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doi:10.2204/iodp.proc.311.110.2006

Basic observations of regional seismic data: detailed site survey information

Proposed Site CAS-04B

We present a discussion of structures observed at proposed Site CAS-04B, which was not drilled during Expedition 311. This site illuminates the nature of the sediments to be accreted and the initial processes by which they are incorporated into the accretionary wedge. As shown along Line 89-08 (Figs. F2, F3A), the incoming sediments consist of a layer of highly reflective turbidites from ~3.2 to 4.8 s two-way traveltime (TWT), or a thickness of ~1.8 km, above a layer of less reflective hemipelagics from ~4.8 to ~5.5 s TWT, or a thickness of ~1.0 km. The total incoming sediment thickness is therefore 2.5–3.0 km, extending from the seafloor at 2.6 km to the top of ocean crust at 5.0–5.5 km depth. The lithology of these Cascadia Basin sediments was sampled at Site 888, ~40 km south of Site CAS-04B. They largely comprise clayey silts with thin beds of fine to coarse sands and gravel and are of late Pleistocene age at the maximum hole depth of 567 m (Westbrook, Carson, Musgrave, et al., 1994).

The low-frequency seismic Line 85-02, ~3 km south of Line 89-08, shows undeformed basin sediments seaward of the large frontal ridge that rises >800 m above the deep seafloor. However, Line 89-08 and the coincident higher-frequency MCS Line ODP-7 (Fig. F3), both ~3 km north of Line 85-02, show that there is significant sediment deformation occurring seaward of this large frontal ridge. Sediment folds peak at shotpoint (SP) 200 and SP 320 along MCS Line ODP-7 (Fig. F3B). For the outer fold at SP 200, the magnitude of folding is significantly smaller above 3.6 s than below 3.6 s TWT. This is also associated with sediment onlap from SP 170 to SP 320 above a TWT of 3.6 s, and the decreasing fold magnitude toward the surface indicates deformation synchronous with sedimentation. Both landward- and seaward-dipping thrust faults occur in this structure, and a clear pop-up feature occurs between the two major faults.

Amplitude blanking or weakening occurs at two locations: in the frontal part of the structure around common depth point (CDP) 330, where deformation is greatest, and on the portion of the structure landward of CDP 680 (Fig. F3A). The exact mechanism for blanking is not clear. Presumably it is associated with some type of penetrative deformation, as proposed by Davis and Hyndman (1989) as an explanation for the rapid landward loss of reflectivity for accreted sediments.

The ODP-7 seismic section (Fig. F3B) captures the image of the frontal fold in the process of formation. We note that the nature of this process varies laterally along the margin. Just 2.5 km south of ODP-7, Line ODP-2 shows little deformation in the incoming sediments seaward of the primary frontal fold, except for incipient folding or faulting. In contrast, along Line ODP-3 just 2.5 km north of ODP-7, the structure has emerged from the seafloor to form the most recent frontal ridge with a height of almost 200 m above the deep ocean basin.

Site U1326

Site U1326 is located at the crest of the first major deformation ridge along the drilling transect and so represents a site with significant uplift. MCS seismic data across the ridge are shown in Figure F4. Velocity analyses of the 1989 MCS data show that the seismic velocities beneath the ridge are nearly the same as velocities in the deep ocean basin at equivalent depths (Yuan et al., 1994). The ridge sediments may therefore be normally compacted. However, velocities in the ocean basin sediments do increase significantly just seaward of the deformation front.

The lower-frequency MCS data (Fig. F4A) clearly image the BSR beneath the crest and on the landward side of the ridge. There is very little reflectivity on the landward side of the ridge, except for very shallow reflectors and a short BSR segment. A BSR is not very evident on the higher-frequency seismic data (Fig. F4B). The weaker BSR at high frequencies indicates that the BSR is not a sharp interface, but rather a velocity gradient zone with a thickness of several meters (Spence et al., 2000; Chapman et al. 2002).

The BSR is well-imaged below the ridge toward the northwest of Line 89-08, for both single-channel and multichannel high-frequency data (Fig. F5). Site U1326 is located 1.4 km northwest of the intersection of Line CAS-03 with Line 89-08. Along Line CAS-03, sediment reflectors dip in both directions away from the region of Site U1326, so there is a general structural culmination in this region. A series of prominent scarps are present at the seafloor; detailed swath bathymetry shows that the scarps are oriented perpendicular to the margin. The scarps are interpreted as the seafloor expression of normal faults dipping to the northwest (Fig. F5B). The faults apparently accommodate slumping or extension parallel to the margin and perpendicular to the direction of compressive stresses responsible for the uplift of the ridge. Since the faults break the seafloor, fault activity has likely occurred recently.

