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PROPOSED SITES

Site MAF-4B
Proposed Site MAF-4B is located on the summit of South Chamorro Seamount in a water depth of 2930 meters below sea level (mbsl) ~70 nmi east of Guam in the western Pacific Ocean. It lies 85 km from the trench where the depth to slab is -26 km, based on Isacks and Barazangi (1977). Side-scan surveys of this seamount show that the southeastern sector of the edifice has collapsed and debris flows of serpentine material (dredged in 1981 and observed on Shinkai 6500 dives in 1995) blanket the inner slope of the trench from the summit of the seamount to the trench axis. The summit knoll sits at the apex of the sector collapse and its formation was most likely initiated in response to the collapse related activity. Submersible observations show the knoll's surface is broken into uplifted slabs of partially cemented serpentine mud (Fryer, 1996) separated by meter deep fissures with cross-cutting orientations. Medium blue-green to dark blue serpentine mud and clasts of metamorphosed rocks are exposed. Seeps in the fissures support a vigorous biological community of mussels, gastropods, worm tubes, and galatheid crabs (Fryer and Mottl, 1992). The mussels are likely of the genus Bathymodiolus, a genus that contains methylotrophic symbionts in their gills and requires high concentrations of methane in the fluids on which they feed (K. Fujikura, pers. comm., 1996). The pore fluid compositions and the presence of reduced materials of a vigorous biological community at the surface suggest the summit knoll is a currently active seep region. The interior of the seamount shows little structure.

It is likely that this seamount is an active serpentine mud volcano similar to Conical Seamount (Fryer et al., in press) and would provide an excellent drill target for studies of the active processes of these mud volcanoes. It has the strongest slab-signature in pore fluids from among the seamounts sampled in 1997 (Wheat and Mottl, submitted) and is second only to Conical Seamount in strength of slab signal.

Site WP-1B
Proposed Site WP-1B is situated on flat seafloor at a water depth of 5640 mbsl ~100 km west of the Kyushu-Palau Ridge axis along the eastern edge of the west Philippine Basin. The site lies on Chron 21, suggesting a basement age of 49 Ma. The sediment section is predicted to be ~400 to 450 m thick based on recent seismic reflection surveys showing a two-way traveltime to basement of 0.5 s. Drilling at other sites in the region on DSDP Legs 31 and 59 (Karig, Ingle, et al., 1975; Kroenke, Scott, et al., 1980) recovered a relatively barren, deep-water section dominated by Holocene to Eocene/Paleocene(?) brown pelagic silty clays overlying basement near the Oki-Daito Ridge (DSDP Sites 294/295). At DSDP Sites 290 and 447 to the south, the section consists of a barren interval of Pliocene clays underlain by Oligocene nannofossil-bearing silty clays mixed with ash. This was underlain by a thick section of polymict and volcanic breccias presumably derived from the Kyushu-Palau Ridge. The underlying basement consists of 80% basalt pillows and 20% dolerite. Because Site WP-1B lies in a similar setting at the foot of the Kyushu-Palau Ridge, it is likely that the section at the proposed site will be similar to that at Sites 290 and 447.

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