DRILLING STRATEGY
The OJP is the world's largest LIP. Despite the considerable geodynamic significance of LIPs, relatively little is known about the composition and origin of large oceanic plateaus, and the primary objective of Leg 192 is to address these questions by recovering rock samples from the basaltic basement of the OJP. The four drilling sites (Figs. 1 and 6) have been selected to maximize the scientific return by sampling as much of the history of the plateau as possible. The first-priority site (Site OJ- 3) is located on the crest of the main part of the plateau. The second priority site (Site OJ-6) is located on the crest of the eastern salient. Next in order of priority are Sites OJ-11, where we aim to sample basalt emplaced on the eastern flank of the main plateau, and OJ-7, where we will sample the southern portion of the eastern salient, which appears to have rifted away from the main part.The specific objectives at each site are similar. Basalt recovery is the priority objective at all sites because we need samples to address the primary questions of the age of the plateau and the composition and temperature of the mantle source. Good basaltic core recovery will also allow us to assess the character and mode of emplacement of the lava flows and to address the important questions of whether volcanism was submarine or subaerial and how far from the eruption site the flows were emplaced. Basement logging at two sites (Sites OJ-3 and OJ-7) will provide additional information on lava-flow morphology and stratigraphy. The planned basement penetration (150 m at Site OJ-3 and 100 m at the other sites) is the minimum needed to yield sufficient flow units for conclusions to be statistically robust. However, sampling basalt alone will not provide all the answers. The age of the sediments immediately overlying basement, the environment of deposition, and the early subsidence history of the OJP are all important for understanding the origin and environmental impact of oceanic LIPs. We therefore plan to core a significant proportion of the sedimentary cover. If all goes well, we should recover enough igneous and sedimentary material to allow us to address all of our objectives. If, however, drilling problems put us behind schedule, we may have to start coring sediment at greater depths than planned and reduce basement logging to achieve our primary goal of basement penetration. In the event that drilling proceeds faster than planned, we shall use extra time for additional coring and logging of the igneous basement.
Leg 192 is scheduled to start and finish in Guam. Transit to the first site (Site OJ-3) is estimated to take 4.7 days, and we plan to spend 18.3 days drilling, casing, and logging this site. Transit to Site OJ-7 should take 1.9 days, followed by 14.7 days of drilling and logging. Corresponding transit and drilling times for the other two sites are, respectively, 1.1 and 4.5 days (Site OJ-6), and 1.2 and 4.0 days (OJ-11). Sailing the 1620 nautical miles back to Guam from Site OJ-11 is estimated take 5.0 days.