Site 1187 is located in 3804 m of water on the eastern edge of the main Ontong Java Plateau. Drilling was focused on the igneous basement and very little sediment was cored (Fig. F7). Coring began at 365.5 mbsf, and basalt was encountered at 372.5 mbsf (366.97 mbsf curated depth; see "Operations"). The sediment recovered is composed of 1.47 m of dark brown, ferruginous claystone that grades downward from burrow mottled to laminated and overlies a 2-cm-thick chalk layer. The chalk contains Aptian microfossils (see "Biostratigraphy"). Rare interbeds of recrystallized limestone are present within the basalt.
The 1.28 m of chalk, limestone, and chert recovered in a wash core (Core 192-1187A-1W) suggests that lithologies in the 365.5-m interval not cored include facies similar to Neogene and Paleogene pelagic sediments elsewhere on the plateau.
Within the cored interval we recognize two lithologic units: an upper unit composed dominantly of claystone and a lower unit composed of basalt with rare recrystallized limestone between flows (see "Igneous Petrology"). To maintain consistency with other sites, we have designated the claystone as Unit III. Pieces of Oligocene foraminifer nannofossil chalk and Eocene siliceous limestone with chert in Core 192-1187A-1W confirm that rocks typical of Units I and II are present in the washed interval (see "Biostratigraphy"" and "Lithostratigraphy" in the "Site 1183" chapter, "Biostratigraphy" and "Lithostratigraphy" in the "Site 1185" chapter, and "Biostratigraphy" and "Lithostratigraphy" in the "Site 1186" chapter).
The top of the unit was not recovered, and the base is placed at the contact with basalt (Section 192-1187A-2R-2, 30 cm; curated depth 366.97 mbsf). The lowest 2 cm of Unit III is white nannofossil chalk of Aptian age (see "Biostratigraphy") that was used completely for paleontological studies. The overlying 1.45 m of Unit III that was recovered is brown claystone containing abundant brown semiopaque grains and 5%-15% zeolite. Biogenic grains include fish debris and rare foraminifers. The upper portion of the claystone is brown (10YR 4/3) and burrow mottled (Fig. F8). The lower portion is very dark brown (10YR 2/2) and dominantly laminated (Fig. F9), although rare small burrows are present. The change in fabric and color is gradational.
The basement unit extends to the bottom of Hole 1187A and consists of basalt flows with rare, intercalated, thin limestone beds. A 5-cm-thick piece of light red, recrystallized limestone with greenish black glass clasts is at 375.2 mbsf curated depth (Sample 192-1187A-3R-1 [Piece 11, 69-74 cm]). A 3-cm-thick piece of light reddish brown recrystallized limestone with greenish black glass clasts is at 376.85 mbsf curated depth (Sample 192-1187A-3R-2 [Piece 4, 90-93 cm]). The basalt is described in "Igneous Petrology" and "Alteration".
Deposition at Site 1187 seems to match predictions based on results from other sites on the Ontong Java Plateau. However, because so little of the sedimentary record was cored, only very limited conclusions can be reached at this time.
A thin layer of chalk overlies the youngest basalt flow, but most of the 1.47 m of Aptian sedimentary rock recovered is noncalcareous. At shallower sites on the main Ontong Java Plateau, the Aptian is dominated by limestone. If the claystone at Site 1187 is representative of Aptian rocks at this paleodepth, then the record would suggest that the Aptian calcite compensation depth (CCD) was between Site 1187 and shallower sites. Alternatively, the apparent dominance of claystone might be a sampling artifact (carbonate-rich Aptian rocks might be common at Site 1187 above the level at which coring began). The transition between chalk and claystone is similarly difficult to interpret. Possible explanations include a change in the depth of the CCD relative to Site 1187 in the late Aptian, unusual depositional and early diagenetic conditions related to the subjacent basalt, or redeposition of shallower-water material.
Within the claystone there seems to be a progressive upward increase in the amount of bioturbation. Laminated claystone is present in other Leg 192 sites, but this is the only observation from Leg 192 of pervasively burrowed intervals in the same lithology. Comparisons of laminated and mottled intervals might be useful in testing paleoceanographic models (i.e., food supply vs. low oxygen) for the origin of the laminated fabric of the basal claystone (see "Lithostratigraphy" in the "Site 1183"" chapter and "Lithostratigraphy" in the "Site 1186" chapter).
Finally, the post-Cretaceous history is probably similar (at a coarse resolution) to the history elsewhere on the main Ontong Java Plateau because lithologies typical of younger lithostratigraphic intervals are present in the wash core. The presence of upper Eocene pelagic limestone (see "Biostratigraphy") indicates that the CCD was lower than the paleodepth of Site 1187 during the late Eocene.