Sandstones

Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Typically quartz and feldspar; lithic fragments are also common. Other minerals may be found in particularly mature sandstone. 

Sandstones are siliciclastic sedimentary rocks that consist mainly of sand-size grains (clast diameters from 2 to 1/16 millimetre) either bonded together by interstitial chemical cement or lithified into a cohesive rock by the compaction of the sand-size framework component together with any interstitial primary (detrital) and secondary (authigenic) finer-grained matrix component. They grade, on the one hand, into the coarser-grained siliciclastic conglomerates and breccias described above, and, on the other hand, into siltstones and the various finer-grained mudrocks described below. Like their coarser analogues—namely, conglomerates and breccias—sand-size (also called arenaceous) sedimentary rocks are not exclusively generated by the physical disintegration of preexisting rocks. Varieties of limestone that contain abundant sand-size allochems like oöids and fossil fragments are, in at least a textural sense, types of sandstones, although they are not terrigenous siliciclastic rocks. Such rocks, called micrites when lithified or carbonate sands when unconsolidated, are more properly discussed as limestones. Also, pyroclastic sandstones or tuffs formed by lithifying explosively produced volcanic ash deposits can be excluded from this discussion because their origin is unrelated to weathering.

Sandstones are significant for a variety of reasons. Volumetrically they constitute between 10 and 20 percent of Earth’s sedimentary rock record. They are resistant to erosion and therefore greatly influence the landscape. When they are folded, they create the backbone of mountain ranges like the Appalachians of eastern North America, the Carpathians of east-central Europe, the Pennines of northern England, and the Apennine Range of Italy; when flat-lying, they form broad plains and plateaus like the Colorado and Allegheny plateaus. Sandstones are economically important as major reservoirs for both petroleum and water, as building materials, and as valuable sources of metallic ores. Most significantly, they are the single most useful sedimentary rock type for deciphering Earth history. Sandstone mineralogy is the best indicator of sedimentary provenance: the nature of a sedimentary rock source area, its composition, relief, and location. Sandstone textures and sedimentary structures also are reliable indexes of the transportational agents and depositional setting.

There are three basic components of sandstones: (1) detrital grains, mainly transported, sand-size minerals such as quartz and feldspar, (2) a detrital matrix of clay or mud, which is absent in “clean” sandstones, and (3) a cement that is chemically precipitated in crystalline form from solution and that serves to fill up original pore spaces.

The colour of a sandstone depends on its detrital grains and bonding material. An abundance of potassium feldspar often gives a pink colour; this is true of many feldspathic arenites, which are feldspar-rich sandstones. Fine-grained, dark-coloured rock fragments, such as pieces of slate, chert, or andesite, however, give a salt-and-pepper appearance to a sandstone. Iron oxide cement imparts tones of yellow, orange, brown, or red, whereas calcite cement imparts a gray colour. A sandstone consisting almost wholly of quartz grains cemented by quartz may be glassy and white. A chloritic clay matrix results in a greenish black colour and extreme hardness; such rocks are wackes.