We cored the sedimentary sequence at Site 1186 from 697.4 mbsf to the top of basaltic basement at 968.6 mbsf (see "Operations"). This Aptian through Eocene succession consists of limestone and chalk composed of foraminifers and calcareous nannofossils with variable amounts of chert. Bioturbation is pervasive. Diagenetic changes include progressive pressure-solution lithification and chert formation. The depositional record has a hiatus spanning most of the Cenomanian through Santonian that appears to be regional in extent at this basement depth.
The first section below summarizes the general features of each lithologic subunit. The second section briefly summarizes the main trends in the sedimentation and diagenetic history. A more detailed discussion of the sedimentation history of the main Ontong Java Plateau and postdepositional diagenetic features is given in "Lithostratigraphy" in the "Site 1183" chapter.
An important caveat to the description of the lithologic subunits and the associated interpretation of sedimentation history is that recovery of the sedimentary sections from Hole 1186A rarely exceeded 10% of each 9.6-m cored interval (Fig. F8). For many cores, the recovery was probably strongly biased toward more indurated lithologies, especially chert, and was probably dominated by the upper portion of each cored interval. For example, estimates from wireline-log velocity data within lithologic Unit II at Leg 130, Site 807, on the northern Ontong Java Plateau indicated a chert content of only 20%, although the sediments recovered were 50% chert (Kroenke, Berger, Janecek, et al., 1991). Recovery of Maastrichtian, Paleocene, and Eocene cored intervals from Hole 1186A was particularly meager, whereas recovery of the Aptian-Albian and Campanian cored intervals was comparatively good.
In order to highlight regional trends in paleoceanography, volcaniclastic influxes and diagenetic processes, stratigraphic divisions assigned at Site 1186 closely parallel those used at Site 1183 (see the "Site 1183" chapter) and are similar to those at other sites on the main Ontong Java Plateau (DSDP Site 289 and ODP Sites 803, 807, and 1185). Neogene-Oligocene chalk and ooze, designated as Unit I at these other sites, were not cored at Site 1186.
The cored part of the lithostratigraphic succession is dominated by carbonate facies and was divided into three units, Units II, III, and basement (Fig. F8; Table T3). Unit III was divided into two subunits. We used the following criteria for subdivisions:
Unit II is chert and white limestone and spans at least 115 m. Recovery of this unit was generally <5%, with some cores containing only a few chert fragments.
Although we did not core from the seafloor to 697.4 mbsf, we assume that the Neogene and late Paleogene sedimentary history was similar to that at other sites on the main Ontong Java Plateau (Shipboard Scientific Party, 1975; Kroenke, Berger, Janecek, et al., 1991; see "Lithostratigraphy" in the "Site 1183" chapter; also see "Lithostratigraphy" in the "Site 1185" chapter). The uppermost sediment cored from Hole 1186A (Core 192-1186A-2R) was limestone with chert. To maintain consistency with the other sites, we have assigned this interval to regional lithologic Unit II. The distinguishing feature of Unit II at other sites is abundant siliceous microfossils or chert within nannofossil chalk. This correlation is supported by the middle Eocene age of the uppermost sediment recovered at Site 1186 (see "Biostratigraphy"). The upper limit of Unit II at other sites is near the end of the Eocene. The relative thickness and age of this portion of the unit in Hole 1186A suggests that the transition to the nonsiliceous chalk of Unit I is probably <50 m above Core 192-1186A-2R.
Limestone and chalk recovered from Unit II are white with faint burrow mottling. Carbonate content ranges from 35% to 97%, depending on the degree of silicification (Table T4). The limestone is composed of 10% to 50% foraminifers in a nannofossil micrite matrix (Fig. F9). The more siliceous limestone has up to 30% radiolarians (see "Site 1186 Sedimentary Thin Sections"). An unusual trend is a downhole decrease in lithification from limestone in Cores 192-1186A-2R through 6R to chalk in Cores 192-1186A-9R through 13R. Chert interbeds are typically dark reddish gray to olive brown. Chert fragments commonly display white rinds and may incorporate small patches of white limestone in their interiors (Fig. F10). Intervals of partially silicified limestone and porcellanite are common (Fig. F11).
