Site 1183 is located near the crest of the main edifice of the Ontong Java Plateau (Figs. F1, F2). We chose this site because it is in the shallowest region of the high plateau, where the seismically defined upper-crustal layer is thickest but the sediment cover is relatively thin (see Neal et al., 1997, and references therein). The original basement depth of the high plateau may have been at its shallowest in this region, and eruptive activity may have been at its most vigorous. We thought it possible that the compositional range of basement lava flows in this central area may have been greater than in previously studied areas located much closer to the margins of the plateau (Malaita, Santa Isabel, and ODP Sites 803 and 807). Also, a distinctive sediment package appears above basement on the multichannel seismic reflection line across this area (Fig. F7), and we thought that this package might correspond to much shallower water deposits than those found elsewhere on the high plateau.
The sedimentary succession (cored from 328.0 to 452.7 meters below the seafloor [mbsf] and from 752.0 mbsf to basement at 1130.4 mbsf; Table T1) is dominated by nannofossil foraminifer chalk and limestone. We divided the sequence into three lithologic units and seven subunits (Fig. F8). Unit I consists of ooze and chalk, with the transition between the two (at 337.6 mbsf) defining the boundary between Subunits IA and IB. However, because we did not core the upper part of the succession, only a single core of ooze (Subunit IA) was recovered. P-wave velocities in the ooze and chalk sections of Unit I increase downward from a mean of 1700 m/s to a mean of 2240 m/s. Oligocene chalk cored from 752.0 to 838.6 mbsf contains abundant volcanic ash layers and is designated Subunit IC.
Unit II is Paleocene to middle Eocene limestone. Conspicuous bands of chert between 838.6 and 958.3 mbsf characterize Subunit IIA, and the presence of zeolite-rich bands (probably altered ash layers) and less pronounced chert bands below 958.3 mbsf define Subunit IIB. The base of Unit II is placed at the lowest significant zeolite-rich horizon (986.6 mbsf), which coincides approximately with the base of the Cenozoic. Bulk density increases sharply in the lowest part of Subunit IIB, and both the water content and porosity decrease. A marked P-wave velocity increase occurs at the boundary between the chert-rich limestone of Subunit IIA (generally <2500 m/s) and the zeolite-rich limestone of Subunit IIB (generally >3000 m/s); the highest velocities of 3500-4400 m/s are at the bottom of the unit. Cretaceous limestone Unit III is subdivided on the basis of color, with the change from a white Subunit IIIA to a mottled gray and pinkish white Subunit IIIB at 1088.8 mbsf, corresponding approximately to a hiatus and condensation of the Cenomanian through upper Coniacian portion of the sequence. Subunit IIIB, directly above basement, contains microfossils (Eprolithis floralis and Leupoldina cabri) restricted to a short interval straddling the boundary between the early and late Aptian.
The lowest 2 m of Subunit IIIB contains two intervals of vitric tuff, the lowermost of which is separated from the underlying basalts by a 25-cm-thick limestone bed. The main component of the tuff intervals is basaltic ash consisting of partly glassy to tachylitic fragments with abundant plagioclase microlites. Texturally, many of the fragments are similar to aphanitic pillow margins in the underlying basalts. Most fragments are nonvesicular, but some have vesicles or scalloped margins. Altered brown glass shards are also present; most are blocky and nonvesicular, but some are moderately vesicular. The tuff is composed of at least eight normally graded beds, several of which have scoured bases. The uppermost layer grades up through parallel-laminated to cross-laminated beds, indicating deposition by turbidity currents or reworking by currents. The minor, moderately vesicular basaltic glass shards in these tuff beds indicate formation by relatively shallow submarine eruptions, whereas the partly glassy basaltic ash that constitutes the dominant component could have been derived from shallow-water to subaerial hydroclastic eruptions or by erosion of a volcanic edifice somewhere in this summit region of the main Ontong Java Plateau. These beds, and possibly a vitric tuff of similar age just above basement basalt at DSDP Site 289 (Andrews, Packham, et al., 1975) are the only evidence to date for any shallow-water or subaerial emplacement on the entire high plateau.
We cored 80.7 m of basaltic basement, from 1130.4 to 1211.1 mbsf, recovering a total of 44.2 m (Table T1) at a low average penetration rate of 1.2 m/hr. We divided the basalt into eight units (Fig. F9), ranging in thickness from 0.36 to 25.70 m, on the basis of the presence of thin interbeds of recrystallized limestone and/or hyaloclastite. Microfossils in the interbeds indicate an age no older than early Aptian. All eight units contain pillow basalt, defined by quenched glassy rims, grain-size variations from aphanitic near pillow margins to fine grained in the interiors, and vesicle patterns. Some of the glassy rims appear to be unaltered even though they are laced with calcite veins. The glass is preserved best in Units 4-7. Except near pillow margins, where small (1-2 mm), elongate vesicles are present, the basalt is essentially nonvesicular, implying a paleodepth >800 m (Moore and Schilling, 1973). Most of it is sparsely olivine ± plagioclase phyric, with a quenched to subophitic groundmass consisting of plagioclase, clinopyroxene, titanomagnetite, glassy mesostasis, and a trace of sulfide. The abundance of olivine phenocrysts increases slightly with depth in the succession, reaching a maximum of ~4%.
