Holes 1158A, 1158B, and 1158C were cored into basement at this site. Hole 1158A penetrated 14.4 m into basement, recovering 0.85 m of basalt core from three sections (Sections 187-1158A-1W-2, 2R-1, and 3R-1), a net recovery of 5.90%. Hole 1158B penetrated 15.0 m into basement, recovering 1.6 m of core from three sections (Sections 187-1158B-2R-1, 3R-1, and 4R-1), a net recovery of 10.67%. Hole 1158C penetrated 9.4 m into basement, recovering 1.61 m of core from three sections (Sections 187-1158C-1W-1, 2R-1, and 2R-2), a net recovery of 17.13%.
All basalt recovered from this hole is included in one lithologic unit: aphyric to sparsely olivine-plagioclase phyric basalt. Glassy rinds, chilled margins, and the wedge-shaped, fracture-controlled morphology of some pieces (e.g., Section 187-1158A-1W-2 [Piece 1]) indicate that these are fragments of pillow lava. Pieces range from ~1 cm to several centimeters in size (i.e., less than the core diameter) and are therefore unoriented. The color of the basalt varies from gray to brownish gray, reflecting local differences in degree of alteration that range from slight to moderate (see "Hole 1158A" in "Alteration").
Olivine phenocrysts are generally equant and euhedral to subhedral; plagioclase phenocrysts are tabular to prismatic. Approximately 20% of the phenocrysts are present as single-phase or two-phase glomerocrysts. Four of the five pieces in Section 187-1158A-1W-2 have between 1.5% and 2.5% total phenocryst content, with olivine and plagioclase in roughly equal amounts (the average size of both phases is ~0.8 mm). One of the nine pieces in Section 187-1158A-2R-1 has a similar pheno-cryst population. The remaining pieces in both sections are aphyric. Section 187-1158A-3R-1 contains ~1% olivine and ~0.5% plagioclase microphenocrysts (~1 mm).
In most cases, the groundmass is fine grained. Section 187-1158A-2R-1 (Piece 3) is unusual in that it is aphyric but has a grainy texture and a coarser groundmass (fine to medium grained; 0.5-1 mm) than the other aphyric pieces in this section. Partial replacement of glassy groundmass by brown clays highlights the spherulitic texture of chilled margins in Section 187-1158A-2R-1. In addition to spherulites, the chilled margins are composed of plagioclase crystals with parallel "box" structures (Figs. F1, F2), indicating rapid cooling. The edges of some subhedral to anhedral prismatic plagioclase phenocrysts in these chilled margins also have overgrowths of box-textured plagioclase that are in optical continuity with the phenocryst (e.g., Sample 187-1158A-2R-1 [Piece 1]; Fig. F3).
Away from the chilled margins, groundmass plagioclase is locally flow aligned on a thin-section scale. In addition, where olivine (or olivine and plagioclase) glomerocrysts are present, some groundmass plagioclase forms snowball textures around the glomerocrysts (e.g., Section 187-1158A-3R-1 [Piece 3]; Fig. F4). In Piece 3, olivine is seriate, ranging from ~1 mm microphenocrysts to groundmass crystals <10 µm in size. Olivine is generally equant and euhedral. About 60% of the microphenocrysts are replaced by Fe oxyhydroxide throughout this core, but it is fresh outside oxidized zones in Section 187-1158A-3R-1. Mn oxide, either as spots or as a continuous coating, is present on weathered (oxidized) uncut surfaces.
All basalts from this hole are included in one lithologic unit: aphyric to sparsely olivine-plagioclase phyric basalt; phenocryst variations are similar to those from Hole 1158A, Unit 1. Color depends on the groundmass grain size and the degree of alteration. Pieces with microcrystalline to very fine grained groundmass range from medium to dark gray where fresh and to whitish gray where altered. Those with a fine-grained groundmass are medium gray where fresh and brownish gray where altered. As in Hole 1158A, most pieces are <3 cm in size and therefore unoriented relative to the core. They have weathered, uncut surfaces with alteration halos. Overall, alteration ranges from slight to moderate (see "Hole 1158B" in "Alteration").
Phenocryst abundance ranges between 0% and 2%, of which olivine comprises ~55% and plagioclase ~45%; both phases are <2 mm in size. Plagioclase phenocrysts are mainly tabular to prismatic and euhedral, although 1- to 2-mm rounded to subrounded plagioclase occurs in some places in Section 187-1158B-4R-1. Glomerocrysts of olivine and plagioclase make up as much as 40% of the phenocryst content in some pieces (e.g., Section 187-1158B-3R-1 [Piece 2]). Many of the plagioclase phenocrysts (~50%) throughout this hole have a wide range of disequilibrium textures, including concentric oscillatory zoning (e.g., Section 187-1158B-2R-1 [Piece 1]; Fig. F5), discontinuous twin planes, and sieve textures.
Groundmass is generally fine grained (<1 mm) but approaches medium grained (~0.9 mm) in some pieces (e.g., Section 187-1158B-2R-1 [Pieces 10 to 14]); the latter have a grainy texture like Section 187-1158A-2R-1 (Piece 3). In Section 187-1158B-2R-1, 10 of the 14 pieces have a similar grainy texture. In many of these pieces, groundmass plagioclase (~0.9 mm) forms a felty texture with clinopyroxene filling the interstices (e.g., Piece 1). Clinopyroxene in the groundmass varies from discrete euhedral to anhedral crystals (e.g., 187-1158B-2R-1 [Piece 1]; Fig. F5) and subophitically encloses plagioclase in the grainy textured pieces (e.g., Section 2R-1 [Piece 13]). Clinopyroxene in the groundmass ranges from ~0.5 to 1.5 mm in size, although varying degrees of strain extinction and areas of small anhedral crystals in which new subgrain boundaries have formed sometimes obscure original grain boundaries. Five pieces in Section 187-1158B-4R-1 have altered glassy rinds.
There are two lithologic units in Hole 1158C. This hole also has the highest percentage (~25%) of oriented pieces at this site.
Unit 1 (Section 187-1158C-1W-1 to Section 2R-1 [Piece 8]) consists of aphyric to sparsely olivine-plagioclase phyric basalt rubble with a range in grain size and textures similar to that seen in Holes 1158A and 1158B (see "Hole 1158A" and "Hole 1158B").
Unit 2 (Section 187-1158C-2R-1 [Piece 8] to Section 2R-2) is a diabase, defined by its subophitic texture and medium grain size (1-5 mm). Thin section shows this to consist of plagioclase and clinopyroxene in roughly equal proportions with ~2% equant to skeletal opaque minerals. The color of this unit is light brownish gray; the brown coloration is due to alteration and Fe staining of plagioclase; overall, alteration ranges from moderate to high (see "Hole 1158C" in "Alteration"). The subophitic texture (Fig. F6) is visible in hand specimen (e.g., Section 187-1158C-2R-1 [Piece 13]). Grain size decreases with depth through Section 187-1158C-2R-2, but a grainy texture persists in hand specimen. Section 187-1158C-2R-2 is also more vesicular (5%-7%) than either the diabase in shallower sections in this hole or the basalts from this and earlier sites. The vesicles are spherical, average 0.7 mm in diameter, and are filled by a cream-tan clay/silicate mixture.
The systematic decrease in groundmass grain size, beginning 30 cm from the bottom of Section 187-1158C-2R-1 and extending through Section 2R-2, may be interpreted as a finer grained marginal facies of Unit 2. There is no evidence indicating whether Unit 2 is a flow or an intrusion.