Holes 1156A and 1156B were cored into igneous basement from 118.2 to 129.6 mbsf and from 181.6 to 215.2 mbsf, respectively. Hole 1156A recovered seven sections (Sections 187-1156A-2R-1 through 3R-3), penetrating 11.4 m into basement, with 6.30 m of recovered core (55.3% recovery). Two lithologic units have been assigned for this hole. Unit 1 is interpreted as a talus breccia. Clasts of basalt and palagonitized glass are present in two generations of micritic limestone matrix, suggesting that brecciation took place in two stages. The basalt clasts are moderately plagioclase-olivine phyric and are found in Sections 187-1156A-2R-1 through 2R-3. Unit 2 is a moderately to sparsely plagioclase-olivine phyric pillow basalt, found in Sections 187-1156A-2R-3 through 3R-3. Calcareous sediment, similar to that in Unit 1, fills interpillow spaces in some pieces (e.g., Sections 187-1156A-3R-2 [Piece 1] and 3R-3 [Piece 2]).
We recovered 12 sections (Sections 187-1156B-2R-1 through 6R-2) from Hole 1156B, penetrating 33.6 m into igneous basement, with 9.92 m of recovered core (29.5% recovery). About 14% of the pieces from this hole have glass rinds and/or chilled margins. All basalts were assigned to a single lithologic unit, a moderately to highly plagioclase-olivine phyric basalt.
The first unit is interpreted as a talus breccia. The breccia includes angular clasts of polymict slightly altered basalt, buff (where weathered) to medium gray (where fresh) (see "Alteration"), palagonitized glass, and micritic limestone (Fig. F1). These clasts are set in a matrix of calcareous sediment composed of two types of micritic carbonate: a pink micritic limestone and a gray micrite cement/matrix. The pink micritic limestone is present as clasts in the gray micrite and as part of some composite basalt-micritic limestone clasts. Alteration halos in some clasts have been truncated by brecciation. Both composite and truncated clasts imply that at least two stages of brecciation have taken place. Later crystalline calcite veins and patches crosscut the breccia (see "Alteration"). Clast size is highly variable, ranging from a few millimeters to several centimeters. There is no sorting either by clast size or density. The matrix is composed of gray micritic carbonate that is extensively replaced by sparry calcite. The relative amount of matrix varies from 0.5% in Section 187-1156A-2R-2 (Piece 20) to 90% in Section 2R-3 (Piece 7).
The angular, centimeter-sized basaltic clasts contain, on average, 4.5% phenocrysts of plagioclase and olivine, with plagioclase being the more abundant phase. The percentage of olivine and plagioclase phenocrysts varies both throughout the unit and on the scale of a single piece. In some pieces, variations in the modal percentage of plagioclase and olivine may reflect flow differentiation (e.g., Section 187-1156A-2R-2 [Piece 11]). Plagioclase is prismatic to tabular and as large as 5 mm in size. Most of the crystals are twinned and show zoning. Many plagioclase phenocrysts are partially resorbed and have irregular glassy melt inclusions. Furthermore, inclusions are concentrated at the rims of some crystals and in the interiors of others, suggesting a complex magmatic evolution. Olivine is euhedral and as large as 3 mm in size. Glassy melt inclusions up to 20 µm across are present in olivine phenocrysts from Section 187-1156A-2R-2 (Piece 15). Single- or mixed-phase glomerocrysts are present throughout the unit. In thin section, the microcrystalline groundmass texture is intersertal, with <1% olivine, 35% skeletal to prismatic plagioclase, 20% clinopyroxene sheaf quench texture, 2% opaque minerals, and 20% mesostasis (indistinguishable quench crystals + glass). Small vesicles, typically <1 mm in size, make up <1% of the rock volume. The basaltic clasts are slightly altered (see "Alteration").
Subangular to rounded fragments of glass and palagonite are present within the carbonate matrix throughout the unit. They vary in size from <1 mm to several centimeters. Cross sections through palagonite rims commonly display concentric zones of alteration products of varying color ranging from yellow-cream to pink-brown (e.g., Section 187-1156A-2R-1 [Piece 8]).
Pink micrite is present both attached to basalt clasts to form composite clasts and as smaller clasts within the gray micritic matrix (e.g., Section 187-1156A-2R-2 [Piece 4]). These micritic clasts are angular or subangular and vary in size from <1 mm to 1.5 cm. Mn oxide coatings, some with dendritic growths into the clast, are generally present. Small Mn oxide nodules are common in the micrite and may form wispy layers.
Unit 2 of Hole 1156A is buff (where weathered) to medium gray (where fresh), sparsely to moderately plagioclase-olivine phyric basalt. Phenocryst abundance varies throughout the core from 3% to 8% with an average of 4%. Some pieces are highly phyric (as much as 20% phenocrysts) and display flow banding. Plagioclase phenocrysts are prismatic to tabular and as large as 6 mm. Some larger plagioclase crystals display rounded shapes, suggesting partial resorption (e.g., Section 187-1156A-3R-2 [Piece 8]). In thin section, plagioclase commonly shows partially resorbed cores overgrown by large euhedral to subhedral rims, suggesting a complex magmatic evolution (Fig. F2). Flow alignment of plagioclase is rare (e.g., Section 187-1156A-3R-1 [Piece 2]). Olivine phenocrysts are equant and as large as 4 mm. Cr spinel inclusions are present in some olivine phenocrysts (e.g., Section 187-1156A-3R-2). Glomerocrysts of plagioclase ± olivine are rare in this unit. Near the chilled margins, the groundmass displays a sheaf quench texture. The sheafs consist predominantly of acicular to skeletal plagioclase intergrown with clinopyroxene. Otherwise, the predominant groundmass texture of the unit is intersertal. Small vesicles <0.5 mm make up ~0.5% of the rock. They are either unfilled or lined with blue cryptocrystalline silica and/or clay. Overall, the unit is slightly altered, except for Sections 187-1156A-3R-2 and 3R-3, where pieces are moderately altered (see "Alteration").
All basalt core recovered from Hole 1156B was assigned to a single lithologic unit. It is a grayish brown (where weathered) to medium gray (where fresh), moderately to highly phyric plagioclase-olivine basalt. Glass, commonly partially altered to palagonite, is present on ~14% of the pieces. Where present, glass rinds range from <0.1 to 1.5 cm in thickness. Phenocryst abundance varies from 3% for some pieces in Section 187-1156B-6R-1 to 25% for some pieces in Section 187-1156B-4R-2. Overall, the average is ~10%, of which 8% are plagioclase phenocrysts as large as 8 mm. Plagioclase varies throughout the unit from prismatic to rounded, with the larger phenocrysts tending to be rounded. In thin section, plagioclase phenocrysts are twinned and have oscillatory zoning; some have sieve-textured cores or, more rarely, aligned inclusions along cleavage or twin planes (Fig. F3). In some cases, they contain melt and fluid inclusions, but melt inclusions are more common. The average olivine phenocryst abundance is ~2%. They are equant, subhedral to euhedral crystals as large as 3 mm. Single- or mixed-phase glomerocrysts are common throughout this unit (Fig. F4).
In thin section, the groundmass is intersertal with plumose quench texture. Small spherical vesicles generally <0.5 mm in size are commonly unfilled. Overall, the unit is slightly to moderately altered (see "Alteration").