Applied to rocks which are silt- to clay-sized sediments (grain size less than 0.0625 mm in diameter). They account for more than 50% of sedimentary rocks and most have a very high clay mineral content. Many contain a high percentage of organic material and can be regarded as potential source rocks for hydrocarbons.
Geography
Clay-like in composition and texture, referring to rocks containing clay minerals and clay-sized particles, for example, shale.