Shipboard major and trace element analyses show that the basalt flows at Site 1183 are tholeiitic and very similar in composition to those forming the >2.7-km-thick Kwaimbaita Formation on Malaita, nearly 1000 km to the south (Figs. F10, F11, F12, F13). Kwaimbaita-type basalt also has been sampled 533 km to the north of Site 1183 at Site 807 (Units C-G), in the single flow penetrated at Site 289, 183 km to the northeast, and at Sites 1185 and 1186 (see "Site 1185" and "Site 1186"). The upper group of basalt flows in Malaita, the ~750-m-thick Singgalo Formation, is compositionally distinct from the Kwaimbaita Formation (Figs. F11, F12, F13); Singgalo-type basalt also is found in Santa Isabel and forms the upper 46 m of flows (Unit A) at Site 807. Its complete absence from Sites 289 and 1183 suggests that basalt of this composition may not be present on the broad crest of the plateau.
Cumulate gabbroic
xenoliths and plagioclase megacrysts are present in Units 2-7 at Site 1183 (Fig.
F14). They are
round to subround and 
3
cm in diameter. Clinopyroxene in the xenoliths is partially to totally resorbed,
whereas the plagioclase shows only minimal signs of reaction with its basaltic
host. Interestingly, similar xenoliths and megacrysts have been found in lava
flows of the Kwaimbaita Formation on Malaita (Tejada et al., in press) and in
the Units C-G flows at Site 807, as well as at Sites 1185 and 1186.
Pervasive low-temperature interaction of the basaltic basement with seawater-derived fluids under anoxic to suboxic conditions has resulted in alteration ranging from <5% to 20% of the rock. Olivine phenocrysts are completely replaced by smectite (probably saponite with subordinate nontronite and celadonite), Fe oxyhydroxide and, more rarely, calcite. Groundmass glass has been partly to completely replaced by the same secondary minerals, with minor amounts of pyrite. A second stage of alteration is marked by the development of black halos, ranging from 2 to 50 mm in thickness. They are seen in hand specimen along surfaces previously exposed to seawater and, less commonly, along the margins of veins and are characteristic of an alteration process initiated during cooling of the lava and completed within 1-2 m.y. (e.g., Honnorez, 1981). Pyrite is associated with the black halos and scattered in the groundmass as far as several centimeters beyond the black alteration front. A third stage of alteration, olive halos containing Fe oxyhydroxide and brown smectite, is common in the upper part of the hole and decreases downhole. This stage of alteration corresponds to halmyrolysis or submarine weathering, which takes place at bottom seawater temperature (i.e., ~2°C) in highly oxidizing conditions and with large water:rock ratio.
Veins are relatively abundant (~20 veins/m) in the basaltic basement. Most result from symmetrical infilling of open cracks with minor or no replacement of the wall rock, and the vast majority contain the following succession of secondary minerals from vein wall to center: smectite and/or celadonite, Fe oxyhydroxide or pyrite, and calcite. Rare, small grains of native copper are present in veins in the upper part of the basement. Veins in the lower part of the basement sequence contain chalcedony and quartz as the final phases precipitated.
The natural remanent magnetization (NRM) of the Miocene ooze and chalk is weak, only slightly above the noise level of the pass-through magnetometer. The Oligocene to Aptian chalk and limestone, with ash layers rich in magnetic minerals, are more strongly magnetized. The NRM of the basalt is strong, but much of the material is broken into small pieces, and reliable magnetic directions are difficult to obtain. However, from detailed sampling of the larger intact pieces, we were able to characterize the intensity and direction of stable remanence. For each core we defined magnetic polarity intervals from consistent values of magnetization and calculated a mean paleoinclination. The combination of polarity intervals and biostratigraphic data yields a magnetic stratigraphy for much of the cored interval below 770 mbsf, including the Cretaceous/Paleogene boundary. All basalt samples measured have normal polarity, consistent with formation of basement during the Aptian. Conversion of paleoinclinations to paleolatitudes, combined with age information, allowed us to construct a drift path for Site 1183 for times between ~120 Ma and the present (Fig. F15). The oldest sedimentary rocks indicate a paleolatitude of 25°-30°S. These values are slightly higher than those determined for basement lava flows at ODP Site 807 (Mayer and Tarduno, 1993) but slightly lower than obtained by Hammond et al. (1975) for basal sedimentary rocks and basement at DSDP Site 289. Results for Sites 1185, 1186, and 1187 are within error of those for Site 1183 (see "Site 1185," "Site 1186," and "Site 1187"). Values for all of these sites are significantly less than both the predicted ~40°S early Aptian paleolatitude of the central plateau in the plate reconstruction of Neal et al. (1997) and the ~50°S latitude of the Louisville hotspot today.
