PRINICIPAL RESULTS

Approximately 12 km east-northeast of Site 1186 the seismic-reflection data show a small body, interpreted to be an igneous intrusion or small volcanic cone, rising into the sedimentary sequence ~500 m above the surrounding acoustic basement (Fig. 29). Emanating from this body toward the drill site is a package of high-amplitude and continuous reflections that could represent a sill, lava flow, or volcaniclastic sediments. Alternatively, it could represent pelagic sediments altered by hydrothermal fluids derived from the igneous body. A secondary reason for selecting Site 1186 was to core this reflection package. If it proved to represent a sill or lava flow, it would provide information on the poorly known late-stage magmatism seen in seismic reflection records across much of the high plateau (Kroenke, 1972; Nixon, 1980) and in the 34-Ma alnöite intrusions and 44-Ma alkalic Maramasike Formation lavas on Malaita (e.g., Davis, 1977; Nixon and Neal, 1987; Tejada et al., 1996).

We began coring at 697.4 mbsf. The lithologic units recognized at this site parallel those at other sites on the main Ontong Java Plateau, except that the Oligocene-Neogene chalk and ooze, designated as Unit I elsewhere on the plateau (and presumed to be present at Site 1186), were not cored. Recovery of Paleocene-Eocene Unit II (697.4-812.7 mbsf; see Fig. 30) was generally <5%. The rocks recovered are white limestone and chalk with faint burrow mottling and dark reddish gray to olive-brown chert interbeds. The lowest core of Unit II contains numerous thin gray beds of zeolite-rich chalk, which were interpreted to be altered volcanic ash layers at other sites on the main plateau. No material indicative of a late-stage lava flow or sill was recovered. Despite the low overall recovery in this unit, the abundance of chert suggests that the seismic reflector package mentioned above represents a particulary chert-rich interval of sediment.

Unit III (812.7 mbsf to the contact with basaltic basement at 968.6 mbsf; Fig. 30) is Cretaceous chalk and limestone. The upper 118 m (Subunit IIIA; Campanian-Maastrichtian) consists of white to brownish white chalk, and the lower 38 m (Subunit IIIB; Aptian-Albian) is mottled light gray and dark brown limestone with minor clay beds. The division between the subunits is marked by a clay-rich band at 930.55 mbsf. As at Site 1183, this band marks a major hiatus (~13 m.y.) in carbonate deposition between the upper Albian and upper Coniacian. Other prominent hiatuses common to both Sites 1186 and 1183 are middle Albian (10 m.y.), uppermost Maastrichtian (2 m.y.), and middle Danian through middle Selandian (4 m.y.). Paleoenvironmental differences between Sites 1183 and 1186 are probably mostly a result of the greater paleodepth of Site 1186 and include a longer period of deposition in the Late Cretaceous below the foraminifer lysocline at Site 1186 (late Albian through early Maastrichtian) than at Site 1183 (late Albian through earliest Campanian).

The lowermost three beds above basement are a yellowish brown limestone overlying a bioturbated transition to a 5-cm-thick interval of dark brown ferruginous claystone, which in turn lies atop a 0.5-cm layer of breccia containing angular basaltic glass fragments. The limestone is late early Aptian in age (upper Leupoldina cabri Zone) and contains a small hiatus marking the absence of the planktonic foraminifer Globigerinelloides ferreolensis and calcareous nannofossil NC7B zones. The 65.4 m of basement penetrated (with 59% average recovery) at Site 1186 consists of basalt lava flows with minor interbeds of yellowish brown sandstone and one interval of reddish brown conglomerate containing rounded limestone clasts (Fig. 31). Fractures in some of the flows are filled with pale brown, partially recrystallized limestone breccia.

Site 1186 was the only hole logged during Leg 192. High-quality logs were acquired in igneous basement and in parts of the cored sedimentary interval. We tentatively interpret a strong seismic reflection in the sedimentary section at 4.57 s two-way traveltime (~720 mbsf; see Fig. 29) as an ~11-m-thick chert or chert-dominated interval. The Aptian-Albian limestone appears to be thinly and regularly bedded. The sharp boundary between sedimentary and igneous rock is well defined on conductivity, porosity, density, and, particulary, Formation MicroScanner logs. Using the same logs, we can distinguish between pillowed and massive intervals within igneous basement.

