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doi:10.2204/iodp.sp.320321.2008

Risks and contingency

One possible operational problem that could be encountered is the presence of chert layers. Although chert is not generally a problem for Neogene sites in the Pacific, drilling and recovery difficulties related to chert layers are a necessary consideration when drilling Paleogene sediments. Cherts have been found frequently in middle and lower Eocene equatorial Pacific sediments. Although early DSDP coring was generally defeated by chert, technological improvements during ODP have somewhat improved the drilling of chert layers. However, this remains a challenge.

Previous legs have shown the general occurrence of cherts in certain parts of the Eocene Pacific. Prior to ODP Legs 198 and 199, the general belief was that sediment that has never been deeply buried, like that found at the sites proposed here, are likely to contain only oozes. However, Leg 199 drilling results indicate that cherts are present even with shallow burial, particularly at the boundary between the lower and middle Eocene. These sediments were still unlithified, as radiolarian ooze was recovered right up to the upper boundary of chert zones and occasionally below the chertified interval.

Understanding the depth and thickness of chert zones will help to minimize sediment loss. Although it is unlikely that the sites in this proposal will avoid chert altogether, the specific planning of this program aims for calcium carbonate (with higher sedimentation rates than chert-bearing radiolarian ooze), as well as a lower abundance of silica-rich formations. One possibility to address recovery problems around the chert layers would be to shoot the APC barrel after offsetting the drill bit by 4–5 m above the bottom of the hole, thus aiming for a half-stroke core (4–5 m). A potential problem with this technique is penetrating the hole obliquely, thus recovering the same sedimentary sequence multiple times, as well as risking bent core barrels. For this reason, expensive nonmagnetic core barrels will not be used for half-stroked cores. Decisions on how to mitigate poor core recovery due to cherts will have to be made during the expedition by considering all of the actual operational factors and risks involved.

During Leg 199, euhedral dolomite often occurred within the chalks just above basement, which we think is linked to Mg-rich fluids from the basement. Because dolomitization resets many of the important geochemical systems used by paleoceanographers, we have chosen sites with basement ages slightly older than the target age. Thus, the target carbonate intervals are not in direct contact with basement rocks. We thus trade off between maximum recovery of target carbonates and avoiding a diagenesis problem.

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