Site 1185 is located at a water depth of 3898.9 m at the eastern edge of the Ontong Java Plateau. Drilling was focused on the igneous basement, and only the lower ~60 m of the sedimentary column was cored (Fig. F8). Rocks in this interval are middle to upper Eocene nannofossil chalk with radiolarians to radiolarian nannofossil chalk with rare chert. The chalks are commonly bioturbated, and synsedimentary slump structures are present in the lower portion of the sedimentary succession. Rare limestone layers and fracture fillings are present in the igneous basement, which is composed of basalt flows.
Two holes were drilled at Site 1185. The sedimentary succession recovered is dominated by chalk and limestone with abundant siliceous microfossils. We recognize a single lithologic unit overlying basement (Table T3).
Although at Site 1185 we did not core the interval from the seafloor to 58 m above basement, we assume that the Tertiary sedimentary history was similar to the history at other sites on the Ontong Java Plateau. To maintain consistency with these other sites, we have designated the recovered interval of sediments as Unit II. The chalk recovered from Site 1185 contains abundant radiolarians and is middle to late Eocene in age. At Site 803, 334 km to the north-northwest, middle to late Eocene nannofossil chalk of Unit II is distinguished from nannofossil chalk of Unit I by its abundant siliceous microfossils. Eocene siliceous chalk is also characteristic of Unit II at DSDP Site 289 (Shipboard Scientific Party, 1975) and ODP Sites 807 (Kroenke, Berger, Janecek, et al., 1991) and 1183 (see "Lithostratigraphy" in the "Site 1183" chapter). Unit II at Site 1185 sits on basalt. Rare interflow limestone beds and sediment-filled fissures are found within the underlying basaltic lava flows.
The cored interval of Unit II in Hole 1185A is 58 m thick and contains middle to late Eocene microfossils (see "Biostratigraphy"). The top of the unit was not cored. The base of the unit is the top of basaltic basement (Section 192-1185A-8R-1, 14 cm; 308.54 mbsf; Section 1185B-2R-2, 0 cm; 309.51 mbsf). The sediment/basalt contact was not recovered.
Unit II consists dominantly of nannofossil chalk with radiolarians to radiolarian nannofossil chalk (Figs. F9, F10). Chert fragments are a minor lithology in Sections 192-1185A-6R-CC through 8R-1 (291.06-308.54 mbsf). The carbonate content ranges from 84.6 to 90.4 wt% (Table T4). The volume proportion of radiolarians estimated from observation of thin sections and smear slides shows a slight increase downhole and is 25%-30% in Cores 192-1185A-5R through 8R (Fig. F8). The chalk is white to very pale brown (10YR 8/2) to light gray (10YR 7/2) and darkens downhole. Bioturbation is common, although the poor recovery, drilling disturbance, and homogeneous nature of the sediments make characterization of the biogenic fabric difficult. Small (<1 mm) black particles (pyrite or Mn oxide) are scattered throughout. Some burrows are surrounded by black particles (Fig. F11). Physical sedimentary structures are rare. Slump folding is observed in the lowest portion of this unit (interval 192-1185A-7R-1, 130 cm, to 7R-CC, 50 cm) (Fig. F12). The upper 20 cm of the slumped material was burrowed after deformation, and the burrows are filled with material similar to the immediately overlying chalk (Fig. F12).
The basement in both holes drilled at Site 1185 consists of basalt flows with rare, intercalated, thin limestone beds. Poorly preserved planktonic foraminifers and nannofossils of Cretaceous age are present in the limestone (see "Biostratigraphy"). The basalt is described in detail in "Igneous Petrology," "Alteration," and "Structural Geology". The contact between chalk of Unit II and basalt basement was not recovered.
Limestone is present in the basalt sequence as rare interbeds, between pillows (Fig. F13), as the matrix of hyaloclastite breccia, and rarely in sediment-filled fissures (Fig. F14). The limestone is composed of micritic calcite with very poorly preserved foraminifers and radiolarians (Fig. F15). Brown semiopaque particles and recrystallized calcite veins are common. Sedimentary structures are difficult to distinguish.
Nannofossil chalk is typical of pelagic sediment on the Ontong Java Plateau. Planktonic foraminifers are very rare in the Eocene chalk of Site 1185, whereas calcareous nannofossils, benthic foraminifers, and calcified radiolarians are abundant. These observations suggest that Site 1185 was at or below the foraminifer lysocline but above the calcite compensation depth (CCD) during deposition of Unit II. No current-generated sedimentary structures were observed.
The folding in the lowest portion of Unit II (interval 192-1185A-7R-1, 130 cm, through 7R-CC, 50 cm) was syndepositional. The elongation of trace fossils within the slump block and the continuous folding imply that there was only a minor amount of soft-sediment deformation. There is no evidence of hardground formation or a major difference in composition across the top of the slump. The uppermost 20 cm of the bed is burrowed, demonstrating that slumping was synsedimentary and that contemporary benthos reworked bottom sediments to at least 20 cm below the sediment/water interface.
Limestone in the underlying basalt is Cretaceous in age, but age resolution in Unit III is poor (see "Biostratigraphy"). The Paleocene-lower Eocene and probably much of the Upper Cretaceous are missing between the youngest limestone and the oldest chalk (see "Biostratigraphy"). The deposition of Unit II near the foraminifer lysocline and a comparison with the stratigraphy of shallower sites (see "Lithostratigraphy" in the "Site 1183" chapter) suggests that the preservation of Eocene chalk began when the regional CCD sank below the depth of Site 1185.