ABSTRACT
Ocean Drilling Program 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, 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 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, Southwest Indian Ridge.
Hammer drill testing was completed at Sites 1104 and 1106 (virtually coincident locations but positioned from different beacons after a hiatus in testing while waiting for resupply). 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, design changes as a result of tests we were able to complete are already under way. We are confident, based on the results of the first sea trial of the hammer drill-in casing system, that continued development will soon allow us to sample the deep ocean crust in environments where, with rare exceptions, we have historically been unable to operate.
Hole 1105A penetrated to a depth of 158 m and cored an interval of 143 m starting 15 mbsf on the Atlantis Bank. We recovered 118.43 m of gabbroic rock representing 82.8% of the cored section. 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, 1.3 km away. This is the first example of two offset holes (Holes 735B and 1105A) in an oceanic gabbroic section in which shipboard results indicate a high probability that specific units, structures, and/or geophysical characteristics may indeed be correlated.
Our 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. The observatory will be installed at a later date and will be part of the future network of seafloor observatories proposed in the International Ocean Network program. Drilling reached a depth of 493.8 mbsf, which was sufficient to create an acceptable borehole below casing for the downhole seismometer installation. The total penetration into basement was 122 m, and total penetration below casing reached 79.4 m. This significant depth of penetration below the casing, as well as the firm attachment to basement should isolate the instrument from noise reported from other ocean floor seismometer installations.