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doi:10.14379/iodp.sp.352.2013 Risks and contingencyThere are a number of risks to achieving the objectives of this program. Time distributionThe main risk in scientific terms would be a failure to penetrate the oldest volcanic rocks at the lava/sheeted dike boundary. To reduce this risk, we aim to drill Site BON-1A first and, if necessary, drill deeper than planned to achieve our objective. Less time would then be spent at Site BON-2A, and any gap in the lava stratigraphy would be filled in by reference to Site 459 in the Mariana fore arc. We do not, however, believe that this risk is high. Site BON-1A is deeper or shallower in the lava sequence than expectedWe need to retain some flexibility in case lavas at the top of Site BON-1A are not at the expected position within the stratigraphic sequence. If higher in the sequence, we might expect to drill deeper at Site BON-1A and shallower at Site BON-2A. If lower in the sequence, we might expect to drill shallower at Site BON-1A and deeper at Site BON-2A. However, in the latter case, drilling rocks unexpectedly deep in the section (e.g., in lower sheeted dikes and gabbros) would also help meet Objectives 1, 2, and 3, and we would expect to drill at least to bit destruction before moving to Site BON-2A. Although very unlikely, we could penetrate an exciting feature (e.g., the Moho) that would require further deepening of the hole if the shipboard scientists support such a plan. Hole stabilityPoor hole conditions will be dealt with by using frequent high-viscosity mud sweeps and or heavy mud to condition the holes. In addition, each deep hole will be cased down to the sediment/basement interface to improve hole stability. In the case of serious technological problems, however, it may be necessary to relocate a site within the same area in order to achieve the required penetration. For this purpose, a series of contingency sites have been identified close to Sites BON-1A and BON-2A (see “Site summaries;” Table T2).
Alternate Site 459 (Mariana fore arc)Sites BON-1A and BON-2A are ideally located, in that they provide the opportunity to spud into sediment and drill the full volcanic sequence in any area that has been well sampled and surveyed. Given the success at Sites 458 and 459 and Hole 786B with 1980s technology, it is unlikely that an alternate site will be needed. However, as noted above, we are fortunate that deeper drilling at Site 459 (Fig. F15) provides an alternate to the Bonin fore arc and that the site already has crossing MCS lines. Having been successfully drilled already, new adjacent holes could be drilled to the depth of the existing holes, cased, and then cored. Note that Site 458 has already cored the transition between boninites and FAB, so only one alternate site is needed. This could be useful in the event that we are unsuccessful at drilling in the Bonin region or for certain geological reasons, for example if Site BON-1A does not provide the deep penetration of the lavas sequence that we hope for. Weather conditionsAugust and September have a high (20%) typhoon risk and may result in suspension of operations for as many as a few days. |
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