The drilling strategy for Hole 801C, an ODP legacy hole drilled during Leg 129 (Lancelot, Larson, et al., 1990), was to reenter and deepen the hole by an additional 250 m (to a maximum of 400 m total basement depth) past the upper oxidative alteration zone of the basaltic crust (Fig. F9; Table T2). During Leg 129 only 63 m of the normal tholeiites was cored. Based on data from Hole 504B and other basement sites with sufficient penetration, the upper oxidative zone of alteration, which contains important element budgets (e.g., K, B), lies in the upper 200-300 m of the basaltic crust (Alt et al., 1986; Staudigel et al., 1995). The transition from volcanics to sheeted dikes may not lie much deeper: 500-600 m in Hole 504B (Detrick et al., 1994); 450 m to Layer 2B (Carbotte et al., 1997); and only a few hundred meters at Hess Deep (Francheteau et al., 1992). By drilling >250 m, the expectation was that we would penetrate the oxidation zone, as well as a significant portion of Layer 2A, and possibly into the dikes.
The drilling strategy at Site 1149 was to core the entire sedimentary section, inferred to be 400-600 m thick, and as far into the upper oxidative alteration zone of the basaltic basement as possible, to a maximum of 430 m (Fig. F9; Table T2). Previous drilling hundreds of kilometers to the east had failed to penetrate successfully through resistant cherts, so most of the sediment column was still unsampled.