Sedimentary Rocks

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Introduction

A ‘sedimentary’ rock is formed as a result of the accumulation and compaction of sediments. The term sedimentary is derived from the Latin word ‘sedimentum’ which means-‘setting down’. Sedimentary rocks are also known as ‘layered’ or stratified rocks because they are characterized by different layers or strata of different types of sediments.

The debris or sediments resulting from the decomposition and disintegration of rocks by the various agents of weathering and erosion are deposited in the water bodies. In this way, layer after layer gets deposited over each other and that increases weight and pressure on the lower layer. The lower layers are therefore consolidated and compacted to form sedimentary rocks.

According to P.G. Worcester (1948), “Sedimentary rocks, as the sediment implies, are composed largely of fragments of older rocks and minerals, that have been more or less thoroughly consolidated and arranged in layers or strata.”

It has been mentioned above that sedimentary rocks are formed by the continuous deposition of sediments in water bodies such as ponds, lakes, rivers, and seas but it is not necessary, because the sedimentary rocks are also formed at the land surface e.g., alluvial fans, loess, dunes, etc.

Characteristics of Sedimentary Rocks

• These rocks are formed by the accumulation of eroded rock particles, plants, and animal remains. Therefore, fossils are found in sedimentary rocks. Analysis of fossils present in the rocks helps to determine the period of formation of sedimentary rocks.

• Sedimentary rocks are formed by the accumulation of eroded rock particles, plants, and animal remains. Therefore, fossils are found in sedimentary rocks. Analysis of fossils present in the rocks helps to determine the period of formation of sedimentary rocks.

• These rocks covered the largest area on ​​the earth’s surface. It is estimated that about 75 per cent of the Earth’s surface is covered by sedimentary rocks and the remaining 25 per cent is covered by igneous and metamorphic rocks. Although sedimentary rocks cover most of the earth’s surface, it constitutes only 5 per cent of the earth’s crust and 95 per cent are made up of igneous and metamorphic rocks. The importance of sedimentary rock is only for the territorial extent, not for the depth of the earth’s crust.

• Layers or strata are found in sedimentary rocks, but these are seldom crystalline rocks.

• Like igneous rocks, sedimentary rocks are not found in massive forms like batholiths, laccoliths, etc.

• These rocks may be consolidated, poorly consolidated, and unconsolidated. The composition of the rock depends on the nature of cementing materials as well as the minerals which are responsible for a rock formation.

• Sedimentary rocks are rarely found in their original and horizontal form because of the lateral compressional and tensional force. Thus, beds in these rocks are found in the form of anticlines and synclines. These forces also create faults in sedimentary rocks.

• These rocks are characterized by joints. These joints are usually found perpendicular to the ‘bedding planes’.

• The connecting plane between two successive layers in a sedimentary rock is called the ‘bedding plane’ or ‘stratified plane’. When two consecutive beds or layers along a bedding plane are uniform or similar in all respect then this relationship is called conformity.

• When the sun’s rays fall on the soft mud or alluvial soils deposited by the rivers, that result in cracks development. These cracks are generally of a polygonal shape. Such cracks are called mud cracks or sun cracks.

• Sedimentary rocks are usually soft, such as clay and mud, but a few are hard, such as sandstone. Sedimentary rocks are mostly permeable and porous, such as sandstone, but a few are impermeable, like clay.

Classification of sedimentary rocks

1. Based on the nature of sediments

2. Based on transporting agents of sediments

1. Based on the nature of sediments

Mechanically formed or clastic sedimentary rock

Sedimentary rocks are the composition of different types of materials and sediments. These rocks break down into fragments of different sizes as a result of disintegration and decomposition. The fragments thus obtained are called fragmental rock materials or ‘clastic materials’. These materials further break down into smaller particles during transportation due to their mutual collision. After being deposited and consolidated at a suitable place these materials transform into sedimentary rocks known as clastic sedimentary rocks.

Sandstone

Sandstones are formed because of the cementation and consolidation of sand grains. Based on size sand grains are divided into five categories as given below

Accumulation and consolidation of sand grains in water bodies formed sandstones. The color of sandstones depends upon the amount and nature of the cementing elements such as iron oxide, silica, calcium, etc. For example, sandstones look Red or Gray if sand grains are tiled by iron oxides, become white or Gray if cemented by calcium carbonates, and become resistant to erosion and hard when cemented by silica. Generally, sandstones are porous, and water can easily percolate through them.

Conglomerate

When the pebbles of various sizes are cemented and consolidated with sand then the resultant form is called conglomerate. If these rock particles are cemented with clay, then they form gravel. When conglomerates are formed because of their cementation by silica, they become hard and resistant to erosion.

Clay rock and shale

Clay rocks are formed by the deposition and cementation of fine sediments. When the size of the grains in the rocks is 1/32 to 1/256 mm in diameter, then they are called ‘silt’. If sediments of grain size of 1/256 to 1/8192 mm are cemented and consolidated, then the formed rock is called ‘cheeka’ or ‘clay’ rock.

Silt and Clay both are soft, but they are impervious. Clay rocks are formed only by kaolin minerals. These rocks are not soluble; therefore, chemical weathering does not affect them. But easily eroded because of mechanical weathering. The pure clay is white but changes its color when mixed with the impurities of other materials.

