Brief notes on the Classification of Sedimentary Rocks

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Sedimentary rocks, as the name implies, are formed from the debris derived from the pre­existing rocks of any origin and from newly formed organic matter. The debris so obtained are carried in suspension and deposited where they are transformed into rock by compaction and cementation.

This process is called lithification. When the debris is transported in water or air, they are known as sediments which are thoroughly consolidated and deposited in layers or strata. That is why sedimentary rocks are also called stratified rocks.

It may be pointed out that some sedimentary rocks involve organic agencies, while some result from the precipitation of minerals from salt waters.

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Sedimentary rocks are called secondary rocks or derived rocks because they are derived from the denudation of other pre-existing primary or parent rocks.

The exogenetic processes of denudation provide the sediments which are deposited mostly in water bodies where the particles are sorted out according to their size. Thus, each layer or stratum has particles of a given size.

Different strata vary in thickness ranging in thickness from a few centimeters to several meters. They may be horizontal or inclined. The various strata are separated by bedding planes.

There are some examples where sediments are deposited on land surfaces as loess, sand dunes or alluvial fans etc.

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It may be noted that the loose particles consist of gravel, sand, silt and clay in varying proportions which get consolidated or compacted into hard rocks by the presence of cementing materials like lime or by the pressure of overlying deposits.

The main agencies which lay down the sediments are rivers, glaciers, and oceans, and all such processes are collectively known as those of denudation. Sedimentary rocks are mostly porous and permeable.

Another property of these rocks is the presence of the remains of plants and animals between the various layers of sediments. These remains get preserved between the strata of sedimentary rocks.

These organic remains or their skeletal impressions are known as fossils which serve the purpose of fixing the relative ages of sedimentary rocks.

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Classification of Sedimentary rocks on the basis of composition:

1. Arenaceous rocks

2. Argillaceous rocks

3. Calcareous rocks

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4. Carbonaceous rocks

1. Arenaceous rocks:

Arenaceous rocks are composed of grains of quartz. They are also called sandy rocks. But fragments of other minerals also enter into their composition. Mechanically formed sedimentary rocks are also called clastic or fragmentary rocks.

These rocks are composed of mechanical sediments derived by the process of denudation of land. Sediments that are derived in this way are known as detrital or clastic sediments.

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These include particles of other rocks such as gravel, sand, silt and clay. Such particles are cemented together to become a sedimentary rock.

Rounded pebbles that are found in the bed of rivers get consolidated to form a rock. Such a type of rock is known as conglomerate.

Breccia is another type of sedimentary rock which is composed of angular fragments of gravel which accumulate at the bottom of a cliff. Sandstone and grit are fine examples of the arenaceous rocks.

Sandstone is the most common sedimen­tary rock. It is well exposed, easily recognised, and resistant to weathering. The sand grains vary in their size ranging from 1/16 to 2 mm in dia­meter.

Sandstones are of different colours depending on the parent materials. However, quartz grains are found in abundance because they are more resistant to weathering, particularly chemical weathering.

The cementing materials in sandstone include calcite, silica or iron oxide (hematite). When the cementing materials are exclusively silica or iron oxide, the rock thus formed is very resistant to erosion and plays important role in the production of peculiar topographic forms. Wind and water action are the main agencies of their formation.

Grit:

It is a very coarse and well-cemented rock. It comprises both sand-grains and small pebbles.

2. Argillaceous rocks:

Argillaceous rocks are clayey rocks. These rocks are made of clay, a hydrated silicate of alumina. Such rocks are formed in the sea or lakes. However, flood deposits are more common.

Fine particles of other minerals may form a deposit which may be called a mud or clay. Clayey rocks are composed chiefly of the mineral kaolinite along with very fine particles of other minerals.

Since clay rocks are composed of kaolin, they are soft and break down easily by mechanical agents. They are easily eroded by various agents of erosion such as wind, streams, glaciers or sea-waves.

But being highly insoluble, they resist chemical weathering to a large extent. Pure clays are white, but on the land-surface clays of various colours are found because of the presence of many colouring materials.

