5.5: Classification of Sedimentary Rocks (2024)

Clastic Sedimentary Rocks (Detrital)

Clastic sedimentary rock (also called ‘detrital’ sedimentary rock) consists of sediment pieces (clasts/rock detritus) that come from weathered bedrock, cemented together to make a new rock. The clastic sediment is often referred to as “siliciclastic” because it is derived from the rocks of the crust which, as we have learned, is largely composed of silicate minerals of igneous rock. Clastic (or detrital) rock is classified and named based on the size and composition of the clasts. Their various sizes, from boulders to clay, is referred to as the sediment grain size. The classification and description of the various clastic rocks appears in the top section of the chart below. The size, shape, and composition of the clasts varies widely based upon the source rock and the length of time the sediment has been moving through the system.

The classification and description of the various clastic sedimentary rock types appears in the top section of the chart below. Photos of each rock type appear beneath the chart.

Chemical, Biochemical and Organic Sedimentary Rock

Chemical sedimentary rock is formed by processes that do not directly involve mechanical weathering and erosion. Chemical weathering contributes the dissolved ions in water that ultimately form the various rock types. Inorganic chemical sedimentary rock is made of minerals precipitated from ions dissolved in solution. Inorganic chemical sedimentary rock forms in environments where ion concentration, dissolved gasses, temperatures, or pressures are changing, which causes minerals to crystallize, such as through the process of evaporation.

Biochemical sedimentary rock is formed from shells and bodies of underwater organisms (such as coral). The living organisms extract chemical components from the water and use them to build shells and other body parts. Biochemical sedimentary rocks are commonly composed of calcite, from a wide variety of sea life, or silica, largely from the single-celled microorganisms called radiolarian.

Organic sedimentary rock contains the remains of organic material that has undergone the lithification process. The source materials are plant and animal remains that are transformed through burial and heat, and end up as coal, oil, and methane (natural gas).

The classification and description of the various chemically formed sedimentary rock types appears in the bottom section of the chart below. Photos of each rock type appear beneath the chart.

¹Fossiliferous Limestone can be further subdivided into Grainstone, Packstone, Wackestone and Mudstone depending on the amount of mud with the lithified lime mud. Grainstone = less than 10% mud \(\rightarrow\) Mudstone = less than 10% fossil organisms.

²Microcrystalline - crystals that are visible only through a high-powered microscope.

Clastic Sedimentary Rock Photos

Conglomerate. A coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification of gravel. Conglomerates are typically poorly sorted and contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Breccia. A coarse-grained clastic sedimentary rock that is composed of a substantial fraction of angular gravel-size clasts larger than 2 mm (0.079 in) in diameter. Breccias are typically poorly sorted and contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Quartz sandstone. A clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) quartz mineral grains. White and red (with iron oxide cement) are most common. Variable rounding and sorting. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Arkose sandstone. A clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) feldspar mineral grains. Pink (potassium feldspar) and white (plagioclase feldspar) are most common. Commonly angular fragments with variable sorting. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.0 Attribution.

Greywacke. A clastic sedimentary that is a variety of sandstone. It is generally characterized by its dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments set in a compact, clay-fine matrix. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Siltstone. A clastic sedimentary rock. As its name implies, it is primarily composed of silt sized particles, defined as grains < 0.06mm. Siltstones differ significantly from sandstones due to the smaller spaces available between the grains (pore space) and often contain a significant amount of clay. Although often mistaken as a shale, siltstone lacks the fissility (easy separation along flat bedding planes) and laminations which are typical of shale. Photo credit: CC BY Attribution 3.0; Callan Bentley, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Shale (with shell and plant fossils). A fine-grained, clastic sedimentary rock, composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz. Shale is characterized by breaks along parallel layering or bedding less than one centimeter in thickness, called fissility. It is the most common sedimentary rock. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Claystone

Claystone. A clastic sedimentary rock composed largely of clay, which measures less than 1/256 of a millimeter in particle size (not visible with the naked eye). Claystone is non-fissile, it does not break easily into layers like shale. Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, licensed under a Creative Commons Attribution 3.0 United States License.

Chemical, Biochemical and Organic Sedimentary Rock Photos

Presented in order on the chart.

