Weakly indurated sediments containing abundant volcanically derived materials are present in Cores 183-1136A-10R to 15R-1, 7 cm (79.90-128.08 mbsf). Postdepositional processes strongly affected these silty to sandy materials (see "Lithostratigraphy"). Underlying these sediments are three igneous units from Cores 183-1136A-15R to 19R (128.08-161.40 mbsf), interpreted to be separate lava flows (see "Igneous Petrology" and "Physical Volcanology").
All basaltic rocks recovered from Hole 1136A are slightly to completely altered to low-temperature secondary phases that partially replace primary minerals, completely replace mesostasis, and partly to completely fill veins and vesicles. The distributions of secondary minerals recorded in the alteration and vein/structure logs (see the "Supplementary Materials" contents list) for Hole 1136A are shown in Figure F28. The most common secondary phases are clays (both Mg-saponite and celadonite), calcite, and zeolites. In general, clay minerals are abundant at all depths, whereas the abundances of calcite and zeolites exhibit more pronounced downhole variations (Fig. F28). In particular, calcite is more common in both veins and vesicles in the upper portions of Unit 1 (130-135 mbsf), near the contact between Units 1 and 2 (140-143 mbsf) and in veins near the contact between Units 2 and 3 (158-160 mbsf). Zeolites are absent or present in only minor amounts in Units 1 and 2 and are only abundant in veins and vesicles near the base of Unit 2 and in Unit 3 (Fig. F28).
The uppermost rocks from Unit 1 (e.g., Sections 183-1136A-15R-1 and 15R-2) are moderately altered and display a distinctive blue-green hue, indicative of celadonite, a potassium ferric iron mica (confirmed by XRD) (see Table T8). Alteration in these rocks is similar to that observed in the carbonate-bearing zeolitic silty clays of Unit V, which directly overlie basaltic basement (e.g., Section 183-1136A-14R-2). In the basaltic basement, celadonite ± Mg-saponite partially replace the groundmass and mesostasis and these phases, together with calcite, fill veins and vesicles. Narrow (2 to 5 mm), late-stage oxidation halos are developed around some veins. Veins have developed by both the filling of open cracks, as well as the alteration of wall rock, with thin slivers of the host basalt having been separated and replaced by celadonite ± Mg-saponite (e.g., Fig. F29). Many veins record multiple generations of mineral precipitation, brecciation, and wall-rock alteration (e.g., Figs. F29, F30). The most common paragenesis observed is clay lining veins with calcite interiors.
The dark gray interiors of Units 1 and 2 (Sections 183-1136A-15R-3, 0 cm, to 19R-2, 93 cm) are slightly to moderately altered. Finer-grained basalts have a pinkish brown color. Light to dark green clay, predominantly Mg-saponite replaces olivine ± clinopyroxene phenocrysts, as well as the mesostasis (see Table T8). Uncommon clay-filled veins display weakly developed green alteration halos. Vesicles are generally lined with dark clay minerals and filled with either lighter colored clays or calcite. Zeolites are absent or only minor constituents of either veins or vesicles from both Units 1 and 2, except from directly above the contact with Unit 3 (e.g., Sections 183-1136A-19R-1 and 19R-2), where clinoptilolite (heulandite) line the margins of both veins and vesicles.
Thin-section observations of basalts from Units 1 and 2 support the hand specimen descriptions and confirm the complete alteration of mesostasis and olivine to brown (in thin section) clay minerals with variable staining by iron oxyhydroxides. All samples display a brownish hue indicative of strongly developed groundmass alteration and oxidation. There is a ubiquitous replacement of interstitial glass and coating of most grain boundaries, cleavage traces, and fractures. Plagioclase laths in both glomerocrysts and groundmass are partially altered to clay minerals. Most groundmass and phenocrystic clinopyroxene is degraded to clay minerals, though uncommon, well-preserved clinopyroxene is present in the fresher, near holocrystalline, fine- to medium-grained basalts from the interior of Unit 2 (e.g., Sample 183-1136A-18R-5 [Piece 1B, 111 to 112 cm]). Vesicle and vein filling include celadonite, brown-green clay (Mg-saponite), calcite, and zeolites (individual zeolites were generally not distinguished in thin section).
Unit 3 comprises a highly to completely altered top of a lava flow, in which abundant glassy material and mesostasis is replaced in situ by black to dark green waxy clay, and zeolites fill wispy crosscutting veinlets and vesicles (e.g., Fig. F31).