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 increased potassium content due to the increased abundance of clay minerals. It has not yet been established how much of the alteration in Hole 1137A can be attributed to interaction of basement rocks with groundwater in a subaerial environment or with seawater in a submarine environment. Many beds are normally graded and others show parallel laminations.
Clasts range from well rounded granules to small boulders.
The depositional environment of Basement Units 5 and 6 appears fluvial, perhaps associated with a braided river. Furthermore, fluvial facies of Units 5 and 6 corroborates our interpretation of subaerial lava flow effusion. We also find single grains of garnet and perthitic alkali feldspar, presumably weathered from sources similar to those of the gneiss and granitoid cobbles, in the sand fraction of Unit 5 and as xenocrysts in parts of the Unit 9 crystal-vitric tuff. It is difficult to imagine anything but an originally continental source for such material. However, we infer that the material was reworked because of the absence of internal stratification, a basal breccia or fine flow top, or normal grading of lithics and crystals.