Sedimentary rocks
Clastic sedimentary rocks are composed of silicate minerals and rock fragments that were transported by moving fluids as bed load, suspended load, or by sediment gravity flow and were deposited when these fluids came to rest. Clastic rocks are composed largely of quartz, feldspar, rock lithic fragments, clay minerals, and mica; numerous other minerals may be present as accessories and may be important locally.
Clastic sediment, and thus clastic sedimentary rocks, are subdivided according to the dominant particle size diameter. Most geologists use the Udden-wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel
2 mm diameter, sand 1/16 to 2 mm diameter, and mud clay is 1/256 mm and silt is between 1/16 and 1/256 mm. The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand, and mudrocks are made mostly of mud. This tripartite subdivision is mirrored by the broad categories of rudites, arenites, and lutites, respectively, in older literature.
Subdivision of these three broad categories is based on differences in clast shape conglomerates and breccias, composition sandstones, grain size and/or texture mudrocks.
Clastic sediment, and thus clastic sedimentary rocks, are subdivided according to the dominant particle size diameter. Most geologists use the Udden-wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel
2 mm diameter, sand 1/16 to 2 mm diameter, and mud clay is 1/256 mm and silt is between 1/16 and 1/256 mm. The classification of clastic sedimentary rocks parallels this scheme; conglomerates and breccias are made mostly of gravel, sandstones are made mostly of sand, and mudrocks are made mostly of mud. This tripartite subdivision is mirrored by the broad categories of rudites, arenites, and lutites, respectively, in older literature.
Subdivision of these three broad categories is based on differences in clast shape conglomerates and breccias, composition sandstones, grain size and/or texture mudrocks.