Introduction :
Sedimentary rock is one of the three main types of rocks, along with 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 sedimentologist 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 produce through the processes of sedimentation not
only within the basin, include outsides also. Its show a clastic
texture.
Endogenetic
rocks: These rocks are produce through the processes of sedimentation
within the basin . Its show an non-clastic texture.
Clastic texture are those , which have an distinct physical appereance. It is produce through physical weathering ,purely mechanical. Example-conglomerate.
Non-clastic texture show an interlocking or crystalline fabric . It is produce through the chemical and biochemical precipitation . Example- Limestone .
Folk classification:
In 1968, Folk introduced a new classification, which have 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, are formed from the accumulation and lithification of sediment derived from pre-existing rocks. 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. Here is the grain size scale commonly used for clastic rocks:
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 this 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
a. 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 rocks, are a type of sedimentary rock that forms through direct chemical precipitation or bio- chemical precipitation within the basin . Unlike clastic rocks, which are composed of particles derived from the erosion and transportation of pre-existing rocks, orthochemical sedimentary rocks form through the precipitation of dissolved minerals. Orthochemical rocks, also known as non- clastic rocks.
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.
Common examples of orthochemical
sedimentary rocks include 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 : They are divided into two groups
A. Chemically deposited
B. Organically deposited
A. Chemically deposited :
Sediments are created /deposited through evaporates and others reaction.
Evaporites :are chemical sedimentary rocks that form as a result of the evaporation of water, leaving behind mineral deposits. Common evaporite minerals include:
a. Rock Salt: Rock salt is 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 they are mention below-
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: Dolostone, also called dolomite, is primarily composed of the mineral dolomite (calcium magnesium carbonate, CaMg(CO3)2). It forms through the replacement of limestone by magnesium-rich fluids.
2. Chert:
Chert is a microcrystalline or cryptocrystalline sedimentary rock primarily composed of the mineral quartz (silica, SiO2). It forms through the precipitation of silica from solutions, often derived from the remains of marine microorganisms like radiolarians or diatoms.
3. Iron-rich Rocks:
Some orthochemical rocks are rich in iron minerals, including:
a. Ironstone: Ironstone is a rock that 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 :
Sediment are produce through the accumulation of organic matter, which are preserved under suitable condition. Here are some classifications of orthochemical rocks based on organic formation:
1. Organic Limestone: Organic limestone forms primarily from the accumulation and lithification of the remains of marine organisms, such as shells, corals, and skeletal fragments of marine organisms. Examples include fossiliferous limestone and coral limestone.
2. Chalk: Chalk is a fine-grained, porous limestone composed mainly of the microscopic remains of marine planktonic algae known as coccolithophores.
3. Coquina: Coquina is a type of limestone composed predominantly of loosely cemented shell fragments and shell debris.
4. 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.
5. 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
rocks but after time when it rework, transported and re-deposited elsewhere
then this rocks are called Allochemical sedimentary rocks .These rocks nature
are non-clastic but texturally clastic.
Example- Fossiliferous
limestone
Conclusion :
sedimentary rocks are a significant component of the Earth's crust and play a important role in understanding Earth's geological history. These rocks are formed through the accumulation and compaction of sediment over long periods of time. They provide valuable information about past environments, climate, and the processes that have shaped our planet.
Sedimentary rocks can be classified into three major types: Terrigenous, Orthochemical,
Allochemical.
One of the key characteristics of
sedimentary rocks is their ability to preserve fossils and other traces of
ancient life. They provide valuable insights into the evolution of life on
Earth and help in reconstructing past ecosystems and understanding biological
processes.
Sedimentary rocks also contain valuable
resources such as coal, oil, and natural gas. These fossil fuels are crucial
for powering our modern society and have significant economic importance.
Moreover, 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.
In summary, sedimentary rocks are of
great significance to geology, paleontology, and various human activities. They
provide a window into the Earth's past, offer insights into ancient
environments, and supply valuable resources that are essential for our modern
way of life. Understanding sedimentary rocks and their formation processes is
essential for comprehending the dynamic history and functioning of our planet.
