Faulting and uncomformity

Faulting

The fracture in the crust along with the tectonic movement of the earth’s crust is known as faulting. When the enormous stress builds and pushes the large intact rock masses beyond their yield limit, then the faulting of the surface is likely to occur. The plane that extends into the earth and along which the slippage occurs is known as fault plane and the angle from the horizontal that the fault plane makes is known as fault dip. The map direction that the fault takes is called the strike and measured east or the west of true north. Generally, the two walls are distinguished, the footwall and the hanging wall. The hanging wall moves horizontally, vertically or in the both directions relative to the footwall. The hanging wall is above the fault plane while the footwall is below. The steep face of an exposed block is called fault scarp whereas the fault line is the trace of the fault along the surface.

Classification of the fault

There are normally three types of faults which are as follows:

1. Normal fault (vertical fault or gravity fault)

If the hanging wall is below the footwall or if the hanging wall is downward with respect to the footwall then, such type of fault is called the normal fault or the vertical fault or the gravity fault.

2. Reverse fault (Thrust Fault)

If the hanging wall is upward with respect to the footwall then, the fault is called the reverse fault or the thrust fault.

3. Strike-slip fault (transform fault)

If the hanging wall neither moves up nor goes down but moves relatively parallel to the foot wall, such type of fault is called the strike-slip fault or the transform fault. In such type of fault, the vertical force and the horizontal force are equal.

Joints

The joint can be defined as the fracture dividing the rocks into two or more sections where there has been no lateral movement in the plane of the fracture. The joints normally have a regular spacing related to either the mechanical properties of the individual rock or the thickness of the layer involved. Generally, the joints occur as the sets, with each set consisting of the joints sub-parallel to each other.

The joints are formed in the solid and hard rocks that are stretched such that its brittle strength is exceeded. When the joints are formed, the rock fractures in the parallel plane to the maximum principle stress and perpendicular to the minimum principal stress which leads to the development of the single sub-parallel joint set. The continued deformation may lead to the development of one or further joint sets. The joints have the relatively constant spacing and the spacing is roughly proportional to the thickness of the layer.

The joints can be marked easily with the weathered rocks especially in the soluble rocks like limestone. The solution by water percolating through the joints has led to the formation of the caves and underground rivers. Normally, the distance between the joints varies from few centimeters to the hundreds of the meters. The sedimentary rocks show the two sets of joints at right angles to one another, in which each extending down perpendicular to the bedding of rocks in which one set extend to the tip and the other set extend to the strike. In the igneous rocks, joints are quite irregular but in the granite, the two vertical sets forming right angles to one another on the top surface and another set of cross joints approximately horizontal occur which is the effect of the weathering.

Types of joints

The joints are generally classified according to their process of formation. The known joints yet are:

Tectonic joints

During the deformation episodes whenever the differential stress is high enough to induce the tensile failure of the rock, irrespective of the tectonic regime the joints formed on such condition is known as tectonic joint. They are often formed at the same time as the faults. The measurement of the tectonic joint patterns can be useful in analyzing the tectonic history of an area because of the information they give on stress orientation at the time of the formation.

Unloading joints

The joints are commonly formed when the uplift and the erosion removes the overlying rocks thereby reducing the compressive and the load and allowing the rock to expand laterally. Joints related to the uplift and the erosional unloading have orientations reflecting the principle stresses during the uplift and the care needs to be taken when attempting to understand the past tectonic stress to discriminate, if possible between the tectonic and the unloading joints.

The exfoliation joints are the special cases of the unloading joints formed at and parallel to the current land surface in the rocks of the high compressive strength.

Cooling joints

The joints formed from the cooling of the hot masses, particularly lava is known as cooling joints. These cooling joint are most commonly expressed as the vertical columnar jointing. Such cooling joint systems are typically polygonal in nature due to the cooling introducing stresses.

Unconformities

During the process of formation of the structure in the earth, many times the rocks are tilted, folded and may be altered and sometimes there may be the break in the rock where no sediments were laid down or the rock may be eroded by the water. Such types of can break representing the erosion or the non-deposition can either be easy or difficult to see, depending upon the types of the rocks being observed. It was first described by the James Hutton in his rock record as he was impressed by the place Siccar Point in Scotland where some of the rocks were laid down horizontally, then tilted and eroded and more rocks were deposited on the top of them. This scenario represented a large group in time between the first set of rocks laid down, and the tilted and the second set of rocks deposited. Such gaps in the stratigraphic record are called uniformities. The unconformity represents the break in sequence in which deposition ends and the erosion begins. These breaks can be relatively shorter in time and can last for the million of the years.

There are different types of unconformities in which two are represented according to the Hutton first saw at Siccar Point in which the first one is angular unconformity and in this unconformity, the sequences of the rocks beneath the unconformity are tilted with respect to the rock sequences above the unconformity. Such type of unconformity usually represents the large gap in the geologic record since the tilting of the rocks and their gradual erosion time.

Disconformity

Generally, if the rock sequences of up and down of the unconformity is more or less parallel then it is said to be disconformity.

Angular disconformity

If the stratification of upper and lower sequence is defined at the certain angle then it is known as angular disconformity. It is the result of the erosion of the tilted strata of sedimentary rock. in this, the tilted beds are covered by horizontal sandstone but the older rocks were not tilted as in disconformity.

Non-conformity

If one or the rock sequence is made up of igneous rock then, this type of unconformity is known as non-conformity. In this sequence, an igneous rock may be younger strata or older strata.

References

Keller, E.A. Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company, Bell, and Howell Company, 1985.

Mahapatra, G.B. Textbook of Physical Geology. Shahadra,Delhi-110032: CBS Publishers and Distributers Pvt.Ltd., 1992.