Introduction :
Sedimentary rock is one of the three main types of rocks, alongsides igneous and metamorphic rocks. It forms through the process of sedimentation, where sediments such as minerals, fragments of other rocks, organic matter and chemical precipitates accumulate over time and become compacted and cemented together.
Types of sedimentary rocks:
Fig.: Classification of Sedimentary rocks
According to the different sedimentologists sedimentary rocks are broadly divided into Exogenetic rocks and Endogenetic rocks on the basis of their formation. These two rocks can be distinguished by their texture.
Exogenetic rocks: These rocks are produced through the processes of sedimentation not only within the basin, include outsides also. Its show a clastic texture.
Endogenetic rocks: These rocks are produced through the processes of sedimentation within the basin. Its shows a non-clastic texture. Clastic and non-clastic texture:
Clastic texture are those, which have a distinct physical appearance. It is produced through physical weathering, purely mechanical. Example-conglomerate.
Non-clastic texture shows an interlocking or crystalline fabric . It is produced through the chemical and biochemical precipitation . Example- Limestone.
Folk classification:
In 1968, Folk introduced a new classification, which has an three main types of classification-
1. Terrigenous
2. Orthochemical
3. Allochemical
1. Terrigenous classification :
The term "terrigenous" comes from the Latin words "terra," meaning earth, and "genus," meaning origin.
Terrigenous rocks also known as clastic rocks, which are formed when sediments from existing rocks accumulate and lithified. They are primarily composed of particles that have been eroded, transported, and deposited by various geological processes such as weathering, erosion, and deposition.
Terrigenous rocks can originate from outside of a basin. The source of the sediment determines whether it is derived from outside sources.
Example of terrigenous rocks are- a. Fragment of volcanic origin
b. Product of crushing by glacier
Classification of Clastic/ terrigenous rocks :
Clastic or terrigenous rocks are classified based on their grain size, composition, and texture. The grain size scale used for classifying clastic rocks is known as the Wentworth scale. It categorizes clastic sedimentary rocks based on the diameter of their constituent particles.
Grain size scale of clastic rocks(commonly used ):
1. Boulder: >256 mm
2. Cobble: 64 - 256 mm
3. Pebble: 4 - 64 mm
4. Granule: 2 - 4 mm
5. Sand: 1/16 - 2 mm
- Very coarse sand: 1-2 mm.
- Coarse sand: 0.5- 1 mm.
- Medium sand: 0.25 -0.5 mm.
- Fine sand: 0.125 - 0.25 mm.
- Very fine sand: 0.0625 -0.125 mm.
6. Silt: 1/256 - 1/16 mm
7. Clay: <1/256 mm
On the basis of the above scale, the clastic rocks are classified as follows :
A. Rudaceous rocks
B. Arenaceous rocks
C. Argillaceous rocks
A . Rudaceous rocks : Very coarse grain where Particle diameter is greater than 2 mm. It also known as rudites, these are transported by rolling or creeping.
Types of Rudaceous rock-
a. Conglomerate : Which are rounded
- b. Breccia : fragments are angular
B. Arenaceous rocks : These rocks particle size range are between 1/16 - 2 mm ,i.e; sand size. Arenaceous rocks also known as arenite. They are transported in saltation.
Example – sandstone, arkose.
C. Argillaceous rocks : Particle size range are <1/16 mm, i.e; clay & silt size. These are transported in suspension. Example – Mudstone, shale
1. 2. Orthochemical Classification :
Orthochemical sedimentary rocks also known as chemical sedimentary and non-clastic rocks that form through direct chemical precipitation or bio-chemical precipitation within the basin. Unlike clastic rocks, orthochemical sedimentary rocks form through the precipitation of dissolved minerals.
The formation of orthochemical sedimentary rocks typically occurs in environments where there is an abundant supply of dissolved minerals in water. These minerals can precipitate out of the water due to a variety of factors such as evaporation, changes in temperature or pressure, or chemical reactions between different solutions.
