7. Site 1187¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

By the time we had penetrated ~50 m of the 65.4 m of basement cored at Site 1186, shipboard inductively coupled plasma-atomic emission spectrometry (ICP-AES) analyses had demonstrated that the basalt was of the widespread, remarkably homogeneous, ~122-Ma Kwaimbaita magma type found at Site 1183 and in the lower 92 m of basement at Site 1185. The bottom of the Kwaimbaita-type lava sequence has not been reached in any of the locations where such lavas have been encountered. For example, at Site 807 this sequence (Units C-G) is >100 m thick (Kroenke, Berger, Janecek, et al., 1991), and on the island of Malaita the Kwaimbaita Formation is >2.7 km thick (Tejada et al., in press). Furthermore, our rate of penetration in basement was low, and reentering (after an imminent drill-bit change) an uncased >900-m hole in chert-rich sediment was risky. These considerations led us to favor drilling a new site to provide fundamental new information about the age, composition, and mantle sources of the Ontong Java Plateau over deepening Site 1186. A site somewhere between Site 803 and Site 1185 (Figs. F1, F2) would be particularly useful because, unlike other sites on the main plateau, Site 803 and probably Site 1185 contain basalt that is significantly younger than 122 Ma. Also, the lava flows in the upper 125 m of basement at Site 1185 are compositionally different from any seen elsewhere on the plateau. We therefore selected Site 1187, near Site 804 (which did not reach basement) on the eastern edge of the main plateau (Fig. F3). Site 1187 is 194 km southeast of Site 803 and 146 km north of Site 1185.

Site 1187 is located at a water depth of ~3804 m (drill pipe measurement) on a single-channel seismic (SCS) reflection profile acquired on cruise TW88-11 of the Thomas Washington (Figs. F2, F3, F4, F5, F6). The location is <3 km from the easternmost point where Ontong Java Plateau basement can be distinguished easily from that of the Nauru Basin. The sedimentary section, interpreted to lie between the seafloor at 5.02 s two-way traveltime (TWT) and the top of high-amplitude, discontinuous reflections at 5.43 s TWT, is characterized by parallel to subparallel reflections of low to moderate continuity. Reflection amplitudes are low to high, and frequency is high. Unconformities and variations in reflection amplitude, continuity, and configuration within the sedimentary section indicate that oceanographic and depositional conditions in the past were occasionally vigorous. The top of acoustic basement is characterized by high-amplitude, discontinuous reflections. Some medium-amplitude and discontinuous intrabasement reflections also are apparent in the seismic-reflection record.

The main objectives at this site were to determine

1. Compositions of basement rocks in order to compare them with those of lavas at other locations where Ontong Java Plateau basement has been sampled, particularly Sites 803 and 1185 on the edge of the eastern high plateau;
2. Age of basement rocks to establish whether basement in this region is 90 Ma, 122 Ma, or some other age;
3. Physical volcanology of basement rocks and the nature of sedimentary interbeds, in order to deduce the eruptive environment (flow types, approximate water depths); and
4. The depositional environment and age of the sedimentary rocks immediately above basement.