Sedimentary Rocks


Staff member
Jun 3, 2020
Sedimentary rocks (from the Latin sedimentum, settlement) are formed by the precipitation and accumulation of mineral matter in a solution or by the compaction of plant and/or animal remains that consolidate into hard rocks. Sediments are deposited, one layer on top of the other, on the surface of the lithosphere at relatively low temperatures and pressures, and may be made up of pre-existing rock fragments of different sizes, resistant minerals, remains of organisms, and products of chemical reactions or evaporation.

A pre-existing rock exposed on the surface of the earth goes through a Sedimentary Process (erosion or weathering, transportation, deposition, compaction and diagenesis) with which it becomes a sedimentary rock. This transformation is known as lithification. As the sedimentary rocks are formed near or on the surface of the earth, their study gives us information about the environment in which they were deposited, the type of transportation agent and, sometimes, the origin from which the sediments were derived.

Sedimentary Rocks - clay, limestone, sandstone, sylvine, chalcedony, coal, phosphorite, conglomerate, bauxite
(Sedimentary Rocks - clay, limestone, sandstone, sylvine, chalcedony, coal, phosphorite, conglomerate, bauxite)

Sedimentary rocks are generally classified, according to the way they are produced, as detrital or clastic, and chemical or nonclastic. The chemical/nonclastic sedimentary rocks are further classified into a subcategory known as biochemicals.

Detrital or Clastic sedimentary rocks
They are mechanical accumulations of particles or sediments of pre-existing rocks called "debris" or "clasts" formed by materials produced by weathering and surface erosion. These are transported and finally deposited, so they have a texture called clastic. These rocks are divided into a wide variety of types, which are characterized by the size of their constituent particles.

The following is the classification of detrital sedimentary rocks:
S. No.SedimentSizeCompositionRock
1Gravel>2mmRounded particlesConglomerate
2Angular particlesBreccia
3Sand1/16 to 2mmPredominantly quartz sandQuartz sandstone
4Quartz with >25% feldsparArkosa
5Quartz with >25% rock fragmentsGraywacke
6Mud<1/16mmPredominantly slimeLimolite
7Slime and clayLimonlite
8Predominantly clayShale and Arcillite

Chemical sedimentary rocks
They are the rocks that originate from materials deposited by chemical means, where the crystals are held together by chemical bonds or intertwined within each other. The materials, already dissolved, are transported and concentrated forming minerals that accumulate in aggregates and are later lithified as in detrital rocks, to form a rock. Almost all of these rocks originate from chemical precipitation in extensions of surface water, either by inorganic chemical processes or by the chemical activity of organisms. Rocks formed by the activity of organisms are known as biochemical sedimentary rocks.

The following is the classification of chemical sedimentary rocks:
S. No.TextureMineral compositionRock
4CrystallineHalitaRock salt

The following is the classification of biochemical sedimentary rocks:
S. No.TextureMineral compositionRock
1ClasticCalcium carbonate shellsLimestone (Chalk, Coquina)
2Generally crystallineAltered microscopic shells of Silicon DioxideFlint
3Generally crystallineMainly carbon from altered plant remainsMineral carbon

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