Leg 179 summary

Abstract

Ocean Drilling Program (ODP) Leg 179 set out with two primary objectives. These objectives were (1) testing the recently developed hammer drill-in casing system on the Atlantis Bank, Southwest Indian Ridge (SWIR), and (2) drilling a cased reentry hole into basaltic basement on the Ninetyeast Ridge to allow future installation of an ocean floor geophysical observatory, the Ninetyeast Ridge Observatory (NERO). Both of these objectives were accomplished during Leg 179. Because of contingencies developed during the leg, certain ancillary objectives were not accomplished, but one contingency drilling site was cored on the Atlantis Bank, SWIR. Hammer drill testing was completed on the Atlantis Bank at Sites 1104 and 1106. These tests provide a wealth of information regarding the viability of the system as a new tool for investigating the solid earth. Although the testing was not completed as planned during Leg 179, design changes as a result of tests that were completed are already under way. Hole 1105A penetrated to a depth of 158 m on the Atlantis Bank, 118.43 m of gabbroic rock representing 82.8% of the cored section was recovered. Gabbro, olivine gabbro, oxide gabbro, and felsic rocks were recovered. Together with logging results that included Formation MicroScanner images of the borehole, this recovery provides a rather complete coverage of the rock types and a comprehensive view of pseudostratigraphy in the gabbroic section that appears related to nearby Hole 735B, 1.2 km away. Second primary objective specifically included drilling a single hole as deep as possible into basement, and installing a reentry cone and casing beyond basement to prepare the Ninetyeast Ridge site as an ocean-bottom observatory