Chert fragments constituted ~40% of the material recovered between 700 and 800 mbsf. FMS resistivity imagery of this interval suggests that 5- to 15-cm-thick high-resistivity bands, which we interpret to be chert layers, comprise ~10% of the in situ sediment. These bands are spaced at 1-m intervals between 700 and 722 mbsf. The most abundant bands are between 722 and 733 mbsf where they comprise ~50% of the logged interval (Cores 192-1186A-4R and 5R; middle Eocene; Fig. F12). This interval (722-733 mbsf) on the geophysical log has the highest average resistivity within the Paleocene-Eocene sediment section (see "Downhole Measurements"). The high-resistivity bands are spaced at ~2-m intervals from 733 to 758 mbsf (Cores 192-1186A-6R and 7R; early Eocene) (see Figs. F12, F13). Below 758 mbsf, the high-resistivity lenses are ~4-5 m apart. Thinner and less continuous chert bands and nodules may be present within these zones.
Core 192-1186A-13R at the base of Unit II contains numerous gray beds, 1-5 cm wide, of zeolitic chalk. The zeolitic chalk zones have diffuse, bioturbated boundaries with the host limestone. Bands with a greater concentration of zeolite are darker and commonly have a microflaser network of seams in an anastomosing pattern surrounding small, elongate chalk nodules (Fig. F14). Zeolitic chalk bands have a relatively high magnetic susceptibility (Fig. F8). Similar zeolite-rich bands were interpreted as altered volcanic ash layers at other sites on the Ontong Java Plateau (see "Sedimentation History of Site 1183" in "Lithostratigraphy" in the "Site 1183" chapter).
We tentatively correlate Cores 192-1186A-2R through 9R with Subunit IIA of Hole 1183A and Cores 192-1186A-10R through 13R with Subunit IIB of Hole 1183A. The distinction between these subunits at Site 1183 is an abundance of chert in Subunit IIA and the presence of zeolitic chalk bands in Subunit IIB (Fig. F8). Cores 192-1186A-2R through 9R are dominated by chert and partially silicified limestone. Core 192-1186A-13R contains several beds of zeolitic chalk, and the chalk within Cores 192-1186A-10R and 11R contains rare stylolite and clay seams, which indicate a higher abundance of clay or a fine-grained zeolite component than in overlying cores. However, the very low recovery (e.g., only a few rock fragments within core catchers for Cores 192-1186A-6R through 8R and 12R) prevents us from formally subdividing Unit II in Hole 1186A.
The base of Unit II in Hole 1183A was assigned to the lowest zeolite-rich bed. We employed this same definition for Hole 1186A and placed the base of Unit II between Core 192-1186A-13R, which contains abundant zeolitic chalk, and Core 14R, which contains white chalk. In Holes 1183A and 1186A, the lowest zeolitic chalk bed is close to the base of the Paleocene.
Unit III in Hole 1186A spans ~156 m from the top of Core 192-1186A-14R to the contact with basalt at 968.6 mbsf (see "Operations"). The contact between sedimentary rock and basalt is within Section 192-1186A-30R-1 at 45 cm, yielding a curated depth of 966.8 mbsf, ~2 m shallower than the true depth based on drilling data. Unit III is Cretaceous in age. The upper 118 m of the succession consist of white to brownish white chalk, and the lower 38 m are mottled light gray and dark brown limestone with minor clay beds. The color difference divides the unit into two subunits. The subunit contact is assigned to the clay-rich band at 930.5 mbsf, which also corresponds to a major hiatus below Campanian strata. These two subunits are recognized at other sites on the main Ontong Java Plateau (see "Lithostratigraphy" in the "Site 1183" chapter).
Subunit IIIA consists of white to brownish white, foraminifer nannofossil chalk to nannofossil chalk with abundant bioturbation. Chert is a minor lithology and is rare in the lower half of the subunit. The subunit spans a 118-m-thick interval from the top of Core 192-1186A-14R (812.7 mbsf) to the top of a clay-rich interval in Section 192-1186A-26R-3, 10 cm (930.5 mbsf).
Subunit IIIA can be divided evenly into two facies on the basis of subtle color variations. The upper ~60 m of Subunit IIIA (Cores 192-1186A-14R through 20R; 813-870 mbsf) consists of white foraminifer nannofossil chalk with well preserved planktonic foraminifers (Fig. F15) and is mainly Maastrichtian in age. Carbonate content of the chalk is 96-98 wt% (Table T4). Bioturbation is pervasive, but burrows are difficult to resolve because of the uniform white color. Chert is present as red bands or nodules. Core 192-1186A-17R in the middle of this white chalk facies has centimeter-scale bands of light greenish gray (Fig. F16). This banded interval correlates with a distinct peak in uranium concentration in the natural gamma ray log of the wireline geophysical tool string; however, this gamma ray peak was not recorded in the natural gamma log of the FMS tool string (see "Downhole Measurements").
