Arkose

Arkose refers to a class of sandstones, which is characterized by a high content of feldspar.

Composition

The most striking feature of the arkoses is their high content of feldspars, which usually exceeds 25%. However, even with arkoses of quartz usually the most abundant mineral. In addition to quartz arkoses contain rock and mineral fragments with varying contents and usually lead kaolinite as weathering residue of feldspars.

Formation

The parent rock of arkoses is usually a feldspatführendes rock such as granite, which has been exposed to weathering and erosion. Then, the transport and the sedimentation of the particles followed. In a normal sandstone are changing in most cases due to the chemical weathering and wear of the grains during transport, the relatively susceptible to weathering feldspars in clay minerals around. Crucial for a significant preservation of feldspar and thus for the formation of arkoses are therefore a short transport, a high accumulation rate, a low degree of chemical weathering and a high feldspar content of the parent rock.

Besides the high feldspar content which often only moderate sorting, and often poor rounding of the grains indicate in arkoses minimize transportation. Ripple marks and oblique stratifications suggest rivers as transport and deposition system. The low fossil content is typical, which favors a slow chemical weathering of feldspars for a continental ( continental ) deposition and an arid climate.

The combination of these characteristics is often found in continental rift zones, in intramontane basins or foreland basin.

Other properties

Arkose weather in greyish and often reddish hue. The ingredients tend to coarse grain sizes ( 0,5-2 mm) that occur sorted just like their parent rock only moderately. The cementation of the grains is usually done by calcite, rarely by iron oxides or silicates. The proportion of the matrix is below 15%.

Examples

• The Uluru (formerly called " Ayers Rock" ) in Australia.
• Fountain Formation of Colorado
• Fulmar sandstone (Central North Sea / rift system in the Jura )
• Tertiary sandstones of the Rhine grave shoulders, for example, Black Forest / Vosges