The major results of drilling at Site 1183 are summarized below:
Site 1184 lies on the unnamed northern ridge of the eastern lobe or salient of the Ontong Java Plateau (Figs. F1, F2). The eastern lobe had not been drilled before Leg 192. As with the dome of the high or main plateau (see "Site 1183"), we thought that this site near the summit of the ridge might be in an area that originally was at relatively shallow water depths. The relationship of the eastern lobe to the high plateau is unknown. It could be contemporaneous with the high plateau or be the trace of the postulated plume tail following the emplacement of the high plateau and, specifically, may be the main locus of 90-Ma eruptions (Tejada et al., 1996). Also, the eastern lobe appears to have been rifted into northern and southern portions that were separated by nearly 300 km of seafloor spreading in the Stewart Basin (Kroenke and Mahoney, 1996). The southern portion, Stewart Arch, is the proposed conjugate feature to the northern ridge. Eruptive products of this poorly understood rifting event may have been preserved along both the northern and southern rift-facing sides of the salient. Furthermore, this part of the plateau passed over the calculated position of the Samoan hotspot ~35-40 Ma (Yan and Kroenke, 1993), and volcanic evidence of this passage might be present.
In the seismic reflection record for this site (Fig. F16), a sedimentary megasequence laps onto the upper surface of a large fault block. Reflection character of the block differs from that of basaltic basement on the main plateau (e.g., Site 1183; Fig. F7). Parallel to subparallel, high-frequency, slightly dipping reflections of limited and variable continuity persist to depths as great as 1.0 s of two-way (P-wave) traveltime beneath the surface of the fault block.
We cored lower Miocene pelagic calcareous ooze and chalk (Unit I) from 134.4 mbsf to the top of the fault block at 201.1 mbsf and volcaniclastic rocks (Unit II) from 201.1 mbsf to the base of the hole at 538.8 mbsf. A 1-cm-thick ferromanganese oxide crust represents the contact between the two units. Paleontological data suggest that deposition of the volcaniclastic succession occurred during the middle Eocene (principally nannofossil Zone NP16) and that deposition of the calcareous ooze began during the earliest Miocene. Little, if any, sedimentary record of events during the late Eocene or Oligocene is preserved.
Unit I is dominated by nannofossil foraminifer ooze with as much as 10% siliceous microfossils; volcanic ash is a minor component. Paleodepths appear to have been bathyal. Grain densities generally lie between 2.3 and 2.6 g/cm3, with a mean of 2.5 g/cm3; porosity averages 66.1%, and the mean bulk density is 1.5 g/cm3. The ooze is weakly magnetic and was badly disturbed by drilling; consequently, we were unable to obtain reliable paleomagnetic data.
The volcaniclastic sequence of Unit II consists of coarse lithic vitric tuff, lapilli tuff, and lapillistone, most of which have a massive texture. Several thin beds of fine ash are also present, but we recovered no pelagic or neritic interbeds. Grain densities in Unit II are significantly more variable than those in Unit I, with a mean of 2.4 g/cm3; bulk densities maintain a nearly constant value of ~1.9 g/cm3, and porosities cluster between 31% and 37%. This unit exhibits normal-polarity magnetization and what appears to be a continuous record of paleosecular variation. The mean inclination (-54°) is much steeper than the expected Eocene inclination and indicates a paleolatitude (35°S) significantly different from that expected for this area in the Eocene (~15°-20°S). Tectonic rotation of the volcaniclastic beds may have taken place after the magnetic remanence was acquired, but it is unlikely that a sufficiently large amount of rotation in the direction required has occurred.
We divided Unit II into five subunits on the basis of changes in grain size, sorting, and sedimentary structures (Fig. F17; Table T2). Wood fragments (Fig. F18) and organic-rich layers were found at the boundaries between four of the subunits (B, C, D, and E) and at the base of the cored part of Subunit IIE, perhaps indicating lulls in volcanic activity. Subunit IID contains numerous thin-bedded intervals with inclined bedding. At ~305 mbsf, where a sharp increase in lapilli size marks the boundary between Subunits IIB and IIC, magnetic susceptibility and P-wave velocity increase abruptly, and mean thermal conductivity decreases slightly. Below 380 mbsf, where a reduction in lapilli size marks the top of Subunit IID, both magnetic susceptibility and velocity decrease and mean thermal conductivity increases slightly.
