Sandstone Classification

Introduction

Classifying sandstones is deceptively difficult. Despite their simple composition - dominated by quartz, feldspars, and rock fragments - geologists have proposed more than 50 different classification schemes for sandstones, none of which has achieved universal acceptance. The core challenge is that mineralogy, matrix content, and grain texture are all geologically significant, yet no single classification captures all three simultaneously without becoming unwieldy.

Descriptive classification of sandstones is based fundamentally on framework mineralogy, though the relative abundance of matrix plays a role in some schemes. Finding a classification suitable for all sandstone types and acceptable to most geologists has proven difficult; indeed, more than 50 different classifications have been proposed, but none has received widespread acceptance. ^[1]

Textural Nomenclature of Mixed Sediments

Unconsolidated siliciclastic sediment is called gravel (>2 mm grains), sand (1/16-2 mm), or mud (<1/16 mm) depending on grain size. The lithified equivalents are conglomerate, sandstone, and shale (mudstone). ^[1]

Because many siliciclastic rocks contain grains of mixed sizes, triangular textural diagrams are used to name them. For sediments containing particles of sand size and smaller, a triangular diagram using sand, silt, and clay as end members is most appropriate. When gravel is also present, a broader textural scheme is needed. Although one might expect the boundary between gravel and sand-mud to be set at 50 percent, many geologists consider a sediment with as little as 30 percent gravel-size fragments to qualify as a gravel. ^[1]

Mineralogical Classification: The QFL Triangle

Most sandstones are made up of mixtures of a very small number of dominant framework components. Quartz, feldspars, and rock fragments such as chert and volcanic clasts are the only framework constituents commonly abundant enough to be important in sandstone classification. ^[1]

Most sandstone classifications use a QFR or QFL plot - a triangular diagram on which quartz (Q), feldspars (F), and rock fragments (R or L) are plotted as end members at the poles of the classification triangle. ^[1]

Arenites vs. Wackes

In the Gilbert (Williams, Turner, and Gilbert, 1982) classification - based on an earlier scheme by Dott (1964) - sandstones effectively free of matrix (<5 percent) are classified as quartz arenites, feldspathic arenites, or lithic arenites depending on the relative abundance of QFL constituents. When matrix is present at or above 5 percent, the terms quartz wacke, feldspathic wacke, and lithic wacke are used instead. ^[1]

A key difference between the Williams-Turner-Gilbert and Dott classifications is that Dott sets the arenite-wacke boundary at 15 percent matrix rather than 5 percent. ^[1]

Arkose and Graywacke

The name arkose is often used informally for any feldspathic arenite that is particularly rich (>~25 percent) in feldspars. ^[1]

The term graywacke is commonly applied to matrix-rich sandstones of any composition that have undergone deep burial, have a chloritic matrix, and are dark gray to dark green, very hard, and dense. The term has been much misused and its continued use is controversial; some geologists advocate abandoning it entirely in favour of the word wacke. In any case, the name is best restricted to field use and should not be used as a petrographic term. ^[1]

Compositional and Textural Maturity

The term maturity is applied to sandstones in two distinct ways. Compositional maturity refers to the relative abundance of stable versus unstable framework grains - a quartz-rich sandstone is compositionally mature; one rich in feldspars or unstable rock fragments is compositionally immature. ^[1]

Textural maturity is determined by the relative abundance of matrix and the degree of rounding and sorting of framework grains. Textural maturity ranges from immature (much clay, poorly sorted and poorly rounded grains) to supermature (little or no clay, well-sorted and well-rounded grains). Textural maturity is said to reflect the degree of sediment transport and reworking, though it may also be affected by diagenetic processes - for example, clay minerals may form in pore spaces during burial diagenesis. ^[1]

The distinction between compositional and textural maturity matters because the two do not always go together. A sandstone can be compositionally mature (dominated by quartz) but texturally immature (poorly sorted, clay-rich matrix) - this happens when quartz grains are derived from a very mature source rock but are deposited rapidly without significant reworking. Conversely, a feldspathic sandstone can be texturally mature if deposited in a high-energy beach environment that removes clay and rounds grains, even though the feldspar content indicates chemical immaturity.

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