Fossiliferous limestone is any type of limestone, made mostly of calcium carbonate (\(\ce{CaCO3}\)) in the form of the minerals calcite, that contains an abundance of fossils or fossil traces. The fossils in these rocks may be of macroscopic or microscopic size. This particular sample contains an abundance of mollusc and gastropod fossils. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Oolitic Limestone is a type of limestone formed from ooids, small, spherical grains composed internally of calcium carbonate (\(\ce{CaCO3}\) – calcite) in concentric layers.
Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Micrite

Micrite is a limestone formed of calcareous (composed of calcite) particles ranging in diameter up to four microns. It is formed by the recrystallization of calcareous mud. “Fossiliferous dismicrite (Nineveh Limestone, Lower Permian; Clark Hill section, Long Ridge, Monroe County, Ohio, USA) 1” by James St. John is licensed under CC BY 2.0 Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Chalk

Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Travertine

Travertine is a form of limestone deposited by mineral springs, especially hot springs. Travertine is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. In the latter, it can form stalactites, stalagmites, and other speleothems (as in the 3D model, above. Grab and rotate the sample to see it in 3D). Travertine can be white, tan, cream-colored, or somewhat rusty in color.

Crystalline Limestone

Crystalline limestone is a carbonate sedimentary rock that is composed of the precipitation of the mineral calcite (\(\ce{CaCO3}\)) from saturated sea water. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (\(\ce{CaCO3}\)). About 10% of sedimentary rocks are limestones. Photo credit: Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, licensed under a Creative Commons Attribution 3.0 United States License. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Dolostone

Dolostone is a sedimentary carbonate rock that contains a high percentage of the mineral dolomite, \(\ce{CaMg(CO3)2}\). Dolomite contains nearly equal amounts of magnesium and calcium. Most dolomites formed as a magnesium replacement of limestone or lime mud before lithification. Dolomite is less soluble than limestone and, for identification purposes, must first be scratched into a small power before it will effervesce with dilute hydrochloric acid. Photo credit: “Dolostone (Silurian; midwestern USA)” by James St. John is licensed under CC BY 2.0. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Chert (and Flint)

Chert is a hard, fine-grained sedimentary rock composed of crystals of quartz (silica \(\ce{SiO2}\)) that are very small (microcrystalline or cryptocrystalline). It often breaks with a conchoidal fracture and was used to make sharp objects like knives and arrowheads in the prehistoric era. Chert is often of biological origin (organic) but may also occur inorganically as a chemical precipitate or a diagenetic replacement (e.g., petrified wood). Geologists use chert as a generic name for any type of microcrystalline or cryptocrystalline quartz. Varieties dark in color are called flint; red varieties are known as jasper; multi-colored and banded varieties are known as agate. Photo credit: Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, licensed under a Creative Commons Attribution 3.0 United States License. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Rock Gypsum

Rock gypsum is composed of the very soft mineral gypsum which defining characteristic is that it can be scratched with a fingernail. It is typically white to very pale pink or tan. Gypsum rock forms within layers of sedimentary rock from the slow evaporation of seawater. James Petts From: Wikimedia is licensed under: Creative Commons Attribution-Share Alike 3.0 license.

Rock salt is composed of the mineral halite (sodium chloride, NaCl, common table salt) which defining characteristics include that it tastes salty and will dissolve readily in water. It is typically clear to white to pale pink or tan. Rock salt forms within layers of sedimentary rock from the slow evaporation of seawater. Photo credit: CC BY Attribution 3.0; Robin Rohrback, Mid-Atlantic Geo-Image Collection (M.A.G.I.C.) on GigaPan. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

Bituminous Coal

Bituminous coal is an organic sedimentary rock formed by diagenesis of peat bog material. Bituminous coal or black coal is a relatively soft coal containing a tar-like substance called bitumen. Its coloration can be black or sometimes dark brown; hard varieties may shine with an iridescent effect. The carbon content of bituminous coal is around 60–80%; the rest is composed of water, air, hydrogen, sulfur, and heavy metals such as lead, mercury, nickel, tin, cadmium, antimony, and arsenic, as well as radio isotopes of thorium and strontium. Nasty stuff! There is no such thing as “clean coal.” Photo credit: Ralph L. Dawes, Ph.D. and Cheryl D. Dawes, licensed under a Creative Commons Attribution 3.0 United States License. Definition credit: Wikipedia licensed under CC BY: 3.O Attribution.

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