Classification of Sedimentary Rocks

Texture		Grain Size < 1/256 mm.
		Crystalline, Clastic, or Amorphous
Composition of Major Fraction		Composition as Indicated in left column (prefix appropriate names for mixtures)	Clay Minerals or Clay-Size Material
Composition of Minor Fraction	< 10% Minor Fraction
	Clay Minerals or Clay-size Materials	Claystone: massive, blocky structure Mudstone: indurated mud; includes claystone and siltstone Shale: finely fissile; may include much silt Clay Shale (Argillaceous shale): chiefly clay minerals Argillite: highly indurated; incipiently recrystallized Bentonite: swells and disaggregates in water
	Silica Opal, Chalcedony, Quartz	Chert: chalcedony or opal; bedded, nodular, massive Diatomite (Diatomaceous earth): diatom tests Radiolarite (Tripoli, in part): radiolarian tests Siliceous Sinter (Geyserite): porous geyser deposit Porcellanite: argillaceous or silty chert	Siliceous Shale Siliceous Claystone Siliceous Mudstone Etc.
	Calcite or Dolomite	Limestone: chiefly calcite, massive Dolomite (Dolostone): chiefly dolomite, massive Chalk: chalky texture Tufa: very porous, friable Travertine: bonded, coherent, denser than tufa Caliche: lime-rich deposit formed near surface	Calcareous Shale: limy shale, etc. Marlstone: 25-75% carbonate
	Iron Minerals Chiefly: Limonite, Siderite, Goethite, Chamosite, Hematite	Hematite Rock: massive hematite Limonite Rock: massive limonite Bog Iron Ore: earthy, impure, limonite Ironstone (Clay ironstone): coherent mixture of iron, silica, clay, and carbonate Siderite (Iron carbonate): massive siderite	Limonitic or Hematitic (Ferruginous) Shale Etc.
	Carbon Humus: Yields carbonaceous derivatives Sapropel: Yields bituminous derivatives	Coal, Bituminous: hackly fracture Coal, Anthracite: conchoidal fracture Asphalt: asphaltic Gilsonite: black, high luster, amorphous	Carbonaceous Shale, Etc.: carbonized remains Bituminous Shale (Oil Shale) Etc.: sapropelic
	Miscellaneous Phosphate (Collophane) Evaporites: Halite and Sylvite, Anhydrite, Gypsum	Phosphorite: phosphate rock Rock Salt: massive halite or sylvite Rock Anhydrite: massive anhydrite Rock Gypsum: massive gypsum	Phosphatic Shale, Etc.

Texture		Grain Size 1/256-2 mm. (part 1)
		Crystalline, Clastic, Bioclastic, Oolitic, Etc.
Composition of Major Fraction		Composition as Indicated in left column (prefix appropriate names for mixtures)	Chiefly Calcite or Dolomite
Composition of Minor Fraction	< 10% Minor Fraction		Limestone, Dolomite, Etc.: All varieties in the Calcite-Dolomite horizontal column are possibe here.
	Clay Minerals or Clay-size Materials		Argillaceous Limestone, Etc.: All varieties in the Calcite-Dolomite hoizontal column are possible here.
	Silica Opal, Chalcedony, Quartz	Siliceous Oolite: > 50% oolitic Oolitic Chert: < 50% oolitic Diatomite: diatom tests Radiolarite: radiolarian tests	Siliceous Limestone: disseminated silica Cherty Limestone: containing chert nodules Etc. All varieties in the Calcite-Dolomite horizontal column are possible here.
	Calcite or Dolomite	Limestone: chiefly calcite, crystalline Dolomite (Dolostone): chiefly dolomite, crystalline Clastic Limestone (Calcarenite or calcite sandstone): clastic Calcareous Oolite: > 50% oolitic Oolitic Limestone: < 50% oolitic Coquina: shells, little cement Organic Limestone: Richly fossiliferous
	Iron Minerals Chiefly: Limonite, Siderite, Goethite, Chamosite, Hematite	Hematite Oolite: > 50% oolitic Limonite Oolite: > 50% oolitic Oolitic Iron Ore: < 50% oolitic Siderite (Iron carbonate): chiefly siderite, crystalline	Limonitic or Hematitic (Ferruginous) Limestone, Etc. All varieties in the Calcite-Dolomite horizontal column are possible here.
	Carbon Humus: Yields carbonaceous derivatives Sapropel: Yields bituminous derivatives	Peat: dark semi-carbonized plant remains Lignite: brown-black well-carbonized plant remains	Carbonaceous Limestone, Bituminous Limestone, Etc. All varieties in the Calcite-Dolomite horizontal column are possible here.
	Miscellaneous Phosphate (Collophane) Evaporites: Halite and Sylvite, Anhydrite, Gypsum	Phosphatic Oolite: > 50% oolitic Rock Salt: crystalline Rock Anhydrite: crystalline Rock Gypsum: crystalline Gypsum Sand: clastic	Phosphatic Limestone, Etc. All varieties in the Calcite-Dolomite horizontal column are possible here.

