Holes 1155A and 1155B were rotary cored into igneous basement from 177.3 to 203.5 and 147.9 to 193.9 mbsf, respectively. Hole 1155A was drilled (Sections 187-1155A-2R-1 through 7R-1) 26.2 m into basement, resulting in 2.34 m of recovered core (equal to 8.93% recovery). Lavas from this hole have been assigned to two units. Unit 1 consists of a sparsely to moderately plagioclase ± olivine phyric basalt, present in Sections 187-1155A-2R-1 through 4R-1; Unit 2 is an aphyric basalt, present in Sections 187-1155A-5R-1 through 7R-1. Unit 2 is dominated by pebble- and cobble-sized fragments with weathered, not drilled, outer surfaces. We interpret Unit 1 as a pillow lava overlying the loose basalt talus of Unit 2.
Hole 1155B was drilled (Sections 187-1155B-2R-1 through 10R-1) 46 m into basement, resulting in 18.18 m of recovered core (equal to 39.52% recovery). Section 187-1155B-10R-1 was drilled 9.6 m into basement with no recovery because of a broken core catcher. Prior to this section, recovery was 49.95%. Lavas from this hole were assigned to a single lithologic unit of moderately plagioclase ± olivine phyric basalt. We interpret this hole as having sampled an intact pillow lava sequence, based on its uniform lithology and the high percentage of pieces with glassy rinds and chilled margins (42.26%: 112 of 265 pieces). Light pink micritic limestone is present in interpillow spaces attached to glassy margins (Fig. F1) and as infill along fractures perpendicular to glassy margins (Sections 187-1155B-6R-1 through 6R-4; Sections 187-1155B-8R-1 through 9R-1) (e.g., Fig. F2).
This unit comprises a medium gray, sparsely to moderately plagioclase-olivine phyric basalt that is slightly altered overall (see "Alteration"). The basalt contains <1% small (<0.5 mm in diameter), rounded vesicles that are typically lined with smectite and have calcite filling the interior (Fig. F3). The unit contains 3% glomerocrysts of plagioclase ± olivine, with plagioclase as the more abundant phase. Plagioclase is prismatic to tabular, as large as 5 mm in size, and commonly twinned. Olivine is equant and as large as 3 mm in size. Subrounded plagioclase—usually with corroded, probably resorbed, rims surrounding uncorroded euhedral cores—is present.
As seen in thin section, the microcrystalline groundmass texture is intersertal with plumose quench textures. The groundmass consists of 45% clinopyroxene (plumose quench growths), 40% prismatic to tabular to microlitic plagioclase with some acicular overgrowths, 1% equant olivine, and 14% mesostasis.
This unit consists predominantly of a slightly to moderately altered (see "Alteration"), medium gray aphyric basalt. Several pieces of moderately plagioclase-olivine phyric basalt were also recovered, but whether these are in situ or fell into the hole from Unit 1 above is unclear. The aphyric basalt contains <1% small (<0.5 mm in diameter), spherical vesicles that are filled with smectite. Very rare anhedral to subhedral plagioclase (up to 2 mm) and anhedral olivine (up to 0.6 mm) are present in <1% of the unit. The few phenocrysts present usually form glomerocrysts of plagioclase ± olivine. The plagioclase is typically twinned and displays oscillatory zoning. Some of the plagioclase is sieve textured with rounded to oval holes, probably due to resorption (Fig. F4). A few pieces from this unit (e.g., Section 187-1155A-6R-1 [Piece 4]) display a typical pillow margin sequence grading from glass rind through a spherulitic band to a more crystalline interior.
The microcrystalline groundmass texture is intersertal (Fig. F4). Lath-like plagioclase and elongate clinopyroxene are present in equal proportions and account for ~85% of the groundmass. The remaining 15% of the groundmass consists of ~2% equant olivine and ~13% glass with cryptocrystalline quench phases; ~70% of each has been altered to Fe oxyhydroxide and palagonite, respectively.
The outsides of most pieces from this unit are fracture surfaces coated by clay and/or calcareous sediment with Mn oxide spots, typically <1 mm in size.
Unit 1 is a buff (where weathered) to medium gray (where fresh), moderately plagioclase-olivine phyric basalt that ranges from highly altered near the top of the hole to slightly altered at the bottom (see "Alteration"). Small (<0.5 mm in diameter), spherical vesicles are rare (<0.5% throughout the unit), and some are filled with clay. Subhedral prismatic to tabular plagioclase phenocrysts as long as 7 mm constitute 5% of the basalt. In some cases they are sieve textured (~15%), probably resorbed, with oval holes (Fig. F5) elongate parallel to fracture and cleavage planes. Subrounded plagioclase with corroded, probably resorbed, rims around euhedral uncorroded cores is also present. In most cases plagioclase is twinned, and some crystals display oscillatory zoning. Cr spinel inclusions are rare in plagioclase phenocrysts (Figs. F6, F7). Equant, subhedral olivine phenocrysts as large as 3 mm make up ~1%-2% of the basalt. Of the olivine phenocrysts, 10%-20% are partially to entirely altered to iddingsite. Glomerocrysts of plagioclase ± olivine are common. Both plagioclase phenocrysts and microphenocrysts are flow aligned near chilled margins in several pieces (e.g., Sections 187-1155B-2R-1 [Piece 9] and 5R-1 [Piece 5]; Fig. F8).
As seen in thin section, the microcrystalline groundmass texture is intersertal (Fig. F7). Plagioclase constitutes 45% of the groundmass, displaying acicular to lath-like quench morphologies. Equant olivine and opaque minerals make up 2% of the groundmass; the olivine is commonly altered to iddingsite. The remaining ~53% is mesostasis, including clinopyroxene quench textures and altered glass.
More than 40% of the pieces in this unit have glassy rinds and chilled margins. Many of these margins show transitions from fresh glass to spherulitic glass to coalesced spherulites (Fig. F9) to more crystalline interiors. Piece 11 in Section 187-1155B-8R-2 is cobble sized and consists of micritic limestone surrounding three ellipsoids of glass and palagonite pillow buds (3-3.5 cm across). The glassy margins of Pieces 1 and 13 are unusual in that they appear to have been folded back into the pillow lava (i.e., fingers of glass + palagonite + micritic limestone protrude into the more crystallized interior). These features may be a result of an eruption of pillow lavas into a slurry of calcareous ooze. However, this same sediment also fills fractures (with no chilled or glassy margin) (Fig. F2) and interpillow spaces. This suggests that some or all of the sediment may have formed after eruption and, through time, filtered into the pillow lava pile, perhaps with one or more significant time breaks.