Sedimentary rocks are secondary rocks, which are constituted of sediments. The sediments are formed by the mechanical or chemical activities of the natural agencies like running water, blowing wind, percolating water (underground), glaciers etc., which causes disintegration as well as decomposition of the pre-existing rocks.
The products of decay are transported to some depositional sites by the natural agencies, where they get deposited and with subsequent compaction form sedimentary rocks. The secondary rocks usually accumulate under a great variety of physicochemical condition and consequently show great variation in mineral, chemical composition as well as in texture. Weathering is the most important process that operate in the formation of sedimentary rocks.
Weathering takes place by three methods as
(i) Mechanical disintegration, associated with physical factors.
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(ii) Rock decomposition, associated with chemical factors.
(iii) Biological weathering associated with the activities of organisms.
(i) Mechanical disintegration:
It is due to frost action, thermal expansion and contraction, aided with gravitational forces. By such mechanical disintegration, reduction of size and desegregation of rocks takes place.
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(ii) Chemical weathering:
The effectiveness of the chemical constituents of the atmosphere (as moisture, carbon dioxide, oxygen etc.) depends on the composition of the rock and the size of the Particles that make up them. Important processes are oxidation, hydration, carbonation etc.
(iii) Biological weathering:
It is due to the physical forces exerted by roots on the adjacent rocks and also due to chemical activity of its products of decay. The decaying products of plants animals produce various types of acids and alkalies which play roles in the decomposition of rocks.
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Products of weathering:
(a) The first product of weathering is a mantle of broken and decomposed material of varying thickness and composition called the regolith, which covers the areas except those from which it is removed as soon as formed.
(b) Soluble salts:
These are carried away along with the transporting media in solutions.
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(c) Colloidal substance:
These are carried by ground water and stream. These are like Al(OH)3, Fe(OH)3 etc.
(d) Insoluble products:
It includes clay minerals, quartz grains, under composed feldspar with some chemical resistant minerals like zircon, tourmaline etc., which are found at the site of weathering and later transported to the sites of deposition by geological agents.
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Deposition:
It may be mechanical or chemical, according to which mechanically transported material gets deposited first and the soluble materials get deposited through the process of chemical precipitation. Sometimes, the activity of bacteria is believed to be effective in the deposition of ferruginous soluble substances like bog- iron ore.
According to the environment of deposition the quality and quantity of the deposition is affected. Besides the assemblages of sedimentary rocks differ from environment to environment. While the continental environment includes-fluviatile, estuarine, lacustrine (fresh and salt water), deltaic, aeolian, glacial etc., the marine environment includes-shallow water, deep water abyssal etc. environment.