Arkosis
Arkose refers to a class of sandstones that is characterized by a high feldspar content .
composition
The most striking feature of the arkoses is their high content of feldspars , which usually exceeds 25%. However, quartz is usually the most common mineral even in arkoses . In addition to quartz, arkoses also contain rock and mineral fragments with changing contents and mostly lead to kaolinite as a weathering residue of the feldspars.
Emergence
The parent rock of Arkosen is usually a feldspar-bearing rock such as granite , which has been exposed to weathering and erosion . This was followed by the transport and sedimentation of the grains. In the case of normal sandstone, in most cases the feldspars, which are relatively susceptible to weathering, are converted into clay minerals due to chemical weathering and the wear and tear of the grains during transport . A short transport route, a high rate of accumulation , a low degree of chemical weathering and a high feldspar content of the parent rock are therefore decisive for the significant preservation of feldspar and thus for the formation of arcoses .
In addition to the high proportion of feldspar, the often only moderate sorting and the often poor rounding of the grains in arkoses indicate short transport routes. Ripple marks and inclined stratifications suggest rivers as a transport and storage system. The low fossil content is typical of a continental (mainland) deposit and an arid climate , which favors the slow chemical weathering of the feldspars.
The combination of these characteristics is often found in continental rift zones , in intramontane basins or in foreland basins .
Other properties
Arcoses weather in a grayish and often reddish color. The constituents tend to have coarse grain sizes (0.5–2 mm) which, like their parent rock, are only moderately sorted. The cementing of the grains is done mostly by calcite , rarely by iron oxides or silicates . The proportion of the matrix is below 15%.
Examples
- Mutitjulu-Arkose of Uluṟu (formerly "Ayers Rock"), Ediacarian / Cambrian , Northern Territory , Australia
- Sandstones of the Fountain Formation, Upper Carboniferous , Colorado , USA
- Fulmar sandstone, Central North Sea Rift System, Upper Jura
- Tertiary sandstones of the Rhine grave shoulders, z. B. Black Forest / Vosges
Web links
Individual evidence
- ↑ Füchtbauer 1988: Füchtbauer, Hans: Sediments und Sedimentgesteine. 4th revised edition. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart 1988, 1141 pp. ISBN 3-510-65138-3