Subarkose

Subarkose is a sandstone classification used in sedimentary petrography to describe a quartz-rich rock that contains a noticeable but non-dominant amount of feldspar, placing it between arkose and quartz arenite. In common classification schemes, subarkose has more quartz overall than feldspar but retains a measurable feldspar fraction, whereas arkose is strongly feldspar-rich and quartz arenite is dominated by quartz with little feldspar or lithic content. The remaining grains, if present, are usually minor lithic fragments.

The mineralogy of subarkose typically features a dominant quartz framework, with feldspar constituting a few percent up to roughly one-quarter of the framework grains. Lithic fragments are generally scarce. Matrix content and diagenetic cement can vary, but cement is commonly quartz or carbonate, and the rock is often well to moderately sorted.

Provenance and depositional settings for subarkose reflect intermediate weathering and transport. It commonly forms from sources that include feldspathic rocks such as granitoids or feldspathic litharenites, with enough transport to remove some, but not all, reactive components. Depositional environments can include fluvial, deltaic, and shoreline settings where sand is derived from relatively proximal, feldspar-bearing sources.

In practice, subarkose is used to indicate an intermediate composition on the maturity spectrum: more quartz-rich than arkose but not as quartz-dominated as quartz arenite. It aids in inferring weathering intensity and sediment source in sandstone studies. See also: quartz arenite, arkose, Folk classification, sedimentary petrography.