All five subunits of Unit II consist predominantly of coarse ash to lapilli-size glass and volcanic lithic fragments (Fig. F19), with less abundant accretionary and armored lapilli, set in a fine ash matrix (Figs. F20, F21). In most of the sequence, glass fragments are more abundant than lithic fragments. However, both the abundance of lithic fragments relative to glass and the proportion of red, oxidized lithic fragments are greatest in the lapilli tuff and lapillistone of Subunit IIC. Oxidation of lapilli probably accounts for the distinctively high magnetic susceptibility of Subunit IIC, and the presence of both hematite and magnetite has been confirmed by X-ray diffraction analysis.
Glass shards in Unit II
volcaniclastic rocks range from <0.1 to ~10 mm and are predominantly
subangular, blocky, and nonvesicular. Slightly to highly vesicular glass shards
are relatively rare. Tachylite clasts are found throughout Unit II and form the
main component of the upper and lower parts of Subunit IIC. Lithic fragments are
mainly subround and subequant to subelongate and principally comprise
nonvesicular and vesicular basalt (generally <10 mm), ranging from partly
glassy to microcrystalline and fine grained, with rare fragments of diabase (20
mm). Rip-up clasts of tuff (65
mm) are also common. Plagioclase and clinopyroxene grains are present as
phenocrysts in basaltic lithic fragments and as discrete clasts; as clasts, they
are generally anhedral, showing signs of mechanical transport and/or fracturing.
Accretionary (Fig. F20)
and armored lapilli (15
mm) are present in all the subunits and are sometimes concentrated in bands.
We interpret the accretionary and armored lapilli, together with abundant blocky glass shards, to indicate that these deposits were formed by explosive hydroclastic eruptions in a shallow-water to emergent eruptive setting (Fig. F22). The presence of nannofossils in finer grained intervals of tuff suggests primary deposition or reworking in a marine environment, and wood fragments and organic-rich layers indicate proximity to a vegetated island. Several features indicate that a component of the volcaniclastic material was derived from subaerial eruptions. These include the presence of vesicular tachylite lapilli throughout the volcaniclastic sequence, two intervals of well-sorted lapillistone (consisting almost entirely of nonvesicular tachylite at the top and bottom of Subunit IIC), and the abundant red, oxidized lithic fragments in Subunit IIC.
The entire 337.7-m volcaniclastic sequence cored from Hole 1184A is altered to varying extents, and the uppermost 8 m is completely altered to pale brown Fe oxyhydroxide, indicative of weathering in an oxidizing (subaerial?) environment. Except for plagioclase and clinopyroxene, almost all of the volcanic components and matrix are heavily altered to smectite, analcime, celadonite, calcite, zeolites, pyrite, and Fe oxyhydroxide. Unaltered glass is present in several cores (most commonly below ~470 mbsf); individual shards are typically rimmed by brown smectite (Fig. F23). From rim to center, the most commonly observed assemblage of secondary minerals in individual glass fragments follows the progression: smectite; analcime and/or other zeolites; rare calcite. The cement between individual clasts is predominantly composed of the same minerals as those replacing glass. Rare pleochroic, blue-green celadonite is also tentatively identified in the cement, filling vesicles in glass and partly replacing individual glassy fragments. The zeolites identified by X-ray diffraction are gmelinite, chabazite, levyne, mordenite, and natrolite, an assemblage rarely found in submarine basalt but common in subaerial environments. Several generations of white, hairline to >5-mm-wide veins cut the cores; these veins are filled with analcime ± other zeolites ± calcite and lined with minor smectite and/or celadonite. Halos in the groundmass adjacent to veins are rare, diffuse, and poorly developed; if present, they typically extend <1 cm into the wall rock and contain smectite and bluish celadonite or brown Fe oxyhydroxide.
Despite the apparent middle Eocene biostratigraphic age of the volcaniclastic rocks, their chemical compositions are similar to those of the 122-Ma Kwaimbaita-type basalt flows and many of the 90-Ma lavas, such as those at Site 803 (Figs. F11, F12, F13). If the biostratigraphic age is correct, then this result would suggest that a fertile portion of the distinctive Kwaimbaita-type mantle source remained beneath this part of the eastern salient for 50-80 m.y. In light of shipboard inductively coupled plasma-atomic emission spectrometry (ICP-AES) analyses, it now seems unlikely that the Samoan hotspot provided much, if any, material for volcanism at Site 1184, although it potentially could have provided a source of heat for melting.
