Drilling Strategy | Table of Contents
PROPOSED SITES
Site WP-2A and Alternate Site WP-2B
Proposed Site WP-2A is situated on flat seafloor at a depth of 5500 m. It lies atop magnetic
lineation M8 in the Japanese lineations northwest of Shatsky Rise (Fig. 4;
Sager et al., 1999;
Nakanishi et al. 1999). The sedimentary section at Site WP-2A is predicted to be 420 m thick
based on seismic sections showing a two-way traveltime of 0.5 s to a reflector thought to represent
igneous basement (see seismic line in "Site Summary" section).
Seismic reflection profiles in the
region typically display an upper transparent layer and a strong smooth reflector. Comparison of
seismic profiles with DSDP and ODP holes in the region (Site 193 [Creager, Scholl, et al., 1973],
Sites 194-197 [Heezen, MacGregor, et al., 1973], Sites 303-304 [Larson, Moberly, et al., 1975],
Sites 578-581 [Heath, Burkle, et al., 1985; Duennebier, Stephen, Gettrust, et al., 1987], and Site
1149 [Plank, Ludden, Escutia, et al., in press]) shows that the upper transparent layer consists of
mid-Miocene to Pliocene silty radiolarian oozes and clays, with some ash layers from nearby arc
volcanoes, that grade downward to pelagic brown clays, often poorly dated because of sparse or
absent microfossils. The strong smooth reflector denotes the occurrence of chert and porcellanite
within Early Cretaceous pelagic clays. These overlie igneous basement, which is usually
recognized in hindsight as a less-prominent reflector 100-200 ms below the top of the cherty layer.
Using lithologic and seismic velocity data from nearby DSDP holes (Sites 303, 304, and 581), we
infer a sediment column at Site WP-2 consisting of an upper layer (~310 m thick) of silty
radiolarian clays and a lower layer (~110 m thick) of cherty clay. The sedimentary section at
alternate Site WP-2B (see seismic line in "Site Summary" section),
located 9 km to the southeast
of Site WP-2A, is similar in thickness.
Site SR-1 and Alternate Site SR-2
Proposed Sites SR-1 and SR-2 (alternate) are located on flat-topped basalt ridges on the southern
(TAMU Massif) and central (Ocean Research Institute [ORI] Massif) volcanic edifices of Shatsky
Rise (Fig. 4; see seismic line in "Site Summary"
section). These ridges were probably formed in
the late stages of the eruptions that formed the basaltic edifices (Sager et al., 1999). SR-1 lies at a
water depth of ~2075 m, whereas Site SR-2 is at a depth of ~3188 m. The summits of both ridges
are inferred to be sediment free using 3.5-kHz records showing a seafloor characterized by strong,
hyperbolic reflectors (see seismic lines in "Site Summary" section).
Site SR-3
Proposed Site SR-3 is tentatively located ~15-20 km east of Site SR-1. The sedimentary cover
over the oceanic plateau at Site SR-3 is ~900 m thick and consists of Late Cretaceous pelagic clays.
