Sedimentary rocks are the rocks resulting from from the consolidation of loose sediments which are derived from the pre-existing rocks & minerals, chemical precipitation from solution and organic remains of plants and animals at or near the earth’s surface. There are mainly two types sedimentary rocks: Clastic and Chemica.
Clastic Sedimentary Rocks:
These are the sedimentary rocks that originate with the accumulation of discrete mineral or rock particles derived from weathering and the erosion of pre-existing rocks (i.e. deposited by mechanical means). Such rocks are called clastic or detrital sedimentary rock. The particles in those rocks called clastic/detrital and sometimes fluvial (deposited by river).
Chemical Sedimentary Rocks:
These are the rocks formed by a variety of processes and are divided into sub-categories including inorganic, and biochemical or organic chemical sedimentary rocks. They are further divide into two categories: Inorganic chemical rocks and biochemical or organic sedimentary rocks.
- Inorganic Chemical Rocks: These are the rocks which form from chemicals that are dissolved in solution, transported, and chemically precipitated out of solution.
- Biochemical Sedimentary Rocks: These types of sedimentary rocks are formed when plant or animal materials are deposited and lithified. These types of rocks involve some form of fossilization or the accumulation of fossilized organism or organism remains, such involve the deposition of plant material and formation of peat and coal deposists.
How Are Sedimentary Rocks Formed
Sedimentary rocks are formed from the consolidation of loose sediments. There are different stages and processes involved in the formation or formation of sedimentary rocks in nature which are described below:
- Stage 1: Pre-existing rocks are broken down by the weathering process i.e. due to decomposition and disintegration of rocks.
- Stage 2: The rocks that are broken down are eroded due to the action of different natural agencies like water, wind, glaciers, etc.
- Stage 3: The eroded materials are transported due to those natural agencies.
- Stage 4: The transported materials are deposited at a different place under a suitable condition.
- Stage 5: The deposited materials are then transfoted into sedimentary rocks due to the processes of diagenesis.
What is Diagenesis
Diagenesis is collectively referred as the physical, chemical, and biological changes which may occur during the formation of sedimentary rocks. Recrystallization, compaction, cementation, lithification, etc are some examples of diagenetic changes. Here are some of the diagenetic processes with their explanation:
- Recrystallization: It occurs when unstable minerals recrystallize to form more stable minerals. Recrystallization most often occurs during the formation of chemical sedimentary limestone rocks that previously contained aragonite a chemically unstable form of calcium carbonate (CaCO3).
- Compaction: It occurs when sediments are progressively deposited on top of one another, and over time the weight of the accumulated sediments increases and compresses the buried sediments. Continued compression of buried sediments reduces pore-spaces and removes excess water, as a result the closely packed individual grains begin to slowly compact into a solid rock.
- Cementation: It involves §a chemical change whereby individual grains are cemented together as minerals are precipitated out of saturated solution that is percolating as a matrix between individual sediments. The accumulation of the precipitated minerals causes the grains to cement together. Cementation can occur in combination with the presence of other minerals, rock fragments, or organic constituents such as fossilized organisms.
- Lithification: It occurs when unconsolidated sediments are cohesively bound to form a solid sedimentary rock. Compaction and/or cementation are generally the precursor to the lithification process.
- Consolidation: It is the process of removing pore water under static load. It is a time dependent phenomenon. For coarse sediment, drainage is sufficient and consolidation is instantaneous
- Dissolution: It is the water moving through the pores that dissolve the grains and reduce the grain size. It depends upon composition of water and grains. Evaporites, limestone are more soluble and form solution channels.
- Pressure Solution: It is a process in which a solid dissolves at its contact with another solid and with pore water; because increased pressure has increased its solubility. Due to this, grains are welded together and the pore space is reduced.
- Replacement: The pore water contain various ions, they may replace the grains and change partially or wholly the composition of grains. Calcites are replaced by silica, phosphorite, magnesium etc.
- Neomorphism: It is the new shape formation due to either grain size increase or decrease.
After the completion of these processes, new sedimentary rock is formed.