26 SEDIMENTARY ROCKS/Mineralogy and Classification been abandoned, as it is now known that sediments are deposited by aeolian, gravitational, and glacial processes, as well as by purely aqueous ones The processes of weathering, erosion, transportation, and deposition are nature’s way of chemically fractionating the Earth’s surface, and lead to a logical classification of sedimentary rocks based largely on their chemistry and mineralogy Serendipitously, this fractionation correlates broadly with their mode of formation Although geologists broadly agree about the definition of the main classes of sedimentary rocks, there is no unanimity about all of them In 1937, Goldschmidt proposed a classification based on five chemical groups, namely: (1) resistates, (2) hydrolysates, (3) oxidates, (4) carbonate precipitates, and (5) evaporites In 1950, Rankama and Sahama added a sixth: reduzates The resulting classification highlights the chemical fractionation that results from sedimentary processes, but produces some very strange and uncouth names Meanwhile, a practical field-based classification had been widely adopted, although with some variation in the fine detail It had long been noted that fractionation on the surface of the earth naturally divided rocks into those that had never gone into solution, and might therefore be termed ‘allochthonous’ or ‘detrital’, and those formed from minerals that had been dissolved in surface water, and had precipitated out These are termed ‘autochthonous’ or ‘chemical’ rocks The allochthonous or detrital sediments are subdivided by grain size The autochthonous or chemical sediments are subdivided by mineralogical (chemical) composition (Table 1) Geopedants will already notice the inconsistency of this classification The detrital sediments are composed of a wide range of minerals, and thus exhibit a diversity of chemistry, which is ignored for the purposes of their classification Similarly, the chemical Table A classification of sedimentary rocks Allochthonous or detrital sediments Classified by grain size Gravel/conglomerate Sand/sandstone Silt/siltstone Clay/claystone (sometimes also termed ‘mudrocks’ or ‘shales’) Autochthonous or chemical sediments Classified by mineralogy Carbonates (limestone and dolomite) Evaporites (gypsum/anhydrite, halite, etc.) Residual (bauxite, laterite, kaolinite) Kerogenous (peat, lignite, coal) Ironstones (haematitic, chamositic, and sideritic) Phosphates (guano) Siliceous (chert, opal) rocks may occur in a wide range of particle size, from boulders of limestone to sapropelic muds Nonetheless, the classification displayed in Table is not for the benefit of geopedants, but for practical use by geologists There is no consensus on the classification of sedimentary rocks proposed in Table Note that the caption reads ‘A classification of sedimentary rocks’ not ‘The classification of sedimentary rocks’ The main groups of sedimentary rocks are now described briefly, pointing the way to articles in this encyclopedia that describe them in more detail Allochthonous or Detrital Sediments As defined earlier, the allochthonous or detrital sediments are the insoluble residue of weathering of pre-existing rocks: igneous, metamorphic, or sedimentary The mineralogy is very varied, depending on the source material and the type and duration of the weathering process (see Weathering) The mineralogy also correlates crudely with the grain size Conglomerates tend to be polymineralic, sandstones are dominated by quartz, and mudrocks are dominated by clay minerals Table shows the subdivision of the allochthonous or detrital rocks by grain size, gravel, sand, silt, and clay being the basis for conglomerate, sandstone, siltstone, and mudstone, respectively The terms ‘rudaceous’, ‘arenaceous’, and ‘argillaceous’ have also been applied to conglomerates, sandstones, and shales, but are little used now The main groups of detrital sediments are now described briefly Conglomerate Conglomerate is composed of particles of gravel, that is to say of particles of greater than mm in diameter, consisting, with increasing size, of granules, pebbles, cobbles, and boulders Collectively, conglomerates have also been known as rudaceous rocks Conglomerates are distinguished from breccias by the fact that the clasts are rounded, whilst those of breccias are angular Because of their large size, conglomerate clasts are composed of many grains or crystals (depending on whether they were derived from earlier sediments or from crystalline igneous or metamorphic rocks) They may thus be composed of a wide range of minerals When derived from igneous or metamorphic rocks, conglomerates may be composed of the wide range of minerals found in the parent rock By contrast, conglomerates derived from sediments will reflect their source mineralogy, but will tend to be composed of a higher percentage of minerals that are stable at the Earth’s surface, rather than in the parent rock The concept of sediments as the insoluble residue of pre-existing rocks is again a useful one to recall This is illustrated