The recently collected swath bathymetry also shows a major slump feature on the steep seaward side of the ridge, just south of the intersection of Lines CAS-03 and 89-08. Drilling on the ridge was originally planned at this intersection point, but with the availability of the new swath bathymetry data, Site U1326 was shifted to the alternate approved location.

Site U1325

Site U1325 is situated in a slope basin that developed between two large ridges of accreted sediments (Figs. F6, F7, F8). As seen in the 3.5 kHz data (Fig. F6), the basin floor is covered with a thin layer of transparent Holocene sediments, ~5–7 m thick. This contrasts with Site U1326 on the frontal ridge, where accreted sediments occur at the seafloor.

Slope basin sediments lie above a buried ridge of accreted sediments, which is located almost in the center of the first slope basin. The slope sediments are highly reflective and continuous. These sediments have only minor deformation in a direction perpendicular to the margin (Fig. F7), although they are somewhat more deformed in a direction parallel to the margin (Fig. F8). The accreted sediments are highly deformed. Their reflectivity and continuity are much reduced relative to the slope basin sediments, and reflection strength is much lower on the high-frequency MCS section (Fig. F7B) relative to the low-frequency section (Fig. F7A). Similarly, the BSR is much less prominent on the high-frequency data, whereas it is very clearly seen on the low-frequency section, mainly within the core of the accreted sediment ridge. A wipe-out zone, or a zone of weakened or chaotic reflectivity, occurs above the crest of the buried ridge; it is particularly evident on high-frequency Line ODP-7 (CDP 7450 in Fig. F7B). The wipe-out zone is also seen in the coincident 3.5 kHz data, although it does not extend quite as high as the seafloor. Site selection was guided by the requirement to avoid this wipe-out zone and the crest of the buried ridge, and so Site U1325 was positioned ~650 m from the top of the ridge toward the northeast along Line ODP-7. At this position, the drill hole penetrates a weak segment of BSR (Fig. F7A) near the northeast limit of its occurrence.

Site U1327

Site U1327 is located along Inline 38 of the pseudo-3-D MCS survey conducted in 1999. Situated only 250 m from the original ODP Hole 889A, the site was selected to provide a tie to the previous drilling so that the previous results could be validated and subsequently extended using the new tools and techniques developed since 1992. In this region, two topographic highs rise ~200 m above the surrounding seafloor. The topographic highs are composed of accreted sediments, whereas the area between the highs forms a 250 m deep trough filled with slope basin sediments. The 3.5 kHz data (Fig. F9) along Inline 38 show that the thin transparent layer of Holocene sediments is absent near the drill site, although it is found ~3 km to the northwest.

The boundary between the accreted and slope sediments is the most prominent feature in the MCS data. The slope sediments are highly reflective and coherent (Fig. F10), and structures within the sediments indicate that deformation and uplift in the region likely occurred during the period that the slope basin sediments were deposited. At Site 889, the uppermost sediments from the seafloor to 87 meters below seafloor (mbsf) comprise mostly clayey silts and silty clays with interbedded thin sand layers (Westbrook, Carson, Musgrave, et al., 1994). With an age from the Holocene to ~450 ka, this unit is interpreted to represent little-deformed slope basin turbidites and pelagics. The lower units, which are seismically incoherent (Fig. F10), mainly consist of clayey silt with a low abundance of sand layers. They are pervasively fractured and are interpreted as typical abyssal plain sediments that were heavily deformed during the accretion process.

The drill sites were positioned so that they intersect a strong BSR. The BSR is well developed in this region, as it is in most accreted sediment sections. Reflection coefficients for the BSR were calculated by comparison to amplitudes for the seafloor reflection, which were scaled to a reflection coefficient using the seafloor primary-to-bubble ratio (Warner, 1990). At Site U1327, the BSR reflection coefficient is ~0.11 (Fig. F11A). To the south in the slope basin, the BSR reflection strength reaches a value of ~0.24 in a crescent-shaped bright spot. The BSR bright spot follows a trough in the boundary surface between the slope sediment and the accreted sediments, as mapped from the pseudo-3-D MCS lines (Fig. F11B). This suggests a structural or tectonic control for the accumulation of gas and/or hydrate within the trough.

