Sedimentary Rocks: Formation, Classification and Uses | Petrology

After reading this article you will learn about:- 1. Formation of Sedimentary Rocks 2. Classification of Sedimentary Rocks 3. Primary Structures 4. Engineering Uses.

Formation of Sedimentary Rocks:

The process of formation of sedimentary rocks is as follows:

(i) Erosion of the existing rocks,

(ii) Transportation of the eroded material, and

(iii) Deposition of transported material into massive rocks by welding or cementation.

Classification of Sedimentary Rocks:

1. Sedimentary rocks may be classified on the basis of grain size into three types:

(a) Rudaceous (in Latin Rudous—pebbles):

Rudaceous rocks are those rocks, which are made up of particles having grain size more than 2 mm in diameter.

Examples:

Breccia, Conglomerate.

(b) Arenaceous (in Latin Arena—sand):

Arenaceous rocks are those rocks, which are made up of particles having the gain size in between 1/10th-2 mm diameter.

Example:

Sandstone.

(c) Argillaceous (in Latin Argilla—clay):

Argillaceous rocks are those rocks, which are made up of finest clay particles i.e., size of particles is less than 1/10 mm in diameter.

Example:

Shale.

2. Sedimentary rocks are classified on the basis of their origin into three types:

(a) Mechanical origin:

All those sedimentary rocks, which are formed by the mechanical action of the denuding agents.

Examples:

Breccia and Conglomerate.

(b) Chemical origin:

All those sedimentary rocks, which are formed by the chemical action of the denuding agents.

Example:

Limestone.

(c) Organic origin:

All those sedimentary rocks, which are formed by the activity of the organisms (both plants and animals).

Example:

Fossiliferous limestone.

Primary Structures in Sedimentary Rocks:

Structural features of sedimentary rocks are of great value in determining their origin.

The important sedimentary structures are as follows:

1. Stratification:

The deposition of sediments into layers or beds is called stratification. The thickness of a single bed may vary from a few cms to many kms.

The plane divides into different beds are bedding planes:

The stratification is formed due to the following:

(a) Difference in kind of material deposited.

(b) Difference in size of particle deposited.

(c) Difference in colour of material deposited.

2. Lamination:

Thin bedding less than one cm in thickness is called lamination. It is usually found in the fine-grained sedimentary rocks like Shale.

3. Cross bedding or Current bedding or false bedding:

It is also called current bedding or false bedding. Cross bedding is the minor bedding or lamination, which lies at an angle to the figure. These structures are found in shallow water and wind formed deposits.

4. Graded bedding:

When sedimentary bed shows a gradation in grain size from coarse below to finer above, it is said to be graded as shown in Fig. 3.17, such type of bedding is called graded bedding. This structure is commonly found in Greywacke.

5. Ripple marks:

Ripple marks are the wavy undulations, which may be seen in the surface of sedimentary deposit. These are produced by the action of the waves and current in shallow water. They may also formed on the deposits from the winds.

Ripple marks are of two types.

They are:

(i) Symmetrical ripple marks or Current ripple marks, and

(ii) Asymmetrical ripple marks or Oscillation ripple marks. Oscillation ripple marks are useful in determination of top and bottom beds.

6. Marks in designated sediments:

The sediments, which have undergone repeated wetting and drying cause mud cracks.

7. Concretion:

These are various shaped mass or nodules of minerals found within sedimentary rocks. Their shape may be elliptical or oval or irregular. Concretions generally consist of CaCO₃, SiO₂ and often possess an internal radiation or concentric structure. They are formed by the deposition of mineral matter from percolation solutions about a nucleus.

Engineering Uses of Sedimentary Rocks:

The strength of sedimentary rocks depends on the strength of its grains or sediments (constituting the rocks) and cementing material. Thus, the selection of a sedimentary rock for any building or project should be done carefully based on the strength, colour and appearance of its grains as well as the cementing material.