Arkose

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Arkose

Arkose is a detrital

Detrital

Detritus is a geological term used to describe particles of rock derived from pre-existing rock through processes of weathering and erosion. Detrital particles can consist of lithic fragments , or of monomineralic fragments ....
sedimentary rock

Sedimentary rock

Sedimentary rock is one of the three main Rock types . Sedimentary rock is formed by deposition and consolidation of mineral and organic material and from precipitation of minerals from solution....
, specifically a type of sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-size mineral or rock Particle size . Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust ....
containing at least 25% feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's Crust .Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock....
., Arkosic sand is sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.As the term is used by geologists, sand particles range in diameter from 0.0625 to 2 millimeters....
that is similarly rich in feldspar, and thus the potential precursor of arkose. The other mineral components may vary, but quartz

Quartz

Quartz is the most abundant mineral in the Earth's continental crust . It is made up of a Crystal structure of silica tetrahedra. Quartz has a hardness of 7 on the Mohs scale and a density of 2.65 g/cm?....
is commonly dominant, and some mica

Mica

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Encyclopedia

Arkose is a detrital

Detrital

Sedimentary rock

Sandstone

Feldspar

Sand

Quartz

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic with a tendency towards pseudo-hexagonal crystals and are similar in chemical composition....
is often present. Apart from the mineral content, rock fragments may also be a significant component. Arkose usually contains small amounts of calcite

Calcite

Calcite is a Carbonate minerals and the most stable Polymorphism of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite....
cement, which causes it to 'fizz' slightly in dilute hydrochloric acid

Hydrochloric acid

Hydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong acid mineral acid and has major industrial uses....
; sometimes the cement also contains iron oxide

Iron oxide

Iron oxides are chemical compounds composed of iron and oxygen. Altogether, there are sixteen known iron oxides and oxyhydroxides....
. Arkose is typically grey to reddish in colour. The sand grains making up an arkose may range from fine to very coarse, but tends toward the coarser end of the scale. Fossil

Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
s 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

Metamorphic

The term Metamorphic can be associated with a number of meanings:*Metamorphic rock: The term for rocks that have been transformed by extreme heat and pressure....
rocks, most commonly granitic rocks

Granite

Granite is a common and widely occurring type of Intrusion , felsic, igneous rock rock . Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as Porphyry ....
, which are primarily composed of quartz and feldspar. 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

Conglomerate (geology)

A conglomerate is a Rock consisting of individual stones that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts....
deposits sourced from granitic terrain and is often found above unconformities

Unconformity

An unconformity is a buried erosion surface separating two Rock masses or Stratum of different ages, indicating that sediment deposition was not continuous....
over such granitic terrain.

The famous central Australian monolith Uluru

Uluru

Uluru, also referred to as Ayers Rock, is a large sandstone rock formation in the southern part of the Northern Territory, central Australia....
(Ayers Rock) is composed of late Neoproterozoic

Neoproterozoic

The Neoproterozoic Era is the unit of geologic time scale from 1,000 to 542 +/- 0.3 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods....
or Cambrian

Cambrian

The Cambrian is a geologic period that began about Mya at the end of the Proterozoic eon and ended about Ma with the beginning of the Ordovician period ....
arkose, deposited in the Amadeus Basin

Amadeus Basin

The Amadeus Basin is a large craton sedimentary basin in central Australia, lying mostly within the southern Northern Territory, but extending into the state of Western Australia....
.