No sediments were recovered from Hole 1243A, and few were recovered from Hole 1243B. Core 203-1243B-1R contains a slurry of mud with a few coherent pieces of nannofossil ooze that had caved into the hole. The pieces are of lithologies and colors like the oozes recovered during Leg 138. They are composed dominantly of coccoliths, with zero to a few percent planktonic foraminifers, discoasters, radiolarians, FeO globules, and glass.
Three intervals within the recovered basalt contain sediment. Section 203-1243B-2R-1 [Piece 6] is a centimeter-sized firm lump of nannofossil ooze that must have caved into the borehole. Section 203-1243B-2R-1 [Piece 9] is an oblate-shaped piece of white limestone with brown palagonite grains and is the main subject of this report. One end of Section 203-1243B-10R-2 [Piece 8] has a thin selvage of pinkish gray limestone with palagonite grains, black specks, and apparent relics of foraminifers adhering to the glassy rind of the basalt similar to that of Core 2R, which is described next.
The white limestone in interval 203-1243B-2R-1 [Piece 9, 45-47 cm] is in proper stratigraphic position, as some of the basalt pieces recovered above and below it are of sufficient diameter to have blocked any vertical shuffling. The 1-m interval from 3984 to 3985 m on borehole logs (1 or 2 m below top of basement) has distinctly lower density and porosity and very likely pinpoints the position of the limestone. This rock is a palagonite- and peloid-bearing foraminiferal limestone. By texture and composition, it is a sparse foraminiferal pelbiomicrite; by its texture at time of deposition, it is a wackestone (Fig. F10). Small dots of a black mineral, up to 5 mm across (probably manganese oxide), mark the white surface on which small circles of finely crystalline calcite outline microfossils. Some flakes of brown palagonite, ~1 mm thick and up to 8 mm wide, are seen on the surface, and additional volcanogenic grains are seen in thin section. The prismatic shape of abundant pores in the rock indicates that authigenic anhydrite had been present in early diagenesis. Presumably, the limestone formed by the diagenesis of peloidal foraminiferal nannofossil ooze that had accumulated in a depression on a basalt surface, perhaps between pillows, and was covered by a later flow.
Calcite is the predominant mineral. The limestone's main component is micrite, based on examination of two thin sections and 650 points counted on one of them, excluding points that fell on voids and plucked areas. Most micrite is matrix between the foraminifers and peloids (44% of the slide), and additional micrite constitutes peloids (15%) and fills some foraminifer tests (7%). Some of the micrite is featureless, but scattered through parts of it are round darker areas or clots ~20 µm across (diameters are reported as observed, not as reduced by 87% to relate random slices to average sphere volumes). Their edges are fuzzy, and light does pass through them; they are dark but not opaque. The micrite is very finely crystalline to aphanocrystalline, with crystal diameters typically 1-2 µm. The ragged feather edge of a thin section, ground as a wedge from 30 to 0 µm, has a few coccoliths among the calcite crystals in the mounting medium. Those nannofossils, and several seen on an acetate peel from the limestone surface, are more numerous than would be expected from laboratory contamination. As nannofossils are the dominant component of the sediment above basement, the micrite of this limestone is thought to have crystallized from nannofossil ooze.
Peloids in this rock are rounded to somewhat irregular masses with a darker border and scattered fuzzy-edged darker clots, as described above, and are held to indicate origin as fecal pellets in modern and ancient peloids. They appear to be bimodal in texture, with modes in medium sand size at ~0.3-0.5 mm and in coarse silt size at ~0.04-0.06 mm. Points were not counted on the second thin section, but by visual estimation, it contains more foraminifers and fewer peloids (ratio of ~3:2) than the approximately equal proportions recorded here.
Walls of foraminifer tests account for 15% of the area of the slide. Many tests appear whole, whereas others are obviously broken. In addition, individual and multiple parallel grains of prismatic calcite within the matrix have lengths similar to the thicknesses of foraminiferal walls and, hence, are assigned as such. Thus, foraminiferal calcite is ~17% of the slide. Virtually all foraminifer walls have the shape of arcs, and many that are sectioned show interiors with globular shapes. Pores may be filled with opaque material, a situation that seems more common in those with globigerinoid-type wall structure. A few biserial, trochoid, and biconvex foraminifers may be present, according to test outlines and the patterns of calcite crystals.
Sparry calcite and straight-edged (commonly square to rectangular) pores fill foraminifer interiors lacking micrite. Spar acts as a patchy cement, with the distinctive pores also present in some areas between the allochemical grains (foraminifers and peloids), in particular between silt-sized peloids. Spar and pores also fill irregular veins and a burrow, and some square voids are randomly distributed in micrite. About 11% of the rock is a mosaic of blocky calcite spar (in crystal sizes mainly <0.15 mm) but, in one instance, 0.25 mm x 0.25 mm. There is little evidence that the spar grew as syntaxial overgrowths on the foraminifers, but there is no doubt that the calcite grew against the earlier mineral now represented by pores. The rectangular pores were recorded on the traverses of points but not included in calculating components of the solid rock. If all were recalculated, the now-absent mineral would have been 6% of the volume of the limestone. The larger of the absent prisms were ~0.20 mm long and 0.06 mm across. Almost certainly, the missing mineral was anhydrite, but no anhydrite was detected by X-ray diffraction (XRD). The paragenesis was growth of euhedral anhydrite in pore spaces, growth of anhedral calcite against anhydrite, and dissolution of anhydrite leaving the distinctive moldic porosity.
The limestone's surface is speckled with black. Typically, when black minerals like these that appear on limestone are analyzed, they are found to be manganese oxides. In thin section, the opaque areas have different forms but provide few clues for their shipboard identification. The most abundant of the opaque grains are 10-25 µm in size and are present in clusters. They display straight sides and multiple sharp angular corners that must represent crystal faces and angles of an aggregate of opaque grains. They lack the bright red color of thin edges of hematite in conoscopic light; these are brownish black to reddish brown. Nor do they have the brassy luster of pyrite in reflected light; these are dull black. The three most common manganese oxide minerals in the marine environment (e.g., in ferromanganese nodules and crusts and in micronodules in pelagic sediments) are vernadite followed by birnessite and todorokite. Only todorokite is represented in the MacDiff XRD database; its lines are not present on the diffractogram. Nor are there any lines for the principal d-spacings given in Burns and Burns (1979) and Roberts et al. (1990) for vernadite (2.39 main; 4.80 and 2.39) and birnessite (7.27 main; 3.56, 2.44, and 1.41). Thus, the actual mineralogy of these opaque minerals remains unidentified, even though they are probably manganese oxides.
The largest totally opaque areas are as wide as 0.25 mm in one section and 0.90 mm in the second. Their edges seem more ragged than edges of the smaller masses mentioned above, but in conoscopic and reflected light their appearance is similar; probably, they are dense accumulations of that mineral. By their size, distribution, and distance apart, these opaque masses represent the black specks seen on the sample.
A very minor constituent of the slide is a cluster of brown, platy, nearly opaque unidentified grains that are ~30 µm long x 7 µm wide. Seen, but not encountered by the cross hairs during the traverses, are a few pieces of fish debris.
Glass with plagioclase microlites was not counted because the traverses avoided the plucked areas where it is present. Laths of plagioclase (to 0.3 mm long) are sheathed in glass, which has been partly replaced by swelling clay and calcite. Plagioclase and smectite are confirmed by small peaks on the X-ray diffractogram and by grains of the feldspar and clumps of the clay in the insoluble residue. The residue also has some glass grains (mainly heavily etched), some spicules, and a mass of zeolites that grew in what may have been a radiolarian test.