An unconformity is a plane of discontinuity that separates two rocks, which differ notably in age.
The younger of these rocks arc nearly always of sedimentary origin and must have been deposited on the surface of the older rock, which is a surface of erosion. Therefore, an unconformity is regarded as a planar structure.
The formation of an unconformity may be attributed to three main processes like erosion, deposition and tectonic activity. Its development involves the following stages
(i) The formation of older rocks.
(ii) Upliftment and subaerial erosion of the older rocks.
(iii) The formation of a younger succession of beds above the surface of erosion.
An unconformity usually shows the following characteristics
(a) The overlying strata differ from the underlying ones with respect to their lithological composition, thickness and order of super-position.
(b) There is a difference in age, indicated by the fossil assemblages, between the overlying and underlying beds.
(c) The attitude (i.e. dip & strike) of the beds above the plane of discontinuity differs from those below it.;
(d) In most cases, a conglomerate horizon is present at the bottom of the younger set of beds.
Types of Unconformities
Four main types of unconformities have been recognized as follows.
(a) Angular unconformity
In this case, the beds beneath the surface of erosion are folded or tilted so that there is angular discordance between the younger and older beds.
Thus, the attitude of the rocks above and' below the plane of discontinuity differs from each other. The contact is known as angular unconformity. Here, both the underlying and over lying rocks are of sedimentary origin.
It is also known as Parallel unconformity as the bedding above the below the plane of discocntinuity are parallel to each other.
The lower as well as the upper series of beds dip at the same amount and in the same direction. This unconformity is developed when there is a lesser magnitude of diastrophism or disturbance between the deposition of the two succession of strata.
This is known as non-depositional unconformity It is similar to disconformity, but is local in extent and therefore the name.
The time involved is also short, so the age difference between the overlying and the underlying beds is very less. Such an unconformity is also known as Diastem.
This term is commonly applied to unconformable structures in which the older formation made up essentially of plutonic rocks, while the younger formations are either sedimentary rocks or lava flows.
It is perhaps the prolonged erosion which must have exposed the intrusive before burial. It does not have any tectonic significance.
The structures formed through tectonic movements often control the local pattern of drainage, the course of rivers, shaping the outlines and details of the coasts etc.
As for example, joints are the planes of weakness in the rocks, which are readily opened up by weathering thus produce topographic effects.