We divided the basement section (968.6-1034.0 mbsf) into four units on the basis of limestone and hyaloclastite interbeds and downward changes in character from massive to pillowed. The units range from 10 to >26 m in thickness (Fig. 32). Basement Unit 1 consists entirely of pillow lava, whereas Units 2-4 have massive interiors. The basalts are sparsely olivine (± plagioclase) phyric. All olivine is altered and usually replaced by dark green clay. The pillows have glassy rims (commonly containing some unaltered glass), aphanitic outer zones, and fine grained interiors. In the massive flows forming most of Units 2-4, the transition from aphanitic flow tops to coarser-grained interiors occurs over several meters and is marked by an intermediate zone with a patchy texture varying between fine grained and aphanitic on a scale of a few millimeters. Olivine phenocrysts are concentrated in the coarser patches and are rare to absent in the aphanitic parts (Fig. 33).

Plagioclase xenocrysts (>2 mm) and, more rarely, plagioclase-rich xenoliths are present throughout the entire basalt section and are more common in the massive intervals. They are similar to the xenocrysts and xenoliths that we reported from Site 1183 and from the lower group of lava flows at Site 1185. Seven bulk-rock lava samples analyzed by shipboard ICP-AES are closely similar in chemical composition to basalt at Site 1183, the lower group at Site 1185B, Units C-G at Site 807, and the Kwaimbaita Formation on the island of Malaita (Fig. 10, Fig. 11, Fig. 12, Fig. 13). The basalt flows all have normal magnetic polarity and give a 23°S paleolatitude, which agrees well with values obtained from basalt of similar age at Sites 1183 and 1185.

The entire section of basaltic basement cored at Site 1186 has undergone low-temperature water-rock interactions resulting in complete replacement of olivine and almost complete replacement of glassy mesostasis. Clinopyroxene and plagioclase are generally unaltered. The overall alteration of the basalt ranges from 5% to 35% by volume, estimated visually by color distribution in hand specimen. On the whole, the alteration is similar to that in the lower group of basalt flows at Site 1185 and especially to that at Site 1183.

The effects of the same three main low-temperature alteration processes are clearly seen:

1. Black and dusky green halos resulted from the replacement of olivine phenocrysts and groundmass glass by celadonite, Fe oxyhydroxide, and, to a minor extent, smectite.

2. Brown halos formed by the complete replacement of olivine and glass by smectite and Fe oxyhydroxide as a result of strongly oxidative alteration at high water:rock ratios.

3.Pervasive alteration under anoxic to reducing conditions and low water:rock ratios has affected the basalt in areas outside the colored halos. Smectite, pyrite, and calcite are the principal secondary minerals formed during this process.

The major results of drilling at Site 1186 are summarized as follows:

1. The sedimentary sequence closely parallels those at other sites on the main Ontong Java Plateau.

2. Biostratigraphy indicates a major hiatus (~13 m.y.) in carbonate deposition between the upper Albian and upper Coniacian, as at Site 1183. Other significant hiatuses at both sites are middle Albian (10 m.y.), uppermost Maastrichtian (2 m.y.), and middle Danian through middle Selandian (4 m.y.).

3. Basement at Site 1186 consists of basaltic pillow lava and massive lava flows. It is probably of similar age to basement at Site 1183; both are immediately overlain by upper lower Aptian (~118 Ma in the time scale of Gradstein et al., 1995) limestone.

4. The basalt is similar in chemical composition to basalt from Site 1183, the lower group of flows at Site 1185, Units C-G at Site 807, and the Kwaimbaita Formation on Malaita. The types and amounts of basalt alteration at Site 1186 are very similar to those at Site 1183.

5. The basalt flows have normal magnetic polarity and yield a paleolatitude of 23°S, which agrees well with that obtained from basalt of similar age at Sites 1183 and 1185.