How the Sedimentary Rocks Serve as an Index of Nature?

Thus the sedimentary rocks serve as an index of the nature of the movements which have occurred.

As a direct result of diastrophism, irrespective of the size or scale, four major groups of structures are produced, which are as follows:

1. Folds

2. Faults.

3. Joints.

4. Unconformities.

Originally the sedimentary rocks arc horizontal, but owing to Ynovements of the earth’s crust, they are often lilted out of their original position. Sometimes the tilting of the beds even takes the originally horizontal beds to a vertical position, but more often the beds, due to unequal uplift or subsidence of the crust (in any particular region) moves into an inclined position. The attitude of an inclined bed is defined by two elements, namely Dip and Strike.

Dip is essentially the angle of inclination of a bed with respect to a horizontal plane. The dip of a bed has got two components like direction as well as magnitude. The angle of dip varies between 0″ (For horizontal beds) to 90° (For vertical beds). The direction of dip is the geographical direction, along which a bed has maximum slope.

Strike the line of intersection of a bed with the horizontal plane.

Folds

Due to compressional forces, rock-strata are crumpled, forming wavy undulations on the surface of the earth which are known as folds.

The process of folding is also known as plicated deformations. Folds are of various size and form and rarely occur as isolated feature. They have widespread occurrences.

(i) Crests and troughs These are the convex and concave portions of the wavy undulation respectively. Thus the wavy undulations are formed of a series of alternate crests and troughs.

(ii) Core is the inner part of a cold.

(iii) Limb is the stretch of the rock-beds lying between any crest and its adjacent trough i.e. the sides of the fold. It is also known as flank.

(iv) Axial plane is an imaginary plane which divides the fold as symmetrically as possible. It is, therefore, that any point on the axial plane is at equidistance from both the limbs. The axial plane may be vertical, inclined or horizontal or even a curved surface.

(v) Hinge is a line running through the points of maximum curva­ture of any of the beds forming the fold. It is also known as fold bend or flexure.

(vi) Plunge of the fold is the angle the fold axis makes with the horizontal. The fold where the axis is not horizontal is known as plunging fold.

(vii) Pitch is the angle between a horizontal line and the azis meas­ured on the axial plane. Pitch and plunge coincide when the axial plane is upright.

(viii) Angle of a fold i,e the interlimb angle is formed by the lines continuing the limbs of the fold upto the place of intersection.

(ix) Height of the fold is the vertical distance between the hinges of the adjacent anticline and syncline. It is also known as the amplitude of a fold.

(x) Width of a fold is the distance between the axial lines of two adjacent anticlines or synclines.

(xii) Nose or curve of the fold is the part of the fold adjacent to the hinge line. It is also known as hinge-area

L = Width of the fold (BC)

H = Height of the fold

A = Axis of the fold

0 = Angle of the fold

AC & AB = Limbs of the fold

B & C = Troughs

Types of Folds

Several types of folds have been recognized on different basis as follows:

(a) Appearance in Cross-section

Folds like antiform, synform, anti- clinorium, synclinorium, anticlinal bend (monocline), synclinal bend (structural terrace), anticline and syncline are included in this category (The details of these folds are beyond the scope of this book). In fact all these folds are simple or complex modification of anticlines and synclines.

Anticline

It is a folded structure where the strata are convexed upwards and the limbs commonly slope away from the axial plane. Here, while relatively younger beds are found upwards, the older rocks constitute the core i.e. the centre of curvature

Syncline

This fold is concave upwards and the limbs commonly dip towards the axial plane. In this case progressively younger beds are found towards the centre of curvature of the fold.

(b) Symmetry of fold.

On this basis folds have been classified as symmetrical, asymmetrical, recumbent, isoclinical, overturned and homoclinal folds. In these folds, the axial plane may be either vertical, inclined or horizontal.

(c) Thickness of limb

Three important forms of fold have been iden­tified on this basis viz. parallel fold, similar fold and suprateneous fold.

(d) Inter-limb angle

Folds like open or gentle fold, closed fold, tight fold and cylindrical fold etc. are recognized on the basis of the interlimb angle while the inter-limb angle is more than 70° in the case of open folds it ranges between 30° to 70° in closed and is below 30° in case of tight folds.

(e) Attitude of folds

On this basis folds like plunging, non plunging doubly-plunging, perclinal folds and reclined folds have been recognized.

(f) Mechanism of folding

According to the mechanism of folding the following four types of folds have been identified, viz. Drag-fold, Flexure-fold. Shear-fold and Flow-folds.