Shales are formed due to the consolidation of silt and clay. Because of the impermeable nature of shale, most of the world’s oil reserves are found above them.

Chemically formed sedimentary rocks

Running water contains chemical ingredients in suspension. Therefore, when this water meets with other rocks (especially below the surface), the soluble materials are removed from the rocks, and when these materials get deposited, compressed, and consolidated form the chemical sedimentary rocks e.g., salt rocks and gypsum.

Organically formed rocks or non-clastic sedimentary rock

When the remains of animals and plants get deposited and consolidated at suitable places, they form organic sedimentary rocks. Based on the amount of lime and carbon, these rocks are divided into three categories as discussed below

Calcareous sedimentary rocks 

Calcareous rocks are formed by the aggregation and consolidation of the sediments derived from the skeleton and remain of those plants and animals which contain a larger portion of lime e.g., limestone.

Limestones are very weak rocks in humid areas because they are less resistant to chemical weathering, while they become very hard and resistant to erosion in hot and dry climates because limestones have homogenous structures and hence, they are not affected by differential contraction and expansion due to temperature changes.

‘Kharia Mitti’ (Chalk) is another form of carbonate rock, but this is softer and more porous than limestone. Kharia or Chalks are formed by the consolidation of calcium carbonate derived from microorganisms like foraminifera.

Carbonaceous sedimentary rock

These rocks formed when the carbonic materials such as remains of vegetation change their form because of their burial during earth movement and consequent weight and pressure on overlying deposits. The initial form of carbonaceous rocks is peat which is dark grey.

Peat, after some time, transforms sequentially into lignite, bituminous, and anthracite coal having a larger proportion of carbon and darker color.

Siliceous sedimentary rock

When the amount of silica dominates in the rocks, then they are called siliceous rocks. It is formed by the aggregation and consolidation of residual parts of Radiolaria organisms, sponges, and diatom plants. ‘Geyserites’ are also deposits of silica around geysers. Geyserite’s color varies from white to grey and pink depending on the impurities present in it.

2. Based on transporting agents of sediments

Sedimentary rocks are also classified based on transporting agents such as running water, wind, ice or glaciers, sea waves, underground water, and oceanic currents. These agents obtain different types of sediments and deposited them in suitable places. After being consolidated these form sedimentary rocks. Based on transporting agents of sediments, the sedimentary rocks are divided into three categories-

Argillaceous or aqueous sedimentary rocks

These rocks are also called aqueous rocks because they form in water bodies. The word aqueous is derived from the Latin word ‘aqua’, which means ‘water’. Aqueous rocks are sometimes called argillaceous rocks because of the predominance of clay in them. Argillaceous rocks are known for their softness and they are essentially impervious rocks. These rocks are further subdivided into three subtypes

Marine sedimentary rocks:

When the sediments are deposited and consolidated in the oceans or seas, then ‘marine sedimentary rock’ are formed. The size of sediments decreases from the coastal land towards the oceans. Thus, you will find boulders, cobbles, pebbles, gravels, sand, silts, clay, and lime in sequential order from coastal lands toward the ocean. Sandstones, limestones, and dolomites are prime examples of this type of rock.

Lacustrine sedimentary rock

When sediments are deposited and consolidated at the bottom of lakes, the rocks thus formed are called ‘lacustrine rock’. These rocks appear on the surface in the following conditions, (1) if the lake becomes dry, (2) if the bottom of the lake is raised because of the earth’s movements, and (3) if the entire lake is filled with sediments.

Riverine sedimentary rock

When sediments are deposited and consolidated in river valleys or on their banks, the rock thus formed is called ‘riverine rock’.

Aeolian sedimentary rock

Due to mechanical weathering in hot and dry regions, sands are formed as a result of the disintegration and decomposition of rocks. The wind picks up the sand from one place and deposits them in other places. The size of the sand particles keeps on getting fine because of their collision during transportation and they are easily blown away by the wind and deposited at remote places.

Thus, continuous deposition of sand particles at a place result in the formation of different types of layers but these layers are not well consolidated as in the case of argillaceous sedimentary rocks. Sometimes there is a complete absence of layers in these rocks. Loess (see fig. 1.1) is the most important rock in this group. Loess is permeable and soft rock, so water easily percolates through them. The largest distribution of loess is found in northern China. Here, the thickness of the loess is several hundred meters. Deposits in these rocks are yellow and rich in lime. Thus, they look like fine loam soils.

Glacial sedimentary rocks  

The materials transported by glaciers include boulders, pebbles, stones, sand particles, soil, etc. are collectively called glacial drifts. The unsorted and unstratified deposits of these glacial drifts are collectively known as tills and the aggregation or accumulation of tills is called moraines.

Based on the place of deposit of moraines, these are divided into four types-

• Terminal moraines: These are formed when glacial drifts are deposited at the line of maximum advance of the glacier.
• Lateral moraines: These are formed by the glacial drifts that accumulate along the sides of the margins of a valley glacier.
• Medial moraines: These are formed when glacial drifts accumulate in the middle of a valley glacier where two glaciers converge.
• Ground moraines: These are formed by the deposition of glacial drifts at the bottom of valley glaciers.