The lower ~60 m of Subunit IIIA (Cores 192-1186A-21R through 26R) display subtle color alternations at a 10-40 cm scale between white and brownish white nannofossil chalk. This interval is mainly Campanian in age. Bioturbation is pervasive and especially visible at the transitions in color. Thin sections indicate that in Core 192-1186A-21R the white chalk contains a slightly higher proportion of foraminifers (~3%) than the brownish white chalk (<2%). Planktonic foraminifers are very rare and poorly preserved in Cores 192-1186A-22R through 26R (Fig. F17). Carbonate content of the nannofossil chalk is 93%-98%. Chert is present as rare black to brown pieces, and its scarcity is probably a factor in the relatively good core recovery in this interval (Fig. F8). Red chert fragments present only at the tops of some cores within this lower interval may have fallen downhole from the Maastrichtian interval above.
Subunit IIIB consists of ~38 m of grayish brown Aptian-Albian nannofossil limestone.
The transition between Subunits IIIB and IIIA is within an interval of bioturbated clayey chalk to claystone spanning ~1 m (interval 192-1186A-26R-2, 80 cm, to 26R-3, 85 cm) (Fig. F18). The clayey chalk has a high magnetic susceptibility (Fig. F8). The relatively dark claystone at 192-1186A-26R-3, 10-15 cm, has 30% carbonate, and a smear slide shows that it contains 30% zeolite. The boundary between Subunits IIIA and IIIB was assigned to the top of this band in Core 192-1186A-26R-3 at 10 cm. The 1-m-thick interval of clayey chalk has a complex fabric with anastomosing laminae and multiple crosscutting mottles and burrows filled with both clay-rich and carbonate-rich material (Fig. F19). Between some burrows are white, 1-cm-diameter irregular clasts or residual fragments of limestone of uncertain origin. Biostratigraphy indicates that this complex, burrowed, clayey interval encompasses nearly 20 m.y., and that the dark clay band selected as the subunit boundary has a late Coniacian age (see "Biostratigraphy"). Planktonic foraminifers are absent from this interval.
Below this transition zone, Subunit IIIB is a light brownish gray limestone with mottles and bands of lighter color (white to light pink to light gray) and darker color (brownish gray to very dark brown). Some of the mottles cut across bioturbation features (Fig. F20). Carbonate content of the limestone ranges from 31 to 88 wt% (Table T4). Planktonic foraminifers increase in abundance downhole (Figs. F21, F22). Magnetic susceptibility in Subunit IIIB displays closely spaced peaks on a generally elevated background relative to other units (Fig. F8). A combination of compacted subhorizontal burrows, subhorizontal streaks, discontinuous planar to irregular laminations, and thin elongate lenses imparts a distinctive woody texture to several intervals within the limestone. A continuum of textures from the woody extreme to microflasers and anastomosing pressure-solution seams is present within Subunit IIIB (Fig. F23). The woody texture is most prevalent in darker intervals. Thin sections and smear slides indicate abundant fine-grained opaque and brownish semiopaque particles, which are probably Fe oxyhydroxide. In Hole 1183A, a similar mottled facies with a condensed woody texture spans 45 m (see "Lithostratigraphy" in the "Site 1183" chapter). FMS resistivity imagery of the limestone indicates discontinuous high-contrast banding at the 5- to 10-cm scale between 933 and 946 mbsf (lower Cores 192-1186A-26R and 27R; 935-945 mbsf; Fig. F24). High-resistivity, ~5-cm-thick bands are spaced at 1- to 5-m intervals from 946 to 965 mbsf near the sediment/basalt contact.
Chert is present as 5-cm-thick bands and as rare small (<1 cm) irregular nodules and stringers within the limestone (Fig. F25). The chert is mainly very dark brown but includes black to dark reddish brown pieces. Calcareous porcellanite with 30% carbonate is also present.
Wireline logs indicate a peak in natural gamma ray intensity from Th that spans ~1.5 m at 950 mbsf within Core 192-1186A-28R (see "Downhole Measurements"). This peak suggests a clay-rich or volcanic-ash-rich band spanning ~1 m, but this feature could not be identified unambiguously in the portion of the core recovered.
