When the sedimentary rock beds are found to have been deposited without interruption, they are said to be “conformable”. An Unconformity are formed when there is break in sedimentation, and it can be defined as a surface of erosion or non-deposition occurring within a sequence of rocks, or an old erosion surface which separates younger series of rocks from the older series of rocks. It indicates a gap or an interval of time or a hiatus in the geological history of the area during which the normal process of deposition was interrupted.
Origin of Unconformity.
An unconformity may be developed due to three main processes like erosion, deposition and tectonic-activity. The stages of its development involves.
- Formation of older rocks.
- Upliftment and erosion of the older rocks.
- Formation of a younger succession of beds above the surface of erosion.
Types of Unconformities
Unconformities have been classified into various types on the basis of the relationship between the underlying and overlying beds of rocks and their attitude. Various type of unconformities are as followed-
- Angular Unconformity: The angular unconformities occur when the older series of beds have been tilted, deformed and eroded before the deposition of younger beds. In this case both the underlying and overlying rocks are of sedimentary origin, but the attitude of the rocks above and below the plane of discontinuity differs from each other.
- Disconformity: It is the type of unconformity in which the beds lying below and above the surface of erosion (or non deposition) are almost parallel. In other words, there is no angular variation in the deposition of the rock of the entire sequence. The are also known as” Parallel-unconformity”.
- Nonconformity: It is commonly seen in the structures in which the older underlying formation are of plutonic igneous rocks and sedimentary or volcanic rocks as the younger overlying rocks. In such cases the surface of contact have different mode of formations.
- Local Unconformity: It is similar to disconformity, but it is local in extent and the time involved is also short. It represent a short period of non-deposition. In this type of unconformity the age difference between the underlying and overlying formations is very less.
- Regional Unconformity: When an unconformity is traced over a large area, extending from hundreds of kilometers, it is called a regional unconformity. It is generally angular unconformity type and is of great significance in historical geology as it establishes the genetic relationship of rocks of a very wide area.
- Blended Unconformity: It is surface of erosion which may be covered by a thick residual soil that grades into the underlying bedrock. Younger sediments deposited above he surface may incorporate some of the residual-soil and asharp contact may be lacking.
How To Recognise Unconformities in Field
The factors which help in recognizing the Unconformities in the field, are as follows:
- Difference in Structure: The group of rocks on either side of an unconformity may show aructural discordance.
- Difference in Fossil Record: Rocks beds lying below an unconformity surface commonly contain faunas which do not occur in the rocks above. The fossils in the younger series may be of higher evolutionary rank.
- Different Environment of Depositions: Association of such rock beds which were formed under contrasting conditions, indicates the presence of unconformities.
- Difference in Grade of Metamorphism: The younger series of rocks are likely to be less metamorphosed than the rock of older series.