The major results of drilling at Site 1184 are summarized below:
Site 1185 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 and Nukumanu atolls into the Nauru Basin (Fig. F1). This part of the plateau is far from sites where basaltic basement crust was sampled previously, the closest being ODP Site 803 (334 km to the north-northwest) and DSDP Site 289 (351 km to the west). We chose this site for two principal reasons. Firstly, the portion of basement volcanic stratigraphy that we could sample by drilling was likely to be different in this part of the plateau from that in more centrally located areas. In particular, only relatively few far-traveled lava flows may have reached the edge of the plateau, and it might be possible to sample deeper stratigraphic levels here than atop the plateau. Secondly, the 26 m of lava flows penetrated at ODP Site 803 (the only other basement site on the eastern side of the high plateau) belongs to the 90-Ma eruptive event (Mahoney et al., 1993). Basement at other sites drilled on the high plateau (Sites 289, 807, and 1183) formed at ~122 Ma. We thought it possible that 90-Ma basement might also be found at Site 1185; indeed, seismic reflection data (Fig. F24) reveal intrabasement reflections in this part of the plateau, suggesting that a carapace of 90-Ma lava flows might overlie 122-Ma basalt. If so, drilling at Site 1185 would provide further insight into the extent, composition, and mantle sources of the poorly understood 90-Ma event, documented previously at Site 803 and far to the south in lava sequences on the islands of Santa Isabel (Tejada et al., 1996; Parkinson et al., 1996) and San Cristobal (Birkhold-VanDyke et al., 1996) and in ash layers at DSDP Site 288 (Andrews, Packham, et al., 1975).
We drilled two holes at Site 1185 (Table T1), the first of which was a pilot hole to determine the depth to basement and the length of casing necessary to attach to a reentry cone at the second hole. In Hole 1185A, we started coring sediments at 250.6 mbsf, contacted basaltic basement at 308.5 mbsf, and cored basement rocks to 328.7 mbsf. In Hole 1185B (20 m west of Hole 1185A), we started coring at 308.0 mbsf, contacted basaltic basement at 309.5 mbsf, and cored basement to 526.1 mbsf. We recovered 14.1 m of the 57.9-m sedimentary section cored in Hole 1185A (Fig. F25). The dominant lithology is middle to upper Eocene radiolarian nannofossil chalk, which gradually darkens downward from white to light gray. Maximum bulk density is 1.6 g/cm3. The most distinctive features of the chalk are its abundant siliceous microfossils and a highly variable abundance (in places, a virtual absence) of planktonic foraminifers, suggesting that deposition was often below the foraminifer lysocline. Foraminifers preserved in the chalk indicate a middle to upper Eocene unconformity that may be associated with a (relative) rise in the CCD. Eocene siliceous chalk has been found at other DSDP and ODP sites on the Ontong Java Plateau.
The basement sequence consists of pillow basalt and massive basalt flows. We did not recover the sediment-basalt contact in either Hole 1185A or 1185B, but rare intercalations of limestone are present between lava flows and in fissures within the basalt. The limestone is composed of micritic calcite with very rare and poorly preserved nannofossils, foraminifers, and radiolarians that provide only rough age control but reveal that limestones of two ages are present. Extremely rare nannofossils in limestone within the upper 15 m of basement indicate a latest Cenomanian to Albian (possibly late Albian) age, whereas recrystallized planktonic foraminifers in thermally metamorphosed limestone 126 m below the top of basement suggest a late Aptian age. This difference in age between the upper and lower parts of the basement section corresponds to differences in basalt petrography, composition, and alteration. The entire basement sequence exhibits normal magnetic polarity, compatible with this range of ages. Paleolatitudes derived from paleoinclination data are similar, within error, to those for Site 1183. The ~50-m.y. hiatus between the lower sediments and basement suggests that this site may have been below the CCD for much of the time from the Cenomanian to middle Eocene.
In Hole 1185A, we cored 16.7 m of pillow basalt (312.0 to 328.7 mbsf; 67% average recovery). The pillows have glassy rims, spherulitic chilled margins, and fine-grained interiors. We divided the basalt into five units (Fig. F26) on the basis of apparent limestone interbeds, some of which may be only interpillow fill. The 216.6 m of basaltic basement penetrated in Hole 1185B (309.5 to 526.1 mbsf; 42% average recovery) exceeds the previous maximum on the plateau of 149 m of lava flows penetrated at Site 807 (Kroenke, Berger, Janecek, et al., 1991). We divided the basement section of Hole 1185B into 12 units ranging in thickness from 1 to 65 m. Units 1, 3, 4, and 6-9 were identified as pillow basalt on the basis of glassy rims and grain-size variations, and Units 2 and 5 are more massive lava flows with pillowed tops and bases. Units 1-9 are separated by thick (as much as 70 cm) intervals of hyaloclastite breccia composed of pillow-rim fragments cemented by carbonate and clay. Units 10-12 are massive flows. The flow tops of Units 10 and 12 are marked by carbonate- and clay-cemented breccia; the top of Unit 11 was not recovered but was inferred from the presence of vesicles, a pronounced change in alteration, and a marked increase in drilling rate over an interval of ~3 m.