The base of Subunit IIIB is placed at the top of the first basalt unit at 968.6 mbsf (Section 192-1186A-30R-1, 45 cm; 966.8 mbsf curated depth). The basal sediments are a compact succession of three facies spanning only 10 cm (Fig. F26). The basal layer is 0.5 cm thick and composed of breccia containing angular basaltic glass fragments (~3 mm across) and rounded coarse-sand-size grains. It is overlain by 5 cm of dark brown ferruginous claystone, which contains a 0.5-cm-thick band of pale blue-green clay, possibly smectite. There may be a break in recovery between the two pieces of ferruginous claystone; therefore, the in situ thickness of this facies may be greater. Rare, small burrows are present within the laminated claystone. In smear slides, the claystone contains >50% brownish semiopaque particles, which are probably Fe oxyhydroxide. The dominance of Fe oxyhydroxide is also indicated by the 44 wt% Fe2O3 in the chemical composition of the claystone (inductively coupled plasma-atomic emission spectrometry [ICP-AES], Sample 192-1186A-30R-1, 40 cm). The claystone chemical composition from ICP-AES also includes 27 wt% SiO2, 16 wt% CaO, 5 wt% Al2O3, 3 wt% MgO, 0.4 wt% TiO2, and 1 wt% each of MnO, K2O, Na2O, and P2O5. The claystone is overlain by yellowish brown limestone, and the transition is partially mixed by bioturbation. The overlying 36 cm of bioturbated limestone in interval 192-1186A-30R-1, 0-36 cm, is reddish yellow to pinkish white.
Basement consists of basalt flows (see "Igneous Petrology" and "Alteration"). Sedimentary interbeds are generally hyaloclastite and volcanogenic sandstone, but some unusual carbonate layers are also present. A reddish brown conglomerate dominated by limestone granules and pebbles is present at interval 192-1186A-32R-3, 76-85 cm (Fig. F27). A thin section indicates that this conglomerate includes rounded clasts of radiolarian wackestone, foraminifer wackestone, hyaloclasts, and concentrations of opaque minerals (Fig. F28). Some fissures and fractures within the basalt are filled with pale brown, partially recrystallized limestone breccia. A thin section from such a fracture in Section 192-1186A-30R-1 (Piece 5, 73-76 cm), indicates an intraclast limestone breccia of sand-size clasts of foraminifer and radiolarian wackestone within a microspar matrix (Fig. F29). The mode of formation of these carbonate breccias and the conglomerate is unknown.
The Aptian-Eocene sedimentation record of the main Ontong Java Plateau is quite uniform. The succession is predominantly limestone and chalk with abundant bioturbation. Regional lithostratigraphic divisions are recognized based on variations in abundance of chert and volcaniclastic material and reflect changes in inputs and preservation of siliceous microfossils and volcanic material. Changes in the ratio of foraminifers to nannofossils and the presence of significant condensed intervals and hiatuses at each site appear to be influenced by the position of the calcite compensation depth (CCD) and the foraminifer lysocline relative to sediment surfaces. Winnowing by bottom currents may play a secondary role in the nannofossil-to-foraminifer ratio by redistributing fine-grained carbonate. The thicknesses of units and subunits in Hole 1186A are nearly identical to those of correlative lithostratigraphic units in Hole 1183A. The regional patterns of lithostratigraphy and postdepositional diagenetic alteration are summarized in "Lithostratigraphy" in the "Site 1183" chapter.
The final stages of plateau volcanism at Site 1186 during the early Aptian produced basalt lava flows separated by rare interbeds of nannofossil foraminifer limestone, redeposited conglomerate with mixed calcareous facies, and volcaniclastic sandstone. Other carbonate ooze seeped into fissures within the basalt flows. The basal sediment above the basalt flows is a 5-cm-thick laminated ferruginous claystone. Basalt alteration and/or local hydrothermal activity may be the source of the Fe oxyhydroxide that characterizes this layer. The bluish gray layer within the brown claystone interval may be a distal volcanic ash that has altered to smectite. Burrowing macrofauna were apparently rare and small during the formation of this claystone. In contrast, bioturbation was pervasive during deposition of all subsequent carbonate sediments.
Foraminifers and nannofossils, with variable amounts of radiolarians and other siliceous organisms, accumulated during the late Aptian and early Albian and from the Campanian through the Eocene (the top of the cored interval from Hole 1186A). The ~25-m.y. interval between these two main episodes of pelagic accumulation is represented by condensed portions of the upper Albian and upper Coniacian. Planktonic foraminifers are not well preserved in Hole 1186A between the lower Albian and the Maastrichtian (~30 m.y.). The record at Site 1183, near the crest of the main plateau, is similar; the records at Sites 1185 and 1187 in deeper water lack carbonate sedimentation from the Aptian through Paleocene. This pattern of preservation suggests that the hiatus and condensation may be associated with an excursion of, first, the regional lysocline and, second, the CCD above the sediment surface at Site 1186 during the Late Cretaceous (see "Lithostratigraphy" in the "Site 1183" chapter.
