Unconformity - opengeology.in

An unconformity is a plane of discontinuity that separates two rocks, which differs in age. The younger of these rocks are nearly always of sedimentary origin and must have been deposited on the surface of the older rock, which is a surface of erosion.

Formation of unconformity :

The formation of unconformity may be defined by three main processes like erosion, deposition and tectonic activity. The stages involves –

The formation of older rocks.
Upliftment and subaerial erosion of the older rocks
The formation of a younger succession of beds above the surface of erosion of older rock

The unconformity can also be defined as a surface of erosion or non-deposition that separates younger strata and older strata. Generally it is a planer feature/structure.

Classification of unconformities :

Unconformities are classified into many types, few important of them are as follows –

Angular unconformity : If the beds beneath the erosional surface are folded or tilted so that there is an angular discordance between the younger and older beds, the contact is called and Angular unconformity. Here both rocks are of sedimentary origin, but the attitudes of older and younger rocks are different.

Disconformity : It is also known as Parallel unconformity as of the fact that the beds above and below the the plane of discontinuity are parallel or having same attitude. This is formed when there is less magnitude of diastrophism or disturbance between the depositional events take place.
Para-conformity : It is a type of unconformity where separation is a bedding plane, also known as pseudo-unconformity.
Non-conformity : This is a kind of discontinuity where older rocks are either igneous or metamorphic origin that is overlain by the sedimentary rock units. The essential concept being that prolonged erosion must have occurred to expose the intrusive before burial. It is also known as Heterolithic Unconformity.
Blended unconformity : It is a surface of erosion, which may be covered by a thick residual soil that grades into the underlying bed rock. Younger sediments deposited above the surface may incorporate some of the residual soil and a sharp contact may be missing.

Besides the above classification, there are some other relationships between the overlying and underlying strata which are also useful in determination of other geological past phenomenon –

On-lap : It is often found that the younger bed completely covers up and advances beyond the limits of the underlying rocks. It is best observed during the marine transgression.
Off-lap : It is reverse of On-lap , lower beds of the upper series extends further than the younger ones due to marine regression.

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