SANDSTONE

lithified accumulation of sand-sized grains (0.063 to 2 mm [0.0025 to 0.08 inch] in diameter). It is the second most common sedimentary rock after shale, constituting about 10 to 20 percent of the sedimentary rocks in the Earth's crust. Because of their abundance, diverse textures, and mineralogy, sandstones are important indicators of erosional and depositional processes. Texturally, sandstones consist of two components: (1) a framework composed of sand-sized grains and (2) interstitial volume between grains, which may be empty, especially in modern sandstones, or, in the case of most ancient sandstones, filled with either a chemical cement of silica or calcium carbonate or a fine-grained matrix. The principal mineral constituents of the framework are quartz, feldspar, and rock fragments. The relative proportions of these have genetic implications, first for the source area and second for the rate of deposition. Texture and mineralogical properties are used for sandstone classification, though considerable debate exists as to which properties to emphasize. Several important classes include orthoquartzite (nearly pure quartz); arkose (feldspar-rich, quartz-poor, with only minor fine-grained component); and graywacke (feldspar- and quartz-rich, but with significant fine-grained component). Sedimentary-rock sequences associated with different sandstone types usually show patterns that reflect the depositional environment and source region. Orthoquartzites, for example, are commonly associated with shallow marine carbonates and shale, whereas graywacke is associated with deep-water shales.