ARKOSE

coarse sandstone (sedimentary rock composed of cemented grains 0.062 millimetres [0.00240.08 inch] in diameter) presumed to have formed by the disintegration of a granite without appreciable decomposition. Arkose thus consists primarily of quartz and feldspar grains together with small amounts of mica, all moderately well sorted, slightly worn, and loosely cemented with calcite or, less commonly, iron oxides or silica. Arkoses are distinguished from the normal quartzose sandstones (orthoquartzites) by their high feldspar content (more than 25 percent of the sand grains) and from the graywackes by their lighter colour. The feldspar content of subarkoses is 525 percent of the sand grains; it is diminished by more extensive weathering or by dilution from nonigneous source rocks. In the absence of stratification, arkose may bear superficial resemblance to granite, and it aptly has been described as reconstituted granite, or granite wash. Like the granites from which they were formed, arkoses are pink or gray. The geological significance of arkose has been much debated. Under normal conditions most of the feldspar decomposes and is converted to clay minerals during weathering of the source rocks, whereas under conditions of extreme dryness or low temperatures, decomposition of the feldspar is inhibited or greatly retarded. Arkoses were, therefore, presumed to be derived from the erosion of a granitic terrane characterized by an arid or glacial climate. Now, however, it is known that the feldspar may escape destruction and thus be transported and deposited with quartz sands if rates of uplift, erosion, and deposition are great enough. Under such conditions, irrespective of the climate, weathering processes are incomplete and the sands derived from such terrane are high in feldspar content. Arkoses, therefore, may be said to indicate either a climatic extreme or high relief. Most ancient arkose deposits seem to be the product of high relief.