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The newly installed digital seismic system on the Glomar Challenger was used to collect data in the Mariana Basin (Site 585), in the Nauru Basin (Site 462), and on the Ontong-Java Plateau (Site 586). The drilling results from Sites 585 and 462 correlate with the seismic data of the basins, allowing multidimensional interpretation of their histories since the middle of the Cretaceous. On the Ontong-Java Plateau, the data suggest the activity of erosional processes. INTRODUCTION Leg 89 was the first cruise of the Glomar Challenger to collect digital seismic data. The computer system and related seismic interfaces were installed on the ship while it was in dry dock in Yokohama, Japan, immediately before the cruise. First tests of any new system can result in failure and frustration, or in success; the tests of this system on Leg 89 yielded all of these. Ultimately, nearly 64 hrs. of data were successfully collected, adding to the existing coverage of the western Pacific. This chapter presents some of the seismic data collected during Leg 89. SEISMIC SYSTEM A Hewlett-Packard 1000 Series E computer was installed on the Glomar Challenger with a seismic interface card designed by the Marine Technology Group at Scripps Institution of Oceanography. Both air guns and a water gun were used as sound sources for this system. Single-channel digital seismic data were recorded in real time on magnetic tapes, using SEG-Y format. In this format, all the data from a single seismic shot are recorded as a single physical record, preceded by additional information in the trace header. The sample rate was 1 kHz. An antialias filter of 1 ms was used, and the shot interval was 10 s. Although variable during the cruise, the speed of the ship was generally 9-10 knots while underway between sites. The software for data acquisition is compatible with the system used on Scripps Institution of Oceanography ships, thus making possible shore-based processing at the SIO campus. DATA COLLECTED Leg 89 began in Yokohama, Japan on 11 October 1982. Three sites (585, 462, and 586) were drilled, in the Mariana Basin, in the Nauru Basin, and on the OntongJava Plateau, respectively (Fig. 1). The final port was Noumea, New Caledonia, reached on 29 November. After initial start-up problems had been solved, recording began during the approach to the first site, Site 585, in the Mariana Basin. During the second portion of the cruise, data were recorded nearly continuously between Site 585 and Site 462 in the Nauru Basin. Various difficulties arose with the seismic interface, and only one additional section of data could be recorded, on 1 Moberly, R., Schlanger, S. O., et al., Init. Repts. DSDP, 89: Washington (U.S. Govt. Printing Office). 2 Address: Geologic Research Division, A-008, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093. the Ontong-Java Plateau southeast of Site 586. Figure 1 and Table 1 indicate the portions of the cruise where data were recorded. The data were processed on the Scripps PRIME computer, using SIOSEIS, a processing and plotting program written by Paul Henkart (copyright 1981). Various balances of filtering, automatic gain control (AGC), and deconvolution were applied to each of the sections of data to optimize the display. In addition, effort was made to show the range of options available with SIOSEIS. Mariana Basin A north-south seismic profile through the Mariana Basin was collected during the approach to and departure from Site 585 (Fig. 2 and 3). Two air guns (60 and 120 in.) were used in this basin, at water depths of about 6100 m. These data complement an east-west profile collected in the Mariana Basin in the fall of 1981 by the Katia Keoki (Shipley et al., 1983; Petersen et al., this volume), which used two 80-in. water guns as the sound source (Fig. 4). Together, these two profiles afford a three-dimensional view of the basin. Drilling at Site 585 recovered a 900-m section composed almost entirely of reworked and redeposited material (Site 585 Chapter, this volume; see summary in Fig. 4). In the airgun seismic sections, reflectors in the interval between 8.10 and 8.74 s are laterally continuous and can be traced across the entire basin to onlap the seamounts to the north and south (Figs. 2 and 3). This portion of the section is correlated with the fine-grained pelagic turbidites (lithologic Units I-V), which become finer-grained upward in the stratigraphic section at this site. Distinct reflectors at 8.4 and 8.6 s are interpreted as the chert horizons recovered in lithologic Units II and IV. The top of the volcaniclastic sediments at 590 m subbottom (the boundary between Units V and VI) is correlated with a continuous, high-amplitude reflector at 8.74 s. This reflector can also be traced to onlap the volcanic highs around the basin. Below the reflector, the sedimentary section is composed of coarse-grained proximal turbidites derived from the constructional phases of the seamounts. The major change in lithology occurs at the transition between acoustic Units V3 and V4, and

Seismic Stratigraphic Sections: East Mariana Basin (Site 585), Nauru Basin (Site 462), and Ontong-Java Plateau (Site 586)