Mudstone, clay stone and shale represent the clayey rocks. Shales are actually impure clays. They are also easily eroded. The particles that constitute the rock are so small that even with the aid of a microscope they are not clearly visible.

Stratification with very thin laminite is very important characteristic of shale rock. Shales that are black in colour are rich in organic material and they accumulate in lagoons and tidal flats. Red shales, on the other hand, owe their colour to iron oxide. They accumulate in stream channels and flood plains. Most of the shales split easily along their bedding planes.

3. Calcareous rocks:

Calcareous rocks or limestones are mainly composed of the shells or skeletons of animals or plants, and are formed of carbonate of lime. Limestones belong to the group of organic rocks. It is the most abundant non-clastic rock.

Chemical and organic processes enter into the formation of limestones. It has so many varieties, e.g. skeletal limestone, oolite and microcrystalline limestone. Many water plants, (algae) and animals build their skeletons from the calcium carbonate.

After their death, their shells accumulate on the floor of the water bodies where they once lived. The shells and skeletons are later on ground to pieces by waves and ocean currents.

These fragments, both coarse and fine, are deposited in layers. When cemented together, limestones are formed. Limestones may occur in thick or thin layers. They have different colours. Since they are soluble, they are easily eroded by chemical weathering.

Below the earth’s surface they produce caves and sinks. By evaporation of rain water dripping from the roofs of limestone caves and falling over their floors, stalactites and stalagmites are formed.

Some of the tiny orga­nisms involved in the formation of calcareous rocks are foraminifera, corals, crinoids, mollusca, and crustacea. Examples of such rocks are globi- gerina ooze, chalk, and coral limestones.

4. Carbonaceous rocks:

Carbonaceous rocks are formed from the remains of plants converted by heat and pressure into coal. Peat, lignite and coal are carbonaceous deposits of organic origin. All these rocks are composed of plant debris in different stages of alteration.

Lignite is also a carbonaceous deposit in which alteration has been incomplete. The carbona­ceous rocks are of different nature, but consist of carbon combined with other elements. It may be pointed out that there is a large variety of hydrocarbons.

They are found in solid, liquid and gaseous from. For example, pitch, asphalt and bituminous coal are the solid forms, crude mineral oils the liquid forms, and natural gas the gaseous form.

Remember that petroleum and natural gas are not rocks, though these are found in the sedimentary strata. In fact, they are com­plex compounds of carbon and hydrogen.

Besides the above broad classification of sedimentary rocks, there are other types such as, chemically precipitated rocks (rock salt), aeolian sedimentary rocks and glacial sedimentary rocks.

The last two classes of these rocks are based on the agents of transportation. Moraines are glacial deposits, whereas loess is formed because of wind action. Loess deposits are found in North China where they are several hundred meters thick.

Loess deposits are characterised by the absence of layers or strata. Being soft and porous, loess is deeply eroded by running water.

Argillaceous sedimentary rocks are further subdivided on the basis of the place of deposition of the rock debris.

Thus, these sedimentary rocks are of the following categories: (i) the marine argillaceous sedimentary rocks, (ii) the lacustrine argillaceous rocks, and (iii) the riverine argillaceous rocks. The marine type argillaceous rocks are formed in marine environment.

The lacustrine type rocks are formed into the lakes, whereas the riverine argillaceous rocks are formed by the deposition of sediments in the river channels and flood plains.

Major characteristics of sedimentary rocks:

1. The sedimentary rocks are formed from the debris obtained from the pre-existing rocks and organic matter.

2. Sedimentary rocks are characterised by layers and bedding planes.

3. Sedimentary rocks contain fossils which are helpful in determining their age.

4. Sedimentary rocks are usually porous and permeable.

5. There are joints in these rocks which are generally perpendicular to the bedding planes.

6. Sedimentary rocks cover about 75 per cent of the land surface.

7. Sedimentary rocks are of great economic importance. They are the storehouse of many useful resources like coal, petroleum and natural gas. These rocks provide many raw materials for various industries – many mineral deposits are mined from sedimentary rocks.

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