Argillaceous rocks

The argillaceous rocks (lutites) include shales, argillites, siltstones, and mudstones; they are clastic sediments whose constituent particles are less than 0.0025 in. or 1/16 mm (if siltstones are included) or less than 0.00015 in. or 1/256 mm (if siltstones are excluded). They are the most abundant sedimentary rock type, varying according to different estimates from 44 to 56% of the total sedimentary rock column. Claystone is indurated clay, which consists dominantly of fine material of which at least a major proportion is clay mineral (hydrous aluminum silicates). Shale is a laminated or fissile claystone or siltstone, in general more consolidated than claystone. Mudstone is a claystone that is blocky and massive. The term argillite is used for rocks which are more indurated than claystone or shale but not metamorphosed to slate. All these argillaceous rocks are consolidated equivalents of muds, oozes, silts, and clays. Loess is a finegrained, unconsolidated, wind-blown deposit. The term shale has been used by many authors generically to denote all of these types of rock. See also Bentonite; Clay; Clay minerals; Loess; Sedimentary rocks; Shale.