The texture of the sedimentary rocks is studied like that of the igneous rocks. Texture means the size, shape, and arrangement of grains in a rock. The particles of the sedimentary rocks are known as the ”Sediments”. As sediments contain particles of various sizes, grain size is an important factor in the description of sedimentary rocks. Depending upon the size, particles of sediments are classified into pebbles, gravels, sand, silt, and clay. Each of these particles gives rise to a particular type of rock. This classification is shown in the following table;
Particle Size in Sediments
|Grain Size||Rock Types|
|10mm or above||
|2mm – 10mm||
|0.1mm – 2 mm||
|0.01 mm – 0.1 mm||
|Clay||Less than 0.01 mm||
The grain size of sand varies from 2 mm to 0.1 mm. They are sub-divided into four groups; (i) very coarse sand, which has a grain size of 1 mm or more. (ii) coarse grain, which has a grain size equal to 1 mm – 0.5 mm (iii) medium sand, which has 0.5mm – 0.25mm sized grains, (iv) fine sand which has a grain size of 0.25mm – 0.01 mm.
Sediments, which contain grains of various grades in nearly equal amounts are said to be “unassorted”. On the other hand, sediments containing mainly grains of one grade only, are said to be “well-assorted” or “graded”. the degree of the assortment may be high in many wind deposits and in sediments deposited on gently sloping sea floors. Stream deposits are commonly less well-graded. Glacial deposits are generally unassorted.
The shape of the constituent grains of sedimentary rocks is of considerable significance in the study of texture. The grains of a rock may be rounded, partially rounded, or angular. Grains, which have been transported to considerable distances commonly show a high degree of rounding, whereas grains that have resulted from disintegration, volcanic explosion, or glacial action are commonly angular. In breccias, the rock fragments are angular, while in the conglomerate, they are rounded.
The chemically formed rocks may contain rounded concretions. If they are of the size of a pin-head (1mm), the texture of the rock is said to be “oolitic”, and if they are of the size of a pea, the texture is described as “pisolitic”.
The texture and mineral composition of sedimentary rocks are of great value in determining the nature of the environment at the time when the sediment was deposited. A conglomerate, for example, indicates a high-energy environment, such as a swiftly flowing stream where only the coarse material can be deposited. The arkose suggests a dry climate where little chemical alteration of felspar is possible. Carbonaceous shale indicates low energy, organic-rich environment, such as a swamp or lagoon.
You may also like to read:
- Introduction to rocks
- Igneous Rocks and their types
- Chemical Composition of Igneous Rocks
- Occurrence of Igneous Rocks
- The Texture of Igneous Rocks
- Structures of Igneous Rocks
- Different Forms of Igneous Bodies/Material
- Mechanics of Intrusion
- Formation of Igneous Rocks
- Origin of Igneous Rocks
- Magmatic Differentiation
- Assimilation of crustal rocks in Magma (Crustal Assimilation)
- Bowen’s Reaction Series
- Types and varieties of Igneous Rocks
- Sedimentary Rocks and their formation
- Classification of Sediments
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