayers. From 31.5 to 212.7 mbsf, the sediments are mainly diatom-bearing clay and silty clay with frequent sand-rich turbidites containing a few glass-rich layers. Included within this deeper sequence is seismic Horizon A, characterized by multiple ash-rich sandy layers between 176 and 183 mbsf. Between 212.7 and 419.3 mbsf, nannofossil-rich claystone and silty claystone with glauconite layers and turbidites are present, underlain by claystone containing thick turbidites and heterogeneous mud clasts.
The precruise 3-D seismic reflection site survey (Figs. F5, F6) and the LWD data (Fig. F13) obtained from Hole 1245A provided a roadmap that was used to guide the sampling and analysis strategy at this site. The logging data, which are of excellent quality, show a marked increase in the amplitude and variability of formation resistivity between 48 and 131 mbsf (logging Unit II). As discussed for Site 1244, this resistivity pattern is interpreted to indicate the zone within which gas hydrates are present. High-resistivity layers are both subhorizontal, indicating accumulation of gas hydrate parallel to bedding, and steeply dipping, indicating that hydrate fills fractures. Archie's Law (Archie, 1942) saturation calculations (Collett and Ladd, 2001) predict a hydrate concentration of ~10%30% of the pore volume in layers distributed throughout this interval.
The estimated depth distribution of gas hydrate obtained from the LWD data was confirmed by several other chemical and physical proxies. Low chloride concentration anomalies (Fig. F14) were detected in samples of interstitial waters from 55 to 125 mbsf (Fig. F14) and are interpreted to reflect in situ hydrate concentrations that are generally below 3%, with one anomaly suggesting a concentration of 15%. Low-temperature anomalies were observed with the IR cameras between 50 and 129 mbsf (Fig. F13). Preliminary estimates of total in situ methane concentration obtained from a PCS at 57 mbsf indicated a concentration very close to in situ saturation, and a PCS located at 120 mbsf indicated that in situ methane concentration just above the BSR is an order of magnitude greater than saturation at in situ conditions (Fig. F16). Two PCS runs below the BSR yielded concentrations that are apparently slightly lower than saturation. The consistency between these multiple independent estimates of the depth range of the zone where hydrate is present gives us considerable confidence in the validity of these estimates. Models will be investigated postcruise to relate the depth of the first occurrence of hydrate to the depth of the SMI (observed at ~7 mbsf at this site) and to more precisely estimate the in situ methane concentration as a function of depth within the hydrate stability zone.
The IR camera was used to rapidly identify the temperature range and potential location of hydrate samples in cores on the catwalk. In addition, the IR data were used to estimate the distribution and texture of gas hydrate downhole. Approximately 80 IR anomalies were identified and classified (Fig. F13). In Hole 1245B, 75% of the anomalies suggested disseminated hydrate and 25% suggested nodular hydrate; in Hole 1245C, 60% of the anomalies suggested disseminated hydrate and 17% suggested nodular hydrate. The IR data were also used select a section of core (Section 204-1245C-7H-5) for an experiment to investigate the relationship between IR imaging, chloride concentration anomalies in pore water, and hydrate distribution in the core. When the core liner was split and removed, a 2-cm-thick, steeply dipping layer of hydrate was found. After allowing the hydrate to dissociate for 90 min, closely spaced sediment samples were taken near the hydrate, including one sample from where the hydrate had been. The chloride concentration anomaly is strongly attenuated 5 cm away from the hydrate sample and has disappeared 10 cm from the hydrate. Since normally only two interstitial water samples are taken in each 9.5-m-long core, the chloride concentration measurements are spatially biased. Frequent low-chloride concentration anomalies downcore must indicate extensive distribution of hydrate. Additional comparison and calibration between data sets with different length scales and sensitivity to hydrate concentration should improve our ability to estimate in situ concentrations from such data.
A major objective at Site 1245 was to sample seismic Horizon A. At Site 1245, as well as at the other three sites where Horizon A was crossed during the LWD phase of Leg 204 (Sites 1247, 1248, and 1250), it is characterized by a very distinctive strong double-peaked low-density anomaly that is 34 m wide. At Site 1245, the density of Horizon A is <1.5 g/cm3 compared to 1.85 g/cm3 in adjacent sediments. Coincidence of this LWD density anomaly with the estimated depth of Horizon A in the seismic data provided confirmation that the velocities used for converting the seismic data to depth were accurate enough to predict the depth of target horizons to within a few meters. This was confirmed by sonic velocity measurements. The direct correlation between the low-density layers and the thick ash layers discussed above was confirmed by bulk density and MS measurements made on the cores.
Another major result of drilling at Site 1245 was the discovery of significant concentrations of higher-order hydrocarbons beneath the BSR. Methane/ethane (C1/C2) ratios in headspace samples reach values <100 between 130 and 180 mbsf (Fig. F15). When combined with the measured in situ temperature gradient (~0.055°C/m) and the observation that the C1/C2 anomaly is due entirely to an increase in ethane concentration, the data suggest that thermogenic hydrocarbons are migrating from deeper in the accretionary complex. Dissolved lithium anomalies observed in association with Horizon A support this interpretation. In addition to C2, enrichments in C3 and other higher-order hydrocarbons were also observed. At Site 1245, the minimum in the C1/C2 ratio is present at ~150 mbsf, ~30 m above Horizon A.
Site 1245 provided confirmation that multiple proxies for in situ hydrate presence (including electrical resistivity measured downhole, core temperatures measured on the catwalk, chloride anomalies measured in interstitial waters, and direct measurements of gas concentration) are generally consistent in predicting the distribution and concentration of gas hydrates in the subsurface. Additional analysis is needed to more precisely understand and calibrate these different proxies. Drilling at this site also demonstrated that seismic Horizon A results from the presence of a pair of low-density ash-bearing sand layers that are likely to be fluid conduits. Shipboard data provide evidence for migration of higher hydrocarbons beneath the GHSZ, although determining details of the role of seismic Horizon A in this migration will only be resolved through integration of data from several sites. Finally, this site provided important lithostratigraphic and biostratigraphic data for reconstructing the geologic history of this hydrate-bearing system, including the rate at which the system formed and lithologic controls on fluid migration and hydrate distribution.
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