3. Site 1183¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1183 is near the crest of the Ontong Java Plateau's main or high plateau (Fig. F1), 183 km west-southwest of Deep Sea Drilling Project (DSDP) Site 289. We chose this site because it is in the shallowest region of the high plateau, where the upper crust is thickest but the sediment cover is relatively thin (1130 m). This broad region is where the lava pile of the high plateau originally may have been shallowest and eruptive activity may have been the greatest. Given its central location, the compositional range of basement lavas here might be greater than in previously studied areas located much closer to the margins of the plateau (Malaita, Santa Isabel, Sites 803 and 807). Also, a distinctive sediment package appears above the acoustic basement in this vicinity that might correspond to shallower water deposits than those found elsewhere on the high plateau.

The site is located on multichannel seismic (MCS) reflection Line 404 of the Hakuho Maru cruise KH98-1, Leg 2, ~18 km east of the intersection of this line with Line 501 (Figs. F2, F3). It lies at a water depth of ~1805 m (drill pipe measurement) in a relatively simple structural setting with coherent basement and no major basement faults nearby (Figs. F4, F5). The 3.5-kHz record shows that the seafloor is rough in places, suggesting a high-energy sedimentary environment. The sedimentary section is interpreted to lie between the seafloor, at 2.4-s two-way traveltime (TWT), and the top of a high-amplitude, continuous reflection at 3.4-s TWT; this section is characterized by parallel to subparallel reflections of high continuity (Figs. F4, F5). Reflection amplitudes are medium to high, and frequency is high. An upper sedimentary megasequence (2.4-3.2 s TWT) appears to be mainly a pelagic drape that exhibits some differential compaction. A lower sedimentary sequence (3.2-3.4 s TWT) varies significantly in thickness and is cut and thinned by a prominent angular unconformity ~7 km west of Site 1183. This unconformity and variations in reflection amplitude, continuity, and configuration within the lower megasequence suggest a dynamic, perhaps relatively shallow-water, paleoenvironment. The top of acoustic basement is characterized by medium- to high-amplitude moderately discontinuous reflections, and some intrabasement reflections are visible. Seismic structure studies of the entire crust and uppermost mantle employing ocean-bottom seismometer data along MCS lines 404 and 501 are in progress, and preliminary results have been reported by Araki et al. (1998) and Mochizuki et al. (1998).

The main objectives at this site were to determine

1. Compositions of basement rocks and compare them with those of basement lavas exposed in Malaita and Santa Isabel and drilled at DSDP Site 289 and Ocean Drilling Program (ODP) Sites 803 and 807;
2. Ages of basement and basal sedimentary rocks to help test the hypothesis that magmatism in most of the high plateau ceased soon after 122 Ma, the age of basement at Site 807 and in Malaita and parts of Santa Isabel;
3. Physical volcanology of basement lavas, the nature of sedimentary interbeds, and the characteristics of basal sedimentary layers in order to deduce the eruptive environment (flow types and approximate depths of eruption);
4. Early subsidence history, as recorded in the basement lava pile and the overlying sedimentary succession;
5. Ages of sequence boundaries observed in the seismic record; and
6. Rates of Cretaceous environmental change as recorded in the sedimentary record.