Geology And Geophysics

Classification of Clastic Sediments and Sedimentary Rocks in Geology and Earth Science



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Geologists classify sediments and sedimentary rocks based upon several parameters, which include their chemical, biological and physical modes of deposition as well as the grain size of the particles comprising the sediment or sedimentary rock.  Clastic sediments are those that are comprised of eroded particles or clasts of previously existing rocks and sediments.

Chemical sediments and sedimentary rocks are those that precipitate out of solution, such as salt (NaCl) and gypsum deposits, which form in evaporation basins called evaporites.  Another chemical sedimentary rock is travertine (stalactites, stalagmites and flowstone), which often forms in caves, caverns or even as the lime scale that forms in water pipes.

Biological sediments and sedimentary rocks are those formed by the life processes and the remains of living things.  Biological sediments include organic peat, which ultimately metamorphoses to become coal.  Coquina, which forms from accumulations of shells and broken shells of marine organisms, serves as a building material in areas where it is abundant, such as in Florida and the Caribbean. 

Think of biological and biochemical sediments as extensions of the chemical sediment classification and, like biology itself, as merely an extension of chemistry.

Before there were plants, there was little or no oxygen in the Earth’s atmosphere and iron remained dissolved in seawater.  With the evolution of photosynthesis, oxygen oxidized the iron in seawater and caused it to precipitate out of solution to form thick iron oxide mud.  This iron rich biochemical sediment eventually metamorphosed to become the iron ore that humans mine today. 

Once all of the iron had precipitated from seawater (by forming thick iron-rich sediments on the sea floor), oxygen began to accumulate in Earth’s atmosphere, and living things then evolved the ability to use this previously toxic (oxidizing) compound.

Limestone is the most common and abundant biochemical sedimentary rock type, and it is subject to a great variety of classification and sub-classification schemes because of its diverse origins and nature.  Limestone deposits form in a variety of ways and in combination; so they may have chemical, biological and biochemical, as well as clastic or clastically derived, components.

Clastic sediments and clastic sedimentary rocks form via the physical erosion, transportation, deposition and accumulation of particles (called clasts) of pre-existing rocks, minerals and shells, usually by fluids such as wind and water.  Thus, chemical and biological sediments and sedimentary rocks erode and become deposited elsewhere as organo-clastic sediments and rocks. 

Micrite is a type of limestone formed by a combination of lime mud particles settling and calcium carbonate precipitation into thick carbonate oozes, which later lithify (turn to stone) to become micritic limestone.  Limestone of many types has chemical, biological and clastic components.

The majority of clastic sediments and sedimentary rocks forms from the erosion and deposition of clasts (particles) of previously existing rocks and sediments.  Geologists base the classification of clastic sediments upon the size of the particles (grain size) of which the rocks and sediments are comprised.  Grain size classification is carried out in addition to the chemical and biochemical parameters discussed previously, and which may further classify or sub-classify clastic sediments.

For engineering purposes and for high technology science, unconsolidated or disaggregated clastic sediments are classified by passing the dried sedimentary material through a series of standard sieves.  The sieved sediments are weighed individually and plotted on a graph of weight versus grain size (as defined by passing through a particular mesh-sized sieve).  The graphic curve thus obtained characterizes and defines the specific clastic sediment in question.

For general observational purposes such as standard field geology and general sediment or sedimentary rock nomenclature, several much less rigorous (but parallel to sieve analysis) systems of classification, such as the Wentworth Scale, are used.  The common clastic sediment size distributions and rock types are summarized and compiled in the table below. 

The common clastic sediment size classifications from coarsest to finest are as follows:

Clasts Size                                 Rock Type

Boulders                   More than 256 mm            Conglomerate

Cobbles                    64 to 256 mm                    Conglomerate

Pebbles                    4 to 64 mm                        Conglomerate

Gravel                       2 to 4 mm                          Conglomerate

Coarse Sand             0.5 to 2.0 mm                    Sandstone   

Medium Sand            0.25 to 0.5 mm                  Sandstone

Fine Sand                  0.125 to 0.25 mm              Sandstone

Silt                            0.002 to 0.125 mm             Siltstone

Clay                          Less than 0.002 mm           Shale

Clastic sediments and sedimentary rocks are characterized by the dominant grain-sized particles they contain.  They are further classified by chemical composition of quartz and feldspar contained, and the adjectives quartic and felsic are applied, respectively.  The black sands on the beaches of Hawaii are classified as mafic because the igneous basalts from which they are eroded are rich in iron and magnesium (mafic) minerals.

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