Sedimentary Rocks: Meaning, Types And Formation

The word ‘sedimentary’ has been derived from the Latin word ‘sedimentum’, which means ‘settling down’. The word settling down indicates solid minerals into a fluid. Most of the sediments are deposited in this fashion.

According to P.G Worcester (1948), “Sedimentary rock, as the word 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.”

According to geologists, sedimentary rocks account for only 5 per cent of the earth’s surface or 16 kilometres (10 miles) of the outer earth’s surface. Each and every alluvial plain on the global earth’s surface is agriculturally more prosperous than igneous and metamorphic rocks.

For example, the Indo-Gangetic Plain in India, the Nile River Plain in Egypt, Mississippi- Missouri Plain in North America, the Amazon River Plain in South America, Hwang Ho- Yangtze Si Kiang Plain in China, the Rhine River Valley in Europe, the Murray- Darling valley in Australia etc.

Many sedimentary rocks are also economically important. Coal is burned to generate and provide energy. Other major resources like petroleum and natural gas are found in pores within sedimentary rocks. Iron, aluminium and manganese are used in construction, while fertilizers are used in agriculture. It contains fossils which can be used as vital evidence in the study of the past. Sedimentary rocks contain most of the earth’s groundwater aquifers which are very essential for life sustenance.

Types of Sedimentary Rocks

Classification on the Basis of the Nature of Sediments

1. Mechanically Formed or Clastic Rocks

The rocks of the lithosphere are decomposed and broken up by mechanical agents like water, wind, glaciers etc. Fragments of many different kinds of rocks and minerals accumulate on the earth’s surface in the form of soils, dust and coarser fragments with variable sizes and shapes. These fragments are classified into five different rocks with respect to their size, shape and contents.

Sandstone- Sandstones are formed mostly due to deposition, cementation and consolidation of sand grains. These rocks are composed of quartz grains. On the basis of their size, they can be divided into the following categories.

Table: Classification of Sands by Grain Size

Sand Types	Grain Size (mm)
Very Coarse Sand	1.0 to 2.0
Coarse Sand	0.5 to 1.0
Medium Sand	0.25 to 0.5
Fine Sand	0.125 to 0.25
Very Fine	0.625 to 0.125

Conglomerates- These rocks are also composed of coarser sand grains with pebbles and boulders of varying sizes. The term ‘conglomerate’ is applied to cemented fragmental rocks containing rounded fragments such as pebbles and boulders; if the fragments are angular in shape, the rock is called breccia. When the rounded fragmented materials are cemented by quartz, the resultant rocks become conglomerates.

Clay Rock- Clay rocks are formed due to the deposition and cementation of sediments. These rocks are composed of fine grains with the size of 0.03 mm to 0.004 mm and are called silts whereas, clays are formed when the sediments of grain size of 0.004 mm to 0.00012 mm are cemented and consolidated. Both the rocks are impervious but they are soft. Clays are composed almost entirely of kaolin.

Shale- Shales are formed of laminae which are easily separated. These rocks are impure clays which contain a considerable proportion of minerals other than kaolinite.

Loess- It is a very fine-grained material which is deposited by the wind on the land. These rocks are very poorly stratified, which means there is an absence of layers. The colour varies from light brown to dull yellow. A peculiar property of loess is its ability to stand on vertical cliffs. Loess is generally poorly consolidated are very prone to erosion. Due to its finely divided condition and the remarkable wealth of soluble mineral plant foods which it contains, loess soils are very fertile.

Representation of Different Mechanically Formed or Clastic Sedimentary Rocks

2. Chemically Formed Sedimentary Rocks

Chemical materials are contained in running water. When such chemically active water comes in contact with the continental rocks, soluble materials are removed from the rocks. These materials are called chemically formed sediments. For example, gypsum and salt rock.

Representation of Different Chemically Formed or Non-Clastic Sedimentary Rocks

3. Organically Formed Sedimentary Rocks

These rocks are formed due to the disintegration and decomposition of sediments by both animals and plants. These sediments, after being deposited and consolidated, form organic sedimentary rocks. On the basis of lime and carbon content, these rocks are divided into three groups.

Calcareous Rocks- These rocks are firmed by sediments which are derived from the skeletons and remains of those animals and plants containing a large portion of lime. For example, Limestone and Chalk.

Carbonaceous Rocks- Unlike other sedimentary rock groups, these are of direct vegetation origin. These rocks are formed due to the transformation of vegetation because of their burial during the earth’s movement and the consequent weight and pressure of overlying deposits. Finally, this results in different grades of coals. For example, Peat, Lignite, Bituminous and Anthracite.

Different Grades of Coal in Successive Stages of Coal Formation

Coals are also found in stratified layers wherein coal layers are known as Coal Seams. Carbonaceous rocks are more important economically than geographically.

Coal Formation and its Depth of Occurrence

Siliceous Rocks- These rocks are formed due to the dominance of silica content. Diatomaceous or infusorial earth is loose in structure and white or grey or brown in colour in colour. Siliceous rocks are formed due to aggregation and compaction wastes derived from sponges, radiolarian organisms and diatom plants. Geyserite is the best example of this group of rocks.

Classification on the basis of Transporting Agents

Argillaceous Rocks

These rocks are also called aqueous rocks due to their formation in water. Aqueous rocks become argillaceous because of the dominance of clay in the rocks. These rocks are characterised by softness and imperviousness. It is further subdivided into three subdivisions i.e., Marine Argillaceous Sedimentary Rocks, Lacustrine Argillaceous Sedimentary Rocks and Riverine Argillaceous Sedimentary Rocks.

Aeolian Sedimentary Rocks

These rocks are the result of the deposition of sand, which is brought down by the wind. They are formed with the absence of layers. Loess is the most important example.

Loess- It is formed due to the accumulation of fine materials of sand. In these rocks, water can easily percolate due to porousness. The most important characteristic of loess rocks is that the entire mass may stand like a vertical cliff or wall. The most extensive loess deposits are found in North China where the Yellow River (Hwang Ho) and its tributaries are the representations of loess rocks.

In India, the best example of loess rocks is found on both banks of paleochannel, and the valley of the Narmada River at Dhunwadhar Falls (Bheraghat) near Jabalpur (Madhya Pradesh), where the loess banks rise 20 to 25 m from the valley floor.

Glacial Rocks

Fine to coarse sediments are deposited by glaciers, and thus, it is called glacial rocks. These rocks are further subdivided into four subdivisions. Lateral Moraines, Medial Moraines, Ground Moraines and Terminal Moraines.

Sedimentary Rocks: Meaning, Types and Formation

Types of Sedimentary Rocks