6. Site 1186¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

The prospectus for Leg 192 included a primary site on Stewart Arch within the territorial waters of the Solomon Islands (proposed Site OJ-7; see Fig. F1). However, the Ocean Drilling Program (ODP) was unable to obtain clearance for this site, partly because the Solomons were in a state of civil and political upheaval throughout much of 2000. By midcruise, it was evident that clearance would not be forthcoming in time (if at all) to drill the site. We therefore chose to drill Site 1186, on the eastern slope of the main Ontong Java Plateau 206 km west of Site 1185, 319 km east of Site 1183, and 149 km east-southeast of Deep Sea Drilling Project (DSDP) Site 289. The very different volcanic stratigraphy at Sites 1183 and 1185, particularly our discovery of high-MgO basalt of probable latest Cenomanian-Albian age at Site 1185, highlighted the importance of a site at a location intermediate between the crest and eastern edge of the main plateau.

Site 1186 lies at a water depth of ~2729 m (drill pipe measurement) on multichannel seismic reflection Line 403 of Leg 2, cruise KH98-1, of the Hakuho Maru (Figs. F1, F2, F3). The seafloor is rough in places, suggesting a high-energy sedimentary environment. The sedimentary section, interpreted to lie between the seafloor (3.64 s two-way traveltime [TWT]) and the top of a high-amplitude, moderately continuous basement reflection (at 4.57 s TWT), is characterized by parallel to subparallel reflections of low to moderate continuity (Fig. F4). Reflection amplitudes in this interval are low to moderate, and frequency is medium to high. Numerous unconformities and variations in reflection amplitude, continuity, and configuration indicate that oceanographic and sedimentary conditions were occasionally vigorous in the past. The most prominent unconformity along Line 403 ascends from west-southwest to east-northeast, crossing Site 1186 at 4.28 s TWT. Changes in reflection character across this unconformity suggest that it delineates the edge of a filled channel.

Approximately 12 km east-northeast of the site, a small body rises into the sedimentary sequence ~500 m above the surrounding acoustic basement; we interpret this body to be an igneous intrusion or small volcanic cone. A package of high-amplitude and continuous reflections emanates from this body to the west-southwest (Fig. F5), decreasing in thickness and number of reflections toward Site 1186. At the site (Figs. F4, F5, F6, F7), a high-amplitude reflection at 4.37 s TWT lies near the western termination of this reflection package, which could represent a sill(s), lava flow(s), or volcaniclastic sediments originating from the igneous body; alternatively, it could represent hydrothermally altered sediments affected by heat and fluids that emanated from the igneous body.

The top of acoustic basement is characterized by a high-amplitude and relatively continuous reflection, and we observe some high-amplitude and relatively continuous intrabasement reflections (e.g., from 4.75 to 4.85 s and 5.35 to 5.45 s TWT beneath Site 1186). Preliminary results of a study of crustal structure as determined from Line 403 seismic and sonobuoy data have been presented by Mochizuki et al. (1998).

The main objectives at this site were to determine

1. Compositions of basement rocks and compare them with those of lavas sampled at Sites 1183 and 1185, DSDP Site 289, ODP Sites 803 and 807, and the islands of Santa Isabel, Malaita, and San Cristobal;
2. Age of basement rocks to establish whether basement in this region is 90 or 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 and approximate water depths);
4. Early subsidence history, as recorded in the basement rocks and the overlying sedimentary succession; and
5. Ages of sequence boundaries observed in the seismic record.