During Leg 203, we accomplished our stated goals: to establish a cased legacy hole with ~100 m basement penetration and to obtain cores and logs representative of the section from the sediment/basement interface to the bottom of the hole. This was accomplished despite the shortening of an already modest operational component of the leg by the decision to reroute the ship to Victoria, British Columbia, Canada, rather than San Francisco, California, and despite two significant technical challenges. The first of these stemmed from the failure to seat 16-in casing into a hole of 18 1/2-in bore. This required us to adapt our operations plans rapidly and led us to complete Hole 1243A by drilling with an 18 1/2-in rotary bit without coring. We then cemented 10 3/4-in casing in place to minimize risk to the integrity of the hole. To do so required the largest cementing job yet attempted by ODP into a formation that proved, near the bottom of the hole, to be porous and fractured.
That this primary objective was achieved so rapidly in the face of this technical challenge is a tribute to the responsiveness and skills of the operations team and rig crew. The decision to case Hole 1243A was made in tandem with the decision to follow cementing of the casing with a trip out and relocation of the ship 600 m east to a location shown in the Leg 203 seismic survey to be equivalent to Hole 1243A in terms of sediment cover and seismic structure. We jetted in at this location (Hole 1243B) to just above the sediment/basement interface and carried out RCB coring through 85 m of basement, achieving a recovery rate of 25% in 10- to 12-Ma mildly altered pillow basalt. This was followed by logging, using triple combo and FMS-sonic tool strings, and by a WST in a VSP configuration. Data were collected successfully. In the case of the first two tool strings, multiple trips to authenticate measurements were possible. The ship was relocated to Hole 1243A, and logs were obtained to confirm that the inclination of the hole was within tolerances for the future seismic package and that the cement bond quality was acceptable. Both conditions were found to be the case.
The rocks recovered from Hole 1243B were largely from pillow basalts and comprised both aphyric and sparsely plagioclase and olivine phyric basalts. Eight lithologic (basement) units were identified, of which seven were igneous and one (lithologic Unit 2) was represented by a single piece of limestone. There was no evidence of thicker massive lava flows in the material recovered or in the log analyses. All igneous units are tholeiitic with the exception of lithologic Unit 4, which consists of alkali basalt. The compressional seismic velocities measured in the samples were high, with a mean of 5.26 km/s. Although the sonic log and VSP velocities were lower, they were consistent with increasing integration of cracks and joints in the increasing wavelengths of the techniques applied. Paleomagnetic measurements indicate that the basaltic cores recovered from Hole 1243B, after the removal of the drilling-induced remagnetization, recorded a stable component of magnetization with both normal and possibly reversed inclinations.
Drilling in Hole 1243A met the objectives of Leg 203. We have a complete cased and cemented legacy hole penetrating nearly 100 m of basement for the installation of broadband seismometers in a future observatory. We have accomplished this in an area of considerable interest to other disciplines in the earth and ocean sciences, with the prospect of providing the infrastructure for a future DEOS multidisciplinary observatory. In addition, we were able to recover basalts in the upper oceanic crust in fast-spreading, young lithosphere in the Pacific, well in excess of the depth drilled during most previous legs.
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