5. Site 1185¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1185 (Figs. F1, F2) is on the eastern edge of the main or high Ontong Java Plateau at the northern side of an enormous submarine canyon system (informally termed the Grand Canyon or Kroenke Canyon) that extends from Ontong Java Atoll and the Nukumanu Islands into the Nauru Basin. This part of the plateau is far from sites where basement crust was sampled previously, the closest of which are Ocean Drilling Program (ODP) Site 803 (334 km to the north-northwest) and Deep Sea Drilling Project (DSDP) Site 289 (351 km to the west). We chose this site for two principal reasons. First, the portion of basement volcanic stratigraphy within reach of the drill bit in this part of the plateau is likely to be different from that in more centrally located areas. In particular, only relatively few, far-traveled lava flows may have reached the edge of the plateau, possibly allowing us to sample a deeper level of the volcanic stratigraphy here than atop the plateau. Second, the 26 m of lava flows penetrated at ODP Site 803, the only other basement site on the eastern side of the high plateau, belong to the 90-Ma eruptive event (Mahoney et al., 1993); basement at other sites drilled on the high plateau (Sites 289, 807, 1183) formed ~122 Ma. We thought that 90-Ma basement might also be found at Site 1185, and drilling this site would then give us a clearer picture of the extent of this later event. Indeed, seismic reflection data (see immediately below) reveal intrabasement reflections in this part of the plateau, suggesting that a carapace of 90-Ma lavas might overlie 122-Ma basement rocks. If so, drilling at Site 1185 would provide further insight into the extent, composition, and mantle sources of the 90-Ma event documented previously at Site 803 and far to the south on the islands of Santa Isabel (Tejada et al., 1996; Parkinson et al., 1996) and San Cristobal (Birkhold-VanDyke et al., 1996).

Site 1185 is located at a water depth of ~3899 m (drill pipe measurement) on multichannel seismic reflection Line 401 of Leg 2, cruise KH98-1, of the Hakuho Maru (Figs. F3, F4, F5, F6, F7). The bathymetric transition between the plateau and the Nauru Basin lies ~35 km to the east-southeast. The site is midway between two seafloor scarps that may represent the heads of slumps or be related to erosion in the canyon system. The seafloor is rough in places, suggesting a high-energy depositional environment. The sediment section, interpreted to lie between the seafloor (at 5.2 s two-way traveltime [TWT]) and the top of a high-amplitude, moderately continuous reflection at 5.55 s TWT, is characterized by parallel to subparallel reflections of moderate to high continuity. Reflection amplitudes are low to high, and frequency is high. Unconformities and variations in reflection amplitude, continuity, and configuration within the sediment section indicate occasionally vigorous oceanographic and sedimentary conditions in the past. The top of acoustic basement is characterized by a high-amplitude, moderately continuous reflection. Several high-amplitude and relatively continuous intrabasement reflections also are apparent (e.g., at 5.9 s TWT beneath Site 1185; Figs. F6, F7). Preliminary results of a study of crustal structure as determined from Line 401 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 for comparison with those of lavas sampled in DSDP Site 289, ODP Sites 803 and 807, the other Leg 192 sites, 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;
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.