Site U1329

As the most landward of the sites in the transect, Site U1329 represents the end-member of gas hydrate formation in an accretionary prism. Near Site U1329, the BSR can be identified as far upslope as CDP 6400 on MCS Line ODP-1 (Fig. F12B). The BSR is also clearly imaged on SCS data collected in 2004 in the northwest–southeast direction parallel to the shelf edge (Fig. F13). On the lower-frequency MCS Line 89-08 (Fig. F12A), the BSR cannot be clearly identified at the drill site, unlike the confident identification on Line ODP-7. Line 89-08, however, shows the more landward structures clearly: strong, near flat-lying sediment reflectors beneath the outer continental shelf (northeast of CDP 3800) increase in dip seaward and become subparallel to the seafloor beneath the steep continental slope.

Near the base of the steep slope section just seaward of Site U1329 (CDP 5600-6000 on Line ODP-1 in Fig. F12B), the seafloor is discontinuous and highly disrupted. This probably corresponds to a region of slump accumulation produced by failure along the steep slope. On the 3.5 kHz data (Fig. F14), the irregular reflectivity at the seafloor contrasts strongly with the normal sedimentation patterns just seaward, where the 5 m thick transparent layer of Holocene sediments drapes over older sediments. Although Site U1329 is located just upslope of this disrupted zone, the site was probably swept by the slumping sediments and the thin transparent layer was removed.

Site U1328

Site U1328 is located within a cold vent, referred to as Bullseye vent, which is part of a larger cold vent field, ~4 km x 2 km in dimension. Vents are identified by the presence of subvertical zones of reduced seismic amplitudes (Figs. F15, F16, F17). These blank zones have been observed over a frequency range from 20 Hz to 4 kHz, and the degree of blanking increases with seismic frequency (Riedel et al., 2002). On the 3.5 kHz data (Fig. F15), the blank zones are seen to extend upward to near the seafloor; at Bullseye vent, a seafloor mound, ~6 m in height, is also observed. The blank zones range from 80 m to several hundred meters in diameter and are typically elongated in the east–west direction. They are associated with near-surface faults and are bound by high-amplitude rims, which in the case of Bullseye appears as a circular feature ~500 m in diameter (see Fig. F11 in Riedel et al., this volume). The rims are artifacts of seismic diffractions that occur at the edges of the blank zones and enhance regular reflectivity by constructive interference (Riedel, 2001).

The blank zones extend through the entire sediment column to about BSR depth. As discussed in Riedel et al. (this volume), there is no evidence for large velocity anomalies within the bank zone. There is no significant velocity pull-up or pull-down and no anomalous velocities from semblance analyses or tomographic inversions (Zykov and Chapman, 2005). Although the BSR in the area of the cold vent is generally weak, sediment reflections are greatly enhanced above and below the BSR to the south and west of Site U1328 (Fig. F16). This feature forms a seismic bright spot for the BSR, as discussed above and shown in Figure F11A. With reflection coefficients as high as 0.3, the high amplitudes indicate either the existence of high concentrations of free gas below the BSR or high gas hydrate concentrations above the BSR. For safety reasons, Site U1328 was positioned outside of the bright spot but as close as possible to it.

Reflection coefficients for the seafloor show a strong correlation with the location of seismic blanking for Bullseye vent. As seen in Figure F17A, the seafloor reflection coefficients are ~0.1 in the circular portion of the blanking zone, which is identified in a time slice of instantaneous amplitude at a depth of 1.752 s (Fig. F17B). Reflectivity within Bullseye vent is less than half of that outside the vent. Presumably, the cause of the reduced reflectivity is produced by reduced near-surface velocities or densities associated with gas released into the sediments from the hydrate cap. Physical property measurements on piston cores, however, show no reduced velocity or density for sediments inside the vent relative to those outside the vent (Riedel et al., 2006).

Within Bullseye vent, a shallow dome-shaped reflector is observed at ~6–11 mbsf (Fig. F17B, F17C). Massive gas hydrate was recovered in piston cores at a depth as shallow as 0.5 mbsf in the center of the vent and dome reflector (Fig. F17B), which supports the interpretation that the reflector represents the cap of a layer of massive hydrate. The thickness of this hydrate layer is not known from the seismic data. Site U1328 was positioned along MCS Inline 27 (Fig. F16A) so that it intersects the hydrate cap reflector near its minimum depth of 6 m.

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