The basalt from Hole 1185A
and in Units 1-9 of Hole 1185B is sparsely to moderately olivine phyric and
generally highly veined. Olivine, the only common phenocryst phase, varies from
fresh, in the glassy and aphanitic rims of pillows, to completely replaced by
smectite, Fe oxyhydroxide, or calcite. Tiny octahedral crystals of chrome spinel
are present, often as inclusions in the olivine phenocrysts (Fig. F27).
Aphanitic pillow margins display a prominent spherulitic texture (Fig. F28)
that grades into variolitic texture in fine-grained pillow interiors. The
massive units also have variolitic texture and are less heavily veined. Units
10-12 in Hole 1185B contain small, sparse phenocrysts of plagioclase and
clinopyroxene in addition to olivine. These rocks are similar in appearance to
the basalt flows at Sites 1183 and 1186 and, like them, contain plagioclase-rich
xenoliths. Shipboard ICP-AES analyses show that Units 10-12 are also very
similar in composition to basalt from Holes 1183A and 1186A (Figs. F10,
F11, F12,
F13). For
example, all have TiO2 
1.1 wt%, Cr
200 ppm, and Zr
60 ppm and appear to belong to the widespread Kwaimbaita magma type. In
contrast, samples of the overlying basalt flows and those in Hole 1185A have the
lowest concentrations of incompatible elements (TiO2
0.7 wt%; Zr
38 ppm; Fig. F12)
and the most primitive, magnesium-rich compositions (MgO 8-10 wt%; Cr
460 ppm) yet found in basalt from the plateau (Figs. F11,
F13). This
combination of elemental characteristics appears to indicate that their parental
magmas formed by even higher total fractions of partial melting than other
Ontong Java Plateau basalts (see "Results
from Previous Sampling of Cretaceous Igneous Basement"in
"Background").
Bulk densities are higher (>2.4 g/cm3) in Units 2, 5, 10, and 11 than in Units 3 and 6-9 (<2.3 g/cm3). Both grain and bulk density decrease downhole in Units 4-9, corresponding to a change from massive to highly veined pillow basalt, and bulk density increases with the change from veined to massive basalt from Unit 9 to Units 10-12. P-wave velocities are generally >5000 m/s in the dense basalt of Units 2 and 10-12 and generally <5000 m/s in the veined basalt of Units 3 and 6-9.
Seawater-derived fluids have interacted pervasively at low temperatures with the basaltic basement, and we can divide the basement section into two groups of flows with different alteration characteristics. One group consists of all the basement units of Hole 1185A and Units 1-9 of Hole 1185B. Alteration in these units occurred under highly oxidizing conditions and with high water:rock ratios, as indicated by light and dark yellow-brown colors near glassy and aphanitic pillow margins; these colors fade to gray-brown and dark gray in coarser grained pillow interiors. The yellow-brown colors are a result of the complete replacement of olivine and pervasive alteration of groundmass by smectite (saponite and nontronite) and Fe oxyhydroxide. Although olivine phenocrysts are commonly completely replaced, rare unaltered olivine is present in aphanitic, dark gray to black areas interpreted as pillow rims. These characteristics are significantly different from the style of alteration in basalt at Site 1183.
A very different type of alteration has affected Units 10-12 at Hole 1185B. The top of Unit 10 marks a dramatic change in alteration character, and the brecciated top of this unit consists of pervasively altered angular basalt fragments. Such severe alteration is likely to be the result of exposure of very permeable basaltic seafloor to bottom seawater for an extended period of time (several million years). Groundmass alteration in Units 10-12 is characterized by pervasive celadonite. Unaltered glass is much rarer in these units than in Units 1-9, but this may be a consequence of the greater flow thickness rather than the level of glass alteration. Broad (centimeter scale) green-gray halos surround veins; wider reduction fronts consisting of scattered pyrite grains in the groundmass extend a few millimeters to a few centimeters beyond these halos. Smaller (millimeter scale) olive halos similar to those from Hole 1183A also are developed near veins, both within green-gray halos and where green-gray halos are absent.
Veins throughout the basement sequence predominantly contain calcite, zeolites (phillipsite), smectite, Fe oxyhydroxide, and rare celadonite and pyrite. Some veins were probably produced by sediment filling open fractures in the basalt; these veins are <5 mm to a few centimeters wide, are filled with pink carbonate and Fe oxyhydroxide, and contain recrystallized foraminifers. They are present both near the boundaries and within the interiors of basaltic units.
