A Classification of Sedimentary Rocks

Sedimentary rocks are the rocks formed by sediments. Sediments are  formed through the destruction and breaking down of previously formed rocks. Sediments are created by the processes of weathering and erosion, undergo transportation, accumulation and deposition and finally undergo a process known as diagenesis to produce Sedimentary Rocks. Physical factors such as Wind (aeolian), Water (alluvial), and Gravity(colluvial) aid the sedimentation process.

Sedimentary rocks are usually characterized by their structure and appearance. features such as bedding planes, ripple marks, mud cracks and fossils not only help identify sedimentary rocks but they also help to determine other properties of the rock such as age, environment of deposition, and the orientation of the original bedding.

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The environments where sedimentation takes place, are classified as Continental environments, Shoreline environments and Marine Environments. According to the method of formation of sedimantary rocks in these environments Sedimentary rocks are classified as follows,

1.  Detrital/Clastic Sedimentary Rocks

Detrital sedimentary rocks are formed by the accumulation of solid particles and debris from pre-formed rocks (detritus). These rocks consist of fragments of rock and minerals. Depending on the Size and Shape of these fragments, several types of rock can be identified.
A cementation of large, rounded pebbles or gravel is identified as a Conglomerate. If the gravel and fragments are sharp and angular, it's called Breccia. According to the grain size of the rock and whether it contains sand sized particles or mud or silt the rocks are named as Sandstone, Siltstone or Shale. These rocks are formed mainly in Continental and Shoreline Environments.

2.  Chemical Sedimentary Rocks
Chemical Sedimentary rocks are formed by the precipitation of material from solutions. These rocks can be formed in water basins as well as in underground formations involving water such as Limestone Caves. According to the chemicals that precipitate to create these rocks are classified into groups such as, Carbonates(Limestone, calc tuffa, travertine), Silicates(Opal, Chalcedony, Agate), Ferruginous(limonite, Goethite), Allitic(laterite, bauxite), Halides(rock salt), Sulphates(Gypsum), and Phosphates(phosphorite).

3.  Biogenic Sedimentary Rocks
Biogenic sedimentary rocks are formed through the vital activity of plants and animals. The building up of animal tissue by using materials in water leads to the formation of these types of rocks. The accumulation of skeletal remains of organisms like marine vertebrae and even diatoms also end up in the formation of biogenic sedimentary rocks. Another point worth noting is that fossils of prehistoric animals can be found only in sedimentary rocks. Carbonates such as fossileferous limestone, siliceous diatomite and caustobioliths such as coal belong to this type. These rocks are primarily formed in Marine environments

4.  Sedimentary Rocks of Mixed Origin
Some sedimentary rocks exhibit properties that belong to more than one of the above types. These rocks are complex and contain detrital, as well as biogenic properties described above to a certain extent. Examples of these rocks are Marls, Calcareous sandstone and Siliceous Clay.