Sedimentary rocks: types, classification and examples

The sedimentary rocks are a type of rocks always in the earth ‘s surface from preexisting rocks passing by the call depositional process , which eventually turn into sedimentary rocks as such. This sedimentation process consists of different large general phases.

1. Erosion: be it biological, chemical or mechanical, this phase of the sedimentary process is characterized by the destruction of the pre-existing solid rock, breaking it down into smaller fragments. Destructive agents can be wind, water or ice.
2. Transport: in this phase, the water in its different states or the wind transport the fragments produced by erosion.
3. Deposit: sediments carried by water, wind or ice are accumulated in depressed areas prone to it, such as deltas, lakes, trenches or basins.
4. Compaction and diagenesis: physical and chemical processes compact accumulated sediments, so that they end up transforming them into sedimentary rocks.

It is due to the nature of this formation process that sedimentary rocks tend to be formed in layers or strata, which allow differentiating the different phenomena that occurred during certain periods of time when studying them.

There are two main ways to classify sedimentary rocks : by their formation and by their composition. On the one hand, the different types of sedimentary rocks according to their formation are classified as:

• Detrital or clastic sedimentary rocks: these are sedimentary rocks that have been formed due to the accumulation over long periods of time of sediments from the erosion of large rocks, slopes or mountains, and deposited by the simple effect of gravity. According to the size of the clasts that compose them, they are classified in turn into conglomerates, sandstones and clay rocks.
• Organogenic rocks : these rocks are made of living things. The most common is to find them formed by the biomineralization of skeletons, but there are also those formed from cellular matter, called organic rocks.
• Chemical sedimentary rocks: when chemical processes such as the evaporation of sea water occur, a residue of salt and other minerals remains. Chemical rocks are formed by the deposition and accumulation of chemical processes like this.
• Marls: This is a type of mixed sedimentary rock that is formed by the accumulation of detrital rocks with chemical rocks. Because of this, some experts prefer not to consider them a type of sedimentary rock per se.

On the other hand, the types of sedimentary rocks according to their composition are classified as:

• Terrigenous: they are formed by terrigenous sediments, mainly granite or siliceous organic particles, and usually with high amounts of quartz.
• Carbonatic. Composed mainly of carbonate minerals, usually calcium. They constitute a third of the total sedimentary rocks.
• Siliceous: These rocks are made up of organic silicon particles or granite weathering.
• Organic: sedimentary rocks of this type have their origin in organic matter of living beings, either from their skeletons or cellular matter.
• Ferro-aluminum: when a ferro -aluminum ore undergoes a weathering process as a result of a volcanic eruption or a similar process, this type of sedimentary rock is created, very rich in metals.
• Phosphatic: these rocks, which are more difficult to find, are created from the lithification process of guano, that is, from bird droppings, although it is not their only way of formation.

There are a lot of sedimentary rocks that we see in our day to day life. Some examples of sedimentary rocks are as follows:

• Coal: You can’t talk about organic sedimentary rocks without mentioning coal, and even oil. These currently very valuable substances have been formed from the lithification of organic remains, and are therefore sedimentary rocks.
• Evaporites: gypsum and rock salt are some of the best known evaporites, formed from the evaporation of seawater and the crystallization of its remains.
• Limestone: limestone rock, used both in construction and gardening, is a carbonate-type sedimentary rock that forms in shallow seawater in tropical areas.

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