The major results of drilling at Site 1185 are summarized below:
The plan for Leg 192 included a site on Stewart Arch within the territorial waters of the Solomon Islands (proposed Site OJ-7; see Fig. F1). However, ODP and the U.S. State Department were unable to obtain clearance for this site before the cruise. By midcruise, it was evident that clearance would not be forthcoming in time (if at all) to drill the site. We therefore chose Site 1186, on the eastern slope of the main Ontong Java Plateau, 206 km west of Site 1185, 319 km east of Site 1183, and 149 km east-southeast of DSDP Site 289 (Figs. F1, F2). The very different volcanic stratigraphy at Sites 1183 and 1185, particularly our discovery of high-Mg, low-Ti basalt of probable latest Cenomanian to Albian age at Site 1185, highlighted the importance of a site at a location intermediate between the crest and eastern edge of the main plateau.
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. Extending from this body toward the drill site is a package of high-amplitude and continuous reflections (Fig. F29) 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 are similar to 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. F30) 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, similar to those 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 particularly chert-rich interval of sediment.
Unit III (812.7 mbsf to the contact with basaltic basement at 968.6 mbsf; Fig. F30) 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) in Hole 1186A consists of basalt lava flows with minor interbeds of yellowish brown sandstone and one interval of reddish brown conglomerate containing rounded limestone clasts (Fig. F31). 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 4.37 s two-way traveltime (~725 mbsf; see Fig. F29) as an ~4-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, particularly, 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. F32). 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. F33).
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 from Hole 1183A, the lower group at Hole 1185B, Units C-G at Site 807, and the Kwaimbaita Formation on the island of Malaita (Figs. F10, F11, F12, F13). 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 from 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 meso-stasis. 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; and (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.
Veins in the basement rocks are lined with smectite or celadonite and filled with calcite. Veined basalt has lower bulk density than does unveined basalt (<2.4 g/cm3 compared with >2.4 g/cm3), lower P-wave velocity (<5000 m/s compared with >5000 m/s), and lower magnetic susceptibility.
The major results of drilling in Hole 1186A are summarized below:
We decided to drill Site 1187 while coring in Hole 1186A. By the time we had penetrated ~50 m into basement at Site 1186, it was clear from shipboard ICP-AES analyses that the basalt was of the widespread, remarkably homogeneous, ~122-Ma Kwaimbaita magma type found in Hole 1183A and in the lower 92 m of basement at Site 1185. The bottom of the Kwaimbaita-type lava sequence has not been reached in any of the locations where such lavas have been encountered. This sequence is >100 m thick at Site 807 (Units C-G) (Kroenke, Berger, Janecek, et al., 1991), and on the island of Malaita the Kwaimbaita Formation is >2.7 km thick (Tejada et al., in press). Furthermore, our rate of penetration in basement was low, and there was risk involved in reentering (after an imminent drill-bit change) an uncased >900-m-deep hole in chert-rich sediment. These considerations led us to favor drilling a new site to provide fundamental new information about the age, composition, and/or mantle sources of the plateau over deepening Hole 1186A.
A site somewhere between Site 1185 and Site 803 would be particularly useful because, unlike other sites on the main plateau, Sites 803 and (on the basis of shipboard biostratigraphic data) 1185 contain basalt that is younger than 122 Ma, and the lava flows in the upper 125 m of basement at Site 1185 are compositionally different from any seen elsewhere on the plateau. We therefore selected Site 1187 (Fig. F1), near Site 804 (which did not reach basement) on the eastern edge of the main plateau. Site 1187 is 194 km southeast of Site 803, 146 km north of Site 1185, and <3 km from the easternmost point where Ontong Java Plateau basement can be distinguished easily, on seismic records, from that of the Nauru Basin (Fig. F34). Our principal objectives at Site 1187 were to establish the composition, age, and eruptive environment of the basement volcanic rocks and to compare them with those of lavas sampled at Sites 803 and 1185. In particular, we wanted to determine whether basement in this region was emplaced during the ~122-Ma or later magmatic events, or both.
