Sandstone Maturity

With reference to quartz arenites, feldspathic arenites and lithic arenites, explain what is meant by “sandstone maturity.”

Mineralogy & Weathering
Supracrustal rocks are those formed at the surface. Subcrustal rocks are those formed below the surface. Where minerals form impact their stability. Unsurprisingly, minerals crystallized at the surface are more stable under surface conditions (pressure, temperature, water content…). The result is that subcrustal rocks like plutonics weather more rapidly at the surface, and supracrustal rocks like volcanics are more resistant to weathering.

Quartz is a highly resistant rock, very hard and stable at surface conditions. This makes quartz a very common component of sedimentary rocks. Feldspar is much softer and unstable at the surface, subject to chemical decomposition. Feldspar usually only survives in conditions where physical weathering rates are faster than chemical weathering rates, so clasts are produced before the mineral rots into clay. This can happen in areas of high source area relief (where physical weathering is faster), or in arid or arctic climates (where chemical weathering rates are slower). Lithics, clasts made of rock fragments, weather depending on the source rock type. Lithic fragments of plutonic rocks are more rare, while fragments of volcanic rocks are more common. These fragments provide the most specific information about the source rock disintegrated into sediments to form the new sedimentary rock.

In terms of mineralogical maturity, a rock is immature when it contains a mix of stable and unstable minerals, and is mature when it contains only stable minerals.

Compositional Ratios
By describing sedimentary rocks in terms of their percentage of quart, feldspar, and lithic fragments (and ignoring accessory minerals), it is possible to use ratios as index markers.

The ratio of feldspar to lithic fragments can serve as an index of sandstone provenance. If a rock has more lithic fragments relative to feldspars, then the source rock is probably resistant supracrustal rocks (like volcanics). If a rock has more feldspars to lithic fragments, then the source rock is probably less resistant subcrustal rocks (like plutons).

The ratio of quartz to other components (feldspar and lithic fragments combined) is an index of compositional maturity. If a rock contains a low proportion of quartz and a higher proportion of softer, unstable elements, it an immature rock. If the sample contains a high proportion of resistant, stable quartz compared to the unstable elements it is a more mature rock.

Texture & Weathering
As clasts are subject to weathering, the grain size decreases. Larger clasts can only be transported by higher energy flows, so larger clasts are transported shorter distances than smaller clasts. The result is increased sorting during transportation: clast size variation decreases with respect to distance from the source. At the same time, grains get tossed and beaten, breaking off sharp corners and becoming more rounded.

In terms of textural maturity, a rock is immature when it contains angular, poorly sorted grains, and it is mature when it contains rounded, well-sorted grains.

Maturity is a method of discussing how stable the sedimentary rock is, particularly in terms of the degree of weathering of its components.

A sedimentary rock is immature when it contains greater than 5% clays or silt. Immature rocks may also contain a larger portion of unstable minerals such as feldspar, and a variety of lithic fragments.

A sedimentary rock is submature when it contains less than 5% clays or silt, with moderately sorted grains. The clasts are mostly angular with potentially a few rounded grains, with some unstable minerals and lithic fragments.

A sedimentary rock is mature when it contains less than 5% clays or silt, and is well sorted. The clasts are mostly subangular to subrounded, with primarily stable minerals (including stable lithics like chert) and only a few feldspar or lithic grains.

A sedimentary rock is supermature when it contains less than 5% clays or silt, and is well-sorted. The clasts are subrounded to rounded, with almost exclusively stable minerals. Supermature sedimentary rocks are indicative of high energy environments.

Sandstone Maturity
Quartz arenites contain 95% quartz and less than 15% matrix material. The high portion of extremely stable materials and the well-bedded texture indicate supermature composition and texture.

Feldspathic arenites (or arkose arenites ) usually contain 40-50% feldspar, a less stable and thus less mature composition. The textures are also usually less mature than quartz arenites, with coarser grains that are moderately sorted and more angular. The exception to this is when feldspathic arenite forms in an environment with slow chemical weathering rates: desert arkose is compositionally immature yet texturally supermature.

Lithich arenites usually contain 30-80% quartz and 5-50% lithic fragments. The compositional maturity can vary broadly depending on the nature of the lithic fragments: chert would be compositionally mature while plutonic rocks would be compositionally immature. The textures commonly range from submature to mature, possibly well-bedded with cross-bedding, ripples, lamination, and lineations.

Wackes are sandstones with more than than 15% matrix material, usually rich in clay and chlorite. This means by definition, wackes contain more than 5% clays and silts, thus are immature sandstones.

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