|IODP Proceedings Volume contents Search|
|Expedition reports Research results Supplementary material Drilling maps Expedition bibliography|
Expedition 311 Scientists2
Site U1329 (proposed Site CAS-05D; Collett et al., 2005) is at the easternmost end of the southwest-northeast-trending, margin-perpendicular transect of sites drilled during Integrated Ocean Drilling Program (IODP) Expedition 311 and is located closest to shore (65 km) at a water depth of 946 m. Newly acquired bathymetry data showed that this site is located in an area characterized by multiple canyons of various sizes where water depth decreases rapidly from 500 to 1300 m over a distance of only 7.5 km with an average slope of 6° (Figs. F3 and F4 in the "Expedition 311 summary" chapter). The canyons appear to form scours that funnel sediment into the deeper basin. At the foot of the steep slope at a water depth of ~1300 m, the seafloor flattens abruptly into a plateau and some of the canyons continue across the plateau and die out in the slightly deeper midslope basin. Some of the slope canyons also appear to be connected to a sediment transport system further upslope as seen in Figure F4 in the "Expedition 311 summary" chapter. Shallow sedimentation at Site U1329 is therefore expected to be dominated by turbidite sequences but could also show signs of more recent erosion.
A map of all the site survey seismic data acquired in preparation for this expedition in the area of Site U1329 is shown in Figure F1. High-resolution 3.5 kHz seismic data do not indicate the presence of the typical Holocene transparent layer described at the other site locations along the transect (Fig. F2). Seismic data do not generally reveal an obvious bottom-simulating reflector (BSR) at this site. The BSR is not detectable on multichannel seismic (MCS) Line 89-08 (Fig. F3); however, a faint BSR-like reflection can be identified along MCS Line PGC9902_ODP-1, but it is difficult to determine the polarity of this reflector (Fig. F4). In the perpendicular direction relative to the previous two lines, a better image of the BSR can be seen on Line PGC0408_CAS05_line03-04 (Fig. F5). Along this line, a clear reflector with polarity opposite to the seafloor is identified at a depth of 154 ms two-way traveltime (TWT). Using the vertical seismic profile (VSP) data from Ocean Drilling Program (ODP) Leg 146 Site 889, an average interval velocity of 1636 m/s was determined for the sediments between the seafloor and the BSR. This average velocity was determined to match observed BSR depths in MCS data with the depth of the BSR in the VSP data. Using this average velocity, the BSR at Site U1329 is ~126 meters below seafloor (mbsf). Using different velocity profiles representing higher velocities (i.e., more gas hydrate) or lower velocities (i.e., less gas hydrate) at depth yielded an error of about ±2.5 m in the BSR depth estimate.
MCS Line 89-08 (low-frequency content <50 Hz) is characterized by strong reflectivity in the sediments below 140 ms TWT (~115 mbsf using the average velocity of 1636 m/s). The frequencies of seismic reflections beneath this depth appear much lower than above, which may indicate the presence of free gas. The top of this high-amplitude, low-frequency horizon forms an erosional unconformity that deepens towards the northeast along MCS Line 89-08. Seismic reflectors above this unconformity are mainly seafloor parallel and form onlaps, whereas underneath the unconformity no clear orientation can be identified.
The objectives of coring and logging this site are tied to completing the transect of scientific drill sites across the northern Cascadia margin. This site is the shallowest and most landward location at the eastward limit of gas hydrate occurrence on this margin. The depth to the BSR rapidly becomes shallower at this site and is only at half the depth compared to Site U1327.
At this eastern end-member site of gas hydrate evolution in the accretionary prism, the objectives include
The operational plan to achieve these objectives was based on a general three-hole concept, which included
Maximum allowable depth for the LWD/MWD logging hole at this site is 270 mbsf. Anticipated total depth for the continuously cored hole is 220 mbsf, with the BSR at 126 mbsf. Wireline logging and tools holes are planned to a total depth of 270 mbsf.