Short essay on the Classification of Sedimentary Rocks


Sedimentary reeks are due to the integration of the products of weathering; so any classification to be acceptable must take into account: (a) the genetic aspect of the sediments, (b) their textural characteristics, (c) their mineralogical composition as well as (d) their structural peculiarities. The present classification which is the most acceptable one has been proposed by E.W. Spencer, and. the basis of classification is the mode of formation of the sediments.

Sometimes the products of weathering are carried down by the natural agencies and sometimes they are found at the place of their origin. Accordingly there are two classes as

1. Residual deposits.


2. Transported deposits.

1. Residual deposits:

These are also known as sedentary- deposits. These are formed due to accumulation and consolidation of those materials which were left as residue during the operation of the weathering processes and transportation. These are the insoluble products of rock weathering which still mantle the rocks from which they have been derived.

They include the following rock types:


(i) Terra-rosa:

These are the insoluble residue of clay and. other matter left behind after solution of limestone.

(ii) Laterite and bauxite:

In tropical and sub-tropical regions a reddish, porous and concretionary material is found to cover vast areas. They generally consist of a mixture of hydrated ferric-oxide with hydroxide of aluminium in varying proportions. These are called laterites.


When the aluminous content predominates the colour lightens- to yellowish or whitish and the rocks become earthy or clay-like. It is called bauxite.

2. Transported deposits:

These are formed from the materials that have been transported both mechanically by traction, and suspension and chemically in solution. Besides, some organic processes also play active roles in the formation of transported deposits. The transported deposits are classified into two groups:

(a) Clastic rocks.


(b) Non-clastic rocks.

(a) Clastic rocks:

These are detrital or fragmental reeks and are carried and deposited by mechanical means. On the basis of mode of transport and grain size, the clastic rocks are classified as follows:

(i) Rudaceous rocks:


Very coarse grained rocks where the sizes of the grains are those of boulders. These are transported in traction, i.e., by rolling or creeping. Also known as rudites.

e.g., (a) Conglomerates-fragments are rounded.

(b) Breccia-fragments are angular.

(ii) Arenaceous rocks:

These rocks consist chiefly particles of sandgrade. They are transported in saltation. Also known as arenites, e.g., sandstone, arkose, graywacke, grits etc.

(iii) Silt rocks:

Here the constituent particles are finer than common sand and coarser than clay. They are transported by suspension, e.g. loess.

(iv) Argillaceous rocks:

These are made up of clay particles, usually transported in suspension, e g, Clay, Mudstone, Shale etc.

(b) Non-clastic rocks:

These are formed due to chemical precipitation as well as by biological means. Thus they are of two types:

(i) Chemically deposited sediments, and

(ii) Organic sediments.

(i) Chemical deposition:

(1) Evaporites. It is only due to evaporation and the deposits rare like salt and gypsum.

(2) Through reaction between the components carried in solution; siliceous, calcareous, ferruginous and carbonates deposits «re produced in this way. The examples are

1. Siliceous deposits:

Chert, flint, siliceous sinter etc.

2. Calcareous and carbonates:

limestone, dolomite, calc- linter or travertine.

3. Ferruginous deposits:

Iron salts, hematite, goethite siderite etc.

(ii) Deposits of organic origin:

These are the products of accumulation of organic matter preserved under suitable conditions.

A rock of organic origin may be built up directly from the beginning aS a quite solid material as in the case of coral rocks and algal limestones. In other cases the deposition may be bio-chemical or bio-mechanical. They are mainly of five types:

1. Siliceous:

Radiolarian ooze; diatoms are lowly plant organisms which secrete silica.

2. Calcareous:

Due to biomechanical processes as well «s biochemical processes these deposits are formed. Fossiliferous lime­stone, chalk, marl etc. are the examples.

3. Phosphatic:

As calcium phosphate is utilized by certain organisms, especially fish and brachiopoda, the remains of these organisms accumulate on the sea-floor forming phosphatic deposits.

‘Guano’ is directly of organic origin.

4. Ferruginous:

By the activities of bacteria, e.g., Bog-iron ore.

5. Carbonaceous:

Coal formations.

Besides the above, the consolidated pyroclastic rocks are also considered as sedimentary rocks.

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