Sedimentary rocks

The sedimentary rocks are those formed by the accumulation of materials or particles, by chemical precipitation or by the growth of organisms, subaerial, under sea or lake-water conditions: the sediments. Generally these are deposited in horizontal layers: the strata. As they are buried, the sediments that are not yet consolidated are compacted by the weight of the overlying deposits and they transform into rocks by cementation. This process is called lithification. The cementation takes place by the precipitation of the minerals dissolved in the fluids that occupy the gaps between the particles, joining them. The sediments often contain remains of organisms, which will become fossils inside the sedimentary rocks.

The sedimentary rocks are classified bearing in mind the process that originated them and their composition. The main groups are the detrital, the carbonatic and the evaporitic rocks.

The detrital rocks are formed by the transport, accumulation and cementation of solid particles derived from the disintegration and weathering of pre-existing rocks. They are classified by the size and morphology of the grains. The most common are, going from large to small grain size, the conglomerates and the breccias, sandstones, siltstones and mudrocks; the lutites are formed by clays and silt. Other classification criteria are the composition of the particles, and the texture or the type of cement.

The rocks formed by chemical precipitation are the result of the crystallization of chemical elements that are dissolved in the sea and in the lakes. They are classified according to their mineralogical composition. The most relevant groups are the evaporites, salts and gypsum, and the carbonates. The latter, have their own classification in which preference is given to the mineral composition (calcite, dolomite) and mainly to the textures of these rocks, especially to the grain size and the type of components they are composed of.

The rocks formed by the growth of organisms are mostly carbonates. It is necessary to differentiate the bio-constructions of reefs from those in which the carbonate is in the form of mud, of grains and particles or of skelletal components. The latter have characteristics in common with the detrital rocks and are quite often associated.

The classification is not strict, since there are numerous rocks or lithological associations that are the result of the intersection of two or all these types.