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Expedition 311 Scientists2
Site U1325 (proposed Site CAS-02C; Collett et al., 2005) is located along the margin-perpendicular transect established during Integrated Ocean Drilling Program (IODP) Expedition 311 and is within the first major slope basin that developed eastward of the deformation front behind a steep ridge of accreted sediments (see Fig. F3 in the "Expedition 311 summary" chapter). The bathymetry data show that the seafloor in the western part of the basin is relatively flat with water depths of ~2200 m (see Fig. F4 in the "Expedition 311 summary" chapter). Around Site U1325, the seafloor becomes gradually shallower before it rises rapidly to the east to form the plateau of the second main accreted ridge at water depths of ~1200 m, on which IODP Site U1327 and Ocean Drilling Program (ODP) Site 889 are located. A map of the seismic site survey data around Site U1325 is shown in Figure F1.
The western part of the slope basin is seismically characterized by a section as thick as 600 ms two-way traveltime (TWT) of relatively undisturbed slope basin–type sediments with almost seafloor-parallel reflectivity (common depth points [CDPs] 1180–1320 in Fig. F2). Toward the eastern part of the basin, a buried ridge of accreted sediments is visible (CDPs 1320–1440 in Fig. F2), covered only by a thin (~20 m) section of more recent slope basin–type sediments. On high-resolution 3.5 kHz subbottom profiler data, Holocene sediments are visible as a 3–5 m thick transparent layer on top of sediments with stronger reflectivity that do not show any further layering (Fig. F3). The top ~20 m thick sediment section is undisturbed and overlies a ~200 m thick section of steeply northeast-dipping sediments. The dip is the result of the uplift associated with an underlying ridge of accreted material, which outcrops at the seafloor ~8 km to the southeast of Site U1325. The sediments forming the ridge of accreted material show laterally limited chaotic reflections that can be traced for only a few hundred meters. The transition between folded, but still coherent, slope sediments to accreted sediments is gradational and not clearly defined at this site. Within the center of the accreted ridge, a section of bright reflections can be seen (CDPs 1350–1360 in Fig. F2; CDPs 7400–7600 in Fig. F4). The short reflection, best imaged in the lower-frequency data of Line 89-08, appears to have a polarity opposite to that of the seafloor, which may indicate the presence of free gas. Interestingly, the high-resolution 3.5 kHz data show a zone of reduced reflectivity (near shotpoint 1650), similar to that observed at cold vent field Site U1328. There may be a connection between the deeper-seated free gas and this near-seafloor expression of potential fluid/gas expulsion. The location of Site U1325 was chosen carefully to avoid this bright reflector and the near-seafloor expression of the potential expulsion feature.
A bottom-simulating reflector (BSR) is clearly visible in the eastern part of the slope basin (Fig. F2), but it fades to the west (CDPs 1180–1280 along multichannel seismic [MCS] Line 89-08) close to the crossing point with MCS Line 89-11. A BSR may be present in the western basin, but it could also be masked by regular seafloor-parallel stratigraphy. The BSR also shows the typical frequency-dependent reflection strength pattern (Chapman et al., 2002). On MCS Line PGC9902_ODP-7 (Fig. F5), virtually identical in location with Line 89-08, the BSR is barely visible (Fig. F4). The data of Line 89-08 are between 10 and 50 Hz, whereas the data of Line PGC9902_ODP-7 contain frequencies twice as high. This frequency-dependent behavior can be explained by a gradient zone of a few meters over which seismic velocity decreases from the gas hydrate–bearing section to the free-gas zone beneath the interface.
At Site U1325, the BSR is less strong (in both the low- and high-frequency data sets) than at the core of the buried ridge of accreted sediments (Figs. F2, F4). The BSR appears to be at ~230 meters below seafloor (mbsf) based on seismic traveltime depth conversion, using an average P-wave velocity of 1636 m/s. This average velocity was determined at ODP Leg 146 Site 889 to match observed traveltime depths of the BSR and vertical seismic profile (VSP) data (MacKay, 1994). We acknowledge that this velocity may not be appropriate in this setting; however, lack of good control in seismic interval velocities from MCS semblance analyses of Lines 89-08 and PGC9902_ODP-7 prohibited a more accurate depth migration of the data. Using an extreme velocity profile, which represents no gas hydrate in the section (Hyndman et al., 2001), yields an average velocity of 1619 m/s, and consequently, the BSR is only 2.5 m shallower. Alternatively, a much higher velocity profile, which represents twice the amount of gas hydrate of what has been seen at Site 889, results in a BSR depth that is 2.5 m deeper.
The primary research objectives for this site are linked to the overriding transect-concept of this expedition. The objectives include
The slope basin is expected to show a different geochemical regime and related geophysical properties than the uplifted ridges of accreted sediments.
The operational plan to achieve these objectives was based on a general three-hole concept, which included