DRILLING STRATEGYDespite the considerable geodynamic significance of LIPs, relatively little is known about the composition and origin of large oceanic plateaus. The Ontong Java Plateau's enormous size and thick blanket of marine sediments constitute particularly formidable obstacles to systematic sampling of basement crust. A widely agreed-upon strategy for plateau basement sampling involves a reconnaissance phase of drilling several holes ~100 m into basement in key areas, followed by a small number of much deeper holes selected on the basis of results from reconnaissance drilling (e.g., Dick et al., 1996). On the two largest plateaus, Ontong Java and Kerguelen-Broken Ridge, the deeper drill holes would consist of one or two holes penetrating ~1 km of basement and one hole with >2 km of basement penetration. Leg 192 was designed to complete the reconnaissance phase of Ontong Java drilling that began with Sites 289, 803, and 807 and was complemented by recent field-based work in the Solomon Islands. We selected four sites that would cover as much as possible (in a 2-month cruise) of the history of major, but previously unsampled, parts of the plateau. Because basement penetration was the main objective, we planned to drill through (i.e., not core) the upper parts of the sedimentary section at all sites.
Site 1183 lies near the crest of the high plateau; Site 1184 is on the northern ridge of the eastern salient; and Site 1185 sits at the edge of the eastern flank of the high plateau, near where it adjoins the Nauru Basin (see Fig. 1). The fourth site (proposed Site OJ-7) was an eastern-salient site located atop the Stewart Arch to the north-northeast of Malaita, in waters claimed by the Solomon Islands. However, because of civil and political turmoil in the Solomons throughout much of 2000, ODP was unable to obtain approval for drilling this site before or during the cruise. At sea, guided by our results from Sites 1183 and 1185, we chose an alternative site, Site 1186. This site is roughly midway between Sites 1183 and 1185, thereby forming a four-site basement transect (including DSDP Site 289) from the crest to the eastern edge of the main plateau. After penetrating ~65.4 m of Kwaimbaita-type basalt flows at Site 1186 (i.e., the same magma type we found at Site 1183 and in the lower 92 m of basement at Site 1185; see below), we decided that the time remaining in the cruise would best be used by drilling a fifth site rather than attempting to deepen Site 1186. On the basis of our results at Site 1185, in particular, we selected Site 1187, on the extreme eastern edge of the plateau north of Site 1185 and southeast of Leg 130 Site 803. Details of each of the five sites drilled during Leg 192 are given in Table 1.
Objectives at all sites were similar. Basement penetration was the priority in order to address the primary questions of the age of the plateau and the range of composition and temperature of the mantle source. We also hoped for good recovery because it would permit assessment of the character and mode of emplacement of basement rocks and allow us to address the question of whether volcanism was submarine or subaerial. Ideally, we also would obtain some information on how far from their eruptive vents the basement rocks were emplaced. Wireline logging of basement was planned at two sites (one of which was the proposed OJ-7 site) in order to obtain additional information on lava flow morphology and volcanic stratigraphy. We planned to core much of the lower portion of the sedimentary cover at each site because the age of the sedimentary rocks immediately overlying basement, their environment of deposition, and the early crustal subsidence history are all important for understanding the origin and environmental impact of the plateau. Moreover, coring of the lower sedimentary section would allow us to ascertain the ages of some of the sequence boundaries observed in the seismic reflection record.
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