(g) Mode of origin

On the basis of the mode of origin folds may be classified as tectonic and non-tectonic folds. While generative folds, culminations and depressions are of tectonic origin, diapiric folds, cambering and valley bulging etc. are folds of non-tectonic origin.

Apart from the above types of folds, there are special types of folds like chevron folds, box or coffer folds, fan-folds, kink-bands, geanticlines etc also.

Faults or Disujunctive Dislocations

These are well-defined cracks along which the rock-masses on either side have relative displacements Faults are known as disjunctive dislocations.

Fault Terminologies

(i) Strike of the fault is the trend of a horizontal line in the plane of the fault.

(ii) Dip of the fault is the angle between a horizontal surface and the plane of the fault. It is measured on a vertical plane that strikes at right angles to the fault.

(iii) Hade is the complementary angle of dip i.e. the angle which the fault plane makes with the vertical plane. In other words, hade = 90°- Dip.

(iv) Fault plane is the plane along which the displacement takes place.

(v) Fault scrap is an upstanding structure with a steep side which is formed due to the relative displacement on either side of the fault line.

(vi) Hanging wall and foot wall. In case of an inclined fault, one of the dislocated block apears to rest on the other.

The former is called the hanging wall and the latter the foot wall. In a more simple way, it can be stated that the fault covers the footwall and lies beneath the hanging wall or in other words, the part above the fault plane is known as the hanging-block and the one below it is the foot wall.

(vii) Up-thrown and down-thrown side

Of the two blocks lying on either side of the fault plane, one appears to have been shifted

downwards in comparison with the other the former is there­fore known as the ‘down thrown side and the latter ‘upthrown side’.

(viii) Throw and heave

The throw of a fault is the vertical component of the apparent displacement of a bed measured along the direction of dip of the fault. Similarly, the horizontal component of the apparent displacement is known as heave or gape.

(ix) Stratigraphic throw

If the same bed occurs twice because of faulting, the perpendicular distance between them measured along a vertical section, at right angles to the strike of the fault is known as stratigraphic throw.

(x) Net-slip is the total displacement due to a fault.

Types of Faults

Faults are classified mainly geometrically or genetically. The geomet­ric classification of faults is strictly based on the attitude of the faults. The major bases of geometric classification are as follows :

(a) Rake of the net-slip (Strike-slip, Dipslip, Diagonal-slip faults).

(b) Attitude of the fault, relative to the attitude of the adjacent rocks (Strike fault, Dip fault, Diagonal fault, Bedding fault, Longitudi­nal and Transverse faults)

(c) Pattern of faults (Parallel, Step, Arcuate, Radial, Enechelon faults).

(d) Angle at which the fault dips (High-angle and Low-angle faults).

(e) Apparent movement on the fault (Normal fault, Reverse fault).

In so far as the genetic classification of faults is concerned, it is assumed that displacements are caused by some stresses. Three types of principal stresses have been assumed, of these two are horizontal and the third one is vertical and due to gravity alone.

The variety of faults depends on the orientation of the three principal stresss, three sets of conditions in which all the stresses are compressional may arise; ac­cordingly three types of faults originate viz. Normal faults, Thrust faults and Transcurrent fault.

Normal Faults :

In these faults, the maximum stress is vertical, and both the mean as well as the minimum stress is horizontal. In such cases, the hanging wall has moved relatively downwards and the fault plane dips toward the downthrown side when the plane of the fault is vertical, the fault is called vertical-fault.

The normal faults are also known as gravity or ‘tensional faults. To this category belong the Horsts, Grabens, Dip-slip faults, Antithetic and Synthetic faults, Parallel, Bedding, Step-faults, etc.

Thrust Faults :

In these faults, the maximum as well as the mean stress are horizontal and the minimum stress is vertical. In such cases, the hanging wall moves relatively over the footwall. where the dip of such faults is more than 45°, they are known as Reverse faults.

In other words, the low angle reverse faults are known as thrust-faults. This type of fault is regarded as a shear deformation under the conditions of compression of the earth’s crust .Nappes, Imbricate or Schuppen structures, etc. belong to this category of faults.

Strike Slip Faults :

These are the faults, where the maximum and minimum stress are horizontal and the mean stress is vertical. In such cases, the displacement remains essentially parallel to the strike of the fault.

These are also known as transcurrrent, transform, wrench or tear- faults. In case of tear faults the strike of the fault is transverse to the strike of the country rock but the displacement is along the strike of the fault plane.

But, in case the strike of the fault plane is parallel to the strike of the adjacent rocks and the displacement is along the strike of the fault plane, it is known as a rift fault.