Texture		Grain Size 1/256-2 mm. (part 2)
		Clastic Unconsolidated: silt, sand Consolidated: siltstone, sandstone Size grades (mm) 1/256-1/16 silt; 1/16-1/8 very fine sand; 1/8-1/4 fine sand; 1/4-1/2 medium sand; 1/2-1 coarse sand; 1-2 very coarse sand
Composition of Major Fraction		Chiefly Quartz			Quartz and > 25% Feldspar	Quartz, Feldspar, Rock Chips, Pelitic Matrix, Angular grains, Tough	Volcanic Ejecta (also in grain size > 2 mm category)
		>90% Quartz	Feldspar 10-25%	Rock Chips > 10%
Composition of Minor Fraction	< 10% Minor Fraction	Quartz Sandstone (Quartzose sandstone)	Feldspathic Sandstone	Lithic Sandstone	Arkose (Arkose sandstone): Normally pink, red, or light gray	Graywacke: normally greenish gray Subgraywacke:low in feldspar, rock chips, or less angular grains. Tendency to chemical cement.	Ash: unconsolidated fragments under 4 mm Tuff: consolidated ash Volcanic Breccia: angular fragments over 4 mm. Agglomerate: large proportion (>25%) of bombs These rocks are classified on the proportions of vitric, crystal (mineral), or lithic material they contain, for example, "vitric lithic ash," or "crystal vitric tuff." If the mineralogy of the crystal or lithic fragments can be determined, the name of the appropriate volcanic rock can be prefixed as,"rhyolite vitric crystal tuff," or simply, "rhyolite tuff."
	Clay Minerals or Clay-size Materials	Argillaceous Quartz Sandstone	Argillaceous Feldspathic Sandstone Loess: fine sand or silt. Massive porous, coherent.	Argillaceous Lithic Sandstone	Argillaceous Arkose	Argillaceous Graywacke Argillaceous Subgraywacke
	Silica Opal, Chalcedony, Quartz	Orthoquartzite (Sedimentary quartzite) (Siliceous quartz sandstone)	Feldspathic Orthoquartzite (Siliceous feldspathic sandstone)	Lithic Orthoquartzite (Siliceous lithic sandstone)	Siliceous Arkose (Quartzitic arkose)	Siliceous Subgraywacke
	Calcite or Dolomite	Calcareous Quartz Sandstone	Calcareous Feldspathic Sandstone	Calcareous Lithic Sandstone	Calcareous Arkose	Calcareous Subgraywacke
	Iron Minerals Chiefly: Limonite, Siderite, Goethite, Chamosite, Hematite	Limonitic or Hematitic (Ferruginous) Quartz Sandstone	Limonitic or Hematitic (Ferruginous) Feldspathic Sandstone	Limonitic or Hematitic (Ferruginous) Lithic Sandstone	Limonitic or Hematitic (Ferruginous) Arkose	Limonitic or Hematitic (Ferruginous) Graywacke Limonitic or Hematitic (Ferruginous) Subgraywacke
	Carbon Humus: Yields carbonaceous derivatives Sapropel: Yields bituminous derivatives	Carbonaceous Quartz Sandstone Bituminous Quartz Sandstone	Carbonaceous Feldspathic Sandstone Bituminous Feldspathic Sandstone	Carbonaceous Lithic Sandstone Bituminous Lithic Sandstone	Carbonaceous Arkose Bituminous Arkose	Carbonaceous Graywacke, Etc. Carbonaceous Subgraywacke, Etc.
	Miscellaneous Phosphate (Collophane) Evaporites: Halite and Sylvite, Anhydrite, Gypsum	Phosphatic Quartz Sandstone, Etc.	Phosphatic Feldspathic Sandstone, Etc.	Phosphatic Lithic Sandstone, Etc.	Phosphatic Arkose, Etc.	Phosphatic Subgraywacke, Etc.

