Greywacke

Greywacke is also known as mixed sandstone or grey sandstone. It includes a variety of sandstones characterised by their hardness and dark colour. Greywacke forms due to diagenesis and primary metamorphism of angular lithic fragments and quartz and feldspar grains. It always contains some clay matrix materials (approximately 15–40%), which are metamorphosed into illite, chlorite and sericite. Greywacke is rich in Al₂O₃, FeO, MgO, and Na₂O. Greywacke has poor grain sorting and low roundness. It also contains sedimentary structures, such as channel moulds and graded bedding. Greywacke can be divided into lithic greywacke and feldspathic quartz sandstone. In the geological record, greywacke is widespread in older rock strata and is mainly related to marine facies. It is a product of orogenic belts, and it is usually present in stable craton areas. Abundant greywackes can be found in the Cenozoic continental strata in China. They are rich in volcanic clasts that were mainly produced in...