Chapter 1 Sandstones 1.1 Sandstone Sandstone is a sedimentary rock. It is formed from mineral grains derived from the erosion of pre-existing rocks, which are transported, then deposited in a sedimentary basin by the action of water, wind or ice. The grains areeither held together with a cement or embedded in a fine grained silty matrix (Figure1.l).The types of mineral grains in a sandstone are highly variable. The most common constituent is quartz. It is the principle constituent of most sandstones and may, in some rare cases, constitute 100%of the rock. Other mineral types commonly found in sandstone incIude feldspars, micas, clay minerals, carbonates and iron oxides. Rock fragments are also relatively common. Sediments can also include organic material, chemical precipitates (salts) and volcanic ashes. Although the list of commonly occurring minerals in sandstones is relatively short, in principle almost any known mineral may occur. The minerals most commonly found in sandstones are those which are most resistant to decay and are best able to survive the processes of erosion, transportation and deposition.
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The formation of sandstones Sedimentary rocks are formed in layers which accumulate on top of each other over long periods of time. Sedimentary units are seldom flat and uniform. There are a wide variety of sedimentary structures which can cause localvariations in the characteristicsof a sandstone. The most common of these are ripples and dunes, formed when sands are deposited by water or wind currents. These structures are seen in sandstones as cross-bedding. Deformational structures can also form after deposition due to movements within the sediment. The size of particles which make up sandstone vary between about 0.06 and 2mmindiameter. Thegrains withinanindividualsandstone may be uniform in their size distribution. More often, a mixture of differently sized particles occurs. As sand grains are transported they become more rounded. Sandstones formed of grains derived from a local source tend to contain a mixture of angular grains. If the grains are transported for longer distances or for a greater length of time before deposition,they become more rounded, and less resistant minerals are lost, leaving the sediment dominated by quartz. After deposition the sediment consists of loose sand grains and pore spaces filled with air or water. As the sediment becomes buried, processes of lithification turn the sediment into sedimentary rock.