The range of brown colors in the Aptian-Albian carbonate sediments is caused by variable concentrations of Fe oxyhydroxide (brown semiopaque particles in smear slides) and possibly variable amounts of other clay-size material. Hole 1183A has a 1.7-m-thick layer of laminated calcareous claystone near the Aptian/Albian boundary. Wireline logs indicate a clay-rich or volcanic-ash-rich zone ~1.5 m thick (at ~950 mbsf) near the Aptian/Albian boundary in Hole 1186A (see "Downhole Measurements"); however, a thick claystone of this age was not observed in the limited core recovered from this interval.
Both the Campanian chalk and the Maastrichtian chalk from Hole 1186A are ~20% thicker and contain a lower relative abundance of foraminifers than Hole 1183A. These differences may result in part from the winnowing of fine-grained carbonate from Site 1183 near the crest of the plateau (thereby thinning the section and concentrating foraminifers), followed by redeposition at deeper sites, such as Site 1186 (thereby thickening the section and diluting the foraminifers).
The Campanian chalk has subtle oscillations between white and brownish white that parallel changes in the relative abundance of foraminifers and may reflect differences in the concentration of trace amounts of noncalcareous material. The Maastrichtian chalk is uniformly white and has a higher abundance of planktonic foraminifers and chert than the Campanian chalk. The chert in the Campanian chalk is black and brown, whereas the Maastrichtian chert is red. This contrast between Campanian and Maastrichtian limestone and chert was also recognized in Hole 1183A (see "Lithostratigraphy" in the "Site 1183" chapter), where each facies spans ~50 m. The core recovered from Hole 1186A indicates that these facies changes have a broad distribution over the main Ontong Java Plateau, and shore-based studies will examine their possible paleoceanographic significance.
In general, the Cretaceous section from Hole 1186A appears to contain a higher abundance of chert than that from Hole 1183A near the crest of the plateau. Core recovery appears to be inversely related to the abundance of chert. The greater chert content in Hole 1186A could be a result of several factors, including an originally higher proportion of siliceous microfossils within the sediments, increased silica preservation at greater depths, or an earlier formation of chert nodules in the porous chalk of Hole 1186A than in the denser limestone of Hole 1183A (e.g., Lancelot, 1973). Siliceous microfossils are rarely preserved within the chalk interbeds, presumably because the siliceous tests underwent complete dissolution followed by silica reprecipitation to form the chert bands and nodules.
The base of the Paleocene coincides with an influx of distal volcanic ash. The ash is now altered and mixed by bioturbation to form zeolitic chalk. At both Sites 1186 and 1183, the highest concentration of volcaniclastic material is in the basal 10 m of the Paleocene, producing a distinctive peak in magnetic susceptibility (Fig. F8; also see Fig. F6 in the "Site 1183" chapter). Zeolite derived from volcanic ash is much less abundant in the upper Paleocene and disappears in the Eocene. Because the original morphology and chemical composition of the glass shards have been destroyed by pervasive alteration to zeolite and clay, no information is available on possible sources of this ash.
Interestingly, the Eocene carbonates from Hole 1186 are limestone, whereas the underlying Campanian-Paleocene carbonates are chalk. This trend is opposite to that expected, because cementation and lithification typically increase with depth in carbonate successions. We noticed a similar but less extreme reversal in induration between Paleocene limestone and Maastrichtian chalk in Hole 1183A (see "Lithostratigraphy" in the "Site 1183"" chapter) and in DSDP Holes 288 and 289 (van der Lingen and Packham, 1975).
The Eocene section at Site 1186 and elsewhere on the Ontong Java Plateau is characterized by abundant chert beds. Possible causes of the increased abundance of siliceous microfossils and some factors in the formation of chert bands in the Eocene sediments are discussed in "Sedimentation History of Site 1183" in "Lithostratigraphy" in the "Site 1183" chapter. The shallowest concentration of chert beds forms a regional seismic reflector (e.g., Berger et al., 1991), which determined our target depth to begin coring at Hole 1186A. The decrease in silica content in upper Eocene sediments in subtropical settings is a global phenomenon.
Following deposition and mixing by bioturbation, the carbonate ooze underwent compaction, progressive pressure-solution lithification, partial silicification, and late-stage redox reactions of iron and manganese compounds. These diagenetic effects created a variety of features, including the following:
These processes progress with increasing depth in Site 1186 and overlap in their effects. Similar diagenetic features are discussed and interpreted in "Lithostratigraphy" in the "Site 1183" chapter.