Basement rocks vary from slightly to completely altered with low-temperature secondary phases replacing primary minerals and mesostasis and filling veins, fractures, and vesicles. Clay minerals are the dominant secondary minerals in all basement units. In downhole logs, highly altered flow tops are characterized by higher potassium content caused by the increased abundance of clay minerals. Alteration of Hole 1137A lavas likely results from both weathering and low-temperature alteration. Unlike postmagmatic submarine alteration of typical oceanic crust, subaerial weathering and low-temperature interaction of basalts and interbedded sediments with groundwater at Site 1137 preceded submarine alteration.
Many beds are normally graded, and others show parallel laminations.
Clasts range from well rounded granules to small boulders. Most intervals are clast supported, but matrix-supported intervals also are present. The depositional environment of Basement Units 5 and 6 appears fluvial, perhaps associated with a braided river. These units represent a significant hiatus of unknown duration between eruptions of basaltic flow Units 1-4 and 7 and 8. Furthermore, fluvial facies of Units 5 and 6 corroborate our interpretation of subaerial lava flow effusion.