Petrography of Common Sands and Sandstones

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There are three major types of sand: terrigenous, carbonate, and pyroclastic. Terrigenous sands are most abundant and are all ultimately derived from outside the basin of deposition by erosion of preexisting crystalline, volcanic, and sedimentary rocks and, except for eolianites, are all deposited by water. Silicates predominate. Carbonate sands are virtually all deposited in marine waters and consist primarily of skeletal grains, oolites plus other coated grains, some locally derived detrital carbonate called intraclasts, and terrigenous carbonate. The latter is really a terrigenous sand, and not abundant except where there is very rapid erosion of thick carbonate sections in orogenic belts or in some glacial situations. Pyroclastic sands are those derived directly from volcanic explosion as, for example, ash, lapilli, and bombs. They may be deposited on either land or in water. Pyroclastic sands are less abundant than either terrigenous or carbonate sands. The more inclusive term volcaniclastic refers to clastic sedimentary materials rich in volcanic debris which may be either of pyroclastic origin or a normal terrigenous (epiclastic) sand derived from an older volcanic terrane (discussed in detail in Chap. 6).