Texture		Grain Size > 2 mm.
		Clastic Unconsolidated: gravel (rounded), rubble (angular) Consolidated: conglomerate (rounded), breccia (angular) Size Grades (mm): 2-4 granules; 4-64 pebbles; 64-256 cobbles; >256 boulders
Composition of Major Fraction		Volcanic Ejecta (also in grain size 1/256-2 mm category)	Chiefly One Constituent Especially quartz, chert, or quartzite. Also shale or limestone. Homogeneous conglomerates and breccias.	Several Constituents Usually including unstable constituents. Mixed conglomerates and breccias.
Composition of Minor Fraction	< 10% Minor Fraction	Ash: unconsolidated fragments under 4 mm Tuff: consolidated ash Volcanic Breccia: angular fragments over 4 mm. Agglomerate: large proportion (>25%) of bombs These rocks are classified on the proportions of vitric, crystal (mineral), or lithic material they contain, for example, "vitric lithic ash," or "crystal vitric tuff." If the mineralogy of the crystal or lithic fragments can be determined, the name of the appropriate volcanic rock can be prefixed as,"rhyolite vitric crystal tuff," or simply, "rhyolite tuff."	Name consists of chief constituent and size grade, for example: Quartz Pebble Conglomerate Chert Cobble Conglomerate Limestone Pebble Breccia Etc.	Name consists of "mixed" or "polycomponent" and size grade, for example: Mixed Pebble Conglomerate Mixed Cobble Conglomerate Name may include composition as Graywacke-Andesite-Chert Pebble Conglomerate
	Clay Minerals or Clay-size Materials		Argillaceous (rock, grade) Conglomerate: uncommon with stable constituent	Argillaceous Mixed (grade) Conglomerate Glacial Till: chaotic mixture of clay, sand, and gravel Tillite: indurated till Fanglomerate: alluvial fan material
	Silica Opal, Chalcedony, Quartz		Siliceous (rock, grade) Conglomerate (Orthoquartzitic [rock, grade] conglomerate)	Siliceous Mixed (grade) Conglomerate
	Calcite or Dolomite		Calcareous (rock, grade) Conglomerate	Calcareous Mixed (grade) Conglomerate
	Iron Minerals Chiefly: Limonite, Siderite, Goethite, Chamosite, Hematite		Limonitic or Hematitic (Ferruginous) (rock, grade) Conglomerate	Limonitic or Hematitic (Ferruginous) Mixed (grade) Conglomerate
	Carbon Humus: Yields carbonaceous derivatives Sapropel: Yields bituminous derivatives		Carbonaceous (rock, grade) Conglomerate Bituminous (rock, grade) Conglomerate	Carbonaceous Mixed (grade) Conglomerate Bituminous Mixed (grade) Conglomerate
	Miscellaneous Phosphate (Collophane) Evaporites: Halite and Sylvite, Anhydrite, Gypsum		Phosphatic (rock, grade) Conglomerate	Phosphatic Mixed (grade) Conglomerate

The names in the above chart are root names and should be preceded by appropriate terms for any significant feature of the rock. The proper order is color, structure, grain size (sandstones only), minor constituents, cement, and root name. Structure includes, for example, "thin-bedded," "massive," "crossbedded," "thinly laminated," etc. The size grade for conglomerates should immediately precede "conglomerate." Some common nonmineral constituent terms are: "tuffaceous," "cherty," "fossiliferous," "crinoidal," "coralline," "clayey (argillaceous)," "silty," "shaly," "sandy (arenaceous)," and "conglomeratic." Minor mineralogy should follow the nonmineral composition and should be restricted to one conspicuous mineral not implied in the root name. It should be applied in adjective form as "micaceous," "chloritic," "glauconitic," and "pyritic." The most common cement terms are given in the chart with the root names. Some typical rock names are "black thin-bedded micaceous shale," "gray massive, medium-grained glauconitic quartz sandstone," "tan siliceous quartz pebble conglomerate," and "white vuggy foraminiferal limestone."

Graywacke currently has two meanings. To some, it denotes a feldspar-quartz sandstone with more than 20 (or 15) percent pelitic matrix. To many others, it has retained its original meaning; that is a very hard, firm sandstone without porosity or chemical cement. In additonal , these "true" graywackes typically have extremely angular grains, a pelitic matrix, and a dark color.