Geochemistry Of Sedimentary Processes
By Tirumala Prasad
Due to extremely high porosity and permeability, the sedimentary rocks have been the major source of water, petroleum, etc resources. Sedimentary rocks do form on the surface of the earth and at low pressure and temperature conditions. The weathered sediments will be converted into sedimentary rocks by various processes. During the process of deposition, some interesting structures will develop in them such as cross bedding, graded bedding, etc.
Weathering, erosion, transportation, deposition, and diagenesis, are the major processes involved in the formation of this type of rocks. These rocks can be formed from igneous, metamorphic and sedimentary rocks themselves.
Mechanical disintegration and decomposition are known as weathering. The intensity of disintegration and decomposition depends upon various factors such as environmental, type of the rocks, organisms, local chemistry, etc. The influence of gravity is also note worthy. Water, wind, glaciers, etc are the major weathering agents. Weathering does effect the rocks insitu, i.e. no transportation is involved. If there is transportation, such type of weathering is known as erosion. Decomposition involves the transformation in the chemical composition of the rocks. This occurs when the exposed rocks react with several types of chemicals. Depending on the variations in the environmental factors, and also the type of rocks, on the surface of the earth, the chemical reactions are variable. The decomposition of feldspar rock would lead to the formation of several types of clay minerals.
Since, water is the major weathering agent, it is required to know the various types of water to study the geochemistry of sedimentary rocks. Rain water, river water, sea water, ground water, etc are the major types of water involved in the formation of sedimentary rocks. Depending upon the composition of the waters, several rock types will be formed. For instance, the total dissolved solid contents are very low in the rain water when compared to other types of water. The total dissolved solid contents are very high in the sea water, hence the sea water does support life for various organisms. Similarly, the surface waters do contain chemicals released from mining, industrial, human activities, etc. Thus the chemistry of the surface waters depend upon social, economic, cultural, political, etc factors. However, the rain water does carry some pollutants along with it to the surface. For instance, sulfur which is present in the air, does mix with rain water. Such pollutants will even enter or percolate into the underground, and contaminate the waters there.
The weathered particles are known as the sediments. Such sediments will be transported by water, wind, glaciers, etc. Size and shape of the fragments are variable depending upon several factors such as the chemical composition of the sediments, climate, duration of transportation, etc. Deposition of the sediments takes place after millions of years of transportation. Diagenesis is the process of conversion of sediments into sedimentary rocks.
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