Arkose

(petrology) A sedimentary rock composed of sand-size fragments that contain a high proportion of feldspar in addition to quartz and other detrital minerals.

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An arenaceous rock that contains a high proportion of feldspar in addition to quartz and other detrital minerals. Arkose is also known as feldspathic sandstone. Although there is no universal agreement, many geologists consider a minimum of 25% feldspar a requisite for calling sandstone an arkose. Other geologists accept a lower value. Arkoses may contain a high proportion of other nonquartz detritus, such as igneous and metamorphic rock fragments, micas, amphiboles, and pyroxenes. Frequently the accessory heavy mineral suite consists of a variety of species.

Sedimentary structures of arkoses are similar in kind to those of the orthoquartzites. Cross-bedding, the major feature, may be displayed on a huge scale, some cross-bedded units being many feet thick. Arkoses are associated with a variety of clastic rocks, dominantly conglomerates, and reddish-colored shales. Arkoses also are found with basic lava flows. Most arkoses are found in geosynclinal areas, but the thin, reworked, granite wash arkoses can be found on stable continental platforms. See also Geosyncline.

The granite-wash arkoses appear to have formed as the result of a transgression of the sea over a land area underlain by granite. The fragmented granite in the soil and mantle rock is incorporated in the basal sediment. In some areas the original granite is changed so slightly that the arkose is called recomposed granite and may be almost indistinguishable from the original granite. Since high relief and climatic extremes generally are associated with orogenic movements, arkoses are usually interpreted as sediments that result from tectonically active regions. See also Arenaceous rocks; Feldspar; Graywacke; Sandstone; Sedimentary rocks.

An arenaceous rock composed of quartz and more than 25% feldspar.

Sandstone containing 25% or more feldspar grains in abundance; used as building stone.

Arkosic sand in the Llano Uplift, Texas, USA with granite outcrops.

Arkose (pronounced /ˈɑrkoʊz/) 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. The other mineral components may vary, but quartz is commonly dominant, 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 '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 tends 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 sandstone found in Slovakia

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. 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 over such granitic terrain.

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

References

1. ^ Folk, R.L. (1974). Petrology of Sedimentary Rocks. Hemphill. ISBN 0-914696-14-9. http://www.lib.utexas.edu/geo/folkready/folkprefrev.html. 
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. 

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