Last updated
Sedimentary rock
Arkose with K-feldspar (pinkish-orangish) and quartz (gray) grains.jpg
Arkose with K-feldspar (pinkish-orangish) and quartz (gray) grains
>25% feldspar

Arkose ( /ˈɑːrˌks,-ˌkz/ ) is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. [1] [2] Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose.

Quartz is commonly the dominant mineral component, and some mica is often present. Apart from the mineral content, rock fragments may also be a significant component. Arkose usually contains small amounts of calcite cement, which causes it to effervesce (fizz) slightly in dilute hydrochloric acid; sometimes the cement also contains iron oxide.

Arkose is typically grey to reddish in colour. The sand grains making up an arkose may range from fine to very coarse, but tend toward the coarser end of the scale. Fossils are rare in arkose, due to the depositional processes that form it, although bedding is frequently visible.

Arkose is generally formed from the weathering of feldspar-rich igneous or metamorphic, most commonly granitic, rocks, which are primarily composed of quartz and feldspar (called 'grus' as a sand). These sediments must be deposited rapidly and/or in a cold or arid environment such that the feldspar does not undergo significant chemical weathering and decomposition; therefore arkose is designated a texturally immature sedimentary rock. Arkose is often associated with conglomerate deposits sourced from granitic terrain and is often found above unconformities in the immediate vicinity of granite terrains.

The central Australian inselberg Uluru (Ayers Rock) is composed of late Neoproterozoic/Cambrian arkose, deposited in the Amadeus Basin. [3]

See also

Related Research Articles

Granite common type of intrusive, felsic, igneous rock with granular structure

Granite is a coarse-grained igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the Earth's continental crust, where it is found in various kinds of igneous intrusions. These range in size from dikes only a few inches across to batholiths exposed over hundreds of square kilometers.

Sandstone Type of sedimentary rock

Sandstone is a clastic sedimentary rock composed mainly of sand-sized silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at the Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Conglomerate (geology) Coarse-grained clastic sedimentary rock with mainly rounded to subangular clasts

Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contain a matrix of finer grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.

Greywacke A hard, dark sandstone with poorly sorted angular grains in a compact, clay-fine matrix

Greywacke or graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found in Paleozoic strata. The larger grains can be sand- to gravel-sized, and matrix materials generally constitute more than 15% of the rock by volume. The term "greywacke" can be confusing, since it can refer to either the immature aspect of the rock or its fine-grained (clay) component.

Quartz arenite

A quartz arenite or quartzarenite is a sandstone composed of greater than 90% detrital quartz, with limited amounts of other framework grains and matrix. It can have higher-than-average amounts of resistant grains, like chert and minerals in the ZTR index.

Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.

The Folk classification, in geology, is a technical descriptive classification of sedimentary rocks devised by Robert L. Folk, an influential sedimentary petrologist and Professor Emeritus at the University of Texas.

Clastic rock Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

Sand Granular material composed of finely divided rock and mineral particles

Sand is a granular material composed of finely divided rock and mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of soil or soil type; i.e., a soil containing more than 85 percent sand-sized particles by mass.

Gazzi-Dickinson method Point-counting technique used in geology

The Gazzi-Dickinson method is a point-counting technique used in geology to statistically measure the components of a sedimentary rock, chiefly sandstone. The main focus part of the technique is counting all sand-sized components as separate grains, regardless of what they are connected to. Gazzi-Dickinson point counting is used in the creation of ternary diagrams, such as QFL diagrams.

Geology of Cape Town Geological formations and their history in the vicinity of Cape Town

Cape Town lies at the south-western corner of the continent of Africa. It is bounded to the south and west by the Atlantic Ocean, and to the north and east by various other municipalities in the Western Cape province of South Africa.

Igneous rocks are found in Bukit Timah, Woodlands, and Pulau Ubin island. Granite makes up the bulk of the igneous rock. Gabbro is also found in the area and is found in an area called Little Guilin, named for its resemblance to Guilin in South China. This area is in Bukit Gombak. Sedimentary rocks are found on the western part of Singapore, which is mainly made of sandstone and mudstones. It also includes the southwestern area. Metamorphic rocks are found in the northeastern part of Singapore, and also on Pulau Tekong, off the east coast of Singapore. The rocks are mainly made up of quartzite, and also make up the Sajahat Formation.

Iron-rich sedimentary rocks Sedimentary rocks containing 15 wt.% or more iron

Iron-rich sedimentary rocks are sedimentary rocks which contain 15 wt.% or more iron. However, most sedimentary rocks contain iron in varying degrees. The majority of these rocks were deposited during specific geologic time periods: The Precambrian, the early Paleozoic, and the middle to late Mesozoic. Overall, they make up a very small portion of the total sedimentary record.

In north European geology, Jotnian sediments are a group of Precambrian rocks more specifically assigned to the Mesoproterozoic Era (Riphean), albeit some might be younger. Jotnian sediments include the oldest known sediments in the Baltic area that have not been subject to metamorphism. Stratigraphically, Jotnian sediments overlie the rapakivi granites and other igneous and metamorphic rocks and are often intruded by younger diabases.

Catoctin Formation

The Catoctin Formation is a geologic formation that expands through Virginia, Maryland, and Pennsylvania. It dates back to the Precambrian and is closely associated with the Harpers Formation, Weverton Formation, and the Loudoun Formation. The Catoctin Formation lies over the a granite basement rock and below the Chilhowee Group making it only exposed on the outer parts of the Blue Ridge. The Catoctin Formation contains metabasalt, metarhyolite, and porphyritic rocks, columnar jointing, low-dipping primary joints, amygdules, sedimentary dikes, and flow breccias. Evidence for past volcanic activity includes columnar basalts and greenstone dikes.

Geology of Ivory Coast

The geology of Ivory Coast is almost entirely extremely ancient metamorphic and igneous crystalline basement rock between 2.1 and more than 3.5 billion years old, comprising part of the stable continental crust of the West African Craton. Near the surface, these ancient rocks have weathered into sediments and soils 20 to 45 meters thick on average, which holds much of Ivory Coast's groundwater. More recent sedimentary rocks are found along the coast. The country has extensive mineral resources such as gold, diamonds, nickel and bauxite as well as offshore oil and gas.

Lilesville Granite Body of granitic rock

The Lilesville Granite, also referred to as the Lilesville pluton, is a ring-shaped body of granitic rock that spans about 94 square miles (240 km2) in Anson, Richmond, and Montgomery Counties in southern North Carolina.


  1. Folk, R.L. (1974). Petrology of Sedimentary Rocks. Hemphill. ISBN   0-914696-14-9. Archived from the original on 2006-02-14. Retrieved 2007-05-05.
  2. Stow, D.A.V. (2005). Sedimentary rocks in the field. Manson. ISBN   1-874545-69-3.
  3. Sweet, I.P.; Crick, I.H. (1991). Uluṟu & Kata Tjuṯa: A Geological History. Australian Geological Survey Organization. ISBN   0-644-25681-8.