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Igneous lithostratigraphy, petrology, alteration, and structural geology

Fragmented pieces of altered basalt were found in sandy sediment at the base of Core 329-U1366F-4H (Fig. F13). The small (<5 mm to 2 cm diameter) fragments are 60%–100% clay minerals that disintegrate when handled. Two fragments, one slightly less altered to clay (interval 329-U1366F-4H-5, 114–116 cm) and one nearly totally altered, were described. A thin section of the totally altered Sample 329-U1366F-4H-5, 110–111 cm, was made.

Based on remnant primary igneous features, Sample 329-U1366F-4H-5, 114–116 cm, was cryptocrystalline and aphyric with clinopyroxene and plagioclase making the bulk of the groundmass. Crystal size fines toward the edges, implying that the sample was a chilled margin. However, intensive alteration near the margin replaced the primary mineralogy, so only alteration rinds remain. The presence of chilled margins and intense alteration suggests that the rock represented a flow top. Alteration rinds on the edges of the basalt fragments are very similar to the lithic sand that hosts the fragment, suggesting that the sand may be basaltic in origin.

Thin section observations of Sample 329-U1366F-4H-5, 110–111 cm, indicate that the basaltic fragments are nearly completely altered to clay minerals, with only a few remnant plagioclase and tiny (<0.05 mm) clinopyroxene crystals visible. Only skeletal primary textural features remain, implying that the rock was subophitic. No phenocrysts or pseudomorphs of phenocrysts were observed. Alteration is characterized by near-complete pale to dark brown replacement of the groundmass (Fig. F13). Alteration includes saponite (55%), iddingsite (35%), iron oxyhydroxides (3%), and minor secondary Fe-Ti oxides, possibly titanomagnetite (1%). No vesicles or veins are present.