We encountered basaltic basement at 372.5 mbsf (Table T1), ~40 m shallower than estimated from the seismic record. We had begun coring at 365.5 mbsf and recovered only 1.47 m of the sedimentary succession above basement. The sediment recovered consists of dark brown ferru-ginous claystone that grades downward from burrow mottled to laminated and overlies an ~2-cm-thick chalk layer. Biostratigraphic analysis of the claystone indicates an age of late Aptian to Albian. The chalk layer, which immediately overlies basalt, contains a late Aptian foraminiferal assemblage, including the planktonic taxa Globigerinelloides ferreolensis, Blowiella duboisi, and Blefuscuiana praetrochoidea, and a lower-slope benthic assemblage. The calcareous nannofossils Eprolithus floralis and Hayesites irregularis, present without any Albian-restricted species, are consistent with a late Aptian age for the chalk. Washed residues of the overlying claystone very rarely contain the planktonic foraminifer Globigerinelloides aptiensis, indicating an age range of late Aptian to middle Albian. A single, questionable specimen of the Albian taxon Blefuscuiana albiana was also recovered. The claystone residues are dominated by fish-bone fragments and small ferromanganese nodules, suggesting slow accumulation below the CCD.
A 70-cm-thick interval of the claystone is reversely magnetized. The late Aptian to Albian biostratigraphic age of the claystone suggests that this interval may be M''-1r'' (also called "ISEA" after the site code used for its initial discovery in Italy), a short reversed-polarity subchron (~115 Ma) within the Cretaceous Normal Superchron. The underlying basalt flows all exhibit normal polarity. Paleoinclination data for the basalt indicate a paleolatitude of ~19°S, essentially the same as at Site 1186 and within error of values for Sites 1183 and 1185.
We cored 135.8 m of basaltic basement, which we divided into 12 units (ranging in thickness from 0.7 to 41.3 m; Fig. F35) on the basis of recrystallized limestone, significant (>10 cm thick) hyaloclastite interbeds, and/or downward changes in lava flow structure from massive to pillowed (e.g., the contact between Units 6 and 7). Most of the sequence consists of pillow-lava flows. The only unequivocally massive portion is the fine-grained, 9-m-thick base of Unit 6; some basalt interpreted to be from pillows >2 m thick also may be from massive flows.
The rims of the pillows
contain both unaltered and altered glass. Basalt inside the rims is aphanitic,
generally altered, and often contains spherulitic zones stained by Fe
oxyhydroxide. In the larger pillows, the grain size coarsens gradually from
aphanitic near the margins to fine grained in pillow interiors. The basalt is
aphyric to moderately olivine phyric. Fresh olivine is present in some of the
least-altered intervals of fine-grained basalt in pillow interiors and in the
massive basalt of Unit 6. Rare, irregular miarolitic cavities (as large as 1 cm
× 2 cm) are present in the interiors of pillows and in the massive portion of
Unit 6. Shipboard ICP-AES analyses (Figs. F10,
F11, F12,
F13) show that
the basalt is relatively primitive (MgO
9 wt%; Cr
485 ppm), like the upper 125 m of lava flows at Site 1185, and virtually
identical to them in its unusually low concentrations of incompatible elements
(e.g., Zr and Ti). The presence of these distinctive lava flows in
>100-m-thick piles at two sites 146 km apart implies that substantial volumes
of this type of magma were erupted on the eastern flank of the high plateau
after the main plateau-forming eruptions at ~122 Ma.
Seawater-derived fluids have interacted at low temperatures with the basaltic basement, resulting in the most pervasive overall alteration observed during Leg 192. This observation is consistent with the high abundance of relatively small pillows. Alteration occurred under highly oxidizing conditions with high water:rock ratios and resulted in the development of light and dark yellow-brown colors through the complete replacement of olivine and the alteration of groundmass to smectite (saponite and nontronite) and Fe oxyhydroxide near the outer zones of pillow margins. The color grades into dark brown and dark gray in the coarser grained pillow interiors. Despite the high average level of alteration in the basalt, unaltered glass is relatively abundant at Site 1187 because of both the large number of individual pillows (i.e., more glassy margins are present per length of core) and the greater thickness of many of the glassy margins compared with those at other Leg 192 sites. Overall, the secondary mineral assemblages and visual characteristics of basalt alteration at all Leg 192 sites are remarkably similar to those seen elsewhere in nonplateau seafloor of varying ages. This similarity indicates that alteration conditions in basement on the Ontong Java Plateau were similar to those operative in typical ocean crust formed at spreading centers.
Veins throughout the basement sequence consist mainly of calcite, zeolites (probably phillipsite with analcime), smectite, Fe oxyhydroxide, and rare celadonite and pyrite. As at Sites 1185 and 1186, the physical properties of basement at Site 1187 strongly reflect the amount of veining and alteration in the basalts. P-wave velocities are high (>5300 m/s) in the dense, relatively sparsely veined basalt of Units 6 and 7 and lower (<5300 m/s) in the more abundantly veined basalt of other units. Areas of high magnetic susceptibility also correlate with the presence of dense, unveined basalt, and the mean bulk densities of sparsely veined intervals are >2.7 vs. <2.7 g/cm3 in abundantly veined intervals.
The major results of drilling in Hole 1187A are summarized below:
