Classification of sedimentary rocks

Classification based on grain size

Clastic (those composed of rock fragments) sedimentary rocks can be classified as:

• conglomerates
• sandstones
• siltstones
• shales
• claystones

Prefixes can also be added to indicate the dominant mineralogy. For example, a quartz-rich sandstone is a quartzose sandstone, a feldspar-rich sandstone is an arkose sandstone, a mica-rich sandstone is a micaceous sandstone, and a lithic-rich sandstone or shale is a greywacke.

Conglomerates

Conglomerates can be further subdivided according to their texture (grain-supported versus matrix-supported), composition (pebbles of one rock type or several different rock types) and source rocks (pebbles derived from within the depositional environment or from outside).

Limestones

Limestones are a different case as they can either be of clastic or chemical origin.

The clastic limestones can contain either:

• rock fragments
• fossils (e.g. fossiliferous limestone)
• ooliths
• pellets

In contrast, the chemically-precipitated limestones consist solely of crystalline carbonates (e.g. microcrystalline limestone). The clastic limestones can be further subdivided according to their grainsize, much like other clastic sedimentary rocks.

Other chemical sedimentary rocks

Other chemical sedimentary rocks include:

• banded-iron formations
• ironstones
• cherts
• evaporites
• silcretes
• calcretes

Zebra rock

Zebra rock or Zebra stone is a distinctive reddish-brown and white banded claystone from the Ord River area of Western Australia where it forms discontinuous lenses within a Late Proterozoic shale sequence. Most occurrences are now submerged beneath the dam waters of Lake Argyle. Since its discovery in 1924, it has been used widely as an ornamental stone. It is composed of small particles of quartz, white mica, clay minerals, hematite and alunite. The striking colour banding was probably formed after the original sediments were deposited by the rhythmic precipitation of hematite-rich bands during alteration of the rock in a highly oxidising environment.

Some examples of sedimentary rocks

• Chalk is a fine-grained limestone, usually formed by compacted microscopic animals called foraminifera.
• Chert is a fine-grained hard sedimentary rock composed of microscopic silica grains, and has a flat fracture.
• Flint is a variety of chert (mostly of upper Cretaceous age) that has a conchoidal fracture.
• Marl is a calcareous mudstone.
• Micrite is a finely-crystalline calcite.
• Mudstone is a fine-grained sedimentary rock that lacks a well-developed bedding plane.
• Shale is a fine-grained sedimentary rock with a well-developed bedding plane.
• Siltstone is similar to mudstones but consists predominantly of silt-sized particles
• Sparite is a coarsely-crystalline calcite.