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doi:10.2204/iodp.pr.306.2005
The (high resolution) stratigraphic goals require high sedimentation rates (>5 cm/ k.y.) at the chosen sites, as well as complete and undisturbed recovery of the stratigraphic sequence. The drilling strategy remained the same as that during Expedition 303 and consisted of APC coring in three or more holes at each site to ensure complete and undisturbed recovery of the stratigraphic section. We used the "drillover" strategy employed during ODP Leg 202 to maximize APC recovery and penetration. Traditionally, the depth limit of APC coring is controlled by the overpull required to retrieve the core barrel. In cases where the full APC stroke is achieved but excessive force is required to retrieve the core barrel (often the limit of APC penetration), the drillover strategy entails advance of the rotary bit to free the APC barrel. APC coring was generally terminated when the pressure gauge on the rig floor indicated that full APC stroke could no longer be achieved. Because of the pivotal role of magnetic studies in the objectives of the proposal, nonmagnetic core barrels were generally used. However, because of the relative fragility and high cost of nonmagnetic core barrels, the normal steel magnetic barrels were used after the initiation of drillover.
Two factors influenced the decision to terminate holes at the limit of the APC and therefore not to utilize extended core barrel (XCB) technique:
- The increase in drilling disturbance associated with the XCB, particularly in the upper part of the XCB section, has not been conducive to the generation of high-resolution PACs. Poor recovery and "biscuiting" are common in poorly consolidated lithologies recovered by XCB.
- At all locations, other than Site U1314, the deeper stratigraphic section was sampled in the region during DSDP Leg 94.