PROPOSED DRILL SITESThe Pollution Prevention and Safety Panel has endorsed four primary sites (Sites OJ-3B, OJ-6B, OJ-7D, and OJ-11C) and three alternates (Site OJ-3C, OJ-6C, and OJ-7E).
Site OJ-3B (OJ-3C, alternate). This site is located near the shallowest part of the high plateau, where the depth to the top of Layer 3A is also the greatest. This region is where the original basement surface of the high plateau was probably shallowest, and also was likely the area of greatest eruptive activity, especially during the 122-Ma period of basement construction. As such, this part of the plateau is where the widest compositional range of basement lava types may be found. For example, at DSDP Site 289, some 200 km northeast of this site, an ash layer containing about 50% K-feldspar was cored just above basement (Andrews, Packham, et al., 1975). A sediment package above basement at Site OJ-3B may include carbonate reefal deposits. Rather than the 100 m of minimum basement penetration planned for the other three sites, a minimum penetration of 150 m is planned at this site.
Site OJ-6B (OJ-6C, alternate). This site is located near the crest of the large eastern lobe of the plateau, which has not been sampled previously. By analogy with Iceland, this lobe could represent a feature like the Reykjanes Ridge. Alternatively, it could be the trace of the plume tail following the emplacement of the high plateau and, specifically, may be the main locus of 90-Ma eruptions. Like the domal region of the high plateau, this area originally may have been at relatively shallow water depths.
Site OJ-7D (OJ-7E alternate). The site sits atop the Stewart Arch between Malaita and DSDP Site 288 (which did not reach basement); this part of the plateau may have been rifted from the eastern salient by spreading in the Stewart Basin following the 90-Ma eruptive event. Basement lavas here may be either 90 or 122 Ma.
Site OJ-11C. Seismic reflection data indicate that a highly reflective carapace of lavas (or possibly sills) may overlie deeper basement in this area. The 26 m of lavas penetrated at ODP Site 803 belong to the 90-Ma eruptive episode, and we postulate that the carapace is also 90 Ma. If so, drilling 100 m into basement here will provide a more complete section of 90 Ma lavas and may penetrate completely through the carapace.
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