Examples - limestone, dolomite, gypsum, and evaporates. Limestone, for instance, forms from the precipitation of calcium carbonate (CaCO3) from water, often in marine environments. Dolomite forms through the replacement of calcium carbonate in limestone by magnesium, resulting in the mineral dolomite (CaMg(CO3)2). Gypsum is another example, formed from the precipitation of calcium sulfate (CaSO4) from evaporating water.
Orthochemical sedimentary rocks can exhibit a wide range of textures and structures, depending on the specific conditions of their formation. They may contain characteristic features such as crystalline textures, layered structures, and distinctive mineral compositions.
Classification of orthochemical rocks :
A. Chemically deposited
B. Organically deposited
A. Chemically deposited :
Evaporites : It is a chemical sedimentary rocks that formed when minerals left behind when water evaporates.
Examples :
a. Rock Salt : This rocks are composed of the mineral halite (sodium chloride, NaCl) and forms in areas of high evaporation such as salt pans or evaporating shallow seas.
b. Gypsum: Gypsum is composed of the mineral gypsum (calcium sulfate dihydrate, CaSO4•2H2O). It forms in arid or semi-arid environments with high evaporation rates.
Through others reaction between the carried component in solution, different types of rocks are formed for example -
1. Carbonate Rocks :
a. Limestone: Limestone is primarily composed of the mineral calcite (calcium carbonate, CaCO3). It can be further classified based on its texture and composition such as fossiliferous limestone (containing visible fossils), chalk (composed of microscopic planktonic organisms), or travertine (formed by precipitation from groundwater).
b. Dolostone : It also known as dolomite that primarily composed of the mineral dolomite (calcium magnesium carbonate, CaMg(CO3)2), forms through the replacement of limestone by magnesium-rich fluids.
2. Chert :
Microcrystalline or cryptocrystalline Chert is a sedimentary rock, which mainly composed of the mineral quartz (silica, SiO2) and forms through the precipitation of silica from solutions, often derived from the remains of marine microorganisms such as radiolarians or diatoms.
3. Iron-rich Rocks :
a. Ironstone : It's contains a significant amount of iron minerals, such as hematite (Fe2O3) or magnetite (Fe3O4).
b. Banded Iron Formation (BIF): BIFs are distinctive sedimentary rocks composed of alternating bands of iron-rich minerals and chert. They are important in understanding Earth's early history and the development of oxygen-rich environments.
B. Organically deposited :
This deposition are from the accumulation of organic matter under a favorable conditions.
Classifications of orthochemical rocks (based on organic formation):
- Organic Limestone : It is primarily from the accumulation and lithification of the remains of marine organisms, such as shells, corals, and skeletal fragments of marine organisms. Examples - fossiliferous limestone and coral limestone.
- Chalk: Chalk is a fine-grained, porous limestone composed mainly of the microscopic remains of marine planktonic algae known as coccolithophores.
- Coquina: Coquina is a type of limestone composed predominantly of loosely cemented shell fragments and shell debris.
- Diatomite: Diatomite is a type of rock composed mainly of the siliceous remains of diatoms, a group of microscopic algae with cell walls made of silica.
- Coal: Although coal is primarily classified as an organic sedimentary rock, it can also be considered an orthochemical rock due to its formation from the accumulation and transformation of organic matter, primarily plant material. Coal forms in swamps and peat bogs through the process of burial and compaction of plant remains.
1. 3. Allochemical Classification : ( allo means false)
Allochemical sedimentary rocks are intermediates of terrigenous and orthochemical rocks. This rocks show a clastic texture .
Initially it’s a Orthochemical sedimentary rock but after time, when it rework, transported and re-deposited elsewhere then this rocks are called Allochemical sedimentary rocks. These rocks nature is non-clastic but texturally clastic.
Example- Fossiliferous limestone
Conclusion :
Sedimentary rocks form over time from compacted sediments and are vital for understanding Earth's history. They hold evidence of past environments, climate changes, and surface processes.
One of the main characteristics of sedimentary rocks is their ability to preserve fossils and other traces of ancient life, also contain valuable resources such as coal, oil and natural gas.
sedimentary rocks are important in various industries, including construction, agriculture, and groundwater management. They serve as reservoirs for groundwater and are often used as building materials and sources of industrial minerals.
